integrated interpretation of transient electromagnetic and ... · integrated interpretation of...

Integrated interpretation of

transient electromagnetic and

seismic data

Joakim Arnfeldt Westergaard

19962527

Department of Earth Sciences,University of Aarhus,DenmarkSupervisors: Kurt I. Sørensen and Esben AukenSeptember 2, 2003

i

Summary

Summary in English

Based on transient electromagnetic data, seismics, exploratory drillings andlogs, a geological interpretation of the area around Tinglev in South Jutlandhas been made. The geology of this area is a combination of Quaternary melt-water sand and till, Miocene clay, silt and sand, and Paleogene clays. Thetop Cretaceous boundary was found at great depth. Indications of salinepore water were found. The area has been highly influenced by the TønderGraben and the salt tectonics involved with this. Layer boundaries south ofthe graben have been found to dip towards this.

Laterally constrained inversion was put to practical use. The inversion rou-tine proved its advantages and could prove very useful in future TEM inter-pretations. Further developments, such as research into constraint factorsand programs to aid in the model building, are however needed before it isready for commercial use.

Referat pa dansk

Baseret pa transient elektromagnetiske data, seismik, boringer og logs, er derer blevet lavet en geologisk tolkning af omradet omkring Tinglev i Sønderjylland.Omradets geologi er en kombination af kvartært smeltevandssand og moræneler,miocænt silt, sand og ler, og palæogent ler. Top Kridt grænsen fandtes pastor dybde. Der blev fundet indikationer pa saltholdigt porevand. Tøndergraven og den dermed involverede salttektonik har haft en stor indflydelsepa omradet. Det blev fundet, at laggrænser syd for graven dykkede op imoddenne.

Laterally constrained inversion blev anvendt. Inversionsrutinen viste sinefordele og kunne vise sig at være meget brugbar i fremtidige TEM tolkninger.Yderligere udvikling, som forskning i band og programmel til opbygning afmodeller, er dog nødvendige, før den er klar til kommerciel brug.

ii

Foreword

This Master of Science Thesis marks the closure of my studies at the De-partment of Earth Sciences, University of Aarhus, a time I know I will lookback on one day with a great feeling of joy. I have had great fun, met manywonderful people, all while learning what I hope will be the foundation ofmy future. It has also been a time of trials and mistakes, especially the factthat I had to give up my previous thesis.

Where I am today is not only a result of my own work but also of helpfrom a number of people, which I an indebted to:

Jens Danielsen, Nikolaj Foged and Anders Vest Christiansen for letting mebenefit from their knowledge of the transient electromagnetic method andtheir equally important their great and never ending help with data inter-pretation.Lars H. Jacobsen for discussions on TEM and for the time we have spenttogether in our far too small office.Tom Hagensen and David Lundbæk Hansen for help with Matlab and Latex.Martin Bak Hansen for letting me benefit from his experience with seismol-ogy.Gitte Aagaard Hollenbo for helping me for three days without pay whengathering data in September 2002, for being a great help in discussing ge-ological aspects with me, for letting me benefit from her critical questions,and last, but certainly not least, for standing by me through the last hectic(and far too hot) summer months while I was writing this thesis.Flemming Jørgensen for recommending me literature and discussing geolog-ical aspects with me.Kurt I. Sørensen and Esben Auken for supervision and support.Steen Thomsen, Sønderjylland County, for helping me obtain articles, dataand various other information.Watertech a/s for letting me use their digital Protem 47 receiver.And last but not least the rest of the employees at the HydroGeophysicsgroup for great coffee breaks.

Aarhus September 2, 2003

Joakim Arnfeldt Westergaard

19962527

iii CONTENTS

Contents

Summary iSummary in English . . . . . . . . . . . . . . . . . . . . . . . . . . iReferat pa dansk . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Foreword ii

1 Introduction 1

2 Electromagnetic theory 32.1 The Maxwell equations . . . . . . . . . . . . . . . . . . . . . . 32.2 The constitutive equations . . . . . . . . . . . . . . . . . . . . 32.3 Maxwell’s equations in frequency domain . . . . . . . . . . . . 42.4 The Wave equations . . . . . . . . . . . . . . . . . . . . . . . 42.5 Boundary conditions . . . . . . . . . . . . . . . . . . . . . . . 62.6 Solutions to the wave equations . . . . . . . . . . . . . . . . . 72.7 Schelkunoff potentials . . . . . . . . . . . . . . . . . . . . . . . 92.8 General solutions . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.8.1 Vertical magnetic dipole . . . . . . . . . . . . . . . . . 102.8.2 Horizontal circular loop . . . . . . . . . . . . . . . . . 102.8.3 Measurements in the center of a horizontal loop . . . . 11

2.9 The transient response . . . . . . . . . . . . . . . . . . . . . . 11

3 The Transient Electromagnetic Method 143.1 Time domain methods in general . . . . . . . . . . . . . . . . 143.2 The Transient Electromagnetic Method . . . . . . . . . . . . . 14

3.2.1 The principle of the TEM Method . . . . . . . . . . . . 153.3 The TEM Method equipment . . . . . . . . . . . . . . . . . . 17

3.3.1 The transmitting system . . . . . . . . . . . . . . . . . 173.3.2 The receiving system . . . . . . . . . . . . . . . . . . . 19

3.4 Filters and waveforms . . . . . . . . . . . . . . . . . . . . . . 193.5 Sources of error . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.5.1 Instrumental effects . . . . . . . . . . . . . . . . . . . . 213.5.2 Geometrical effects . . . . . . . . . . . . . . . . . . . . 213.5.3 Electromagnetic noise . . . . . . . . . . . . . . . . . . 213.5.4 Geological noise . . . . . . . . . . . . . . . . . . . . . . 223.5.5 Cultural noise . . . . . . . . . . . . . . . . . . . . . . . 23

3.6 The HMTEM method . . . . . . . . . . . . . . . . . . . . . . 253.6.1 The HMTEM Method equipment . . . . . . . . . . . . 26

iv CONTENTS

4 The seismic method 274.1 Seismology in general . . . . . . . . . . . . . . . . . . . . . . . 274.2 General discussion on reflection seismology . . . . . . . . . . . 284.3 Sources of errors in reflection seismology . . . . . . . . . . . . 30

4.3.1 Problems concerning field work and processing . . . . . 304.3.2 Multiples . . . . . . . . . . . . . . . . . . . . . . . . . 304.3.3 Dipping reflectors . . . . . . . . . . . . . . . . . . . . . 314.3.4 Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.4 Reflection seismic data processing . . . . . . . . . . . . . . . . 334.4.1 Muting . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.4.2 Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . 334.4.3 Migration . . . . . . . . . . . . . . . . . . . . . . . . . 344.4.4 CDP stacking . . . . . . . . . . . . . . . . . . . . . . . 344.4.5 NMO correction . . . . . . . . . . . . . . . . . . . . . . 34

5 Exploratory drillings and logs 355.1 Exploratory drillings . . . . . . . . . . . . . . . . . . . . . . . 355.2 Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.2.1 Gamma-ray logs . . . . . . . . . . . . . . . . . . . . . . 355.2.2 Resistivity logs . . . . . . . . . . . . . . . . . . . . . . 365.2.3 VSP-logs . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6 Inversion of electromagnetic data 376.1 The linear case . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.1.1 Estimation errors . . . . . . . . . . . . . . . . . . . . . 386.1.2 Prior information and constraints in the linear case . . 39

6.2 The non-linear case . . . . . . . . . . . . . . . . . . . . . . . . 406.2.1 Prior information in the non-linear case . . . . . . . . . 416.2.2 Inversion . . . . . . . . . . . . . . . . . . . . . . . . . . 416.2.3 Prior depth values and constraints . . . . . . . . . . . 42

6.3 Em1Dinv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

7 Data and data acquisition 457.1 HMTEM soundings . . . . . . . . . . . . . . . . . . . . . . . . 467.2 Seismic data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487.3 Exploratory drillings . . . . . . . . . . . . . . . . . . . . . . . 487.4 Logs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8 Programs 518.1 SiTEM and Semdi . . . . . . . . . . . . . . . . . . . . . . . . 518.2 Read emo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

v CONTENTS

8.3 MapInfo and Geobase . . . . . . . . . . . . . . . . . . . . . . . 548.3.1 The MapInfo workspaces . . . . . . . . . . . . . . . . . 55

9 Geology of southern Jutland 569.1 The Pre-Cretaceous period . . . . . . . . . . . . . . . . . . . . 569.2 The Cretaceous period . . . . . . . . . . . . . . . . . . . . . . 569.3 The Tertiary period . . . . . . . . . . . . . . . . . . . . . . . . 57

9.3.1 The Paleogene period . . . . . . . . . . . . . . . . . . . 599.3.2 The Neogene period . . . . . . . . . . . . . . . . . . . 60

9.4 The Quaternary period . . . . . . . . . . . . . . . . . . . . . . 649.4.1 The Saalian Ice Age . . . . . . . . . . . . . . . . . . . 659.4.2 The Eemian interglacial stage . . . . . . . . . . . . . . 669.4.3 The Weichselian Ice Age . . . . . . . . . . . . . . . . . 669.4.4 The Post Glacial period (Holocene) . . . . . . . . . . . 68

10 Geophysical interpretation of TEM data 7010.1 An introduction to MCI and LCI . . . . . . . . . . . . . . . . 7010.2 Results from the interpretations - general discussion . . . . . . 7110.3 Results from MCI and single-site interpretations . . . . . . . . 73

10.3.1 Interpretation of profiles 1, 2 and 4 using MCI . . . . . 7310.3.2 Interpretation of profile 3 using MCI . . . . . . . . . . 76

10.4 Results from LCI interpretations . . . . . . . . . . . . . . . . 7710.4.1 Interpretation of profiles 1, 2 and 4 using LCI . . . . . 7910.4.2 Interpretation of profile 3 using LCI . . . . . . . . . . . 8010.4.3 Interpretation of profile 5 using LCI . . . . . . . . . . . 82

10.5 Results from the interpretations - an overview . . . . . . . . . 82

11 Integrated interpretations 8411.1 The logs of DGU no. 168.1378 . . . . . . . . . . . . . . . . . . 8511.2 Geological interpretation of profiles 1, 2 and 4 . . . . . . . . . 8811.3 Geological interpretation of profile 3 . . . . . . . . . . . . . . . 9411.4 Geological interpretation of the field area - an overview . . . . 96

12 Conclusions and discussions 9912.1 The geological interpretation . . . . . . . . . . . . . . . . . . . 99

12.1.1 Suggestions for future work . . . . . . . . . . . . . . . 10012.2 Experiences gained from using LCI . . . . . . . . . . . . . . . 101

vi CONTENTS

Appendices

A Position of the soundings 107

B Position of exploratory drillings 108

C Details about the equipment 109C.1 Gate Center Times for the UH, VH and Hi Segments . . . . . 109C.2 Other parameters . . . . . . . . . . . . . . . . . . . . . . . . . 110

D The CD-ROM 112

E 2D sections with TEM soundings, seismics and exploratorydrillings 113E.1 Figure of the model area including profiles . . . . . . . . . . . 113E.2 MCI and single-site interpretations of profiles 1 through 4 . . . 113E.3 LCI interpretations of profiles 1 through 4 . . . . . . . . . . . 113E.4 LCI interpretations of profile 5 . . . . . . . . . . . . . . . . . . 113E.5 The geophysical interpretation of TEM data . . . . . . . . . . 113E.6 The geological interpretation . . . . . . . . . . . . . . . . . . . 113

vii LIST OF FIGURES

List of Figures

2.1 Electric and magnetic fields as a function of time. . . . . . . . 92.2 Dipole moment of a large, horizontal loop. . . . . . . . . . . . 112.3 The step and impulse response. . . . . . . . . . . . . . . . . . 123.1 Contour plots of the current maximum. . . . . . . . . . . . . . 163.2 The transmitted current pattern. . . . . . . . . . . . . . . . . 173.3 Central loop and offset arrays. . . . . . . . . . . . . . . . . . . 193.4 The waveform. . . . . . . . . . . . . . . . . . . . . . . . . . . 203.5 The effect of a polarizable layer on a TEM profile. . . . . . . . 233.6 Capacitive and galvanic couplings. . . . . . . . . . . . . . . . . 243.7 Picture of the HMTEM equipment and sketch of the configu-

ration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.1 Particle motion of P- and S-waves. . . . . . . . . . . . . . . . 284.2 Ray paths of reflected and refracted waves. . . . . . . . . . . . 294.3 Different types of multiples. . . . . . . . . . . . . . . . . . . . 314.4 A dipping reflector. . . . . . . . . . . . . . . . . . . . . . . . . 324.5 The principle of diffraction. . . . . . . . . . . . . . . . . . . . 337.1 The field area. . . . . . . . . . . . . . . . . . . . . . . . . . . . 457.2 Diverging sounding curves of Ti010. . . . . . . . . . . . . . . . 467.3 Sounding Ti102 which had to be abandoned. . . . . . . . . . . 477.4 The VSP-log from the deep exploratory drilling. . . . . . . . . 498.1 View of the Read emo data residual window. . . . . . . . . . . 549.1 Stratigraphic charts of sediments found in Denmark in general

and specifically in South Jutland. . . . . . . . . . . . . . . . . 589.2 Miocene deposits. . . . . . . . . . . . . . . . . . . . . . . . . . 629.3 Tertiary transgressions and regressions. . . . . . . . . . . . . . 639.4 The Ice Ages and interglacial stages found in Denmark. . . . . 649.5 Map of the landscapes in southern Jutland and on Funen. . . 6810.1 The principle behind LCI. . . . . . . . . . . . . . . . . . . . . 7110.2 The five units of the geophysical model. . . . . . . . . . . . . . 7211.1 The logs from the deep exploratory drilling. . . . . . . . . . . 8711.2 The depth to the Tertiary deposits. . . . . . . . . . . . . . . . 93E.1 The division of soundings among the profiles. . . . . . . . . . 114

1

1 Introduction

The present Master of Science Thesis is based on data from transient elec-tromagnetic soundings, seismics and explanatory drillings. It is the result ofa close co-operation with the HydroGeophysics Group, the Department ofEarth Sciences, University of Aarhus.

Two of the new larger surveys in Denmark are the mapping of the MioceneRibe Fm and the project ”Grundvand til Sønderjylland og Schleswig - etgrænseoverskridende projekt”, whose results are presented in RingkøbingAmt et al. (1999) and Friborg et al. (2002), respectively. The former projectis a co-operation between six counties in Jutland, the latter between Sønder-jylland County’s department of groundwater and Landesamt fur Natur undUmwelt des Landes Schleswig-Holstein (henceforth referred to as LANU inthis thesis). The latter project concerned the same area and was partly basedon the same data as this thesis.

The chief purpose of this thesis is to make a geological interpretation ofthe field area using integrated interpretation of data from transient electro-magnetical soundings, seismics, logs and exploratory drillings.

Applied geophysics have been used in numerous surveys in Denmark, andalso in the above mentioned. The Transient Electromagnetic Method (TEM)was introduced in Denmark by the Department of Earth Sciences, Universityof Aarhus in the early 1990’s, and has been much used since (Jørgensen et al.(2003a), Danielsen et al. (2003), HydroGeophysics Group (2000)). It is oneof the preferred methods for locating aquifers and buried Quaternary valleys(Jørgensen et al. (2003c), Jørgensen et al. (2003b)).

Significant new progresses, e.g. the new and promising SkyTEM method,are made concerning the TEM method at present, but also the inversion ofthe data is undergoing vast developments. One of the new features is the im-plementation of Laterally Constrained Inversion (LCI), described in Aukenet al. (2003a), and the further development of this, as presented in (Chris-tiansen et al., 2002).

The secondary purpose of the thesis is to put LCI to practical use, andthereby gain experiences, which can be used in the further development.

The first two chapters of the thesis will give an introduction to the backboneof electromagnetic and seismic measurements - the theory. These theoretical

2

studies are followed by a description of the methods. Hence chapters 3, 4and 5 are dedicated to the principles behind transient electromagnetic mea-surements, seismics, exploratory drillings and logs, including various sourcesof error. Chapter 6 moves back to the theory again - only this time inversiontheory. This is the chapter where the inversion procedure, with emphasison laterally constrained inversion, is described. An introduction to the data,how it was acquired and some practical notes concerning it, will be given inchapter 7, while chapter 8 will deal with the most frequently used programs.A description of the Read emo macro, which was written for this thesis isfound here.

The two most important issues in this thesis are the geology and the inter-pretation of TEM data. Chapter 9 describes the regional geology of southernJutland. A geophysical interpretation of TEM data alone will be given inchapter 10 along with an introduction to the concepts of mutually and lat-erally constrained inversion. These chapter will form the foundation of theintegrated interpretations of chapter 11. It is in this chapter that TEM,seismics, exploratory drillings, logs and prior geological knowledge will beintegrated into a geological interpretation of the area. Finally, in chapter 12,conclusions and discussions will be given.

3

2 Electromagnetic theory

The purpose of this chapter is to give an introduction to the basic theory ofelectromagnetism. Here the derivation of the electromagnetic theory, whichis the backbone of TEM-method, towards the responses will be studied.

2.1 The Maxwell equations

The Maxwell equations describe the electromagnetic phenomenons. An elec-tromagnetic field is defined by the five following vector functions: e (electricfield intensity [V/m]), h (magnetic field intensity [A/m]), b (magnetic induc-tion [Wb/m2] or Tesla), d (dielectric displacement [C/m2]) and j (electriccurrent density). In time domain these equations can, with the aid of theabove mentioned vector functions, be written as (Ward and Hohmann, 1988):

Faraday’s law: ∇× e = −∂b

∂t(2.1)

Ampere’s law: ∇× h = j +∂d

∂t(2.2)

Gauss’ law for the electric field: ∇ · d = ρ (2.3)

Gauss’ law for the magnetic field: ∇ · b = 0 (2.4)

where ρ is electric charge density [C/m3]

2.2 The constitutive equations

Maxwell’s equations are uncoupled in time domain, but not in the frequencydomain where they can be found via a one-dimensional Fourier transformusing the constitutive equations, which, in frequency domain, can be writtenas (Ward and Hohmann, 1988):

D = ε(ω,E, r, t, T, P, . . . ) · E (2.5)

B = µ(ω,H, r, t, T, P, . . . ) · H (2.6)

4 2.3 Maxwell’s equations in frequency domain

J = σ(ω,E, r, t, T, P, . . . ) · E (2.7)

where ε, µ and σ are the dielectric permittivity, the magnetic permeabilityand the electric conductivity as functions of angular frequency ω, position r,time t, temperature T , pressure P and the electric or magnetic field intensityE1 or H. ε, µ and σ are complex which allows the phases of D and E, of Hand B and of J and E to be different.

In most cases the following assumptions are made when working with elec-tromagnetic problems (Ward and Hohmann, 1988):

1. All Media are linear, isotropic, homogenous, and possess electrical prop-erties which are independent of time, temperature, or pressure.

2. The magnetic permeability is assumed to be that of free space, i.e.µ = µ0

2.3 Maxwell’s equations in frequency domain

A one-dimensional Fourier transform of Faraday’s and Ampere’s laws (equa-tions 2.1 and 2.2) using the constitutive equations yield Maxwell’s equationsin frequency domain (Ward and Hohmann, 1988):

∇× E + zH = 0 (2.8)

∇× H − yE = 0 (2.9)

where z = iµω and y = σ + iεω is the impedivity and the admittivity.

2.4 The Wave equations

The wave equations are derived from Maxwell’s equations, and show howelectric and magnetic fields propagate through the earth. By taking the curlof equations 2.1 and 2.2 you find (Ward and Hohmann, 1988):

∇× (∇× e) + ∇×(∂b

∂t

)= 0 (2.10)

1Please note that capital letters are used when operating in the frequency domain.

5 2.4 The Wave equations

∇× (∇× h) −∇×(∂d

∂t

)= ∇× j (2.11)

As mentioned before, the Maxwell equations are uncoupled in time domain.They are, however, coupled through the constitutive equations in the nondis-persive case, i.e. where µ, ε and σ are independent of time:

d = eε (2.12)

b = hµ (2.13)

j = eσ (2.14)

Using these simplifications, Faraday’s and Ampere’s laws, and assuming thate and h are piecewise continuous and possess continuous first and secondderivatives, equations 2.10 and 2.11 reduce to:

∇×∇× e + µε(∂2e

∂t2

)+ µσ

∂e

dt= 0 (2.15)

∇×∇× h + µε(∂2h

∂t2

)+ µσ

∂h

dt= 0 (2.16)

If we use that ∇ × ∇ × f ≡ ∇∇ · f − ∇2f and the fact that ∇ · e = 0 and∇ · h = 0 for homogeneous regions, equations 2.15 and 2.16 reduce to thewave equations for electric and magnetic fields in time domain:

∇2e − µε∂2e

∂t2− µε

∂e

∂t= 0 (2.17)

∇2h − µε∂2h

∂t2− µε

∂h

∂t= 0 (2.18)

The wave equations in frequency domain, also known as the Helmholtz equa-tions, can be found via Fourier transformation with respect to time:

∇2E + (µεω2 − iµσω)E = 0 (2.19)

6 2.5 Boundary conditions

∇2H + (µεω2 − iµσω)H = 0 (2.20)

Ward and Hohmann (1988) define the wave number, k, as k2 = µεω2− iµσω.The displacement current is much smaller than the conduction current forearth materials and for frequencies lower than 105 Hz, meaning that µεω2 µσω so that k2 ≈ −iµσω. If applied to equations 2.17 through 2.20 we havein time domain:

∇2e − µσ∂e

dt= 0 (2.21)

∇2h − µσ∂h

dt= 0 (2.22)

and in frequency domain:

∇2E − iµσωE = 0 (2.23)

∇2H − iµσωH = 0 (2.24)

Equations 2.21 through 2.24 are diffusion equations, meaning that they ap-ply for earth materials. By making these approximations, the resolution ofresistors deteriorates, since the displacement current is a result of a small dis-placement in the electrons in the double layer of the resistor. This is calledthe quasi-static approximation.

2.5 Boundary conditions

Electromagnetic measurements are based on a primary field which is the re-sult of a direct current. When this current is turned off, it gives rise to asecondary distribution of charges and currents, hence a secondary magneticfield is created. Both fields must satisfy Maxwell’s equations, the equationsderived therefrom and the conditions given at the boundaries between thehomogenous regions involved in the problem, i.e. the layer boundaries.

Boundary conditions are derived from the integral form of Maxwell’s equa-tions. The complete derivation will not be stated here, but can be found inappendix A.1.2 in Ward and Hohmann (1988)

7 2.6 Solutions to the wave equations

Normal B: The normal component, Bn, of B is continuous across an inter-face separating medium 1 from medium 2:

Bn1 = Bn2 (2.25)

Normal D: The normal component, Dn, of D is continuous across an inter-face due to the accumulation of surface charge density ρs:

Dn1 − Dn2 = ρs (2.26)

Tangential E: The tangential component, Et, of E is continuous across aninterface:

Et1 = Et2 (2.27)

Tangential H: The tangential component, Ht, of H is continuous across aninterface, if there is no surface current:

Ht1 = Ht2 (2.28)

Current density J: The normal component, Jn, of J is continuous acrossan interface:

Jn1 = Jn2 (2.29)

2.6 Solutions to the wave equations

The dimensional wave equations are second order, linear, homogenous dif-ferential equations to which Ward and Hohmann (1988) focus on two setsof basic solutions. The first one is for plane waves with a sinusoidal timedependance, eiωt:

e = e+0 e−i(kz−ωt) + e−

0 e−i(kz+ωt) (2.30)

h = h+0 e−i(kz−ωt) + h−

0 e−i(kz+ωt) (2.31)

Since k is complex, it is written:

k = α − iβ (2.32)

in which α and β are both real. e+0 and e−

0 are the amplitudes of the electricwave propagating at time t = 0 in the positive and negative z-direction, re-spectively. The same indices apply for the magnetic wave given by h+

0 andh−

0 .

8 2.6 Solutions to the wave equations

In the quasi-static case, i.e. where conduction currents dominate over dis-placement currents, α and β are both real, and are given by:

α = β =(ωµσ

2

)1/2

(2.33)

This means that in the quasi-static case, waves propagating in the positivez-direction are given by:

e = e+0 e−iαz e−βzeiωt (2.34)

h = h+0 e−iαze−βzeiωt (2.35)

Since β is real, e−βz decreases as z increases. This shows that the amplitudeof an electromagnetic wave is reduced by a factor of 1/e at a distance δ, alsocalled the skin depth:

δ =2

ωµσ

1/2

= 503( 1

fσ

)1/2

(2.36)

As seen in equation 2.36, the skin depth decreases with increasing frequencyand conductivity.

The other type of solutions Ward and Hohmann (1988) focus on is that ofimpulsive electric and magnetic fields at z = 0, i.e. a plane wave propagatingin a whole space. If displacement currents are neglected (in the quasi-staticcase) inverse Fourier transformation of equations 2.30 and 2.31 yields thesolutions: (

e

h

)=

(e+

0

h+

0

) (µσ)1/2z

2π1/2t3/2e

−µσz2

4t (2.37)

for a wave propagating in the positive z-direction.

Figures 2.1a and 2.1b show the field as a function of time and distance,respectively. As seen in figure 2.1a, the field exhibits a maximum followedby a long tail. Setting the time derivative equal to 0 gives the time at whichthe peak occurs:

tmax =µσz2

6(2.38)

As seen in figure 2.1b the field exhibits the same behaviour with distanceas it does with time; it has a maximum followed by a long tail. Setting the

9 2.7 Schelkunoff potentials

derivative with respect to distance equal 0 gives the distance at which thepeak occurs, also called the penetration depth, Zd:

Zd =( 2t

µσ

)1/2

(2.39)

Figure 2.1: (a) Electric or magnetic field as a function of time 100 m froma 1D impulse in a 100 Ωm whole space. (b) Electric or magnetic fieldas a function of distance at time = 0.03 ms (after Ward and Hohmann(1988), fig. 1.2)

The velocity at which the maximum travels by is given by:

V =Zd

dt=

1

(2µσt)1/2(2.40)

2.7 Schelkunoff potentials

Maxwell’s equations are homogenous in the frequency domain and applyonly to source-free regions. If the region contains a source they must bereplaced by the following, which takes into account the inhomogeneity thatis introduced:

∇× E + zH = −JSm (2.41)

10 2.8 General solutions

∇× H − yY = JSe (2.42)

in which JSm and JS

e are magnetic and electric currents, respectively. Theimpedivity z = iµω and the admittivity y = σ + iεω have been mentionedbefore (see section 2.3).

According to Ward and Hohmann (1988), these equations can be solved forinfinite homogenous regions provided JS

m and JSe can be described explicitly.

If E and H are described in terms of potentials, i.e functions from which Eand H may be solved by differentiation, the equations are more easily solved.This is where Schelkunoff potentials can be useful.

2.8 General solutions

In the TEM, a short electric pulse is emitted in a square transmitter loopand a transverse electric field is transmitted. Therefore, the F-potential is ofinterest. Due to the difficulties in working with theory on a square loop, acircular loop of the same area is used instead. This is a good approximation.When working with plane 1D models, either containing or not containingsources, one needs to know the character of the source.

2.8.1 Vertical magnetic dipole

A vertical magnetic dipole is a good approximation of a circular or squareloop, and the Schelkunoff potential of this may be written:

F (ρ, z) =zom

4π

∫∞

0

[e−u0(z+h) + rTEeu0(z−h)

] λ

u0

J0(λρ)dλ (2.43)

where m is the magnetic moment of the dipole, λ = (k2x + k2

y)1/2, ρ = (x2 +

y2)1/2, un = (λ2 −k2n)2 and Jn is the Bessel function of order n. If we assume

that the magnetic permeability is that of free space, the reflection coefficientcan be written rTE = λ−u1

λ+u1(Ward and Hohmann, 1988).

2.8.2 Horizontal circular loop

When working with a large, horizontal circular loop, the dipole momentshown in figure 2.2 is:

dm = Iρ′dφdρ′ (2.44)

where I is the current in the loop, and ρ′dφdρ′ is the contribution from theloop area. If this is substituted into equation 2.43 and one integrates over

11 2.9 The transient response

the loop, the result is the TE potential:

F (ρ, z) =z0Ia

2

∫∞

0

F (λ, z)J1(λa)J0(λρ)dλ (2.45)

Figure 2.2: The dipole moment of a circular loop positioned at the surfaceof the earth (after Ward and Hohmann (1988), fig. 4.6)

Using Schelkunoff potentials permits us to write the vertical component ofthe magnetic field as:

Hz =Ia

2

∫∞

0

[e−u0(z+h) + rTEeu0(z−h)

]λ2

u0

J1(λa)J0(λρ)dλ (2.46)

2.8.3 Measurements in the center of a horizontal loop

If measurements are made in the center of a horizontal loop, the responsecan be found by setting ρ = 0. Furthermore, if the measurements are madeon the surface of a homogeneous earth, equation 2.46 becomes (Ward andHohmann, 1988):

Hz = Ia

∫∞

0

λ2

λ + uJ1(λa)dλ (2.47)

and so the vertical component of the magnetic field at the surface of theearth is:

Hz = −I

k2a3

[3 − (3 + 3ika − k2a2)e−ika

](2.48)

2.9 The transient response

The step response of this field can be found by inverse Laplace transformationof equation 2.48 divided by iω (Ward and Hohmann, 1988):

hz =I

2a

[ 3

π1/2θae−θ2a2

+ (1 −3

2θ2a2)erf(θa)

](2.49)


in which erf is the error function, and θ =(

µ0σ4t

)1/2

.

The time derivative of the field, also called the impulse response, can befound by differentiation with respect to time:

∂hz

∂t= −

I

µ0σa3

[3erf(θa) −

2

π1/2θa(3 + 2θ2a2)e−θ2a2

](2.50)

The behaviour of the magnetic field intensity and the derivative of this, givenin equations 2.49 and 2.50, can be seen in figure 2.3. At late times θ becomessmall and the field intensity decays with a t−3/2-dependency while the timederivative decays with a t−5/2-dependency.

Figure 2.3: The behaviour of the magnetic field and its derivative at thecenter of a circular loop with radius 50 m on a 100 Ωm homogeneousearth. A 1A current has been turned of abruptly at t=0 (after Wardand Hohmann (1988), fig. 4.8).


Equations 2.49 and 2.50 can be approximated by:

hz ≈Iσ3/2µ

3/20 a2

30π1/2t−3/2 (2.51)

and∂hz

∂t≈ −

Iσ3/2µ3/20 a2

20π1/2t−5/2 (2.52)

In time-domain methods an induction-coil censor usually records the decay-ing field. It is therefore appropriate not to speak of magnetic field intensity,h, but magnetic induction, b, which is easily done as the relation betweenthese is given by b = µ0h. Equations 2.49 and 2.50 becomes:

bz =µ0I

2a

[ 3

π1/2θae−θ2a2

+ (1 −3

2θ2a2)erf(θa)

](2.53)

and∂bz

∂t= −

I

σa3

[3erf(θa) −

2

π1/2θa(3 + 2θ2a2)e−θ2a2

](2.54)

while the late time approximations, equations 2.51 and 2.52, becomes:

bz ≈Iσ3/2µ

5/20 a2

30π1/2t−3/2 (2.55)

and∂bz

∂t≈ −

Iσ3/2µ5/20 a2

20π1/2t−5/2 (2.56)

The apparent resistivity is derived from the approximation of the impulseresponse in equation 2.56 (Christensen, 1995):

ρa =( Ia2

20∂bz

∂t

)1/2 µ5/30

π1/3t−5/3 (2.57)

14

3 The Transient Electromagnetic Method

The purpose of this chapter is to give an introduction to the Transient Elec-tromagnetic Method (TEM). The basic principles, equipment and sources oferror will be presented here. Electromagnetic measurements are divided intotwo categories: Frequency domain methods and time domain methods. Onlythe latter will be described here.

3.1 Time domain methods in general

Time domain measurements are made by measuring the amplitude of a signalas a function of time. A pulse is transmitted by abruptly turning off acurrent, and the measurements start after the disappearance of the primaryfield. This means that only the secondary field is measured. On one handthis removes all the difficulties concerning the separation of the two fieldcomponents involved in frequency domain methods, but on the other handit makes it necessary to measure over a longer period of time, causing themagnitude of the secondary field to vary much more, i.e. the dynamic rangeincreases. Furthermore, it is necessary to stack the measurements since theycontain a wide range of frequencies, making it difficult to filter out undesirednoise (Christensen, 1995).

3.2 The Transient Electromagnetic Method

TEM is a time domain method which was originally designed in the sev-enties as a way of finding ore deposits, mainly sulfides. It was introducedin Denmark by the Department of Earth Sciences, University of Aarhus,and has since become very popular for hydrogeological mapping. Approx-imately 60.000 soundings has been carried out in Denmark. Its ability tofind low-resistivity layers has made it one of the preferred methods for lo-cating the bottom of aquifers. According to Jørgensen et al. (2003c), layerswith resistivities higher than 80 Ωm - 120 Ωm can not be expected to bemapped correctly if they are surrounded by layers with a lower resistivity.At present TEM is under continuous development, which has resulted in thedevelopment of the airborne SkyTEM method, the continuous TEM method(PATEM) and the high moment TEM method, which will be described insection 3.6 (the high moment TEM method will be denoted HMTEM here.The term HiTEM is used in some literature as well).

15 3.2 The Transient Electromagnetic Method

3.2.1 The principle of the TEM Method

As mentioned in section 3.1, a TEM measurement is a measurement of theamplitude of a transmitted signal with respect to time. Faraday’s law (equa-tion 3.1) states that a varying electric field induces a magnetic field and viceversa:

∇× e = −∂b

∂t(3.1)

e is the electric field and b is the magnetic induction.

This means that a direct current running in a coil induces a static mag-netic field, the primary field. This contains no information of the earth, butis proportional to the magnetic moment, M :

M = InAloop (3.2)

where I is the direct current, n is number of turns and Aloop is the area ofthe loop.

After a period of time, the current is turned of abruptly and the primarymagnetic field begins to decay, causing an electric field to be induced andeddy currents to begin running in the ground near the coil. Due to ohmic lossthese currents will soon begin to decay, thereby inducing a secondary mag-netic field. Immediately after the current has been turned off, the strength ofthe secondary and primary fields are equal. Since the decay of these currentsis a function of the conductivity of the earth so is the magnitude of the sec-ondary magnetic field. The decay of the secondary magnetic field induces anelectromotive force in the receiver coil. The decay of this current is measuredas a function of time (Christensen, 1995).

As time passes by, the eddy currents diffuse down- and outwards at an angleof approximately 300 through the ground as seen in figure 3.1. The penetra-tion depth, Zd, at a given time, t, is given by:

Zd =( 2t

µσ

)1/2

(3.3)

µ is the magnetic permeability and σ is the conductivity of the media. Themaximum depth to which measurements can be obtained, Zlast, is not onlya function of the moment, but also of the noise level, Vnoise:

Zlast = (2

25π3)1/10(

M

σVnoise

)1/5 (3.4)

16 3.2 The Transient Electromagnetic Method

At this depth the measurements are so influenced by noise that they areunsatisfactory. According to Christensen (1995) a common limit is when thesignal-noise ratio, S/N, equals 1. As seen the maximum penetration depthdecreases with increasing conductivity.

Figure 3.1: Contour plots of the current maximum. Notice how it movesdown- and outwards in a smoke ring-like manner (after West and Mac-nae (1991), fig. 19).

The time of diffusion, td, is easily derived from equation 3.3:

td =µσZ2

d

2(3.5)

and the time of the last measurement, tlast is then:

tlast = µ( M

20Vnoise

)2/5(σ

π

)3/5(3.6)

As mentioned before, the maximum penetration depth is a function of themagnetic moment. It is seen from equation 3.4 that Zlast is proportional toM1/5, so a dramatic increase in magnetic moment is needed to increase themaximum penetration depth. Under normal Danish conditions, the conven-tional TEM system has a penetration depth of approximately 130 m, up to150 m under ideal conditions (Jørgensen et al., 2003b).

17 3.3 The TEM Method equipment

3.3 The TEM Method equipment

Apart from cables, the conventional TEM system used in Denmark consistsof the following:

1. A Protem 47 receiver.

2. A Protem 47 transmitter.

3. A transmitter loop consisting of a wire. For practical reasons, the loopis usually square.

4. A receiver coil.

The transmitting and receiving systems will be discussed in the followingsections.

3.3.1 The transmitting system

The transmitter is usually a Protem 47 transmitter. The current is trans-mitted in a pattern as shown in figure 3.2. It takes a certain amount oftime, ton, before the current reaches the desired level, after which it remainsconstant. After a period of time, the current is turned off abruptly and itbegins to decrease. The time it takes for the current to reach zero after ithas been turned off, is called the turn-off time, toff . This is a function of theself-inductance of the coil (Geonics Limited, 1991)2.

Figure 3.2: The current is transmitted with shifting polarity. The turn-offtime is denoted T/O time (after Geonics Limited (1991), fig. 6).

As seen in equation 3.2, the moment is a function of the loop size and thenumber of turns, but the increase in area or number of turns is not neces-sarily proportional with the increase in magnetic moment, since increasing

2According to Young (1992), the change in current with respect to time is dI/dt = V/L,so toff = Imax

dIdt = Imax

LV . Since increasing the number of turns and length of loop sides

causes the self-inductance to increase, the turn-off time must also increase.

18 3.3 The TEM Method equipment

the length of the transmitter wire also increases the ohmic loss. Increasingthe area of the transmitter loop also leads to a declining resolution of near-surface structures since it increases toff , thereby allowing the current systemto diffuse to a certain depth before the measurements start3. The conclusionmust be that one needs to consider whether a high penetration depth orresolution of the top layers is more important.

This cycle of turning the current on, keeping it constant, turning it off againand measuring is repeated with different frequencies. The most commonlyused in Danish surveys are listed in table 3.1. Furthermore, the gate centertimes for the analogue Protem 47 receiver can be seen in appendix C.1. Themeasurement cycle period is denoted Tx Period in figure 3.2.

Segment Ultra High Very High High(UH) (VH) (Hi)

Frequency [Hz] 237.5 62.5 25

Table 3.1: Repetition frequencies in countries using 50 Hz power line fre-quencies according to Geonics Limited (1991).

These frequencies are relevant for countries using 50 Hz power line frequen-cies and have been chosen to help filter out undesired cultural noise. Protem47 also allows measurements on medium (MD) and low (LO) frequencies,but these are rarely used because of the relatively small dipole moment ofthe system.

In conventional TEM soundings the transmitter coil consists of a 40 m times40 m wire in which a current, typically of 1 A or 3 A, is transmitted. Onlythe central loop configuration, in which the receiver is placed in the centerof the transmitter loop, is used and only the vertical component, Hz, of themagnetic field is measured. This may be modified such as in the HMTEMmethod in which both a central loop configuration and an offset configurationare used (for further information on the HMTEM method, see section 3.6).The receiver is placed outside the transmitter in the latter. Both configura-tions can be seen in figure 3.3. In addition all components of the magneticfield are measured in the airborne electromagnetic method.

3As mentioned in section 3.1, the measurements begin after the current is turned offand the primary magnetic field is gone. The transmitter does, however, apply a voltageto damp self-induction (Geonics Limited, 1991).

19 3.4 Filters and waveforms

Figure 3.3: Sketch of the central loop array (a) and offset array (b) (afterHydroGeophysics Group (2000), fig. 2.7).

3.3.2 The receiving system

The receiver, a Protem 47, records the signal in limited time intervals calledgates. The signal is then integrated over each gate resulting in a single mea-suring point for each gate. The width of these is not constant but increasesexponentially with time. They are narrow at early times to accurately mea-sure the transient when the magnetic field varies rapidly, and wide at latertimes when the magnetic field varies slowly (Geonics Limited, 1991). Foreach segment there are 20 gates. The VH segment overlaps with gates 9-20of the UH segment and gates 1-16 of the HI segment. The gate values ofeach measurement cycle are stacked and averaged in order to reduce the in-fluence of random noise. Each measurement cycle for the analogue Protem47 receiver lasts typically 45 seconds. The transmitter and the receiver areconnected through a cable in order to synchronise them.

The receiver coil consists of a copper wire wound up to form a coil. This isencapsulated in an insulating material to protect it from electric fields andcast in fibre glass. As mentioned in section 3.2.1, the receiver measures theelectromotive force in the receiver coil. According to Faraday’s law of induc-tion the electromotive force induced in a closed loop is a function of the areaof the coil, the angle between this and the incoming field and dB/dt (Young,1992):

ε = −dB

dtA sin θ (3.7)

3.4 Filters and waveforms

As all other physical instruments, TEM equipments have a band-limited re-sponse. Low pass filters are employed in most commercial TEM receivers

20 3.4 Filters and waveforms

and also in this thesis (filter settings can be seen in appendix C.2)4.

Low pass filters were earlier thought to be of little effect, but studies byEffersø et al. (1999) have proved that this is not the case. AM and VLFtransmitters cause high-amplitude input signals. The standard deviation ofthis noise exhibits a t−1 proportionality5, and the level of the AM transmitternoise is often orders of magnitude higher than the background noise level,effectively lowering the signal-noise ratio at early and medium decay times.A low pass filter allows the passage of signals below a cut-off frequency, ωc,and suppresses signals with higher frequencies. This means that the TEMresponses will be distorted if the applied filters are not accounted for in theinversion scheme. The distortion depends on the cut-off frequency but otherfactors such as subsurface resistivity plays a part as well. A higher resistivitywill increase the distortion. The before mentioned integration of the decaysignal serves two purposes: Firstly it reduces the dynamic range of the inputsignal and secondly it reduces the bandwidth, thereby functioning as a lowpass filter with a frequency response of a sinc-function (Effersø et al., 1999).

As mentioned in section 3.3.1, the current is transmitted with shifting po-larity, and in a manner as seen in figure 3.2. A close up of the transmitterwaveform is seen in figure 3.4.

Figure 3.4: Sketch of the transmitter waveform and how it is approximatedwith a piecewise linear function (after HydroGeophysics Group).

The turn-on ramp has an exponential behaviour which is modelled by twolinear steps. The two points R1 and R2 are given as a percentage of theamplitude. Typical values are 63 % and 86 %. The turn-off ramp is of both

4High pass filters are not relevant for conventional TEM or HMTEM, but they helpfilter out undesired noise in for example PATEM.

5A white noise spectrum is expected to show a t−1/2 proportionality.

21 3.5 Sources of error

linear and exponential behaviour. The end of the linear piece, which is calledthe ”avelance”, is denoted R3 in the figure. Up to three piecewise, linear,continuous functions are used to model the exponential part. A typical valuefor R4 is 37 % (HydroGeophysics Group).

3.5 Sources of error

Knowing the sources of error is crucial in knowing your data - without anestimate of the error involved with them, data are worth very little. Thereare several sources of error in the TEM method such as instrumental orgeometrical effects and electromagnetic, geological and cultural noise. Thesewill be discussed in the following sections.

3.5.1 Instrumental effects

The two most important instrumental errors are clock drift and inability tohandle the large dynamic range, but instrumental errors also include lackof calibration facility. Clock drift can be checked by doing measurementsat a station near the transmitter loop or by measuring an auxiliary R-Cor R-L calibration unit. Overdriven amplifiers are usually a problem whenatmospheric or cultural noise is high. Furthermore, it is crucial to know theshape of the transmitted waveform (Spies and Frischknecht, 1991).

3.5.2 Geometrical effects

As mentioned before, the traditional configuration involves a 40 m times 40m square transmitter loop with a receiver placed either in the middle of thisor in offset. Geometrical errors are introduced if the transmitter loop is notsquare or the receiver is not placed as intended. Since measurements aremade during transmitter off time, geometrical errors are negligible in centralloop TEM measurements, but are on the other hand enhanced for offsetconfigurations. Geometrical errors may also be induced due to topographybut even if the ground is not flat the smoke rings will soon begin to traveldownwards as if it was (Nabighian and Macnae, 1991).

3.5.3 Electromagnetic noise

Electromagnetic noise is naturally generated. It may be a result of spherics,an effect coming from lightning discharges all over the world. This type ofnoise is of random character but is more frequent in equatorial regions suchas Brazil, Central Africa or Malaysia. Usually noise from spherics has a fre-quency above 1 Hz. The fluctuations in the Earth’s magnetic field due to the


solar wind, Earth’s rotation and the magnetic field are large scale, but do notinterfere significantly with the measurements since they contribute only tothe low frequency noise below 1 Hz (Spies and Frischknecht, 1991). By stack-ing measurements with shifting polarity, as seen in figure 3.2, electromagneticnoise of random character may be removed.

3.5.4 Geological noise

Geological noise include anisotropy, frequency-dependent conductivity, dip-ping layers and near-surface inhomogeneities.

Layers are usually assumed to be homogeneous and parallel to the surface,and as this is obviously not always the case, errors are introduced. Smallinhomogeneities influence the measurements more at early times than at latetimes due to the smaller earth volume covered by the measurements at earlytimes. It is impossible to do inversions of TEM data in more than one di-mension, making it impossible to describe a 3D world by a 1D model (Spiesand Frischknecht, 1991). Mapping of valley structures is a good example ofthis. The success of the imaging depends on the slope of the valley sidesand ultimately the resistivity contrast between the valley fill and the valleyfloor; the lower the resistivity of the valley floor, the larger the slope canbe. According to Jørgensen et al. (2003c) valleys with slopes less than 30-40degrees may be correctly imaged given an ideal resistivity contrast. On theother hand only slopes of less than 20 degrees can be imaged correctly if thevalley floor has a higher resistivity than the valley fill.

Conductivity may even vary with direction, as it is seen in sedimentary rockswhere it is typically higher parallel to bedding than perpendicular to bed-ding. This is called anisotropy and a measure of this is the anisotropy factor(Spies and Frischknecht, 1991):

λ =√

σ/σt (3.8)

where σ and σt are longitudinal and transverse conductivity, respectively.

Furthermore, conductivity is not always independent of frequency, givingrise to induced polarization (IP) effects. The instant currents are turnedoff in the transmitter loop, vortex currents are induced in the ground be-neath it. As a result of these currents, ions in the pore space fluids willbegin to move in the same direction, thereby creating an additional current,the polarization current. The result is a decreasing resistivity at early delaytimes. These ions will begin ”piling up” at places of low mobility, what Flis


et al. (1989) refer to as the ”charged state”. As the vortex currents beginto decay, these ions will move back into their original place, the equilibriumposition, only this time at a slower rate. This movement gives rise to a sec-ond polarization current, this time of opposite direction. Given a sufficientlypolarizable earth the polarization current may dominate the vortex currentat late times, thereby possibly reversing the signal (Flis et al., 1989). As seenin figure 3.5, a polarizable layer increases the TEM response outside the loopand decreases it inside the loop (Spies and Frischknecht, 1991).

Figure 3.5: The effect of a thin polarizable layer on a TEM profile. Dashedlines represent a non-polarizable layer and solid lines a polarizablelayer (after Spies and Frischknecht (1991), fig. 23).

3.5.5 Cultural noise

Cultural noise is man-made, its main contributor being the 50 Hz mainpower-lines. In developed countries like Denmark this constitutes a harmonicsignal, making it removable by measuring with the appropriate frequenciesseen in table 3.1. Transients from switching on and off electric equipment isa problem in densely populated areas.

VLF transmitters are also a source of errors producing strong signals around15 kHz-24 kHz. AM radio transmitters produce signals from around 200 kHz,which may overload the system if they are not filtered out by low pass filters(see section 3.4) (Spies and Frischknecht, 1991).

Another major source of error is coupling noise from metallic conductorssuch as fences, buried pipes, power lines etc. These function as conduc-tors, resulting in a secondary magnetic field if they are in the vicinity ofthe sounding. The time constants of direct induction in these features areusually negligible as a result of the relatively small cross-section, so the main


contribution comes from the ability to channel currents induced in the sur-rounding media (Danielsen et al. (2003); Spies and Frischknecht (1991)). Asseen in figure 3.6, coupling noise can be either capacitive or galvanic.

Figure 3.6: Soundings showing galvanic (a) and capacitive couplings (b)(after Danielsen et al. (2003), fig. 4 and 5).

The former happens when the conductor is isolated, i.e. there is no gal-vanic contact between this and the surrounding media. The earth-insulator-conductor system works as a L-C-R electrical circuit giving rise to secondarymagnetic fields in the conductor. Capacitive couplings may be easily rec-ognized as fluctuations in the sounding curves. They are related to mod-ern high-power lines, insulated underground cables and telephone cables(Danielsen et al. (2003); Spies and Frischknecht (1991)).

Galvanic coupling requires galvanic contact with the conductor. The cur-rent runs from the surrounding media into the conductor and back, so the

25 3.6 The HMTEM method

system can be regarded as an L-R electrical circuit. Galvanic couplings resultonly in a bump in the sounding curve, making them difficult to recognize,although the resulting inverse model often contains an anomalously high con-ductivity. They are related to metal pipes or old type power cables with ametallic protection shielding (Danielsen et al. (2003); Spies and Frischknecht(1991)).

The effect of a coupling depends on the shape of the conductor and thedistance between this and the transmitter, r. The shape of conductors maybe three-dimensional (3D), two-dimensional (2D) or elongated with ends (re-ferred to as 2.5D). The response of a 3D conductor decays proportional tor6 while the response of 2.5D and 2D conductors decays proportional to r4

and r2, respectively. A safety distance of 125 m to 150 m from roads withunderground cables, pipelines, etc. is often suggested. The safety distanceincreases with increasing resistivity (Danielsen et al., 2003).

3.6 The HMTEM method

The HMTEM (High Moment TEM) method is relatively new in Denmark. Itwas introduced in spring 2001 and since then more than 2000 soundings havebeen carried out. It is based on the same principles as the conventional TEMmethod, but measurements are made using both a high and a low moment.In practice this is done by varying the current in the transmitter coil, whichhas a size of 30 m times 30 m. A current of up to 75 A is used for high momentmeasurements and 2.4 A for low moment measurements. For comparison aHMTEM sounding has a moment of maximum 30m*30m*75A = 67500 Am

2

,while a conventional TEM sounding has a moment of 40m*40m*3A = 4800Am2 (Danielsen et al., 2002). As seen in equation 3.4, the maximum penetra-tion depth is a function of the moment, making this greater in HMTEM thanin conventional TEM, approximately 250 m under normal Danish conditionsand up to 300 m under ideal conditions (Jørgensen et al., 2003b).

Using a central loop configuration, receiver amplifiers are saturated due to thehigh moment. An offset configuration is therefore used during high momentmeasurements, while a central loop configuration is used during low momentmeasurements. The offset configuration is sensitive towards near-surface in-homogeneities at early times and there is a sign change in the measurements.The array geometry has a great influence on when this occurs. A 30 % errorin the response near the sign change will be introduced if the receiver coil islocated 71 m instead of 70 m from the transmitter for a 60 Ωm half-space.This effect becomes negligible after the sign change. These factors make in-

26 3.6 The HMTEM method

terpretation of early time offset measurements problematic (Danielsen et al.,2002).

IP effects are minimized by using the offset configuration. As the distancebetween the receiver and transmitter coil increases, the IP effect moves tolater times and ultimately outside the measurement window (Danielsen et al.,2003). The large earth volume covered makes coupling with cultural fea-tures more pronounced in HMTEM soundings than in conventional TEM(Danielsen et al., 2002).

3.6.1 The HMTEM Method equipment

The equipment has been modified in several ways in HMTEM to make it ableto cope with the large moment required. As mentioned in section 3.6, thetransmitter can transmit up to 75 A. In order to do this, it has been necessaryto increase the thickness of the transmitter loop wire from 2.5 mm2, whichis used in conventional TEM measurements, to 10 mm2. It has also beennecessary to employ a number of car batteries, a cooling system and a smallbelt-driven tractor, as seen in figure 3.7. While the receiver is still a Protem47, the transmitter is usually a Groundwater Instruments TEM-TX, designedat the Department of Earth Sciences, University of Aarhus.

Figure 3.7: To the left a picture of the HMTEM equipment, and to theright a sketch of both the offset and central loop configurations (afterDanielsen et al. (2002), fig. 1).

27

4 The seismic method

A large portion of the data used in this thesis is reflection seismic data. Thepurpose of this chapter is to give an introduction to the reflection seismicmethod and the most common sources of errors involved with it. The gath-ering and processing of the seismic data were done by Holger Lykke-Andersenand Egon Nørmark, Department of Earth Sciences, University of Aarhus.

4.1 Seismology in general

Exploration seismology deals with artificially generated elastic waves propa-gating through the earth (body waves). While seismology does not provideknowledge about the exact lithology of the sediments, it does provide usefulinformation about the structure and distribution of rock types. It is mainlyused to locate ores, geothermal reservoirs, aquifers, hydrocarbons etc. (Sher-iff and Geldart, 1995). The latter is probably its most important feature inDenmark.

Exploration seismology is actually an offspring of a natural phenomenon,earthquake seismology. When an earthquake occurs, the earth is fracturedand rocks on opposite sides of the fracture move relatively to each other thusgenerating seismic waves travelling outward from the site (the epi-center).These waves are recorded on seismographs at various locations around theworld, thereby giving an idea of the type of rock through which the waveshave travelled. Seismic waves are divided into P-waves (P is for pressure orpush-pull) and S-waves (S is for secondary, shear or shake), both of whichcan be seen in figure 4.1. P-wave particle motion is longitudinal and involvescompression and rarefaction of the material (not rotation). S-wave particlemotion is on the other hand transverse and involves shearing and rotation ofthe material - not volume change. S-waves can not travel through liquid ma-terial, such as in the outer core, and they always travel slower than P-waves(Fowler, 1990).

The source is not natural in exploration seismology, but man made and mov-able. It is often a small charge of explosives placed in a vertical hole drilledinto the ground, but others types, e.g. vibrating mechanisms or air guns,are also used. When the explosive is set off, the result is waves propagatingoutwards from the blast site. These are reflected or refracted when comingin contact with density contrasts in the earth, i.e. layer boundaries.

28 4.2 General discussion on reflection seismology

Figure 4.1: The particle motion is longitudinal in P-waves (a) and trans-verse in S-waves (b) (after Balling (1998), fig. 2.1).

The distance between recording points, which consist of a number of seismo-graphs placed with the same distance to each other along a line, is relativelysmall. Data consist of measurements of the time required for the waves totravel through the medium, the travel time. They are recorded digitally,making it possible to enhance signals with respect to noise by processing(Sheriff and Geldart, 1995).

Ray paths can be divided into two main categories which can be seen infigure 4.2; refracted waves (or head waves) and reflected waves. The path ispredominantly horizontal in the former as the waves travel along subsurfaceinterfaces. It has not been employed in this thesis and will therefore not bediscussed further. According to Fowler (1990), one of the advantages of thelatter is the high resolution, which makes it the preferred method in oil andgroundwater exploration. It will be presented in sections 4.2 through 4.4.

4.2 General discussion on reflection seismology

When waves come in contact with layer boundaries separating two media ofdifferent elastic properties, reflected and refracted waves are produced. Incase of reflection the energy stays in the same medium as the original energy,while the energy is refracted into the other medium with an abrupt changein the direction of propagation in case of refraction. Snell’s law states:

sin θ1

α1

=sin θ2

α2

= p (4.1)

29 4.2 General discussion on reflection seismology

where α1 and α2 are seismic velocities, θ1 is the angle of reflection, θ2 is theangle of refraction and p is the raypath parameter. When α2 > α1, θ2 > θ1.When θ2 reaches 900, θ1 = sin−1(α1/α2) and θ1 is called the critical angle, θc.For this angle, the reflected wave travels along the interface. For angles ofincidence greater than θc, it is impossible to satisfy Snell’s law and 100 % ofthe energy is reflected, either as P-waves or as converted S-waves and/or asevanescent waves. The relationship between the waves on either side of theboundary can be found from the relations between the stresses and displace-ments on either side, which must be continuous at the boundary (Sheriff andGeldart, 1995).

Direct waves travel directly from the source (S) to the receiver (R) (seefigure 4.2) and are therefore the first to arrive. The time it takes for thiswave to reach the receiver is:

t = x/α1 (4.2)

where x is the source-receiver distance and α1 is the velocity of layer 1.

Figure 4.2: The behaviour of reflected and refracted waves. S is the source,R the receiver. The former takes the path SCR, the latter SABR (afterFowler (1990), fig. 4.8).

Reflected waves travel downwards from the source and are at some pointreflected back to the surface where they are recorded by the receiver. Itenters and exits at the same angle measured to the normal of the boundary- angle of incidence equals angle of reflection. The time it takes for the waveto reach the receiver is:

t =SC

α1

+CR

α1

(4.3)

30 4.3 Sources of errors in reflection seismology

Since |SC| = |CR|, this can be written as:

t =2

α1

√z21 +

x2

4(4.4)

which constitutes a hyperbola on a time-distance graph. If t2 is insteadplotted against x2, equation 4.4 is the equation of a straight line with slope1/α2

1 and intercept t20=4Z21/α

21 on the t2 axis (Fowler, 1990). Since the paths

are predominately vertical, source-receiver offsets need not be as large as forrefraction seismology (Sheriff and Geldart, 1995).

4.3 Sources of errors in reflection seismology

Reflection seismology is generally based on the assumption of plane and hor-izontal layer boundaries. As this is naturally not always the case, errors willbe introduced. Further errors may be introduced by problems with the fieldwork, during processing, by variations in seismic velocity or by ray pathsthat differ from the ideal path. It is necessary to be aware of these errors.

4.3.1 Problems concerning field work and processing

It is quite common during land based field work that a number of shot pointsmust be skipped owing to for example impassable terrain. This lowers thecoverage of the section (number of receivers divided by twice the electrodespacing), and thus decreases the signal-noise ratio in the area. The resultis a decreasing resolution of week and deep reflectors (Sheriff and Geldart,1995). Errors may also be introduced during processing, especially duringmuting which is a process used to remove all other signals than those fromreflections. Muting is done manually, and is therefore subject to personalerrors or cases of bad judgement. A further discussion on reflection seismicdata processing can be found in section 4.4

4.3.2 Multiples

Waves may be reflected more than once before being recorded. This problemis usually more pronounced in marine surveys, since the multiple reflected atthe sea surface and seabed is very strong. Fortunately, multiples are periodic,enabling them to be filtered out during processing (Fowler, 1990). Only thelargest impedance contrasts will generate multiples that are strong enoughto be recognized as distinctive events. This is because the amplitude of themultiples is proportional to the product of the reflection coefficients for eachreflector involved and these are generally small for most interfaces (Sheriff


and Geldart, 1995).

Based on the travel path of the multiple compared to that of the primaryreflections from the same deep interfaces, Sheriff and Geldart (1995) dividemultiples into short-path and long-path multiples. The former arrive shortlyafter the primary reflection and interferes with the signal from this, therebychanging the wave shape. Examples of short-path multiples are ghosts, nearsurface multiples and peg-leg multiples. Long-path multiples, on the otherhand, arrive so late that they appear as separate events on a seismic recordinstead. Figure 4.3 shows different types of short-path and long-path multi-ples.

Figure 4.3: Different types of short-path and long-path multiples (afterSheriff and Geldart (1995), fig. 6.29).

Since multiples spend most of their time in the shallower sections, bounc-ing off the interface at the low velocity zone, they have smaller stackingvelocities and do not align on a continuous velocity log. This makes themrelatively easy to identify and attenuate using CDP stacking (CDP is shortfor Common Depth Point and is discussed in section 4.4.4).

4.3.3 Dipping reflectors

Dipping interfaces constitute a very common problem in seismology. Signalsare distorted on a CDP section since the reflection point is not verticallybelow the source / receiver position. This is illustrated in figure 4.4. A wavetravelling from source A or B is reflected, not vertically below these but at


points A′ and B′ on a reflector with dip δ. z1 and z2 are the vertical depthsbeneath points A and B. While the reflector actually has a dip of:

δ = tan−1(z − z1

x

)(4.5)

it seems to have a dip of:

δ′

= tan−1(BB

′

− AA′

x

)= tan−1

(z cos δ − z1 cos δ

x

)(4.6)

This shows that tanδ′

= sin δ, so the apparent dip δ′

is smaller than the actualdip δ (except for very small angles where sin δ ≈ tan δ) (Fowler, 1990).

Figure 4.4: A dipping reflector distorts the signal. The dashed line showsthe apparent reflector, A and B are sources and A′ and B′ are reflectionpoints (modified from Balling (1998), fig. 2.46).

4.3.4 Diffraction

If a reflector ends abruptly, as may be the case with for example faults, thewave will be diffracted. According to Huygens’ principle every point on awavefront acts as a point source, generating spherical waves. The wavefrontat a later time is the sum of these point sources. If a reflector terminates ata point P, as shown in figure 4.5, it will act as a point source when struckby a seismic wave, thereby sending out seismic waves in all directions. Thetravel time for the wave travelling from C to P and back again will be:

t =2

α1

√x2 + z2

1 (4.7)

On a time-distance graph this constitutes a hyperbola with greater curvaturethan the hyperbola of equation 4.4. According to Fowler (1990), the diffrac-tion branch recorded to the left of B will be 1800 out of phase with the onerecorded to the right. This permits the location of the edge of the reflectorby detailed analysis of the seismogram.

33 4.4 Reflection seismic data processing

Figure 4.5: A reflector that ends abruptly will cause diffraction (after Fowler(1990), fig. 4.116a).

4.4 Reflection seismic data processing

Raw data are often very noise filled and therefore unusable. It is necessaryto pluck out usable information and remove everything else. This is a veryindividual discipline called processing, to which the following sections willgive an introduction. Processing starts already while gathering data, whenthese are saved in the desired format and the geometry of the seismic sectionis attached to them.

4.4.1 Muting

Muting is removing undesired signals. These may either be unwanted becauseof a high noise level, because focus lies elsewhere or because they are aresult of refracted waves (which are normally unwanted in reflection seismicsurveys). It is also possible to remove signals from a distinct receiver if thereis a problem with it.

4.4.2 Filtering

Filtering is a way of removing undesired frequencies. Those around 100 Hzare often desired in shallow seismic surveys. Lower frequencies are often aresult of noise; typically the 50 Hz power lines or nearby windmills. Filteringworks as a transformation of the signal via a Fourier transformation fromtime domain and into frequency domain. A reasonably periodic function,g(t), can be expressed by a Fourier transform as a superposition of sinus-

34 4.4 Reflection seismic data processing

and cosinus functions (Sheriff and Geldart, 1995):

g(t) =1

2a0 +

∞∑n=1

(an cos 2πνnt + bn sin 2πνnt)

=∞∑−∞

αnej2πνnt (4.8)

By using Euler’s formula, eix = cos x + i sin x, and letting the number offrequencies go towards infinity, the Fourier series can be written as (Sheriffand Geldart, 1995):

g(t) =

∫∞

−∞

G(ν)ej2πνntdν ↔ G(ν) =

∫∞

−∞

g(t)e−j2πνntdt (4.9)

This transform is done on a computer and is a reasonably fast process. Theresult is an amplitude spectrum showing every frequency. The usable fre-quencies are then multiplied by 1 while the undesired are multiplied by 0,effectively removing these.

4.4.3 Migration

Structural variations in the third dimension are generally assumed to nonex-istent. Migration tries to deal with these problems and move the reflectorsback into their correct location, thus removing diffractions, multiples or ef-fects resulting from curved reflectors or dipping layers (Fowler, 1990).

4.4.4 CDP stacking

To enhance reflections and reduce background noise, it is advantageous tostack signals from nearby receivers. The most common way of stacking inseismology is a process known as CDP stacking (also known as CMP stack-ing), in which every recording of reflections from the same reflection point isstacked (Fowler, 1990).

4.4.5 NMO correction

As a result of the different offset distances of the measurements, data havedifferent travel times. The CDP gathers must therefore be corrected byNMO (Normal MoveOut) correction during processing in order to stack them.NMO correction is an extensive process that will not be presented furtherhere, but additional information can be found in Sheriff and Geldart (1995)or (Fowler, 1990).

35

5 Exploratory drillings and logs

The purpose of this chapter is to give an introduction to exploratory drillingsand the type of logs used in the thesis.

5.1 Exploratory drillings

Exploratory drillings are a very good source of information. One must, how-ever, remember that they constitute a 1D source - they only provide in-formation of the geology at that specific location. On the other hand thisinformation is usually very reliable. TEM soundings, seismic data etc. areoften used to correlate between drillings.

5.2 Logs

To supplement the lithological logs in the exploratory drillings, geophysicallogs are often used. One of the advantages of these is their ability to establishlayer boundaries, may often be difficult given only sediment samples.

Several logs were used in the borehole; a Vertical Seismic Profiling log (VSP-log), a spectral gamma-ray log (SNG-log), a natural gamma-ray log (NG-log)and a focused electrical log (FEL-log). These will be described in the follow-ing sections. Joint interpretation of the above mentioned logs will usuallygive a rather good indication of the type of sediment found in the drilling.A sandy sediment would for example give rise to a high reading on the re-sistivity log and a low reading on the gamma-ray log, while it would be theother way around for a clayey sediment.

5.2.1 Gamma-ray logs

A natural gamma-ray log shows the total intensity of natural gamma ra-diation in a sediment. Gamma-rays are emitted during radioactive decayof uranium-238 (U) or thorium-232 (Th) and their radioactive daughters orpotassium-40 (K), the latter normally being the largest source of radioactiv-ity in the Earth’s crust. Measurable quantities of radioactive isotopes areoften associated with the presence of clay minerals (Ellis, 1987).

Instead of just having a measure of the total intensity of gamma-ray de-cay in the sediment, one might be interested in knowing which isotopes itis connected to as well. A SNG-log is from uranium and daughters, thosecoming from thorium and daughters and those coming from potassium. This

36 5.2 Logs

makes it a very powerful tool for interpreting, since K, U and Th concentra-tions depends on the sediment type. Research has shown that U is associatedwith organic matter. It is the only component in clean carbonate rocks, whilemarl contains small amounts of K and Th as well. Th is mainly associatedwith clay minerals, but a high Th level may also be connected to heavy min-erals. K is the largest source of natural gamma-ray activity in the Earth’scrust (Engell-Jensen et al., 1984).

5.2.2 Resistivity logs

A resistivity log measures the resistivity of the sediment. Archie’s low pro-poses that the resistivity of a clay free sediment is a function of porosity, φ,water saturation, SW , and pore water resistivity, RW (Ellis, 1987)6:

Rt =aRw

φmS1/nw

(5.1)

n is the saturation coefficient. According to Ellis (1987), values of 1, 2 and2 are typically used for a, m and n.

A borehole is coated with conductive mud, which will often migrate intothe sediment, thereby lowering the effective resistivity considerably and in-fluencing the measurement. The presence of this mud may also alter thecurrent paths, as current will tend to flow in the low-resistivity mud, ratherthan in the sediment. The amount of mud that migrates into the formationdepends primarily on the porosity of the sediment, the viscosity of the mudand the pressure in the borehole compared to the pressure in the sediment.

5.2.3 VSP-logs

A VSP-log is a log which shows a vertical profile of the seismic velocity.Since the seismic waves need not travel as far as they do when received atthe surface, VSP-log data are often more credible than ordinary seismic data,thus giving a better resolution (Sheriff and Geldart, 1995). VSP-logs may beof three types: Zero-offset, offset or walkaway. With zero-offset VSP-logs, thesource is placed more or less directly above the borehole7, with offset VSP-logs the source is placed in offset and with walkaway VSP-logs the source ismoved to successively larger distances from the borehole.

6A clay content will contribute to the conductivity of the sediment, and must thereforebe taken into account. Different models of how to do this has been made, e.g. the Waxman& Smith model (Ellis, 1987).

7Placing the source directly above the borehole may cause problems as many boreholeshave metal coating, which may be set in motion by the blast.

37

6 Inversion of electromagnetic data

In applied geophysics, data as a result of the underlying earth are measured.This is what you might call a forward problem, a model is given and the dataresponse is calculated: from model to data. It is, however, often the otherway around; data has been acquired and we wish to find the model, whatmay be described as an inverse problem: from data to model.

The purpose of the chapter is to give an introduction to the inversion ofelectromagnetic data. Sections 6.1 through 6.2 are meant to give an in-troduction to the theoretical side of inversion. Inversion of electromagneticdata is a non-linear problem, but to understand it fully, the chapter willbegin with the linear case, which, with relatively small changes, will work inthe non-linear case. Special attention will be given to Laterally ConstrainedInversion (LCI) on a layered 1D earth model, but Mutually Constrained In-version (MCI) is generally based on the same principles. A more generaldiscussion on the inverse modelling code Em1Dinv will be given in section6.3. MCI and LCI will be presented in section 10.1.

Data measured by a geophysical instrument, in for example a TEM sound-ing, are given in a data vector dobs = (d1,d2, ...,dN), where N is the numberof data points. The observational error, eobs, of this data vector is assumedto be unbiased, i.e. errors are independent and the expectation value is zero.

6.1 The linear case

Where nothing else is stated, the linear case is based on Jacobsen (2000).

In the linear case dobs is connected to the true model, mtrue, through:

dobs = Gmtrue + eobs (6.1)

where eobs is the data error and G is the Jacobian matrix. Cobs is the covari-ance matrix.

In the linear case, the model can be found from data in one step. Theleast squares estimate is a simple and often used way of determining whichmodel fits data best. The least squares, QLSQ, is a measure of the quality ofthe model:

QLSQ ≡N∑

i=1

(dobs,i − gi(m))2 = ‖dobs − Gm‖ = ‖emisfit‖ (6.2)

38 6.1 The linear case

emisfit is the misfit between the linear model response and the observed data.If mLSQ is the best model estimate, QLSQ(dobs,mLSQ)≤QLSQ(dobs,m) for anymodel. Expressing the misfit in terms of least squares is called the L2-norm.This system may be written as:

(GTG)m − GTdobs = 0 (6.3)

T denotes vector transpose. If the number of data exceeds the number ofmodel parameters, the problem is under-determined and there will be aninfinite number of solutions. If, on the other hand, the number of modelparameters equals the number of data, there is exactly one solution given as:

mLSQ ≡ (GTG)−1GTdobs (6.4)

The least squares estimate of equation 6.4 is not always the best solution.Other types of least squares estimates include the minimum length estimate(the L1-norm), the weighted least squares estimate, the damped lest squaresestimate. These will not be discussed here, but further information on themcan be found in Menke (1989). One of the more useful estimates is the BestLinear Unbiased Estimate (or BLUE) given by:

mBLUE = (GTC−1obsG)−1GTC−1

obsdobs (6.5)

where the associated best linear unbiased inverse mapping is:

HBLUE = (GTC−1obsG)−1GTC−1

obs (6.6)

6.1.1 Estimation errors

Only knowing the model estimate is unsatisfactory - one must also know theestimation errors related to it.

We regard mtrue as being constant and eobs as a stochastic vector with meanvalue zero and covariance matrix Cobs. This yields an estimation error in theform of a stochastic vector with covariance matrix:

Cest = HinvCobsHTinv (6.7)

From the linear mapping Hinv. The variances on distinct model parametersare given by the diagonal elements of this covariance matrix, while the stan-dard deviations are the square roots of these.

39 6.1 The linear case

Following equation 6.7, the covariance matrix of the maximum likelihoodestimation error, or equivalently the BLUE error is:

CBLUE = HBLUECobsHTBLUE

= (GTC−1obsG)−1GTC−1

obsCobsC−1obsG(GTC−1

obsG)−1

= (GTC−1obsG)−1 (6.8)

6.1.2 Prior information and constraints in the linear case

One may wish to include prior information from other soundings, explanatorydrillings etc. in the inversion. This prior information may be regarded as anew data set, mprior, with error eprior in the form of a stochastic vector sothat (Jacobsen, 1993):

mprior = mtrue + eprior (6.9)

Cpriori,j= cov(epriori

, epriorj)

We expect the mean value E[eprior] to be zero.

Constraints may be included as well. Two examples are MCI and LCI inver-sions where horizontal constraints on resistivities and depths are commonlyused. For different reasons one may also wish to apply a very strong verticalconstraint on resistivities, thereby creating a ”pseudo layer”. If constraintsare included in the inversion one may write (Jacobsen, 1993):

r + ec = Rmtrue (6.10)

Cc = cov(epriori, epriorj

)

We still expect the mean value E[ec] to be zero. Since the meaning of theconstraints are that the constrained parameters should be the same, r is azero-vector.

Combining both constraints and prior information we get:⎡⎣ G

IR

⎤⎦mtrue =

⎡⎣ dobs

mest

r

⎤⎦ +

⎡⎣ eobs

eprior

ec

⎤⎦ (6.11)

40 6.2 The non-linear case

The roughening matrix may be written as:

Rp =

⎡⎢⎢⎢⎣

1 0 · · · 0 −1 0 · · · 0 0 00 1 0 · · · 0 −1 0 · · · 0 0...

......

0 0 0 · · · 0 1 0 · · · 0 −1

⎤⎥⎥⎥⎦ (6.12)

This matrix contains the constraints at the location of the 1’s and -1’s. Thecovariance matrix, Crp, of this is assumed to be a diagonal matrix (Aukenet al., 2003a).

The joint covariance matrix is:

C =

⎛⎝ Cobs 0

¯0¯

0¯

Cc 0¯

0¯

0¯

Cprior

⎞⎠ (6.13)

According to Jacobsen (1993), equation 6.11 may easily be expanded to thefull solution of a linear inversion problem containing both data, prior valuesand constraints:

mBLUE = (GTC−1obsG + C−1

prior + BTC−1c B)−1 ·

(GTC−1obsdobs + C−1

priormprior + BTC−1c r) (6.14)

6.2 The non-linear case

Unless otherwise stated, the non-linear case is based on Auken et al. (2003a).

In the linear case, the model can be found from data in one step. This isnot possible in the non-linear case, where the model, mtrue, is found througha stepwise linear approximation based on a reference model, mref . This isalso called a Gauss-Newton approximation. Equivalently to equation 6.1, thegeneral non-linear problem may be expressed as:

dobs∼= g(mref ) + G(mtrue − mref ) + eobs (6.15)

or in short:δdobs = Gδmtrue + eobs (6.16)

g is the non-linear mapping of the model to data space. It is a vector contain-ing non-linear but typically differentiable coordinate functions - the ”mildlynon-linear case”. G is the Jacobian matrix. It contains all the partial deriva-tives of the mapping. We assume the observational errors, eobs, to be unbi-ased, so the covariance matrix, Cobs, is diagonal.


6.2.1 Prior information in the non-linear case

The application of prior information on primary parameters for a mildlylinear problem is analogue to the application of these in the linear case, i.e.as the inclusion of an extra data set:

Ppδmtrue = δmprior + eprior (6.17)

in which eprior is the error on the prior model with expectation value zero, Pp

is the identity matrix of same dimension as the model vector and δmprior =mprior − mref . Subscript p will be used to denote primary parameters, thatis resistivities and thicknesses.Covariance matrix Cprior describes the variance of the prior model. Errorsin prior guesses are likely to be correlated, so Cprior could have off diagonalelements, as opposed to the covariance matrix for the data vector. This isovercome by introducing lateral constraints, which are connected to the truemodel as:

Rpδmtrue = δrp + erp (6.18)

where erp is the error on the constraints, the expected value of which is zero,and

δrp = −Rpmref (6.19)

thereby making the effective roughening:

Rpmtrue + erp = 0 (6.20)

This is equivalent to equation 6.10. The roughening matrix is equivalent toequation 6.12.

6.2.2 Inversion

Similar to equation 6.11, the inversion problem may so far be written as:⎡⎣ G

Pp

Rp

⎤⎦ · δmtrue =

⎡⎣ δdobs

δmprior

δrp

⎤⎦ +

⎡⎣ eobs

eprior

erp

⎤⎦ (6.21)

or:G′ · δmtrue = δd′ + e′ (6.22)

where G resembles the partial derivatives and d’ the data. The covariancematrix for the joint observation error, e’, is:

C′ =

⎡⎣ Cobs 0

Cprior

0 Crp

⎤⎦ (6.23)


If only diagonal error covariances are used, the least squares sum may bewritten as:

Q =

(1

N + M + A

N+M+A∑i=1

[δd2

i

var(e′

i)

]) 1

2

(6.24)

where A is the number of constraints on primary parameters, and M thenumber of primary model parameters. Similarly to equation 6.5, equation6.24 is minimized through the model estimate:

δmest =[G′TC′−1G′

]−1

G′TC′−1δd′ (6.25)

6.2.3 Prior depth values and constraints

Lateral constraints on depths may be advantageous under Danish conditions,but including these proposes a separate problem. This is because the forwardmodelling code is based on the logarithm of the primary parameters and

log(zj) =

j∑n=1

log(tn) (6.26)

where zj is the depth to the jth interface

Adding prior information on depths, the solution may be written as:

Pzδmtrue = δmz−prior + ez−prior (6.27)

but since δmz−prior = zprior − Pzmref , equation 6.27 is effectively

Pzmtrue = zprior + ez−prior (6.28)

The vector, zprior contains the logarithm to the values to which the individ-ual depths are constrained. ez−prior is the error on prior data, again withexpectation value zero. The variances in prior data are given in Cz−prior,which is once more assumed to be a diagonal matrix.In other words lateral constraints on depths are included in much the sameway as lateral constraints on primary parameters.

Including prior depth values and constraints, equation 6.21 may be expandedto: ⎡

⎢⎢⎢⎢⎣GPp

Pz

Rp

Rz

⎤⎥⎥⎥⎥⎦ · δmtrue =

⎡⎢⎢⎢⎢⎣

δdobs

δmprior

δmz−prior

δrp

δrz

⎤⎥⎥⎥⎥⎦ +

⎡⎢⎢⎢⎢⎣

eobs

eprior

ez−prior

erp

erz

⎤⎥⎥⎥⎥⎦ (6.29)

43 6.3 Em1Dinv

or in short:G′ · δmtrue = δd′ + e′ (6.30)

The total covariance matrix for the system becomes

C′ =

⎡⎢⎢⎢⎢⎣

Cobs

Cprior 0Cz−prior

0 Crp

Crz

⎤⎥⎥⎥⎥⎦ (6.31)

Variances on lateral constraints are given in Crz.

Writing the model estimate given in equation 6.25 as an update at the nth

iteration yields:

mn+1 = mn + ([G′Tn C′−1G′

n + λnI]−1 · [G′T

n C′−1δd′

n]) (6.32)

λ is the Marquart damping factor.

Expanding 6.31 with respect to 6.29 we get:

mn+1 = mn +([

GTC−1obsG + C−1

prior + PTz C−1

z−priorPz + RTp C−1

rp Rp+

RTz C−1

rz Rz + λnI]−1

·[GTC−1

obs(dobs − g(mn))+

C−1prior(mprior − mn) + PzC

−1z−prior(zprior − Pzmn)+

RTp C−1

rp (−Rpmn) + RTz C−1

rz (−Rzmn)])

(6.33)

where g(mn) is the non-linear forward response of the nth model.

6.3 Em1Dinv

Em1Dinv is an 1D inverse modelling code used to analyse and invert electro-magnetic and geoelectric measurements designed by Esben Auken, Hydro-Geophysics Group at the University of Aarhus. It is based on a 1D model,and responses can be calculated in both time and frequency domain. In theformer, both high and low pass filters are supported. Both the receiver andthe transmitter can be placed in the earth (only in the case of the verticalmagnetic or electric dipole for the transmitter), on the ground or in the air.Furthermore, the receiver can be of random polarization (Hx, Hy, Hz or Ex,Ey, Ez). The inversion is done in different modules, meaning that it can beused for joint inversion as well as for MCI and LCI. The inversion is carried

44 6.3 Em1Dinv

out as an iterative least squares approach, as seen in equation 6.33.

Three residuals are calculated in the inversion; a data residual, RESdata,a residual related to the vertical constraints, RESvertical, and a residual re-lated to the lateral constraints on primary parameters, RESlateral. These aregiven in equations 6.34 through 6.36:

RESdata =1

Nd

[ Nd∑i=1

(dobsi − dmodel

i )2

var(dobsi )

]1/2

(6.34)

RESvertical =1

NM · (NP − 1)/2

[ NM∑i=1

(NP−1)/2∑j=1

(mi,j − mi,j+1)2

var(mi,j)

]1/2

(6.35)

RESlateral =1

(NM − 1) · NP

[ NM−1∑i=1

NP∑j=1

(mi,j − mi+1,j)2

var(mi,j)

]1/2

(6.36)

Nd is the sum of the number of data and lateral constraints on depths, NM

is the number of models and NP is the number of parameters.

A residual showing the weighted sum of the three residuals mentioned aboveis also calculated:

Restot =RES2

dataNd + RES2verticalNM(NP − 1)/2 + RES2

lateral(NM − 1)NP

NP + NM(NP − 1)/2 + (NM − 1)NP

(6.37)It is the above residual, which is equivalent to equation 6.24, that is mini-mized in the inversion.

According to Auken et al. (2003a), the Marquart modification via the pa-rameter λn and an adaptive damping on the step size for the model update isused to stabilize the inversion. This step size is evaluated after each iteration,and determined based on the success of the previous iteration. This is seenin equation 6.33. These two factors should help make the inversion processrobust. Safe convergence is achieved by starting the iteration from horizontallayers with equal resistivities. The inversion is carried out on the logarith-mic to the model parameters. The standard deviations given in this thesishave been converted, thereby giving a value that is more easily understoodintuitively - one that you can divide or multiply with.

45

7 Data and data acquisition

This chapter contains a description of the data used in this thesis and theacquisition of them.

The field area is located in South Jutland, close to Tinglev. A map showingthe area including the location of the seismic section, HMTEM soundingsand exploratory drillings is seen in figure 7.1.

Figure 7.1: The field area including the seismic section, HMTEM soundingsand exploratory drillings.

These data were originally gathered as part of the project ”Grundvand tilSønderjylland og Schleswig - et grænseoverskridende projekt”, a co-operationbetween Sønderjylland County’s department of groundwater and LANU. Thisproject is partly funded by the EU via the INTERREG II program. Thepurpose of the project is to ensure the water supply of South Jutland andSchleswig.

46 7.1 HMTEM soundings

7.1 HMTEM soundings

The high moment TEM (HMTEM) data were gathered in June 2001 by my-self and fellow students using an analogue Protem 47 receiver. 66 soundingswere carried out, including 5 test soundings8. Soundings were placed as closeto the seismic section as possible while trying to place them 250 m apart.Maps showing the location of buried gas pipes, electrical wires etc. had beenobtained in advance and was used when choosing where to place the sound-ings. Nevertheless, as the field area consists mainly of farmlands with manyfences, 14 of the 61 ordinary HMTEM soundings had to be completely aban-doned due to noise. Furthermore, 9 of the remaining 47 soundings hold onlyone interpretable segment. The field area is mainly flat, with topographyranging from 20 m to 26 m. The topographic levels of the measurements arebased on digital topographic maps from the Danish KMS (Kort og Matrikel-styrelsen).

A general problem concerning a biased signal was recognized in all sound-ings. At late delay times, the earth response is so small that the biased signal,which was alternately subtracted from and added to the earth response asthe reference cables were shifted between measurements , could cause diverg-ing sounding curves. A close-up of sounding Ti010 is seen in figure 7.2

Figure 7.2: Close-up of the diverging sounding curves at late times of sound-ing Ti010.

The problem was not so severe that data were un-interpretable, but did,

8A test sounding is a sounding which is repeated in the same place each day to testthe state of the equipment.

47 7.1 HMTEM soundings

however, necessitate that data were cut off at earlier delay times that wouldotherwise have been necessary.

A report containing data processing and strictly geophysical interpretation,was previously made by the HydroGeophysics Group, University of Aarhus.The results were delivered to Sønderjylland County. The coordinates of thesoundings from June 2001 can be seen in appendix A.

An additional 13 soundings were carried out in September 2002. The mainpurpose of these soundings was to achieve a greater penetration depth. Thus,these were carried out not only in the conventional manner, but also witha 40 m times 40 m transmitter loop consisting of a thicker wire than usual(16 mm2). This should result in a greater magnetic moment, thereby in-creasing the penetration depth. These soundings were placed in areas wherean increased penetration depth was desired and preferably where more datawas needed. Based on experiences from the survey of June 2001 sites withmuch cultural noise were avoided. Data were recorded with a digital Pro-tem 47 receiver, kindly put at my disposal by Watertech a/s, Aarhus. Therewas a problem with this equipment, which was so severe that data were un-interpretable, and they had to be completely abandoned: sounding curvesdid not overlap, and in some cases sounding curves from different segmentscrossed. Sounding curves from one of these soundings, Ti102, are shown infigure 7.3. Later examinations of the equipment showed that the problemconcerned moisture in the components.

Figure 7.3: Sounding Ti102; one of the soundings from September 2002which had to be abandoned.

48 7.2 Seismic data

7.2 Seismic data

Reflection seismic data were gathered in spring and summer 2001 by theDepartment of Earth Sciences, University of Aarhus under the supervisionof Holger Lykke-Andersen. Data were gathered along a 20 km long pro-file running from the Danish-German border in the south to Tinglev andfrom thereon west towards Heds. The receiver was a 48 channel GeometricsStrataview receiver. A spacing of 5 m between receivers was used. Seis-mic sources were in the form of 50 gram of explosives placed in two meterdeep holes drilled into the ground, except in the part of the profile that ranthrough Tinglev, where a seismic vibrator device was used.

Further seismic data were gathered south of the border by Institut fur Ge-owissenschaften at Christian-Albrechts University of Kiel.

7.3 Exploratory drillings

As most exploratory drillings in the area are quite shallow, at least comparedto the penetration depth of the HMTEM soundings, only data from a fewof them have been used as a source of information in this thesis. One ofthese, DGU no. 168.1378, was drilled as part of the project ”Grundvand tilSønderjylland og Schleswig - et grænseoverskridende projekt” just south ofTinglev. It is 415 m deep and is located very close to the seismic section andapproximately 200 m from soundings Ti003 and Ti004. Lithologic sampleswere taken for every meter of the drilling, thereby giving a very good descrip-tion of the sediment. The lithological log (as a drilling data summary sheet)has been used in the interpretations. Water samples were taken from twofilters at depths of approximately 340 m and 370 m. Analysis showed thatit was saline with a concentration of approximately 3000 mg/l and 74 mg/l,respectively. For comparison, a typical concentration found in sea water is30.000 mg/l. According to Thomsen (2003), this is probably residual water.

A total of six other deep exploratory drillings were used in the interpre-tations: DGU no. 167.234A and 167.234B, located in the most western partof the profile (depths 596 m and 172 m, respectively), DGU no. 168.1228,168.1193 and 168.16B, located in Tinglev (depths 221 m, 241 m and 119m, respectively) and LANU no. 1121/53 which is just south of the border toGermany (depth 240 m). Only lithological logs were available from these, theLANU drilling only as depths to formations. The locations of the exploratorydrillings can be seen in appendix B.

49 7.4 Logs

7.4 Logs

A vertical seismic profiling log (VSP-log), a spectral gamma-ray log (SNG-log), a natural gamma-ray log (NG-log) and a focussed electrical log (FEL-log) were used in drilling DGU no. 168.1378. A plot of the resistivity andgamma-ray log data is seen in figure 11.1, in connection with the interpreta-tion of these, while the VSP-log is shown in figure 7.4. VSP-log data weregathered by the Department of Earth Sciences, University of Aarhus, SNG-log data by Uffe Korsbech, DTU, and gamma-ray log and FEL-log data byLANU.

The penetration depth of the focused electrical log was as low as 20 to 50cm, probably closest to the former. Both gamma-ray logs average over aball shaped volume of earth with a diameter of 50-60 cm giving a penetra-tion depth of 25-30 cm. This focusing method effects the resolution of layerboundaries which will not be found as sharp boundaries (Korsbech, 2003).Caution should be taken when interpreting data from the SNG-log from be-low a depth of 402 m, since there are indications that the log had alreadyreached the bottom of the borehole at that point (Korsbech, 2003).

VSP-log data were gathered by lowering a 21 m long streamer into the bore-hole, below the groundwater level. Attached to it was 12 hydrophones usedto record the seismic waves from the source. This was placed almost di-rectly above the borehole (a zero offset VSP-log). After recording data, thestreamer was lowered further, and the process was repeated until the bottomof the borehole was reached. A plot of the VSP-log is seen in figure 7.4.According to the log the seismic velocity increases from approximately 1750m/s to 1950 m/s from the top of the borehole to the bottom. The VSP-logis not actually used in the interpretations, but it is a very good source ofinformation about the seismic velocities in the vicinity of the borehole.

Figure 7.4: The VSP-log from the deep exploratory drilling with DGU no.168.1378 (full page figure on the following page).

51

8 Programs

The purpose of this chapter is to give an introduction to the most frequentlyused programs in working on this thesis.

8.1 SiTEM and Semdi

SiTEM and Semdi are commercial programs developed by the HydroGeo-physics Group at the Department of Earth Sciences, University of Aarhus.

SiTEM is used for processing transient electromagnetic data. It importsraw data from TEM receivers, and merge this with information about setupgeometry, recording processes and instrument filters. With SiTEM one canremove bad data or add of extra noise to it. Data plot windows show TEMdata converted in various ways such as dB/dt or late-time apparent resistiv-ity, making it very easy to work with. Output data are saved as a .tem-file,which among other things contain information about the geometry of thearray. This means that when HMTEM measurements are processed, twodifferent files must be saved (one for the central loop data set and one forthe offset data set).

Semdi is short for Single site Electromagnetic Data Inversion. It is a graph-ical user interface to the inversion engine Em1Dinv, discussed in section 6.3.With Semdi, one can set up model files for inversions (.mod-file), see theresults and possibly edit them for reinterpretation. Multiple windows showrecorded data, model parameters, model parameter analysis and forwarddata. Semdi supports single-site inversion and MCI.

8.2 Read emo

Semdi would normally be the natural choice when working with single-siteand MCI interpretations. Semdi is, however, not designed for use with LCIinterpretations, so a program that is, was needed.

Read emo is a macro designed for use with the program Matlab. To be-gin with, Read emo was designed only to find the true models from the LCIinterpretation, i.e average the central loop and offset data sets, but it soonbecame clear that new functions were needed:

1. The user interface should be clear and easy to use since working on thisthesis would involve frequent use of the macro.

52 8.2 Read emo

2. Since data residuals play a major role in interpreting TEM data, themacro should be able to calculate these for every data set of the LCImodel.

3. The macro should be able to plot input and output TEM data alongwith error bars to help with the interpretation.

4. The data from September 2002 held three data sets per sounding9,so the macro should be able to cope with this, while finding the truemodels.

5. The data residuals and true models, the analysis file and model file,should be written in a text file, the latter formatted in a way to allowit to be imported directly into an Access database and from there onto Geobase (see section 8.3).

Read emo reads from an .emo-file (output file from Em1Dinv, see section6.3) specified by the user. The macro reads the number of data sets and thenumber of iterations. From these it determines where in the .emo-file to readfurther data from automatically.

After reading this information, Read emo proceeds to the actual processof finding the true models. The order of data in the .mod file (input modelfile from Em1Dinv) is based on the geographical location of the soundings,and hence so is the order of the data sets in the .emo-file. As some datamay have been abandoned, this means that data are not always given in analternating cycle of central loop and offset data sets. By evaluating the firstcharacters of the .tem-file-names, the macro identifies which data sets belongto the same sounding.

The statistically true standard deviations (in short STD) of two parameters,Da and Db, are based on the values of these and their standard deviations,STDa and STDb:

STDtruea= DaSTDa

(8.1)

STDtrueb= DbSTDb

The true mean values, Dtrue and STDtrue, are based on STDtrueaand STDtrueb

:

Dtrue =Da/STD2

truea+ Db/STD2

trueb

1/STD2truea

+ 1/STD2trueb

(8.2)

9Three different array geometries were used in these measurements. As mentioned insection 7.1, data were later abandoned.

53 8.2 Read emo

STDtrue =1

1/STD2truea

+ 1/STD2trueb

(8.3)

Standard deviations can either be formatted as 0.xx or 1.xx10. The user se-lects how with the setting STD format. Furthermore, by settingConvert STD = 1, a standard deviation of -1 will be shown as 99 - as it isin Semdi.

There are, of course, no depths, thicknesses and standard deviations of theseparameters for the last layers, but since a parameter value is needed for ev-ery layer, dummy values are given. The thickness of the last layer is setto 50 m (Zlast = Zlast−1 + 50m) if it is not a pseudo layer. If it is, theaccumulated thickness of the pseudo layers is set to 50 m. Since standarddeviations of depths and thicknesses inside a pseudo layer have no meaning,the user has the option to assign a dummy value to these too (by settingConstraint pseudolayer = x, where x denotes the vertical constraint on re-sistivities between pseudo layers in the model file).

The user has the option to cut off soundings holding only the central loop dataset or both data sets at a selected depth by setting Penetration depth LM =z or Penetration depth HM = z, respectively, where z denotes the selecteddepth.

After finding the true models, Read emo goes on to read the parametersrequired to calculate RESdata, given in equation 6.34. All parameters in-volved in this calculation are read from the .emo-file. An option in the macro(setting Use standard deviation = 0) allows the user to base RESdata on astandard deviation of 0.05, instead of the one read from the emo-file. Dataresiduals are plotted in a figure (as seen in figure 8.1), and the user is giventhe option to see input and output TEM data for either a single data set, orfor those belonging to a distinct sounding.

By setting Write model file = 1 and Write analysis file = 1, it is possi-ble to have the model and analysis files written as text files. These will bydefault be placed in the same directory as the .emo-file, and their names willbe based on the name of this.

10At the department of geophysics, standard deviations are usually not given as factorsyou can divide or multiply with.

54 8.3 MapInfo and Geobase

Figure 8.1: Data residuals are plotted in a window so the user can choosewhich dataplots to see.

Various information such as resistivities, thicknesses, depths, standarddeviations, prior informations, vertical and lateral constraints and names of.tem-files are stored for the user to see if desired. Read emo is included onthe CD-ROM.

8.3 MapInfo and Geobase

MapInfo is a Geographical Information Systems (GIS) program - a mappingsolution. It can be used to create maps showing different data such as roads,shops, pipe lines, power lines etc.

Geobase is a MapInfo application. It was developed by Watertech a/s forVejle County to help make a geological model of the county. When addingdata sets such as geophysical data, hydraulic head data, exploratory drillingsetc. in MapInfo, Geobase makes it possible to create 2D sections onto whichthese data can be projected. While working with Geobase a previously un-known error was encountered. A large projection distance could cause datato be projected onto a wrong place on the section, but tests showed it couldbe overcome by lowering the projection distance.

55 8.3 MapInfo and Geobase

8.3.1 The MapInfo workspaces

MapInfo workspaces showing the HMTEM soundings, seismics and exploratorydrillings on a 2D section have been created using Geobase. These are avail-able on the CD-ROM, and as printouts in appendix E.

The seismic data were processed to a level of zero. A velocity of 1800 m/swas used in the depth conversion of the data, and therefore also to registerthe data in Geobase.

The position of the southernmost drilling, LANU no. 1121/53, has beenchanged in order to show it on the 2D section. Its location is approximately1400 m further south-southwest (for correct position, see appendix B).

A little note concerning the layouts with model parameter analysis: Standarddeviations of layer boundaries and resistivities have been added to these, theformer as diamonds located at layer boundaries and the latter as bars ontop of the ordinary sounding bars. The model parameter analysis bars coverthe same depth interval as the sounding bars, but have been made a littlenarrower, in order to make the latter visible.

56

9 Geology of southern Jutland

The regional geology of southern Jutland will be described in this section.Unfortunately, the amount of literature on this specific subject is rather lim-ited, and as a result of this, the majority of literature used is concerned withthe country as a whole, not southern Jutland specifically. Pre-Cretaceousdeposits will only be presented briefly in section 9.1, as they are only foundat great depth in the area.

9.1 The Pre-Cretaceous period

In the Permian, Triassic and Jurassic periods the Danish area was divided intwo depositional areas; the Danish Bassin to the north and the North GermanBassin to the south. These were more or less divided by the Ringkøbing-FynHigh through the Permian and Triassic. Pangaea broke up in the Jurassicperiod, and the Danish area including the Ringkøbing-Fyn High was upliftedresulting in increasing erosion. Sediments from these periods are clayey, siltyand sandy. The material came from the north and northeast, and becomegradually more fine grained towards the south.

Diapirism connected with Permian salt deposits has played a great part inthe area. In the Triassic period, these deposits were deformed plastically andstarted to elevate as a result of their low density compared to the overlyingsediments. This process is to some extent still going on today (Michelsen andBertelsen, 1978). Salt diapirs are especially common in Schleswig-Holstein,where they have had a great effect on Tertiary sedimentation (Friborg et al.,2002).

9.2 The Cretaceous period

The Cretaceous period began approximately 135 Ma ago with a transgres-sion; in the Upper Cretaceous 82 % of the Earth’s surface was covered bywater compared to 71 % today. The result was that in the middle of theLower Cretaceous period the sediments changed gradually from predomi-nantly terrestrial clays and sands to biogenic, pelagic limestones. Tuxen Fm,Sola Fm and Rødby Fm belong to this period (Thomsen, 1995).

In the Upper Cretaceous limestone continued to be deposited. First as hardcalcareous clays and marls, but later, in Campanian and Maastrichtian, assoft chalk with a very low content of terrestrial material. In Denmark thisis known as Skrivekridtet. Apart from having a very high content of lime, it

57 9.3 The Tertiary period

contains a large amount of fossils. It seems to have been a tectonically calmperiod with warm and dry climate and high eustatic water level (Thomsen,1995).

It was, however, also in the Cretaceous period that an active spreading ofthe North Atlantic sea bottom began; 83 Ma ago the sea bottom betweenGreenland and North America started to spread and 57 Ma ago spreadingbegan between Greenland and Europe. While the former gradually stoppedduring the Eocene period, the latter is still active today (Bjørslev Nielsen,1995). On a more global scale the collision between the African and Eu-ropean plates started in the Upper Cretaceous. They also started to moveaway from the South and North American plates. This collision resulted invast deformations, bassins and depositions with the Alpine mountain rangeas one of the results (Bjørslev Nielsen, 1995).

9.3 The Tertiary period

The Tertiary began with the mass extinction 65 Ma ago and a general low-ering of the sea level. It is divided into the Lower - and Upper Tertiary,also known as the Paleogene and Neogene period. The former is again sub-divided into the Paleocene, Eocene and Oligocene periods, while the latteris subdivided into the Miocene and Pliocene periods. A stratigraphic chartshowing the Tertiary sediments found in the southern part of Jutland and inDenmark in general can be seen in figure 9.1.

The geology of southern Jutland, and Denmark in general, has been in-fluenced by the Scandinavian uplifting and the sinking of the North Sea areawhich, together with sedimentation, resulted in alternating series of trans-gressions and regressions throughout the Tertiary (Skovbjerg Rasmussen(1996); Skovbjerg Rasmussen (1998)).

The Tønder Graben is a northwest-southeast going graben structure. Thenorth side is step-like system of faults with a few hundred meters offset in thebasement, while the south side is a more classical graben structure formed bysalt tectonics. It was active during the Tertiary which is assumably linked tothe Alpine folding (Sorgenfrei (1966); Lykke-Andersen (2003); Friborg et al.(2002); Thomsen (2003)).


Figure 9.1: Stratigraphic charts of sediments found in Denmark in general(a) and specifically in South Jutland (b) (from Larsen and Kronborg(1994), fig. 3 and Skovbjerg Rasmussen (1996), fig. 2).


9.3.1 The Paleogene period

At the beginning of the Paleocene period, a 2-3 cm thick, fossil poor layer ofclay, which is known as fiskeleret in Denmark, was deposited. Otherwise, thedeposition of limestone continued in the Paleocene. The limestone from Da-nian is composed of skeletons from colonial organisms called bryozoans andit contains flint nodules. There is a tendency towards a more fine grained,fossil poor limestone in the southern part of the country. Lellinge GrønsandFm and Kerteminde Marl Fm were deposited in Selandian. While these sed-iments are still calcareous, they contain more terrestrial material as clay andsilt than the Cretaceous deposits do (Thomsen, 1995).

According to Heilmann-Clausen (1995), the shift from clean, biogenic chalkto clays is connected to the major geographical events of the Danian period:Thermic uplifting of the British Isles connected to the development of theIcelandic hot spot and the volcanic activities around Greenland, the FaraoeIslands and Scotland. In addition, both the connections with the CentralEuropean oceans and the Atlantic Sea north of Scotland and via the En-glish Channel were cut off, leaving only the connection with the cold seasto the north open. The latter was connected to the Laramidic tectonics ofthe Alpine area. Thus, the biogenic calcareous sedimentation being put toa halt (Heilmann-Clausen (1995); Larsen (2002)). As a result of the beforementioned uplifting of the British Isles, the amount of clastic material in theoceans from the British area was rather large in this period, but materialfrom Fenno-Scandia was also abundant (Heilmann-Clausen, 1995).

After Danian a transgression took place and the Danish area was totallycovered by water, probably of great depth. The nearest land area was theFenno-Scandia Shield, but even that was far away. Furthermore, it was prob-ably of low relief, at least in the Eocene period.

The Upper Paleocene Æbelø Fm and Holmehus Fm are both clays. The for-mer is a silty, non-calcareous, dark gray clay that overlies Danske Kalken11

in some areas of Denmark (Heilmann-Clausen, 1995).

Holmehus Fm is a more fine-grained, virtually lime free, heavy clay. BothÆbelø Fm and Holmehus Fm contain considerable amounts of smectite ofvolcanic origin; occasionally above 90% for the latter. The colour of Holme-hus Fm is greenish, bluish or reddish (Heilmann-Clausen, 1995).

11Danian limestone is also called ”Danske Kalken”.


During a short regression in the Upper Paleocene, Østerrende Clay, a siltyclay, was deposited, but the transgression soon took over again and the non-calcareous Ølst Fm from the Upper Paleocene to Lower Eocene period wasdeposited at open sea. Its lower part (also called ”Stolle Klint Clay”) is ahighly laminated clay that contains layers of volcanic ashes from the openingof the North Atlantic Ocean between Greenland and Norway. The upper partof Ølst Fm is silty and not quite as laminated. Its ash layers are consideredto be a result of volcanism in the Greenland area. Ølst Fm is not found inthe northwestern part of Jutland where the contemporary Fur Fm is foundinstead (Heilmann-Clausen, 1995).

Whereas Østerrende Clay and Ølst Fm are both lime free, Røsnæs Clayfrom Eocene contains lime. Its colour is predominantly red as a result ofiron-rich minerals, but it contains green ash layers. It was deposited in a pe-riod with low sedimentation rate and can be found as far south as Hamburg(Heilmann-Clausen, 1995).

No lime is, however, present in the Eocene Lillebælt Clay, a clay that inthe lower part is greenish-reddish and dark greenish-gray in the upper part.Its few ash layers represent the last volcanic activity involved with the NorthAtlantic Sea spreading. More sandy, near-shore layers of clay and marl aregradually introduced in Schleswig-Holstein (Heilmann-Clausen, 1995).

A new connection between the North Sea and the Tethys Sea through theEast European areas seemed to have opened up in the Upper Eocene, therebyreintroducing warmer water and biogenic lime production. The result wasthe deposition of Søvind Marl, an Upper Eocene, light gray (almost white),clayey marl deposited in deep, open waters (Heilmann-Clausen, 1995).

In the Oligocene there was a drop in sea level which, combined with thegeneral uplifting of the Scandinavian area, led to extensive erosion. ViborgFm and Branden Clay, both deposited in Lower Oligocene, were the first sed-iments to introduce mica in noticeable amounts, but are only found in thearea around Viborg and in the Danish Sub Basin today (Heilmann-Clausen(1995); Larsen (2002)).

9.3.2 The Neogene period

Vejle Fjord Fm is found throughout the middle and southern parts of Jutland.It was deposited in the Upper Oligocene to Lower Miocene in a regressiveperiod with a humid and temperate climate. It contains both mica, shell


fragments and organic material in the form of plant residue. Vejle Fjord Fmis usually divided into three facies, although Skovbjerg Rasmussen (1995)suggests that a fourth is needed. The three units are called the BrejningClay, the Vejle Fjord Clay and the Vejle Fjord Sand.

Despite being a mica clay, Brejning Clay is believed to have been depositedpartly in a shelf environment and partly in a quiet, tidally-influenced lagoonalenvironment. It is the latter part, which is more silty, that Skovbjerg Ras-mussen (1995) proposes as the fourth facies. The material was probablytransported to the Danish area very quickly by rivers from Fenno-Scandia,but most of it is believed to have been deposited before reaching the Jutlandarea and the sedimentation rate is believed to have been low. Vejle FjordClay and Vejle Fjord Sand are also believed to have been deposited in la-goonal environments. The former is dark and mica-rich and was depositedin the intertidal environment of the lagoon. Its sandy elements are believedto have been deposited when water broke through the barrier islands duringstorms. Vejle Fjord Sand consists of sand from these barrier islands, washoverfans and beach deposits at or near the barrier islands (Skovbjerg Rasmussen(1995); Dybkær and Skovbjerg Rasmussen (2000); Heilmann-Clausen (1995);Friis (1995)).

The continental Ribe Fm from the Lower Miocene period consists of coarsesand to fine gravel. It is found in the southern and southwestern part ofJutland. In the southwestern part there is a change in facies through deltaicsediments to the marine Arnum Fm, a mica-rich sediment consisting of clay,sand and gravel. The two formations are found both overlayering and inter-twining with each another. It has been proposed that Ribe Fm is partly thecontinental equivalent of Arnum Fm, and that the latter consists of materialthat was first deposited in flood plains (Ribe Fm) and then washed out to sea.Arnum Fm and Ribe Fm were deposited in a period of limited transgressionsand regressions. The varying amount of heavy minerals in the former sug-gests varying levels of erosion (Friis, 1995). In mid and southern Jutland asand layer, Bastrup Sand, is found in the Arnum Fm (Skovbjerg Rasmussen,2001). The extent of the Ribe Fm has lately been established in RingkøbingAmt et al. (1999).

Odderup Fm is a common denominator for a series of non-marine, coal bear-ing deposits from the mid Miocene to Pliocene period that were depositedin the eastern parts of the country. It contains a wide variety of lithologiesspanning from heavy, lacustrine clays to gravels and sands deposited in rivers(Friis, 1995).


During a transgression in mid Miocene, the sediments shifted towards a black,marine mica clay known as Hodde Fm. The basis contains sandy and grav-elly elements that is connected with this transgression. Hodde Fm is foundin the western part of Jutland. It has been shown that the transgressionhappened gradually, the sediments found in the southwestern part being theoldest. Fossils found in the sediment point towards an oxygen-poor bottomenvironment (Friis, 1995).

This transgression continued, and in the Upper Miocene and Pliocene moreopen waters with oxygen-rich bottom environments were found. In the west-ern and southwestern parts of Jutland, heavy, dark, marine clay called GramFm, was deposited. This becomes more silty upwards, and in the uppermostpart, belonging to the Pliocene period, an influence from fresh water hasbeen found in southwest Jutland. No coastal sediments corresponding to themarine sediments have been found, thereby implying that the extent of thisformation is a result of erosion (Friis, 1995). Figure 9.2 shows the Neogenedeposits.

Figure 9.2: The Neogene deposits (after Dybkær and Skovbjerg Rasmussen(2000), fig. 2).


Sæd Fm is a series of fluviatile and marine sediments that are local to thesouthwestern part of Denmark. It has not been found in the area and willnot be discussed further here. A figure showing the dynamics of the LateOligocene to Pliocene transgressions and regressions in South Jutland can beseen in figure 9.3.

Figure 9.3: The dynamics of the Tertiary transgressions and regressionsfrom Late Oligocene to Pliocene in South Jutland (modified fromSkovbjerg Rasmussen (1996), fig. 9).

64 9.4 The Quaternary period

9.4 The Quaternary period

The Quaternary period began approximately 1.7 Ma ago. It has been a pe-riod with alternating cold and warm periods. In the cold periods, the IceAges, glaciers appeared in the Scandinavian highland, later spreading to thesurrounding lowlands including Denmark. During the interglacial stages, theice disappeared. Indications of four Ice Ages and three interglacial stageshave been found in Denmark. The Ice Ages are called Menap, Elster, Saaleand Weichsel, and the interglacials stages Cromer, Holstein and Eem (seefigure 9.4).

Figure 9.4: Indications of four Ice Ages (Menap, Elster, Saale and Weichsel)and three interglacial stages (Cromer, Holstein and Eem) have beenfound in Denmark (based on Kronborg (1995), fig. 9).

The oldest Quaternary marine sediments found in Denmark have usuallybeen considered to be of Elsterian age, but according to Knudsen (1995) itis likely that at least some of them are older. The majority of Quaternarydeposits found in South Jutland and Schleswig-Holstein are from the Saalianand Weichselian Ice Ages and are found in three main landscape forms: hillislands12, outwash plains and Weichselian moraines. Elsterian sediments arealso found, but not in the vicinity of the field area (Friborg et al., 2002).The Eemian interglacial stage and the Ice Ages Saale and Weichsel will bepresented in sections 9.4.1 through 9.4.3, with emphasis on the latter.

Hill islands are remnants from Saale. They were originally topographic highsthat were surrounded and partly covered by glacial deposits during the We-

12Hill islands are relict glacial hills. In danish they are called ”bakkeøer”.


ichselian Ice Age. During and after this, they were eroded through slumpingand other periglacial processes.

Moraines may be deposited as ice-marginal moraines (e.g. as glacitectoniclandforms, push and squeeze moraines, dump moraines and latero-frontalfans and ramps) or as subglacial moraines (e.g. flutes, drumlins or flat tillplains). These will not be discussed here, but further information on themcan be found in Benn and Evans (1998).

Outwash plains are almost flat areas consisting of mainly sandy outwashmaterial. This was deposited by braided and meandering rivers of meltwa-ter from the glaciers. They are found in the low lying areas of the Saalianlandscape. The material has been sorted by the meltwater and the finest sed-iments are found furthest away from the location of the glacier front. Tinglevoutwash plain is found in the field area. According to Friborg (1996), thiscan actually be separated in two outwash plains, the upper deposited duringthe Weichselian and the lower during the Saalian. These are separated byEemian deposits which are of marine origin in the western part and of limnicorigin the eastern part.

Another important glacial landscape is the tunnel valleys; valleys lying par-allel to former ice flow direction. They are interpreted as valleys eroded bysubglacial meltwater, but some of them may also be a product of pre-glacialvalleys that were deepened by the glaciers or faulting. They often have un-dulating valley floors and vary between 0.5 km and 4 km in width and 25 mto 350 m in depth (Benn and Evans (1998); Jørgensen et al. (2003b)).

9.4.1 The Saalian Ice Age

According to Kronborg (1995), at least four ice advances are thought to havetaken place in Denmark in Saale. In chronological order, these are repre-sented by Haar I Till, Haar II Till, Hinnerup Till and Asklev Till, which arefound in Mid Jutland. They were deposited by ice advances coming from thenorth, except Haar II Till, which was deposited by an ice advance comingfrom the northeast. Two Saalian interstadials, Vejlby I and Vejlby II, areknown (Houmark-Nielsen, 1987).

Houmark-Nielsen (1987) recognizes three ice advances in the central partof Denmark, represented by the Trelde Næs Till, Ashoved Till and LillebæltTill. In chronological order, these ice advances are called the Norwegian IceAdvance, which invaded the land from the north and probably terminated


south of the Danish-German border, the Middle Saalian Advance, invadingthe land from the northeast, and the Paleobaltic Advance, which came fromthe east-southeast. According to Houmark-Nielsen (1987), there has beensome discussion on the extent of the two latter, but he suggests that the Mid-dle Saalian ice sheet may be equivalent to one that covered the larger partof the Netherlands, while the Paleobaltic Advance is likely to have reachedwestern Jutland.

During Saale Ice Age, South Jutland was completely covered by ice. To-day, Saalian sediments are found in the form of hill islands and sedimentsthat have been redeposited in Weichsel (Friborg et al., 2002). There is a slightdip of 1:1000 towards the west-southwest in the Saalian outwash plains ofSouth Jutland (Friborg, 1996), thereby indicating the position of the paleo-ice margin

9.4.2 The Eemian interglacial stage

The Eemian interglacial started about 130.000 years ago. The low-lyingparts of the country were flooded during Eem and marine sediments, suchas Cyprina Clay, are found in North and South Jutland. The distribution ofland and sea became somewhat similar to that of today (Houmark-Nielsen(1987); Kronborg (1995)). Animal bones, which were broken in order to gainaccess to the bone marrow, have been found at Hollerup in the central partof Jutland, showing the presence of humans at that time (Kronborg, 1995).According to Friborg (1996) and Knudsen (1995), both marine and limnicEemian deposits have been found in South Jutland, the former in an areastretching from the west coast to Jejsing Bakkeø in the central part and thelatter north and south of Jejsing Bakkeø.

9.4.3 The Weichselian Ice Age

The Weichselian Ice Age began approximately 115.000 years ago (Knudsen,1995). When not covered by ice, Denmark was a tundra plain, sufferingperiglacial conditions. The northern part of Jutland was, however, coveredby water. This led to vast erosion of the post-Saalian landscapes throughslumping and to the formation of ice wedges. According to Kronborg (1995),at least four ice advances are recognized in Denmark. In chronological orderthese are called the Kattegat Ice Stream, the Old Baltic Advance, the NorthEastern Advance and the Young Baltic Advance. Houmark-Nielsen (1987)proposes a different situation for the central part of Denmark, although thefour main ice advances correspond to those of Kronborg (1995).


The Kattegat Ice Stream was deposited by a glacier from southern Norway.It covered only the northern part of Denmark (Houmark-Nielsen, 2003).

The Old Baltic Advance came from the southeast through the Baltic Sea.It has been believed to have covered only the Danish Islands and parts ofsouthern Jutland, but later studies have suggested that it reached West Jut-land, where it covered the hill islands (Kronborg, 1995). Houmark-Nielsen(1987) does, however, propose that only the eastern part of South Jutlandwas covered in Weichsel.

Since the melting away of the ice from the Old Baltic Ice Advance, Den-mark was an arctic desert for around 50.000 years. Then, around 18.000years ago, a new ice advance, the North Eastern Ice Advance13, came fromcentral Sweden. It formed the main line of stagnation in Jutland, or ”Hove-dopholdslinien”, as it is known as in Denmark (Kronborg, 1995). Houmark-Nielsen (2003) dates the advance back to 21.000-20.000 years BP. Accordingto Houmark-Nielsen (1987), there were two readvances, the Fyn-WesternLimfjord Readvance and the Storebælt-North-Jylland Readvance, before theNorth Eastern ice sheet finally melted away.

The last major ice advance of the Weichselian, the Young Baltic Ice Ad-vance, took place about 15.000 years ago, a few thousand years after the icefrom the North Eastern Ice Advance had melted away. It came as far as east-ern Jutland. Melting away was interrupted by several smaller ice advances(Kronborg, 1995). Kjær et al. (2003) divides the Young Baltic Ice Advanceinto the East Jylland and Bælthav advances, and proposes that these tookplace 18.000 and 15.000 years ago.

Sediments from the previously ice covered areas of South Jutland are usu-ally glacially reworked meltwater sediments and clayey tills, while meltwatersands were deposited on the outwash plains in front of the glacier. The lateglacial deposits in the area consist of fluviatile clay, sand and gravel (Friborget al., 2002). A map showing the landscapes in southern Jutland and onFunen along with the main stagnation line can be seen in figure 9.5. Theclimate was generally cooler in Late Glacial time compared to today, butthere were warmer periods, the interstadials, such as Bølling and Allerød.

13Houmark-Nielsen (1987) terms this the Main Weichselian ice sheet.


Some of the first plants to invade the country were dryas14, arctic willowand sea buckthorn, while (white) birch, mountain ash and later willow flour-ished in the interstadials (Kronborg (1995); Larsen (2002)). Vegetation was,however, sparse and water abundant in the interstadials, so extensive erosiontook place.

9.4.4 The Post Glacial period (Holocene)

When the ice melted away and the Holocene started 11.500 years ago, twothings happened. Having been relieved of the massive weight of the ice sheet,an isostatic elevation of the land began. This was most pronounced in thenorthern parts of the country, as the ice had been thicker there. On the sametime, large amounts of meltwater caused an eustatic change in water level.At first, the latter dominated, as this process is more or less instantaneous,while isostatic elevation is a rather slow process. In the Preboreal period, alsoknown as the Continental period, the elevation of the land took over to anextent so that Denmark, England and Sweden were connected. However, thetemperature continued to rise and the Scandinavian and North American icecaps almost melted entirely away 8-9.000 years ago, in the Atlantic period.This caused the sea level to rise rapidly, and the land soon became floodedagain. This is known as the Flandrian Transgression and the sea as theLittorina Sea (or the Stone Age Sea). The many shells found in the sedimentsindicate very good living conditions. The Holocene was also a period whenmany bogs were often formed in topographic lows and often in places wheredead ice had been left behind. Marine marsh and tidal deposits have beenfound in southwest Denmark (Knudsen (1995); Larsen (2002); Friborg et al.(2002)). Later on, the uplifting of the land took over again and the coastlinebegan to take its present form. In the warmer periods the Stone Age manhad lived as a hunter and gatherer, but as the climate became colder again inthe Subboreal interval, he was forced to clear the forests and start cultivatingthe earth. During the Subatlantic period (Iron Age to modern times), thetemperatures continued to drop, finally reaching a level as today (Kronborg(1995); Larsen (2002); Larsen and Kronborg (1994); Friborg et al. (2002)).

Figure 9.5: Map of the landscapes in southern Jutland and on Funen alongwith the main stagnation line (full page figure on the following page).

14These cold periods are also called Older and Younger Dryas, respectively, after thisvegetation type.

70

10 Geophysical interpretation of TEM data

The purpose of this chapter is to present and discuss the results of the MCI,single-site and LCI interpretations. General patterns in the models are rec-ognized, but specific interesting examples will be presented. This providesthe frame of a geological interpretation of the model area, given in chapter11. An introduction to the concept of MCI and LCI interpretation will begiven in section 10.1, and a general discussion on interpretation and results ofthe interpretations in section 10.2. Geobase and MapInfo have been used tocreate 2D sections with TEM soundings, seismics and exploratory drillings.Printouts of these are found in appendix E. As mentioned in section 8.3.1, theposition of the southernmost drilling, LANU no. 1121/53, has been changedon the 2D section.

As a rule of thumb, HydroGeophysics Group (2001) labels the parameterresolution intervals as seen in table 10.1:

STD Parameter resolution<1.2 Well resolved

1.2-1.5 Fairly well resolved1.5-2.0 Poorly resolved>2.0 Unresolved

Table 10.1: The parameter resolution is based on HydroGeophysics Group(2001).

10.1 An introduction to MCI and LCI

MCI is short for Mutually Constrained Inversion. With MCI two distinctdata sets can be inverted as one system, thereby creating two related mod-els. The data sets may interpretationally be the same, but differ in that themethods are sensitive towards different physical properties of the earth. MCIis used to interpret HMTEM data which consist of two data sets; one fromthe central loop measurements and one from the offset measurements. MCIinterprets these data sets concurrently by applying a mutual constraint, theconstraint factor, on one or more corresponding parameters. Joint inversionand MCI have many of the same features, but one advantage of the latteris that it is more robust. Both resulting models can be independently eval-uated. By using MCI, parameter resolution may be enhanced and invisiblelayers may appear (Auken et al. (2003b); Yang, C.-H. et al. (1999)).

71 10.2 Results from the interpretations - general discussion

LCI is short for Laterally Constrained Inversion. It allows the user to puta lateral constraint on a number of 1D models and then invert them as onesystem. A method that very much resembles MCI, and in fact the inversionroutines are similar. Resistivities, depths, thicknesses, or a combination ofthese may be constrained, thereby obtaining a smooth transition in modelparameters along the profile. This is shown in figure 10.1. 1D LCI worksunder the assumption of a 1D model, but even areas with faults or buriedvalleys may be modelled, as long as this is accounted for in the model, e.g.by splitting this up in different profiles.

Figure 10.1: The principle behind LCI (after Christiansen et al. (2002)).

Constraint factors are relative. In simple terms this means that by using aconstraint factor of 1.1, a depth of 20 m would be allowed to vary between18.2 m and 22 m, while a depth of 200 m could vary between 182 and 220 m.

It is not the purpose of this thesis to give a detailed analysis of constraint fac-tors. Therefore standard values based on the level of depths and resistivitieshave been used.

10.2 Results from the interpretations - general discus-sion

Generally, the interpreter should choose the simplest model that fits data.This would, among other things, be the model with fewest layers, but priorinformation from nearby soundings or geology play a part in the choice ofmodel as well. If prior information indicates that the inversion routine hasfound a local minimum instead of a global one may choose to alter the start-ing model to see how the inversion reacts. Generally, the starting modelshould differ somewhat from the resulting model in order to let the inversionroutine find the result. An unlucky choice of starting model may result in badresults as the inversion routine may find a local minimum instead of a global.

72 10.2 Results from the interpretations - general discussion

As mentioned before, only 47 out of the 61 original soundings are inter-pretable, and 9 of these hold only one interpretable segment. This leaves38 soundings to be interpreted with MCI and 9 with single-site interpreta-tion. All 47 soundings are used in LCI interpretations. Generally speaking,soundings which hold both central loop and offset data, require four or fivelayer models, while soundings holding only central loop data require mod-els with fewer layers. In some soundings, e.g. Ti056, a five layer model waschosen even though a four layer model fits data very well. This is based onprior information from adjacent soundings, which have all been interpretedwith five layers. Ti029 and Ti030 hold only central loop data and havebeen interpreted with four layer models. If no other considerations but thoseto data were taken, three layer models would have been chosen. However,the models from adjacent soundings suggest that four layer models with amedium-resistive second layer are preferable. In other words, it is not thedata from Ti029 and Ti030, specifically, that requires a four layer model, butprior information from nearby soundings. This fourth layer is included in themodels of Ti029 and Ti030.

Five distinguishable layers of similar, or at least comparable, resistivities andthicknesses have been found in the majority of the soundings. To ease thepresentation of the results, a general geophysical model consisting of thesefive layers have been constructed (see figure 10.2). They will henceforth bereferred to as unit 1 to unit 5, whereas the term layer will be used to referto a distinct layer in a sounding. This means that a sounding may have beeninterpreted with four layers, layers 1, 2, 3 and 4, but these may correspondto units 1, 2, 3 and 5.

Figure 10.2: A geophysical model consisting of five units has been con-structed to ease the presentation of the results.

73 10.3 Results from MCI and single-site interpretations

Because the inversion is based on the assumption of a 1D-model, the sound-ings have been divided into five profiles in the LCI models. This divisionis primarily based on the results from MCI and single-site interpretations,but seismic data and prior knowledge of the geological model were also takeninto consideration. Table 10.2 and figure E.1 (found in appendix E) showthe division of soundings into profiles 1 to 5.

Profile SoundingsTi044, Ti043, Ti035, Ti036, Ti037, Ti038,

Profile 1 Ti033, Ti032, Ti031, Ti030, Ti029, Ti026,Ti025, Ti024, Ti023, Ti022, Ti021, Ti019

Profile 2 Ti018, Ti016, Ti017, Ti015, Ti014, Ti013Ti010, Ti009, Ti008, Ti007

Profile 3 Ti006, Ti004, Ti003, Ti002, Ti001, Ti045,Ti046, Ti047, Ti048

Profile 4 Ti049, Ti050, Ti051, Ti052, Ti053, Ti056,Ti057, Ti058, Ti059, Ti060

Profile 5 Includes soundings from profiles 1 and 2

Table 10.2: The division of soundings among the five profiles, starting withthe southernmost sounding.

Profile 5 is a combination of profiles 1 and 2. The purpose is to see howinformation from one profile would affect the other.

10.3 Results from MCI and single-site interpretations

This section is divided into subsections based on the overall models. As thesamme pattern is found in the models throughout the greater part of profile1, 2 and 4, these will be presented together. 2D sections with MCI and single-site interpretations with and without the model parameter analysis can beseen in appendix E.2. The corresponding workspaces, along with .emo-filesand .mod-files from Semdi, are also found on the CD-ROM.

Lateral constraints of 1.1 on both resistivities and depths between centralloop and offset data sets were used.

10.3.1 Interpretation of profiles 1, 2 and 4 using MCI

The soundings on profile 1 are divided into two groups. The southern groupwith soundings Ti044-Ti031 have been interpreted with four layer models,


except in the case of Ti038, which lacks offset data. Unit 3 is not foundin this group. Moving north along the profile, soundings Ti030-Ti019 havebeen interpreted with five layer models, or in the case of Ti030, Ti029 andTi026 in which offset data were abandoned, four layer models. They distinctthemselves from the first group in that unit 3 is found.

The soundings on profiles 2 and 4 (apart from Ti060) show the same patternas Ti030-Ti019; they are all interpreted with four or five layer models (ifthey hold one or two data sets, respectively), and especially the resistivitiesand depths of the upper layers are comparable. It seems that units 1 to 4are found throughout these profiles, and that unit 5 is also found on profile 4.

Four layers would have been sufficient for Ti049 and Ti050, but based oninformation from adjacent soundings five layer models were chosen.

The surface layer of the three profiles corresponds to unit 1 - the high-resistivity surface layer. Its resistivity varies, but is generally found to bearound 150 Ωm. It is unresolved to poorly resolved. It is generally not ex-pected that the uppermost part of the model is well resolved. The reasonsare that the current system diffuses to a certain depth before the measure-ments start and that the resistivities of surface layers are often very high.The thickness of unit 1 is generally between 20 and 30 m.

The second layer is medium-resistive, R2 ≈ 25 − 40 Ωm15, and correspondsto unit 2. Once more, the resistivities and thicknesses vary, the latter beingbetween 20 and 35 m in most cases. Both parameters are poorly resolved tounresolved in most soundings, but the resolution is much better where thelayer is thick, e.g. Ti013-Ti007.

As mentioned before, unit 3 is not found in soundings Ti044-Ti031, butin the rest of the soundings a layer with a resistivity of approximately 10 Ωmis found at a depth of approximately 50-60 m. The thickness of this layer,which corresponds to unit 3, varies a lot: It is almost non-existent in Ti030,but increases to 30 m in Ti019 and Ti022. It seems to be thinner in theeastern part of profile 4 than in the western part. The depth to unit 3 isgenerally well resolved, which was expected, as it is the depth to a conductivelayer. There are difficulties in resolving the resistivity and thickness of thelayer, probably because the layer is quite thin.

15Rn denotes the resistivity of layer n, while Zn denotes the nth layer boundary.


The next layer, which corresponds to unit 4, is found at depths around 80-90m, generally at shallower depths in the southern end of profile 1. This pa-rameter is not quite as well resolved as the the depth to unit 3, but it is inmost cases fairly well to well resolved. Unit 4 is high-resistive, but the pa-rameter varies a lot and no pattern is seen in most soundings. It is, however,lower in the southern end of profile 1, where it varies between 75 Ωm and103 Ωm, than in the northern end, where it is approximately 130 Ωm. Thereis also a general tendency that this parameter is better resolved in the southend. The thickness of unit 4 is around 170-200 m on profile 1, but seems toincrease slightly southwards. The thickness varies much more on profiles 2and 4.

Unit 5 has a resistivity of approximately 5-10 Ωm in the south end of profile1, but it is lower in the north end. The same pattern is seen on profile 4,where is decreases towards the west, although it is not as obvious as on profile1. The resistivity is generally poorly resolved or even unresolved, probablybecause the depth is about the maximum penetration depth at this locality.An average of 5-6 data points have been removed from the Hi segment ow-ing to noise and the before mentioned biased signal (see section 7.1). Eventhough the resistivity is unresolved, it is noticeably low in the north end,around 1-4 Ωm. No pattern is seen in the resistivity of layer 5 on profile 2.The depth to the bottom layer is well resolved. It is approximately 220-240m in most soundings on profiles 1 and 4, where it corresponds to unit 5. Aremarkable difference in the depth to the low-resistive layer is seen on profile2, as it varies much more, from 164 m in Ti016 to 285 m in Ti008. It is,however, generally deeper than on profiles 1 and 4. The poor resolution ofthe resistivities is probably a result of the great depth.

It is interesting to take a closer look at the model parameter analysis ofthe depths to the low-resistive bottom layer for the two soundings mentionedabove. The standard deviations are 1.06 and 1.36, respectively. This meansthat in 66 % of the cases, Z4 would be between 155 m and 174 m in Ti016,and between 210 m and 388 m in Ti00816. There is no overlap between thesespans, making it less likely that it is the same layer. There do not seem tobe any conclusive answer to whether the low-resistive bottom layer of profile2 corresponds to unit 5 or not.

Some soundings distinguish themselves from the others, e.g. Ti052, in which

16The concept of model parameter analysis is complex, and it is built on the assumptionof a linear analysis.


R5 is noticeably high. Based on prior information from adjacent soundings,inversions were carried out in which R5 was constrained to 8 Ωm by a fac-tor of 1.5. The result was R5 = 8 Ωm and Z4 = 264 m - a rather largedepth compared to that of adjacent soundings. It seems the inversion triesto push the layer further down to a point where it is not in conflict with data.

Focus should also be given to Ti043 from profile 1, in which the bottomlayer has a considerably higher resistivity than usual (24 Ωm) and is foundat much greater depth than elsewhere (300 m). The depth is well resolvedwhile the resistivity is unresolved.

Ti060 differs from the rest of the soundings as the medium-resistivity secondlayer is very thick and unit 3 is missing. Whether the former corresponds tounit 2 or not is questionable.

10.3.2 Interpretation of profile 3 using MCI

The soundings of profile 3 are located in the Tønder Graben and their modelsdiffer fundamentally from the ones discussed above. It is questionable howmany units from the before mentioned geophysical model can be recognized,and there are even rather large discrepancies internally in the profile. Asa consequence, the subsurface layers will be presented in groups accordingto their model, rather than be described all together from top to bottom.Except in the case of Ti006 and Ti045, five layer models have been used tointerpret the soundings on profile 3.

The surface layer is an exception as unit 1 is found in all soundings. Bothresistivities and thicknesses of this layer are comparable to that of profiles1 and 2. R1 is a relatively low in Ti003, but since only offset data wereinterpretable in this sounding, it is unresolved.

The models of Ti046, Ti047 and Ti048 are alike. They have all been in-terpreted with five layer models in which layer 2 is a well resolved medium-resistive layer, layer 3 is low to medium-resistive (around 15 Ωm) layer andlayer 4 low-resistive (around 5 Ωm). The fifth layer is a high-resistivity layerfound at great depth, around 190 m. As expected, this high resistivity isundetermined. Layer 2 is likely to correspond to unit 2.

Ti045 differs from the above mentioned as layer 3 is found at lower depth.Furthermore, it is thinner and overlies a medium-resistivity layer. A fourlayer model has been used.

77 10.4 Results from LCI interpretations

A medium-resistive second layer is also found in Ti001 and Ti002. It islikely to correspond to unit 2, even though it is thinner than seen elsewhere.Layer 3 is high-resistive, while layer 4 and 5 have resistivities of around 7Ωm and 3 Ωm, respectively, in both soundings.

As previously mentioned, the offset data of Ti003 had to be abandoned,but the deeper part of the model bears some resemblance to Ti006, Ti004,Ti002 and Ti001. There is, however, a high-resistivity bottom layer, just likein the models of Ti046, Ti047 and Ti048.

Apart from the second, thin, low-resistive layer of Ti006 and Ti004, there issome resemblance between these and Ti001, Ti002 and Ti003. The resistiv-ity of the third layer is lower, but it is questionable how much confidence toput in this, since it is not well resolved. Layer 2 may correspond to unit 3,but it is found at considerably lower depth than on profiles 1 and 2. Ti006is interpreted with a four layer model which does not find a high-resistivitybottom layer, while Ti004 does.

10.4 Results from LCI interpretations

The results of LCI interpretations will be described in this section with em-phasis on what is achieved by using LCI, rather than a detailed discussionon parameters.

As previously mentioned, five profiles, of which profile 5 is a combinationof profiles 1 and 2, were used in the LCI interpretations. The division ofthe soundings among these profiles is seen in table 10.2. 2D sections withLCI interpretations of profiles 1 to 4 and of profile 5, both with and withoutmodel parameter analysis can be seen in appendixes E.3 and E.4. The corre-sponding MapInfo workspaces are included on the CD-ROM along with themodel and .emo-files from the preliminary and final LCI models.

The standard deviations of the model parameter analysis used in this thesisare coupled. This means that they are a function of both data fit and con-straints. This means that the parameters are generally well resolved usingLCI, and as a consequence of this, parameters which are well resolved willgenerally not be discussed.

All soundings must be interpreted with the same number of layers in LCI.The desired number of layers is on the other hand normally based on the


results from MCI and single-site inversions, and are therefore not always thesame. In practice, this problem is overcome by introducing ”pseudo layers”;layers that have a very hard vertical constraint on resistivity imposed onthem17, thereby creating two layers with identical resistivities. The thick-ness of the ”true” layer is the sum of the thicknesses of the pseudo layers.Where to introduce pseudo layers needs careful consideration, as it is highlydependent on which lateral constraints are introduced in the model.

The LCI models presented in this thesis are the result of a number of pre-liminary models; in model number 1, one set of parameters (for example theresistivity depth of one layer or a layer boundary) was tied together, in num-ber 2 two sets of parameters, and in the final model all desired parametersare constrained. The purpose of this stepwise procedure is to determine theeffect of the newly introduced constraint. Furthermore, it is easier to deter-mine whether some constraint factors need adjustment. Only the results ofthe final models will be presented.

Units 1, 2 and 3 are found at shallower depths, where the geology is presumedto change more quickly, predominantly due to the fact that they consist ofQuaternary sediments and/or have been subject to glacial erosion. There-fore, the lateral constraints applied on them are weaker than those on unit5. The method also offers better resolution of the upper part. Table 10.3gives an overview of the general lateral constraints used in the LCI models,although these may have been modified in some models, e.g. where a layerboundary that seems to dip have been found. Lateral distances betweensoundings have to some extent been taken into consideration as well. Thesoundings were placed 250 m apart, but as some were abandoned, this dis-tance may be longer in practice.

Constraint factors Constraint factorson resistivities on depths

Layers 1-4 1.2 1.2Layer 5 1.05 1.1

Table 10.3: The general constraints used in the LCI models

If other constraints than those of table 10.3 have been used, a brief outline ofthese will be given in the beginning of the section. In case of any ambiguities,I refer to the final LCI model files.

17A vertical constraint of 1.01 on resistivities has been used to create pseudo layers.


The lateral constraints between central loop and offset data sets were thestandard for MCI, which are 1.1 on both resistivities and depths.

10.4.1 Interpretation of profiles 1, 2 and 4 using LCI

While the models of Ti049-Ti058 were quite alike in the MCI interpretations,those of Ti059 and Ti060 were somewhat different. As a consequence, thetwo latter soundings were only tied together with Ti049-Ti058 on the depthsto the low-resistive bottom layer and its resistivities. The lateral constraintsbetween Ti035 and Ti043 on profile 1, and between Ti053 and Ti056 on pro-file 4 have been loosened by 50 % because of the large distance between these.

While the resistivities of unit 1 varied rather much in the MCI interpre-tations, a more constant level of 150 Ωm and a smoothing of the depths isachieved by using LCI. The large difference is the result of the poor resolu-tion of the layer in the MCI and single-site interpretations.

A good example of how information from a well resolved parameter migratesto a poorly resolved parameter is seen in the surface layer resistivities ofTi053 and Ti056. Using MCI these were 116 Ωm and 235 Ωm, respectively.The former parameter is well resolved while the latter is unresolved. UsingLCI yields R1 = 120 Ωm in Ti053 and R1 = 174 Ωm in Ti056. The fact thata resistivity this high is well resolved in the former sounding using MCI isconnected to the large thickness of the layer.

Relatively little change is seen in units 2 and 3. This was more or lessto be expected, as the parameters of these were pretty similar to begin withand the lateral constraints imposed on them relatively weak. Unit 3 seems tothin out towards the east on profile 4. This layer was included in soundingsTi049 and Ti050 because of information from adjacent soundings. The factthat it is very thin indicates that it is not a true reflection of the geology.This is a very positive feature that shows the robustness of LCI; data pro-vides no background for the layer which was introduced, and the inversionresult implies that it should not have been included. Furthermore, a layerthat thin would not be resolvable using TEM.

The resistivity of unit 4 is much more constant than was the case with MCI,but there is a tendency towards lower values in the north end of profile 1.It decreases from a general level around 90 Ωm in the south end to approxi-mately 70 Ωm in the north end. The same pattern of decreasing resistivities


of unit 4 is seen on profile 2 eastwards. Note also that the resistivity of thisunit is lower on profile 2 than in the northernmost soundings on profile 1.The resistivities are generally fairly well resolved in the northern part of pro-files 1, 2 and in the western part of profile 4. Given the parameter resolution,it can be interpreted as the same layer.

Using LCI seems to have had a rather large effect on unit 5 as well. Thedepth to it is kept at a pretty constant level of 240-260 m throughout profiles1 and 4 (apart from a dip in Ti049-Ti051), yet the difference between theresistivities in the southern and the northern end of the former is remarkable;it is 8 Ωm in Ti044-Ti037, 4-5 Ωm in Ti033-Ti031 and 1 Ωm in Ti025-Ti019.The resistivities are fairly well resolved on profile 1 and fairly well resolvedto resolved on profile 4. The depths are well resolved.

A rather large change is seen in the low-resistive bottom layer of profile2, as resistivities and depths are smoothed considerably. The resistivities arefairly well to poorly resolved. Using LCI seems to indicate that this layerdoes not correspond to unit 5, seen in the soundings immediately to thesouth. The depths to the low-resistive layers of the two profiles are very dif-ferent, and in fact, the southernmost soundings of profile 2 are the ones thatdeviate the most from profile 1. Unfortunately, the offset data of soundingsTi015-Ti013 had to be abandoned, so they contain no information on layer 5.

Another example of the smoothing effect of using LCI is the bottom layerof Ti052. In the MCI interpretations, this had a relatively high resistivitycompared to nearby of the soundings, and the parameters were generallyunresolved. The layer was also found at a little shallower depth. This pa-rameter was poorly resolved in Ti052, but well resolved elsewhere. UsingLCI both parameters are smoothed. The resulting depths are based primar-ily on the well resolved parameters, while the resistivities are more of an evencombination.

10.4.2 Interpretation of profile 3 using LCI

The general lateral constraints used in this profile differ slightly from thosementioned in table 10.3, in that they are 1.2 on all parameters, a choicebased on the apparently large internal differences of the profile. The lateralconstraints on Z2 between Ti046, Ti047 and Ti048 have been loosened to 1.3,owing to seismic data, which showed a dipping reflector in this area.

While the MCI models of profile 3 showed rather large discrepancies inter-


nally in the profile, structures are enhanced using LCI. In the constructionof the LCI model an additional feature was included: A sixth layer was in-troduced in all soundings (with the exception of Ti045). As it was based onprior information from exploratory drilling DGU no. 168.1378 combined withthe high-resistivity bottom layers of Ti003 and Ti004, found using MCI, theinclusion of this layer is a good example of how the geological interpretationis starting to influence the geophysical interpretation. The resistivity wasconstrained to 70 Ωm by a constraint factor of 1.2 in sounding Ti004.

The soundings are still divided into two groups, one with soundings Ti006-Ti001 and one with Ti046-Ti048. There are no lateral constrains betweenthese groups. Ti045 falls out of category, but as it will be shown later, itbears resemblance to the latter group. Once more, the surface layer (unit 1)is seen in all soundings with resistivities smoothed by LCI.

The general trend of Ti006-Ti001 is models in which layers 1 and 3 havehigh resistivities, layer 2 a medium resistivity, layers 4 and 5 low resistivitiesand layer 6 a medium to high resistivity. Layer 5 is a pseudo layer in Ti006.In Ti004, which was located close to the drilling, the resistivity of layer 6was constrained to 70 Ωm, by a factor of 1.2. The result was R6 = 58 Ωm.This was a little higher than the LCI inversion ended up with, if no con-straints were used. This layer is, however, based on little TEM-data. Theparameter resolution is generally not as good in these soundings as elsewhere.

Ti046-Ti048 show some of these features as well, but the medium-resistivitysecond layer is thicker and is found directly on top of the two low-resistive lay-ers. Below another medium-resistivity layer is introduced. This last feature isalso the major difference between the MCI models and the LCI models. Fur-thermore, R6 is higher than seen in Ti006-001. Both the resistivity of layer 5and 6 are undetermined. The high-resistivity third layers from Ti006-Ti001are not found in soundings Ti046-Ti048.

Ti045 shows the same features in the top of the model, but layers 4, 5 and 6are all medium-resistive, and could be interpreted as one layer. The modelparameter resolution of this sounding is generally poor as it is not tied toother soundings.

Whether other layers than the surface layer of profile 3 can be correlatedto units from the geophysical model or not is still questionable. It seemslikely that the medium-resistivity second layer corresponds to unit 2, butotherwise there are no clear answers.

82 10.5 Results from the interpretations - an overview

10.4.3 Interpretation of profile 5 using LCI

As mentioned before, profile 5 is a combination of profiles 1 and 2. Thepurpose of creating profile 5 was to use information from the profiles on eachother. It was especially interesting to see, how information from the north-ernmost soundings of profile 1 would affect the southernmost soundings onprofile 2 and vice versa.

The lateral constraints between Ti035 and Ti043 have been loosened by 50% because of the large distance between these.

The most interesting feature was to see whether the low-resistive bottomlayers from the two profiles corresponded to each other, but as the depth tothis layer was well resolved to begin with, it seemed not to be moved consid-erably by the introduction of the constraints. As a result of this, these wasleft out in the final model. The conclusion must be that the layers do notcorrespond to each other.

One could also have expected that the resistivity of unit 4 in the two profileswould have been brought closer together. These resistivities were, however,generally undetermined in both profiles using MCI. This means that there isno certain information that can be used to migrate between the soundings,and the resistivity increased slightly on profile 1, whereas it remained overallconstant on profile 2.

10.5 Results from the interpretations - an overview

A geophysical model of five units was created to ease the presentation ofthe interpretations. A printout of the 2D section with LCI interpretationsof profiles 1 to 4 and an overview of the geophysical model can be found inappendix E.5. The corresponding workspaces are included on the CD-ROMalong with .emo-files and .mod-files.

Profile 3 is located in the Tønder Graben, where the geological setting seemsto differ fundamentally from what is seen elsewhere.

Unit 1 is high-resistive. It seems to be the only unit which is certainlyfound throughout the profiles.

Unit 2 is medium-resistive. It is found on profiles 1, 2 and 4 and mostlikely also on profile 3. The thickness varies significantly.

83 10.5 Results from the interpretations - an overview

Unit 3, the low-resistive layer, is found in the northern part of profile 1(soundings Ti030-Ti019) and on profile 2. It is found in some parts of profile4 as well. The soundings on profile 3 find low-resistivity layers, but they aregenerally thicker and found at greater depths. Without information fromadditional data, it is uncertain whether they correspond to unit 3.

Unit 4 has a high resistivity and is found on profile 1, 2 and 4. There isan interesting pattern in the resistivity of this unit, as it generally decreasestowards the Tønder Graben on profiles 1, 2 and 4.

Unit 5 is a low-resistive layer that is found on profiles 1 and 4. The re-sistivity is extremely as low as 1 Ωm some areas, while the depth is ratherconstant compared to that of the top layers. The latter is expected to be theresult of glacial disturbance of the upper layers. It seems unlikely that thebottom layer on profile 2 corresponds to unit 5, as it is generally found atmore shallow depths.

84

11 Integrated interpretations

A geological interpretation of part of the model area was earlier presentedin Friborg et al. (2002), an interpretation that also implemented small-scalegravimetrical data. LCI of the TEM data was however not used and thenature of this chapter is therefore threefold:

1. To present a geological interpretation of the model area by integratingTEM data, seismics, exploratory drillings and logs.

2. To discuss what a fresh look at the soundings and the implementationof LCI yields.

3. And finally to discuss where there is a conflict in data and where appar-ent discrepancies can be explained by uncertainties, errors or physicaldifferences in the mode of operation of the methods.

Appendix E.6 shows a 2D section of the LCI interpretations of profiles 1through 4, seismics, explanatory drillings and a geological interpretation, thelatter as lines representing layer boundaries. It is also available as MapInfoworkspace ”Layout of geological interpretation”. The geological interpreta-tions of the TEM data are based on the resistivity intervals of table 11.1:

Deposit Resistivity [Ωm]Clayey till 20-50 (typically 40)

Sandy or gravelly till > 40Meltwater gravel > 80Meltwater sand > 60Meltwater silt 40-80Meltwater clay 10-40 (typically 25)

Miocene quartz or mica sand > 80Miocene mica silt 40-80

Miocene-Oligocene mica clay 10-40Paleogene-Eocene heavy clay

(Lillebælt Clay Fm/Røsnæs Clay Fm) 1-3

Table 11.1: A selection of resistivity intervals from Jørgensen et al. (2003a),on which the geological interpretation of HMTEM soundings arebased.

The chapter is divided into four sections. Results of the logs from the deepexplanatory drilling, DGU no. 168.1378, will be presented in section 11.1,

85 11.1 The logs of DGU no. 168.1378

before the results of the geological interpretation. The geological models ofprofiles 1, 2 and 4 are quite similar, and the geological interpretation of thesewill consequently be described together in section 11.2, while the geologicalinterpretation of profile 3 will be described in section 11.3. An overview ofthe geological interpretation will be given in section 11.4.

Three well defined reflectors are seen on profiles 1, 2 and 3. These willhenceforth be referred to as reflectors 1 through 3, and are marked with pur-ple, green and yellow lines, respectively, on the 2D section. They are foundat depths of approximately 160 m, 250 m and 500 m in the south end ofprofile 1 (X=2000), but dip towards the Tønder Graben. Reflections in thetop 100 m are abundant, but general patterns are hard to see. The reflectionson profile 4 are not as clear as in those profiles 1 and 2, which is most likelyconnected with the relatively small distance to the north flank of the TønderGraben. The two best defined reflectors on profile 4, reflectors 1 and 2, arefound at depths of 250 m and 380 m. Furthermore, weak traces of reflector3 are found at approximately 600 m depth.

11.1 The logs of DGU no. 168.1378

The focused electrical log (FEL-log), natural gamma-ray log (NG-log) andspectral gamma-ray log (SNG-log) show distinct features which, togetherwith the detailed lithological log, can be used in the geological interpreta-tion. Before giving a geological interpretation of the profiles, it is necessary topresent these logging data and their importance to the interpretation. Plotsof the logs can be seen in figure 11.1. The focus depth of the FEL-log wasrather small compared to the diameter of the borehole, and considering thesediment, it seems likely that the resistivities are highly influenced by theborehole mud. The layer boundaries are however trustworthy.

0-46 m: A high reading on the FEL-log combined with a low reading onthe gamma-ray logs indicate a sandy sediment. This is in perfect accordancewith the lithological log, which shows deposits of meltwater sand in this in-terval. A brief spell of lower resistivities around 25 m can be correlated withclayey till deposits.

46-67 m: The high reading on the FEL-log and rather low reading onthe gamma-ray logs show the upper part of the marine Gram Fm, whichis silty and in some cases even sandy. A deceasing resistivity and an increas-ing gamma-ray activity, especially the Th-component of the SNG-log, in thelower part of the interval show that the sediment is becoming gradually more

86 11.1 The logs of DGU no. 168.1378

fine grained.

67-196 m: The FEL-log shows a very low resistivity, while the gamma-ray activity is high in this interval. The lithological log shows that heavy,marine clay, the Gram Fm and Hodde Fm, is found in this interval. A peakin the U-component in the SNG-log at a depth of 168 m marks the boundarybetween these. The fluctuations in resistivity in the upper part is a result ofsilty layers.

196-346 m: While the gamma-ray activity remains at a rather constant lowlevel throughout this interval, the resistivity decreases significantly. Odd-erup Fm and Arnum Fm are found in this interval, the latter primarily inthe lower part, but there is a transition period in which these intertwine.Odderup Fm is found as limnic, medium to coarse grained sand, graduallybecoming more fine grained downwards, and Arnum Fm as marine, ratherheavy, dark clays to fine sands. The resistivity fluctuations may be a resultof the alternating cycles of marine and limnic sediments. A salt concentra-tion of 3000 mg/l was found in water samples taken at a depth of 340 m,in the Arnum Fm. As the gamma-ray logs show no indication of a highand/or increasing clay content, this salt content is likely to explain the de-creasing resistivity. According to Jørgensen et al. (2003b), the resistivity ofsandy sediments is inversely proportional to the ion content of the pore water.

346-414 m: The FEL-log remains at a low, constant level in this inter-val, in which primarily the marine Arnum Fm is found. The Th-componentincreases slightly, but shows large fluctuations. The low resistivity is proba-bly a result of salt water rather than clayey materials. Water samples froma depth of 370 m showed a salt concentration of 74 mg/l. This sample wastaken in a limnic sediment, explaining the relatively low salinity. Accordingto Friis (1995), heavy minerals are abundant in the more fine-grained partsof the Arnum Fm, which explains the fluctuations of the SNG-log. BastrupSand was found in the deepest part of this interval.

87 11.1 The logs of DGU no. 168.1378

Figure 11.1: The FEL-log, NG-log and SNG-log from the deep exploratorydrilling with DGU no. 168.1378.

88 11.2 Geological interpretation of profiles 1, 2 and 4

11.2 Geological interpretation of profiles 1, 2 and 4

Profiles 1 and 2 stretch from the German-Danish border and northwards to-wards Tinglev. Profile 4 is found on the east-west going part of the profile,which follows the direction of the Tønder Graben. Based on the data, it isfound immediately north of the northern flank.

Only one deep drilling, LANU no. 1121/53, is available south of the TønderGraben (X=100 m)18, and this is only to be used with caution. Primar-ily because it is located nearly one and a half kilometers south of the firstsoundings, but also because only depths to formations, not actual lithologi-cal information, are available. The drilling shows a 35 m thick surface layerof Quaternary deposits on top of the Gram Fm/Hodde Fm (no distinctionbetween these have been made in the lithological log). Odderup Fm is foundat a depth of 92 m, and below that the Arnum Fm, including a 10 m thicklayer of Bastrup Sand, found at 192 m depth.

All soundings show a surface layer with resistivities around 150 Ωm. Itcorresponds to unit 1 (see the description of the geophysical model in chap-ter 10), and is interpreted as meltwater sand - the Tinglev outwash plain.Friborg (1996) mentions that this is actually two distinct outwash plainsseparated by Eemian deposits, the upper deposited in the Weichselian andthe lower in the Saalian. This is not seen in the soundings on profiles 1,2 and 4, but there may be indications in some of the drillings, e.g. DGUno. 168.1378 (X=11.000 m) and 167.234A (X=19.800 m), where two sandymeltwater units are found. These are, however, separated by till, not byEemian deposits. Seismic reflections are as previously mentioned abundant,but large-scale patterns corresponding to the lower interface of the unit arenot seen.

Unit 2 is medium-resistive (around 30-40 Ωm), which is also found in allsoundings. The thickness of this layer varies significantly: It is quite thickaround soundings Ti031-Ti023, becomes thinner north of these and thenagain thicker northwards in soundings Ti014-Ti007. The same pattern ofvarying thicknesses is seen on profile 4. Unit 2 is interpreted as clayey till,but in some parts of the profile it may very well include the upper part of theGram Fm. As mentioned previously, this is more silty in the upper parts.According to table 11.1, the resistivity of Miocene mica silt is 40-80 Ωm, andas this interval overlaps with that of clayey tills, it is possible that they may

18X will henceforth be used to denote the location on the 2D section. As mentionedbefore, this does not present the true location of the LANU drilling.


not be distinguished based on resistivities alone. No conclusion can be drawnfrom the seismic data either. Given the resistivity alone, another possibleinterpretation is that unit 2 is meltwater clay and silt, but this seems unlikelyto be found in such a large area. As unit 2 seems to be found throughoutall profiles, including profile 3, the lithological log from drilling DGU no.168.1378 has been used. Clayey till is found in this drilling at a depth of 25m, underlain by the silty upper part of the Gram Fm. Deposits of clayey tillare seen in other drillings as well, including some shallow drillings that havenot been shown on the 2D section. Again there are numerous reflections,but none common and well defined. The lower boundary of the Quaternarysediments is marked with a red line on the 2D section.

Unit 3 is low-resistive, 5-10 Ωm. It is not found in the southern part ofprofile 1 and in the westernmost part of profile 4 (Ti060), but otherwisethroughout profiles 1, 2 and 4. It grows thicker northwards from Ti030 (thesouthernmost sounding where it is found) and then thinner again throughprofile 2. It is significantly thin on the other side of the Tønder Graben, insoundings Ti049-Ti51, but becomes thicker westwards. The layer was onlyincluded in soundings Ti049 and Ti050 because of prior information fromadjacent soundings, and given the very low thickness found by the inversionit is doubtful whether it represents a true picture of the geology. Again, theresistivity alone is not sufficient to tell whether this is Miocene mica clay orclayey till, but based on information from drillings, it is interpreted as theformer, the Gram Fm and/or Hodde Fm. None of the remaining drillingsare of much use in this interpretation. There are seismic reflectors that maycorrespond to the bottom of this layer, but they are scattered and not alwayscoherent with the TEM soundings. The lower boundary of the Gram Fm andHodde Fm is marked with a blue line on the 2D section.

Gram Fm and Hodde Fm are found overlying Odderup Fm and Arnum Fmin the LANU drilling. The two former are both deposits of various grainsizes, but according to the lithological log of drilling DGU no. 168.1378, theyhave predominantly been found as fine-grained sands and are intertwiningwith each other. In some parts of the profiles, one would expect higher re-sistivities, above 80 Ωm, of such a sediment, but given the poor resolution ofthis parameter in the MCI interpretations, unit 4 is still interpreted as theOdderup Fm and Arnum Fm.

Reflector 1 is found in the middle of unit 4 on profile 1, but is not recognizedin the soundings. It is interpreted as the Bastrup Sand, in the drillings foundinternally in the Arnum Fm. The thickness of this layer is only approximately


20 m, explaining its double reflection appearance. As the resistivity contrastis small and the contrast in acoustic impedance large, it could explain whyno indications of it is found in the electromagnetic data, while a well definedreflection are seen in the seismics.

An offset as large as 170 m between reflector 2 and the depth to layer 5is seen on profile 2. Inversions were carried out in which the low-resistivebottom layers of profiles 1 and 2 were tied together, but this seemed as an”unreasonable” constraint. They did of course converge, but only when tiedtight together. This indicates that they are not the same layer.

However, the depth to the low-resistive bottom layer corresponds to reflector1 on profile 4 and in soundings Ti010-Ti007 on profile 2. The resistivity isunresolved in the MCI interpretations and it is not possible to say exactlywhat it is, only that it is low - nevertheless surprising as the reflector is in-terpreted as the Bastrup Sand. This is interpreted as a result of saline porewater. It may be that the relatively large permeability of the layer facilitatessalt water intrusion from the Tønder Graben. Being a sandy sediment, thepore water resistivity would have a large effect on the formation resistivity.According to Jørgensen et al. (2003a), formation factors of 5 are typical forDanish sands and gravels19. A salt concentration of 3000 mg/l was foundin the pore water from drilling DGU no. 168.1378. The resistivity of thispore water would be 1-2 Ωm. Combined with a formation factor of 5, theresulting formation resistivity would be 5-10 Ωm. It is only in the sound-ings closest to the Tønder Graben that a distinction between the BastrupSand and the Arnum Fm can be made in the soundings. This supports theinterpretation of salt water intrusion: The resistivity contrast is too smallfarther away from the Tønder Graben, where the pore water is fresh. It ispossible that this salt water intrudes into the Tønder Graben from Flensburg.

Using TEM data alone, the low-resistivity bottom layers of profiles 1 and4 would probably have been interpreted as the same layer. That they arenot, is seen by using integrated interpretation of different data types - a goodexample of the importance of this.

Friborg et al. (2002) mention the presence of Vejle Fjord Fm on top of seismicreflector 2, but questions the distribution of it. A rather weak reflector couldshow the top of this formation, but the soundings show no indications of this

19The formation factor, F, was earlier on described by Archie, and is a function of theporosity, φ: F = a/φn. The values a = 1 and n = 2 are usually used


layer boundary - once more it is seen that the methods react on differentproperties. Further traces of the reflector are hardly recognizable south ofthe Tønder Graben, although it is likely to correspond to a better definedreflector found at only a little shallower depths on profile 4, on the other sideof the Tønder Graben. No drillings are available to support this interpreta-tion.

Unit 5 is the low-resistive layer found at depths of approximately 250 min the TEM soundings. In the southern end of profile 1, the top of this corre-sponds to reflector 2, but while the layer boundaries found in the soundingsremains at a rather constant level, the reflector dips towards the north. Thereis a general decreasing trend in resistivities towards the north as well, butthe poor resolution of this parameter makes it impossible to say whether thisis a reflection of the geological setting. Layer 5 is interpreted as Paleogeneclay. The discrepancy between the TEM soundings and seismics could bea combination of several things. First of all, the large depth is important.Even though the depths found in the TEM soundings are well resolved, theycould be moved 20 m up or down, and a similar uncertainty applies for theseismic data. Secondly, the result may be influenced by the Vejle Fjord Fm,which is found on top of the Paleogene sediments. Last but not least thediscrepancy could be caused by saline pore water. Although the resistiv-ity of unit 4 shows that its pore water is generally fresh, dense saline porewater could accumulate in the lower part of the sediment, on top of the in-permeable clay. The logging data of drilling DGU no. 168.1378 imply thatthe salt water concentration increases with depth throughout the OdderupFm and Arnum Fm. Furthermore, there seems to be a connection betweenthe resistivity and the size of the gap between the methods, but as TEMdata to base the low-resistive layer on are sparse and resistivities unresolvedin the MCI interpretations, this interpretation is open for discussion. DrillingDGU no. 167.234A shows that the uppermost Paleogene sediments, found ata depth of 370 m, consist of the Eocene Røsnæs Clay Fm. According to table11.1, the expected resistivity of this formation is 1-3 Ωm.

Reflector 3 is interpreted as the top Cretaceous boundary. Limestone is foundat a depth of almost 600 m in drilling DGU no. 167.234A, below Selandiansilts and shales. There are no clear reflectors corresponding to this depthon profile 4, but there are traces below soundings Ti052 and Ti053. Somecare should be taken with the above mentioned drilling. Primarily becauseit as a rotary drilling, meaning that there are some uncertainties primarilyconcerning depths, and secondly because the drilling report mentions thatthere were were signs of caving in the drilling.


The interpretation that reflectors 2 and 3 correspond to the top of the Paleo-gene and Cretaceous deposits, respectively, is supported by Ringkøbing Amtet al. (1999), in which maps of the depth to these are found.

Two buried Quaternary valleys are seen in the seismic data on profiles 1 and2, a rather broad one around Rødebæk (X=5-7.000) and one with ”double-appearance” around Gardeby Mark (X=9-10.000). They are not as pro-nounced in the TEM soundings, neither using MCI or LCI, where a thick-ening of the two upper units is seen, but not to the extent that could beexpected from the seismics. This implies that it is not the function of tootight constraints between the soundings. It seems likely to be an exampleof how a resistivity contrast is not always linked to a contrast in acousticimpedance. A map of the depth to the Tertiary sediments is seen in figure11.2. It shows that these valleys are part of a large system of buried Qua-ternary valleys, running from Flensburg towards Tinglev, coherent with theTønder Graben.

Figure 11.2 shows that there is also a Buried Quaternary valley in the west-ernmost part of profile 4, and that this is part of the before mentioned largesystem of valleys. The seismic data quality is low, but there seems to bea dipping reflector around X=19.600 that could show the presence of this,something that the poor seismic resolution may actually be an indicationof. Two drillings are located in the area, drilling DGU no. 167.234A in thewesternmost end of the profile and 167.234B 300 m further east. Both showvarying Quaternary sediments to a depth of approximately 150 m. Thisdepth is in good correspondence with the seismic reflector. Beneath that,Miocene sediments of varying grain sizes are found in the drillings. Thehigh-resistive layer of Ti060 is the meltwater sands of Tinglev outwash plain.It is likely that layers 2 and 3 are a combination of the Quaternary clayey tilland the Miocene sediments. The boundary between these is not seen in thesoundings because of a low resistivity contrast between the sediments. Thelow-resistive bottom layer of soundings Ti060 is interpreted as the BastrupSand.


Figure 11.2: The depth to the Tertiary deposits (after Friborg et al. (2002)).

94 11.3 Geological interpretation of profile 3

11.3 Geological interpretation of profile 3

Profile 3 runs in a south-north going direction starting just south of Tinglev.Ti006-Ti001 are found in this part. Inside the town it takes a bend towardsthe west, the part with soundings Ti045-Ti048.

Exploratory drillings DGU no. 168.1228, 168.1193 and 168.16B are foundinside the town (all at X=12.500), while DGU no. 168.1378 is found justsouth of it. There are of course no soundings in the vicinity of the threeformer.

The soundings on profile 3 are divided into two groups showing differentmodels, one with Ti006-Ti001 and one with Ti046-Ti048. The remainingsounding, Ti045, pretty much falls out of category.

The high-resistive surface layer, interpreted as Tinglev outwash plain, andthe medium-resistive layer beneath, interpreted as clayey till are found in allsoundings. The thickness of the former is approximately 25 m, which corre-sponds very well to that of the upper meltwater deposits, found in DGU no.168.1378.

Soundings Ti006-Ti001 are located on the south-north going part of the pro-file, just north of the southern flank of the Tønder Graben. They all find ahigh-resistive third layer that seems to correspond to each other, althoughthey are of varying thicknesses. Based on DGU no. 168.1378, it is likely to bethe lower meltwater unit, but while there is a good correspondence betweenthe top boundaries and the drilling, there are large discrepancies in the lowerinterfaces. It is possible that the layer in the TEM soundings is an integrationof the lower meltwater unit and the sandy and silty upper part of the GramFm. This interpretation seems to be supported by the fact that the lowerboundary of the latter corresponds very well to the lower boundary of layer 3.

Layers 4 and 5 are both low-resistive. They are interpreted as the clayeyGram Fm and Hodde Fm. The boundary between these is found at 168 mdepth in drilling DGU no. 168.1378, while those between layers 4 and 5 arefound at shallower depths (around 120 m in Ti004 and Ti003). It may bethat TEM has difficulties with distinguishing between these owing to a lowresistivity contrast. That the resistivity of layer 5 is lower than the resis-tivity of layer 4 is in accordance with the FEL-log, which shows a generaldecreasing resistivity with depth in the interval where the above mentionedformations are found.

95 11.3 Geological interpretation of profile 3

The depths to layer 6 varies significantly in soundings Ti006-Ti001, but isgenerally coherent with a bulging seismic reflector. The layer is interpretedas Odderup Fm and Arnum Fm. It is found at approximately 190 m depthin sounding Ti003, which is the same depth as that to the before mentionedformation in drilling DGU no. 168.1378. Layer 6 is unresolved in the sound-ings, but this was to be expected as it is found beneath a relatively thickpack of clayey sediments and consequently based on little data.

Soundings Ti006-Ti001 are separated from Ti045-Ti048 by a buried Qua-ternary valley (X=13.000), on which Ti045 seems to be on the fringe of.There is a dipping reflector around sounding Ti046, but otherwise the struc-ture is seen primarily because other reflectors are cut off, not because thevalley floor constitutes a well defined reflector itself. This cutting off of thereflectors is probably more a function of deteriorating data quality, than anactual physical phenomenon. The presence of this valley is supported by thethree drillings inside Tinglev, DGU no. 168.1228, 168.1193 and 168.16B, inwhich the Quaternary deposits are much thicker than usual, and the depthto the Tertiary sediments (see figure 11.2).

Ti046-Ti048 are found on the other side of this valley. The lower meltwaterunit is not found beneath units 1 and 2 here. Layers 3 and 4 are low-resistiveand layer 5 is medium to high-resistive, similar to layers 4, 5 and 6 of Ti006-Ti001. The low-resistive layers are consequently interpreted as Gram Fmand Hodde Fm, and layer 5 as Odderup Fm and Arnum Fm. Layer 6 has ahigher resistivity than layer 5, but as the resolution of the former parameteris a function of the constraints rather than data, it is not necessarily a truereflection of the geological setting. However, a rather weak reflector found tobe coincident with the boundary to layer 6 indicates that it is. This reflectorcan be correlated to a reflector below soundings Ti004-Ti001. It is markedwith a wine red, dashed line on the 2D section.

The location of Ti045 seems to be on the fringe of the valley. Althoughthe thicknesses are different, both the upper meltwater unit (Tinglev out-wash plain) and clayey till seem to be found, but the deeper layers of themodel differ fundamentally from the other soundings. As seen before, layer 3is low-resistive, but it is much thinner and overlays medium-resistive layers20.There are no drillings that can be used directly in the interpretation of thissounding, but given the geological setting and the resistivity of the sediment,

20Layer 5 is a pseudo layer

96 11.4 Geological interpretation of the field area - an overview

it is likely that they represent a series of valley-fill sediments, possibly withMiocene sediments in the deeper parts.

The Tønder Graben is a complex structure which is not yet fully investi-gated and understood. Seismic surveys have shown the northern flank to bea step-like system of normal faults going as far down as the basement. Thesouth side on the other hand seems to have a totally different explanation,which primarily involves salt tectonics. It is well known from for examplegravimetrical surveys that there is a large salt pillow south of it, the upliftingof which is believed to be responsible the large offset of the sediments acrossthe flank and the dipping layer boundaries south of it. It is possible thatsalt from the graben migrated southwards, thereby facilitating subsidenceand consequently the graben. That there seems to be no salt in the actualgraben supports this. The fact that the Late Tertiary deposits are thicker inthe Tønder Graben than elsewhere is an indication that it was active duringthe Tertiary period. This is in accordance with the interpretation of Friborget al. (2002).

The Bastrup Sand that was found in drilling DGU no. 168.1378 is seen as areflector at a depth of approximately 380 m, 140 m deeper than on the southside of the flank.

11.4 Geological interpretation of the field area - anoverview

Based on the previous discussions, a summary of the geological interpreta-tion will be given in this section.

A high-resistive surface layer with a thickness of 25-30 m is found throughoutthe entire profile. This is interpreted as the sandy meltwater sediments ofTinglev outwash plain, which was deposited in the Weichselian.

Beneath this, clayey till is found. The lower boundary of this varies, therebyindicating the presence of Quaternary erosional structures. There is, how-ever, a possibility that the lower parts if the layer in the TEM soundings mayinclude the silty upper part of the Gram Fm in some parts of the profile. Alower sandy meltwater unit is found in the Tønder Graben beneath the till.

A number of buried Quaternary valleys have been found in the model area.They are all connected to the large valley system that follows the direc-


tion of the Tønder Graben. Starting from the south they are found aroundRødebæk in the northern part of profile 1 (X=5-7.000), Gardeby Mark onprofile 2 (X=9-10.000), inside Tinglev on profile 3 (X=13.000) and in thewesternmost part of profile 4, around Heds (X=19.600).

A low-resistive layer, interpreted as the Miocene clayey Gram Fm and/orHodde Fm, is found underneath the Quaternary sediments in the larger partof the profile. It is not found in the south end of the profile 1 and proba-bly not in the eastern and westernmost parts of profile 4 either. Is it onlypossible to distinguish between these on profile 3, where drilling DGU no.168.1378 is found.

Sandy Miocene deposits as the marine Arnum Fm and limnic Odderup Fmunderly the Gram Fm and Hodde Fm, or, in the areas where the Mioceneclays are missing, the Quaternary deposits.

The thin layer of Bastrup Sand is found in the seismics inside the Arnum Fm.It is also recognized as a low-resistive bottom layer in the TEM-soundings ofprofiles 2 and 4, which is most likely linked with saline pore water. This maybe intruding from the Tønder Graben, and could be linked to an intrusionfrom Flensburg Fjord.

A low-resistive layer, found at a depth of approximately 250 m, is inter-preted as Paleogene clay. On profile 1, this layer boundary is coincident witha well defined reflector, which also marks the top of the sediment on profile4. Rather weak reflectors indicate the presence of the Vejle Fjord Fm on topof this. The uppermost Paleogene sediments consist of the Røsnæs Clay Fm.

Discrepancies between the seismics and TEM soundings on profile 1 couldbe the result of a number of the before mentioned Vejle Fjord Fm or simplyjust uncertainties in data. Based on the resistivity of the Odderup Fm andArnum Fm, found in the soundings, the pore water must be overall fresh.These discrepancies could, however, also be a result of saline pore water, thepresence of which the logging implies.

The Tønder Graben seems to have been active in the Tertiary period, whichis indicated by the relatively thick Late Tertiary sediments found there. Thegraben itself is partly a result of salt tectonics (south flank) and partly aresult of faulting (north flank).

A salt pillow has pushed the sediments upwards on the south side of the


Tønder Graben, thereby creating an offset of approximately 140 m in theBastrup Sand. These salt tectonics are also echoed in the depth to theCretaceous, which is approximately 500 m north of the graben and 600 msouth of this, and in that the layer boundaries found south of the graben diptowards the north.

99

12 Conclusions and discussions

The purposes of this thesis were:

1. To make a geological interpretation of the field area using integratedinterpretation of data from transient electromagnetical soundings, seis-mics, logs and exploratory drillings.

2. To put Laterally Constrained Inversion (LCI) to practical use.

The conclusions to be drawn are the following:

12.1 The geological interpretation

Based on the discussions in the previous chapters, a summary of the geolog-ical interpretation will be given in this section.

Quaternary sediments are found throughout the entire profile. The surfacelayer consists of 25-30 m of meltwater sand, the Weichselian Tinglev outwashplain, and beneath this clayey till. A lower unit of meltwater sand was foundin the Tønder Graben beneath the till.

Four buried Quaternary valleys have been found in the model area. Allfour are connected to the large valley system that follows the direction of theTønder Graben.

A low-resistive layer, interpreted as the Miocene clayey Gram Fm and/orHodde Fm, is found underneath the Quaternary sediments in the larger partof the profile. It is not found in the south end of profile 1 and probably notin the eastern and westernmost parts of profile 4 either. It is only possibleto distinguish between the two formations on profile 3, where drilling DGUno. 168.1378 is found.

Sandy Miocene deposits such as the marine Arnum Fm and limnic Odd-erup Fm underlie the Gram Fm and Hodde Fm, or, in the areas where theMiocene clays are missing, the Quaternary deposits. The Bastrup Sand isfound internally in the Arnum Fm.

In the southern part of profile 1, Paleogene clay, consisting of the EoceneRøsnæs Clay Fm, is found at a depth of approximately 250 m and increas-ingly deeper northwards towards the Tønder Graben. North of this it isfound at approximately 370 m depth.

100 12.1 The geological interpretation

Discrepancies between the seismics and TEM soundings on profile 1 could bethe result of the possible presence of the Vejle Fjord Fm, saline pore wateror simply just uncertainties in data. The pore water found in the OdderupFm and Arnum Fm is otherwise fresh.

The top Cretaceous interface have been recognized as a reflector. This isfound at approximately 500 m depth south of the Tønder Graben and 600m depth north of it.

Indications of saline pore water are seen in the low-resistive bottom layerin the TEM-soundings of profiles 2 and 4, a layer that corresponds to theBastrup Sand.

The Tønder Graben and the salt tectonics involved with this seem to haveplayed a great role in the geology of the area. Layer boundaries south of thegraben have been found to dip towards this. The offset across the south flankof the graben is approximately 140 m in the Bastrup Sand. Furthermore, thesaline pore water could be connected to an intrusion from the graben. Therelatively thick Late Tertiary sediments found in the Tønder Graben suggeststhat it was active in the Tertiary period.

The importance of using integrated interpretation is illustrated in the inter-pretation that the low-resistivity bottom layer on profile 1 is the Paleogeneclay, while that on profile 4 is a result saline pore water. Using TEM dataalone, these would probably have been interpreted as the same layer

12.1.1 Suggestions for future work

From a scientific point of view, deep drillings are very interesting due to thefact that they may help increase the understanding of the geology of the area.A deep drilling south of the Tønder Graben would remove some of the uncer-tainties concerning this area. Placing one in the south end of profile 1, couldhelp answer the questions of whether the pore water in the lower parts ofArnum Fm and Odderup Fm is saline, whether the Miocene clays and VejleFjord Fm are found in this area, and if they are, what the distribution ofthese are. Furthermore, placing the drilling in the south end of profile 1 willlower the costs as well, since the sediments are found at shallower depth here.

Concerning the matter of ensuring the water supply, the thick Neogene sed-iments in the Tønder Graben may prove to be good aquifers. Furthermore,

101 12.2 Experiences gained from using LCI

the Gram Fm and Hodde Fm may provide a good clay cover, thereby pro-tecting the aquifers from pollutants.

Future geophysical investigations in the area might include a new attemptto increase the penetration depth of the HMTEM soundings.

12.2 Experiences gained from using LCI

LCI is a very useful inversion method which will surely be used in years tocome, especially with the continuing development of new methods as theSkyTEM method. LCI enhances structural features. Information from adja-cent soundings migrate between these, thereby yielding a more trustworthyoverall model. The inversion result is primarily based on the well resolvedparameters. As long as one is careful in building the LCI model, these fea-tures seem very credible. The inversion method has in examples shown tobe be robust. Well resolved parameters are not as influenced by the lateralconstraints as poorly resolved parameters. Poorly resolved parameters be-come more reliable as the inversion result is based on more than one dataset. LCI may therefore especially improve the interpretation of the deepestpart of the model.

The rigidness of the system may be viewed as a drawback. The inversiontries to minimize the total residual of the model instead of the individual.This may make it necessary to change the starting parameters a number oftimes in order to get an acceptable result. Especially the thicknesses fromMCI and single-site interpretations were used as an inspiration in this thesis,but in some cases resistivities needed tuning as well. It seems to be a com-promise between enhancing single soundings or the overall model.

Suggested developments of LCI involve:

Further study on the size of constraint factors and their dependence on lat-eral distance is needed.

Programs to be used in the model building and interpretation of data areneeded.

The implementation of 2D and ultimately 3D inversion.

Furthermore, it should be considered whether lateral constraints on depthsshould be made absolute instead of relative.

102 REFERENCES

References

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Auken, E., Sørensen, K. I., and Pellerin, L.: 2003b, Mutually ConstrainedInversion (MCI) of Electrical and Electromagnetic Data, Paper submittedto Geophysical Prospecting, January 2003, pp 21

Balling, N.: 1998, Seismiske metoder. Lecture notes, Department of EarthSciences, University of Aarhus, Chapt. 2, pp 2/1-2/62

Benn, D. I. and Evans, D. J. A.: 1998, Glaciers and glaciation, ArnoldPublishing, Ltd, pp 734

Bjørslev Nielsen, O.: 1995, Danmarks geologi fra Kridt til i dag, Departmentof Earth Sciences, University of Aarhus, Chapt. 1, pp 3-18

Christensen, N. B.: 1995, Den transiente elektromagnetiske sonderingsme-tode. Lecture notes, Department of Earth Sciences, University of Aarhus,pp 38

Christiansen, A. V., Auken, E., Sørensen, K. I., and Smith, J. T.: 2002, 2-Dlaterally constrained inversion (LCI) of resistivity data, Presentation, The8th EEGS-ES meeting, Aveiro, Portugal

Danielsen, J. E., Auken, E., Jørgensen, F., Søndergaard, V., and Sørensen,K. I.: 2003, The application of the Transient Electromagnetic method in hy-drogeophysical surveys, Paper submitted to journal of Applied Geophysics,March 2003, pp 22

Danielsen, J. E., Auken, E., and Sørensen, K. I.: 2002, HiTEM - a highmoment / high production TEM system, Proceedings of the 8th EEGS-ESmeeting, Aveiro, Portugal, pp 4

Dybkær, K. and Skovbjerg Rasmussen, E.: 2000, Palynological dating of theOligocene-Miocene successions in the Lille Bælt area, Denmark, Bulletinof the Geological Society of Denmark, Vol. 47, pp 87-103

Effersø, M., Auken, E., and Sørensen, K. I.: 1999, Inversion of band-limitedTEM responses, Geophysical Prospecting, Vol. 47, pp 551-564

Ellis, D. E.: 1987, Well logging for earth scientists, Elsevier Science Pub-lishing Co., Inc., Chapt. 4, 9-10, pp 57-72, 181-226

103 REFERENCES

Engell-Jensen, M., Korsbech, U., and Madsen, F. E.: 1984, U, Th and Kin Upper Cretaceous and Tertiary sediments in Denmark, Bulletin of theGeological Society of Denmark, Vol. 32, pp 107-120

Flis, M. F., Newman, G. A., and Hohmann, G. W.: 1989, Induced-polarization effects in time-domain electromagnetic measurements, Geo-physics, Vol. 54, pp 514-523

Fowler, J.: 1990, The solid earth - an introduction to global geophysics,Cambridge University press, Chapt. 4, pp 76-156

Friborg, R.: 1996, The landscape below the Tinglev outwash plain: a re-construction, Bulletin of the Geological Society of Denmark, Vol. 43, pp34-40

Friborg, R., Kirsch, R., Scheer, W., Stoepker, K., and Thomsen, S.: 2002,Grundvand til Sønderjylland og Schleswig, Sønderjylland County andLANU, pp 93

Friis, H.: 1995, Danmarks geologi fra Kridt til i dag, Department of EarthSciences, University of Aarhus, Chapt. 6, pp 115-128

Geonics Limited: 1991, Protem 47 Operating Manual, pp 113

Heilmann-Clausen, C.: 1995, Danmarks geologi fra Kridt til i dag, Depart-ment of Earth Sciences, University of Aarhus, Chapt. 5, pp 69-114

Houmark-Nielsen, M.: 1987, Pleistocene stratigraphy and glacial history ofthe central part of Denmark, Bulletin of the Geological Society of Denmark,Vol. 36, pp 1-189

Houmark-Nielsen, M.: 2003, Signature and timing of the Kattegat Ice Stream:onset of the Last Glacial Maximum sequence at the southwestern marginof the Scandinavian Ice Sheet, Boreas, Vol. 32, pp 227-241

HydroGeophysics Group, SiTEM help file, HydroGeophysics Group

HydroGeophysics Group: 2000, Grundvandskortlægning. Lecture notes, Oc-tober 2000, pp 111

HydroGeophysics Group: 2001, Processering og tolkning af TEM data, Lec-ture notes, March 2001, pp 21

Jacobsen, B. H.: 1993, Practical methods of covariance specification for geo-physical inversion, Proceedings of the Interdisciplinary Inversion Workshop2, Copenhagen 1993, pp 1-10

104 REFERENCES

Jacobsen, B. H.: 2000, Geophysical inverse modelling. Lecture notes, De-partment of Earth Sciences, University of Aarhus, Chapt. 3-8

Jørgensen, F., Auken, E., and Sørensen, K. I.: 2003a, Anvendelsen af TEM-metoden ved geologisk kortlægning, HydroGeophysics Group, pp 72

Jørgensen, F., Lykke-Andersen, H., Sandersen, B. E., Auken, E., andNørmark, E.: 2003b, Geophysical investigations of buried Quaternary val-leys in Denmark: An integrated application of transient electromagneticsoundings, reflection seismic surveys and exploratory drillings, Paper sub-mitted to journal of Applied Geophysics, March 2003, pp 21

Jørgensen, F., Sandersen, B. E., and Auken, E.: 2003c, Imaging BuriedQuaternary Valleys using the Transient Electromagnetic Method, Papersubmitted to journal of Applied Geophysics, March 2003, pp 26

Kjær, K., Houmark-Nielsen, M., and Richart, N.: 2003, Ice flow patterns anddispersal of erratics at the southwestern margin of the last ScandinavianIce Sheet: signature of Palaeo-ice streams, Boreas, Vol. 32, pp 130-148

Knudsen, K. L.: 1995, Danmarks geologi fra Kridt til i dag, Department ofEarth Sciences, University of Aarhus, Chapt. 12, pp 247-270

Korsbech, U.: 2003, Personal communication, Associate Professor at DTU

Kronborg, C.: 1995, Danmarks geologi fra Kridt til i dag, Department ofEarth Sciences, University of Aarhus, Chapt. 13, pp 271-290

Larsen, G.: 2002, Geologisk set - Fyn og øerne, Geografforlaget, pp 144

Larsen, G. and Kronborg, C.: 1994, Geologisk set - Det mellemste Jylland,Geografforlaget, pp 272

Lykke-Andersen, H.: 2003, Personal communication, Department of EarthSciences, University of Aarhus

Menke, W.: 1989, Geophysical Data Analysis: Discrete Inverse Theory,Elsevier Science Publishing Co., Inc., pp 289

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Nabighian, M. N. and Macnae, J. C.: 1991, Electromagnetic Methods inApplied Geophysics, Volume 2, Application, Parts A and B, Society ofExploration Geophysicists, Chapt. 6, pp 427-479

105 REFERENCES

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Skovbjerg Rasmussen, E.: 1995, Vejle Fjord Formation: Mineralogy andgeochemistry, Bulletin of the Geological Society of Denmark, Vol. 42, pp57-67

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Spies, B. R. and Frischknecht, F. C.: 1991, Electromagnetic Methods inApplied Geophysics, Vol. 2, Application, Parts A and B, Society of Explo-ration Geophysicists, Chapt. 5, pp 285-378

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Thomsen, S.: 2003, Personal communication, Sønderjylland County

Ward, S. H. and Hohmann, G. W.: 1988, Electromagnetic Methods in AppliedGeophysics, Vol. 1, Theory, Society of Exploration Geophysicists, Chapt.4, pp 131-250

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Young, H. D.: 1992, University physics, Vol. 2, Addison-Wesley PublishingCompany

107

A Position of the soundings

Sounding UTMx UTMy Sounding UTMx UTMyTi001 516568 6086988 Ti032 515830 6080789Ti002 516305 6086823 Ti033 515788 6080669Ti003 516323 6086549 Ti034 515880 6079282Ti004 516442 6086260 Ti035 515867 6079443Ti005 516414 6086027 Ti036 515744 6079645Ti006 516719 6085812 Ti037 515729 6079752Ti007 516833 6085445 Ti038 515769 6079914Ti008 516932 6085200 Ti039 515617 6080225Ti009 516956 6084922 Ti040 515740 6078996Ti010 516900 6084756 Ti041 515820 6078781Ti011 516931 6084491 Ti042 515445 6078644Ti012 517021 6084177 Ti043 515661 6078446Ti013 517048 6084078 Ti044 515408 6078455Ti014 517165 6083718 Ti045 515418 6088350Ti015 517255 6083487 Ti046 514953 6088116Ti016 517230 6083218 Ti047 514707 6088292Ti017 517064 6083250 Ti048 514556 6088499Ti018 517027 6083104 Ti049 513527 6088300Ti019 517146 6082915 Ti050 513333 6088415Ti020 516791 6082826 Ti051 513083 6088309Ti021 516705 6082509 Ti052 512652 6088709Ti022 516682 6082342 Ti053 512465 6088945Ti023 516631 6082128 Ti054 513865 6088305Ti024 516533 6081936 Ti055 512187 6088954Ti025 516412 6081914 Ti056 511881 6089341Ti026 516100 6081907 Ti057 511701 6089575Ti027 516264 6081492 Ti058 511264 6089755Ti028 516257 6081317 Ti059 510795 6089895Ti029 515953 6081186 Ti060 510341 6090377Ti030 515858 6081110 Ti061 510264 6090486Ti031 516029 6080972

Table A.1: Position of the soundings from June 2001. Soundings withbold letters were interpretable, while the others had to be abandoned.Coordinates are given in UTM 32 ED50.

108

B Position of exploratory drillings

Drilling archive number UTMx UTMyDGU no. 168.1378 516523 6086479DGU no. 167.234A 510379 6090511DGU no. 167.234B 510392 6090484DGU no. 168.1228 516297 6087850DGU no. 168.1193 516295 6087900DGU no. 168.16B 516293 6087949LANU no. 1121/53 513306 6075844

Table B.1: Position of the exploratory drillings. Coordinates are given inUTM 32 ED50.

109

C Details about the equipment

C.1 Gate Center Times for the UH, VH and Hi Seg-ments

Gate UH [µs] VH [ms] HI [ms]1 6.9 0.49 0.1012 9.0 0.57 0.1223 12.1 0.70 0.1524 16.0 0.084 0.1885 20.2 0.100 0.2306 26.3 0.126 0.2917 33.8 0.155 0.3678 42.5 0.190 0.4559 54.7 0.240 0.57510 69.3 0.300 0.72011 86.0 0.365 0.88012 107 0.450 1.0813 138 0.580 1.3814 175 0.730 1.7515 219 0.900 2.1916 280 1.14 2.8217 354 1.44 3.5618 441 1.79 4.3719 561 2.26 5.5420 707 2.85 7.04

Table C.1: Gate center times for the analogue Protem 47 receiver (GeonicsLimited, 1991).

110 C.2 Other parameters

C.2 Other parameters

Receiver coil

Serial number: 16Cut-off frequency: 490 kHz. 1. orderArea: 31.4 m2

Offset: 72.5 mArea: 9000 m2

Transmitter

Groundwater InstrumentsSerial number: 4Current, low moment: Approx. 2.4 ACurrent, high moment: Approx. 74 AT/O time configuration: 1.0 µsRx/Tx delay: 5.6 µs

Turn-on settings for the transmitter, low moment

Exp. Dec. Const.: 4.0e+41. ramp [pct.]: 662. ramp [pct.]: 96

Turn-off settings for the transmitter, low moment

Toff [Amp/s]: 5.8e+5End Avelance [Amp]: 0.4Exp. Dec. Const.: 1.0e+61. ramp [pct.]: 372. ramp [pct.]: 13

Turn-on settings for the transmitter, high moment

Exp. Dec. Const.: 1.3e+31. ramp [pct.]: 662. ramp [pct.]: 96

Turn-off settings for the transmitter, high moment

111 C.2 Other parameters

Toff [Amp/s]: 2.8e+6End Avelance [Amp]: 0.5Exp. Dec. Const.: 4.7e+51. ramp [pct.]: 372. ramp [pct.]: 13

Receiver

Geonics Protem 47analogue serial number: 900603

Cut-off frequency, 1.UH: 240 kHz 1. orderCut-off frequency, 2.UH: 620 kHz 1. orderCut-off frequency, 1.H: 37 kHz 1. orderCut-off frequency, 1.H: 420 kHz 1. order

UTM zone

GPS datum: European ED 1950

Processing

Uniform STD (dB/dt): 5 %Noise processing: Based on noise recordsMedian filters: Not used

112

D The CD-ROM

The CD-ROM contains the files involved with this thesis. The followingthings are found in the directories:

\Data\ contains rawdata from the TEM soundings and the SiTEMproject, both concerning data from June 2001 and September 2002.

\Interpretations\ contains files from the interpretations:.emo-files and .mod-files from the MCI and single-site interpretations..emo-files and .mod-files from the LCI interpretations.The Em1Dinv configuration file which was used..tem-files from Semdi.An Access database containing drillings and interpretations of TEMsoundings.

\MapInfo\ contains the MapInfo layouts, and files used by these.

\Read emo\ contains a copy og the Read emo macro, the sub-macroesand a readme file.

\Thesis\ contains the thesis and MapInfo layouts as pdf-files.

Furthermore, the thesis is included on the CD-ROM as a .pdf-file.

113

E 2D sections with TEM soundings, seismics

and exploratory drillings

E.1 Figure of the model area including profiles

E.2 MCI and single-site interpretations of profiles 1through 4

E.3 LCI interpretations of profiles 1 through 4

E.4 LCI interpretations of profile 5

E.5 The geophysical interpretation of TEM data

E.6 The geological interpretation

Profile SoundingsTi044, Ti043, Ti035, Ti036, Ti037, Ti038,

Profile 1 Ti033, Ti032, Ti031, Ti030, Ti029, Ti026,Ti025, Ti024, Ti023, Ti022, Ti021, Ti019

Profile 2 Ti018, Ti016, Ti017, Ti015, Ti014, Ti013Ti010, Ti009, Ti008, Ti007

Profile 3 Ti006, Ti004, Ti003, Ti002, Ti001, Ti045,Ti046, Ti047, Ti048

Profile 4 Ti049, Ti050, Ti051, Ti052, Ti053, Ti056,Ti057, Ti058, Ti059, Ti060

Profile 5 Includes soundings from profiles 1 and 2

Table E.1: The division of soundings among the five profiles, starting withthe southernmost sounding.

114 E.1 Figure of the model area including profiles

Figure E.1: The division of soundings among the profiles.

1

F Erratum

The three residuals on page 44, given in equations 6.34, 6.35 and 6.36, shouldread:

RESdata =[ 1

Nd

Nd∑i=1

(dobsi − dmodel

i )2

var(dobsi )

]1/2

(F.1)

RESvertical =[ 1

NM · (NP − 1)/2

NM∑i=1

(NP−1)/2∑j=1

(mi,j − mi,j+1)2

var(mi,j)

]1/2

(F.2)

RESlateral =[ 1

(NM − 1) · NP

NM−1∑i=1

NP∑j=1

(mi,j − mi+1,j)2

var(mi,j)

]1/2

(F.3)

integrated interpretation of transient electromagnetic and ... · integrated interpretation of...

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