tdem useful tol for identifying and monitoring fresh-saltwater interface

5/22/2018 TDEM Useful Tol for Identifying and Monitoring Fresh-saltwater Interface

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TDEM: A USEFUL TOOL FOR IDENTIFYING AND MONITORING THE

FRESH-SALTWATER INTERFACE

Giovanni BARROCU & Gaetano RANIERIDepartment of Territorial Engineering

Faculty of Engineering - University of Cagliari, ItalyE-mail: [email protected] [email protected]

AbstractOne of the main problems in studying saltwater intrusion processes is the temporal and

spatial monitoring of the fresh-saltwater interface. In the past geophysical methods have beenused to study its evolutionary trend. Time Domain Electromagnetic (TDEM) methods have provento be most efficacious, producing better results than the traditional galvanic techniques. In fact, the

major advantage of this method is that deep soundings can be performed in a relatively short time.TDEM sounding appears to be particularly useful for discriminating between layers having lowresistivity and consequently for monitoring salt water intrusion and diffusion, as well as artificialrecharge procedures. However, interpretative limitations related to the principle of equivalencearise when intermediate resistive layers occur, together with problems of induced polarisation (IP)and superparamagnetism (SPM) associated with the presence of highly resistive layers at the baseof the stratigraphic sequence. This method has been experimentally tested in well characterisedtest sites where geophysical investigations (reflection seismics, SEV, IP, gravimetry), drillings, aswell as chemical and isotopic analyses of the water had already been conducted. The resultsconfirm that the method is practical, economic and perfectly reliable, especially at large depths

(down to 100 m) and in the fast acquisition version, using windows from 4 s to some ms.

IntroductionDeveloped in Russia in the early

eighties for studying deep structures, since1985 the TDEM has found wide application ingeological, engineering and environmentalspheres and today represents a potentiallypromising method for investigating electricalparameters of the subsoil.

Much has been published on theTDEM, including applications inhydrogeological research (Sorensen K.I.,1996; Fittermann D.V. and Stewart M.T.,1986; Christensen N.B. and Sorensen K.I.,1994; Meju M.A., 1995), and for studying thespecific problem of salt water intrusion (YuhrL. and Benson R.C., 1995; Hoekstra P. andal. 1996; Richards R.T. and al. 1995;Goldmann M., 1996).

Other workers (Nabighian M.N. andMcnae J.C., 1991; Auken E., 1995; SorensenK.I., 1998; Christensen N.B., 1997; Meju

M.A., 1994,1994 ) have studied variousaspects of the application, processing and

interpretation of TDEM soundings and its usein combination with other geophysicaltechniques.

A variety of geophysical methods hasbeen adopted in saltwater intrusioninvestigations. Conventional DC resistivitytechniques have long been used tocharacterise shallow aquifers, gravity andmagnetic methods have been applied toreconstruct deep aquifers and the bedrock.Barbieri G. et al., 1986 applied the I.P.method to describe the temporal evolution ofsaltwater intrusion. Other applications haveinvolved electromagnetic and seismicmethods.

Here we examine the mainadvantages and disadvantages of TDEM withrespect to other techniques.


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Basic concepts of TDEM soundingsA typical TDEM array consists of a

transmitter loop and a receiver coil. A verystrong current (from 1 to 20 A), produced by abattery or motor generator, is injected into asquare (usually single turn) loop of groundedwire and connected directly to the transmitter.The transmitter current, though still periodic,is a modified symmetrical square wave, asshown in Fig. 1.

After every second quarter-period thetransmitter current is abruptly reduced tozero for one quarter period, whereupon itflows in the opposite direction. In a receiverloop (coincident or inside the transmitter loop)when the current is switched off a voltagepulse is recorded. In the coincident loopconfiguration, the length of the loop side isapproximately equal to the desired depth ofpenetration. Generally the loop side variesfrom a few to hundreds of meters. A single ormulti turn receiver coil, placed in the middle ofthe transmitter loop or coincident with thetransmitter loop, is connected to the receiverby means of a short cable.

As shown by Mc Neill J.D., 1996, theprinciples of TDEM resistivity sounding arerelatively easily understood.The process of abruptly reducing thetransmitter current to zero induces, accordingto Faraday's law, a short duration voltagepulse in the ground, which causes a loop of

current to flow in the immediate vicinity of thetransmitter wire (see Fig.2).In fact, immediately after the transmittercurrent is turned off, the current loop passesinto the ground immediately below thetransmitter and, on account of the finiteresistivity of the ground, the current amplitudeimmediately starts to decay. Similarly thedecaying current induces a voltagepulse which causes more current to flow at agreater distance from the transmitter loop,and also at greater depth (Mc Neill J.D,1996).

The deeper current flow also decaysdue to finite resistivity of the ground, inducingeven deeper current flow and so forth.

To determine the voltage produced bythe decaying magnetic field at the receivercoil at successively later times,measurements are made of the current flowand thus also of the electrical resistivity of theearth at increasingly greater depths, and it isthis process that forms the basis of resistivity

sounding in the time domain. The outputvoltage of the receiver loop is shown in fig.3.

Fig. 2 - Current flowing in the ground.

The decay characteristic of the voltage in thereceiver is determined for a number of timegates, each measuring and recording the

amplitude of decaying voltage. The timegating differs with time. In fact, to minimisemeasurement distortions, the early timegates, which are located where the transientchanges rapidly with time, are very narrow,whereas the later gates, where the amplitudeof the transient decay diminishes, are muchbroader to enhance the signal-to-noise ratio.

Fig.3 Gate locations


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Only the (two) transients that occurwhen the transmitter current has just beenshut off are measured.

Lastly, particularly for sounding atshallower depths, where it is not necessary tomeasure the transient characteristics out tovery late time, the period is typically of theorder of one millisecond or less, which meansthat in a total measurement time of a fewseconds, measurements can be made andstacked on several thousand transientresponses to improve signal-to-noise ratio. Toincrease depth the variations must berecorded at a later time (by some seconds).

Apparent resistivity in TDEM soundingsThe voltage response can be divided into anearly stage(where the response is constantwith time), an

intermediate stage(response

shape continually varying with time) and alate stage(response is a straight line on log-log plot).The response varies quite simply with timeand conductivity as :

2/5

2/3

1)(t

MktV

= (1)

where k1 = a constant

M = product of T current (amps) xarea (m2)

= terrain conductivity (Siemens/m)t = time (seconds)

and V(t) = output voltage from a single turnreceiver coil of 1 m2area.

Note that unlike conventionalresistivity measurements, where themeasured voltage varies linearly with terrainresistivity, for TDEM the measured voltage,

V(t) varies as 3/2 , so the TDEM isintrinsically more sensitive to small variationsin conductivity than conventional resistivity

soundings.It can also be shown for layered earth,

that the shape of the induced curves is verysimilar. To make the curves morerepresentative of the resistive structure, wecan convert the voltage curves to apparentresistivity.

As observed earlier as time increases,so too does the depth to the current loopsand we can carry out resistivity soundingwith depth.

Since =1/, from equation (1)

( )( ) 3/53/2

3/2

2

tte

Mkt

a=

(2)

Summing up, except for the early timedescending branch and the intermediateanomalous region described above, thesounding behaviour of TDEM is analogous toconventional DC resistivity if we let thepassage of time achieve the increasing depthof exploration rather than increasing inter-electrode spacing . However, at very latetime there is no signal and no asymptote tothe bedrock resistivity in the curve can beobserved.

Practical indications from theoryThe voltage induced in the receiver coil is theproduct of the receiver coil Moment Mr (areatimes the number of turns) multiplied by thetime derivative of the vertical magnetic fluxdensity:

2/3

)4

(5

)(

a

t

r tt

M

M

tV

= (3)

Where is the magnetic permeabilityand Mt is the transmitter loop moment =L

2 I,

with L length of the side .The equation describes some major

points concerning transient soundings.Because V(t)/Mr is inversely proportional totime and the current diffuses downwards withtime, it is more difficult to sound at greaterdepths unless the transmitter moment isincreased. To increase the transmittermoment, we can increase the transmittercurrent or the loop area or the wire turns orboth.

The depth of investigation isdependent upon the geolectrical section: withtwo-layered sections with the same first-layerresistivity , we can sound more deeply withthe same current in the section with the moreconductive basement.

In investigations at shallow depths, itis very important to measure the voltage inthe early and intermediate stages.

Early stage measuring is rather delicateand complex, but in latter years severaldevices have been developed for measuring


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during a short time after the transmittercurrent is cut-off. Soundings obtained withthis instrument are called TEM-FAST and area very useful tool for hydrologists for studyingthe shallower subsoil .

By contrast to DC methods, in TDEMsounding the form of apparent resistivitycurves depends on the size of receiving-

transmitting antenna. At early times (t)appears to be overestimated the smaller theloop size is.

Practical recommendationsBased on the experience gained from usingTDEM sounding in many differentapplications, some practicalrecommendations are summarised below:1. The maximum depth of investigation is

determined by the maximal time t, in

which it is possible to reliably register asignal V(t)/Iand not exceed the loop sideby 3 times;

2. the TDEM technique performs effectivelyin sections with high conductivity: the

layers, for example, with = 1 and =1.5 Ohm m are reliably stratified;

3. it is practically impossible to distinguishlayers with very high resistivity;

4. super-small antennas (less than 10-15meters) for stratifying layers at shallowdepths can only be used for rocks withlow resistivity;

5. at high levels of resistivity it is necessaryto use large antennas;

6. the most favourable range of specificresistivity for stratification of rock is 10

Ohm m < < 300 Ohm m;7. mean resolution is about 1/10 of the loop

side.

For optimum sizing of the antenna it shouldbe remembered that large antennas increaselimiting depth of the investigation, but they donot allow to stratify subsurface horizons.Estimations of depths h, for which reliable

interpretation of the results is possible, at useof coincide antennas with the sideTR=REC=L usually fall within the range:hmin> L/10 and hmax< 3L.

Recently, vertical and lateral resolutionproblems of TDEM soundings have beenstudied on the Poetto beach in Cagliari(Deidda G.P. and Ranieri G., 2000). Figure 4shows a 66 m long apparent resistivitypseudo-section, reconstructed from 23soundings performed using a 6.25m loop side

with 3 m spacing. The profile was carried outparallel to the coast line, 50 m far . Thelayered section, with the very low resistivitycontrast indicated, demonstrates thepossibility of discriminating low resistivity fromvery low resistivity. The layered increasedresistivities towards the surface showdifferent salt content (salt water zone,diffusion zone, brackish water zone, freshwater zone)

Fig.4 Apparent resistivity pseudosectionshowing different conductive layers

Natural phenomena affecting TEMmeasurements

In practice, two physical phenomena existthat can essentially effect the efficiency of

geological interpretation of TDEM soundings.Both these phenomena are associated withfrequency dispersion of electromagneticproperties of rocks:

Superparamagnetic effect (SPMeffect)

Induced polarisation effect(IPeffect)However, depending on the desired

outcome, both effects can be consideredeither "harmful" (noise), or "useful", as theycontaining further information on the structurebeing investigated.Studies of SPM effects using TEM-FAST invarious parts of the world have shown the

following: The most intensive SPM effects exist in

areas of effusive and volcanicsedimentary rocks. Surface clayformations covering parent rocks are themost superparamagnetic.

SPM effects are produced in conditions oflong term permafrost and are usuallylocated on the edges of zones of thawingfrozen rocks.

0 10 20 30 40 50 60

(m)

-20.00

-15.00

-10.00

-5.00

Apparentdepth(m)

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Apparent Resistivity Pseudo-sectionPoetto beach

Apparent resistivity (Ohm m)


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Significant SPM effects are observed onglaciers or very resistive rocks.

IP effects are produced in

Thin well-conducting horizon of surface

clay deposits with 300-500 Ohm m. The late stage will bedeformed.

Glaciers and permafrost rocks.

Surface deposits severely polluted withindustrial waste, (including petroleumpollution)

Crustal erosion in crystal rocks and faultzones

Noise SourcesOther sources of noise for TDEM soundingsinclude :

circuit noise(depending on the goodness

of the device). radiated and induced noise by radio and

radar transmitters and also by lightning;

presence of magnetic field produced by50/60 Hz power lines.

presence of nearby metallic structures.

Data reduction and interpretationApparent resistivity curves have been

numerically calculated for a variety of layeredsoils , particularly in Russia where thetechnique was developed. The field data arecompared with a selection of curves, fromwhich the actual geo-electric section isdetermined. More recently the advent of fastcomputer inversion programs allows the fieldtransient data to be automatically inverted toa layered earth geometry in a matter ofminutes. Like all electrical soundingtechniques, TDEM interpretation also suffersto a greater or lesser extent fromequivalence. In many cases the automaticinversion program giving the real thicknessand resistivities of the different layers is not

always usefully applied.When the layers have very similar resistivity itis more convenient to transform the voltagedata into apparent resistivity versus depth.Godio et al. 1999 show that in many cases itis more reliable to transform only the V(t)/I

versus time data into a versus h data.Figures 5,6 and 7 show the apparentresistivity pseudo-sections obtained in thearea of Capoterra (south-west Sardinia), for aprofile perpendicular to the coast line and two

profiles parallel and perpendicular to saltworks. The apparent resistivity sectionsdescribe saltwater intrusion more accuratelythan the true resistivity sections, which arevery irregular (and unnatural) on account ofthe equivalence effect in the interpretation.

fig.5- Apparent resistivity pseudosectionshowing the salt water intrusion far from thecoastal line.

Fig. 6- Apparent resistivity pseudosection in aprofile perpendicular to salt-works showingsalt water intrusion and the "spray" effect

Fig.7- Apparent resistivity pseudosection in aprofile parallel to salt-works.

Advantages of TDEM

TDEM geo-electric sounding offers significantadvantages over conventional DC resistivitysounding, namely:

improved operating speed (it is possibleto perform about 30 soundings per day, ina very confined space, while in

100 200 300 400 500 600 700 800 900 1000 1100 1200

Distance [ m ]

-100

-80

-60

-40

-20

0

Depth

[m]

50 150 250 350 450 550 650 750 850

[Ohm m]

Sea coast-line Land

S1

Macchiareddu - Cagliari (Italy)

Apparent resistivity pseudosectionProfile: n.3 Cardiga

0 25 50 75 100 125 150 175 200 225 250

Distance [ m ]

-70

-60

-50

-40

-30

-20

-10

Depth

[m

]

100 200 300 400 500


Apparent resistivity pseudosectionProfile: n.1 - P488Ohm m

East West

0 50 100 150 200 250 300 350

Distance [ m ]

-80

-70

-60

-50

-40

-30

-20

-10

Depth[m]

50 100 150 200 250 300 350

North South

Ohm m


Apparent resistivity pseudosectionProfile: 2 - P488_S


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conventional DC soundings the AB linecan be over 2 km long when the surfacelayers are highly conductive for soundingdown to 100 m)

improved lateral resolution;

improved resolution of conductiveelectrical equivalence;

simple to inject current into a resistivesurface layer

possibility of using the instrument in urbanareas;

great synergy with other geophysicalmethods

The latter point has been clearlydemonstrated by Deidda G.P. et al. (2000).The TDEM can be conveniently integratedwith the seismic reflection method to providea more complete interpretation of thegeological section. In fact the seismic data,

though they provide a very good descriptionin terms of geometry and elasticcharacteristic of a layered subsoil, are unableto identify salt water and fresh water content.The two sections obtained by superposingTEM FAST results and TEM results on theseismic reflection, show excellent agreementof most of the interfaces obtained via TDEMand seismics. And allow to assign to thedifferent layers recognised by seismics,electrical resistivities indicating the saltcontents of those layers. In the sectionssaltwater intrusion effects have been

observed along what are probably fresh watercanals and aquifers, laterally confined byclay beds, at a depth of over 100 m (figg.8a,b and 9a,b).

Fig.8a,b- Resistivity section obtainedrespectively from a TEM FAST survey and a

TEM survey.

Fig. 9a Superposition of TEM FAST resultsand HR reflection seismic section

Fig. 9b Superposition of TEM results andseismic section

The disadvantages of TDEM soundings are:

they do not perform well in highly resistive

soils the interpretation programs are limited to

1D or 2D structures

the equipment is rather expensive

ConclusionsThe TDEM method is a potentially useful toolfor hydrogeologists, particularly for studyingsalt water intrusion phenomena . It cancomplement conventional DC resistivitymethods and FDEM methods. Thecombination with other high resolutiongeophysical method such as reflection

seismics and a knowledge of a givengeological feature gained from a fewboreholes can be very usuful for describingthe subsoil and to define the aquifergeometry and groundwater quality.

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1.5

3

5

20

40

60

80

100

200

400

600

800

1000

1400

1800

2500

3500

4500

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13

13.793.172

3.092

3.111

115.9

453.5

133.8

13.083.883

2.568

2.686

41.68

421.5

18.36

3.8559.337

2.156

3.017

2026

3.8559.337

2.156

3.017

2026

3.09712.2

1.815

32.83

1.519

742.8

2.5611.115.68

2.04

33.47

1.33

745.7

4.98

1.97

1.08

899.2

5.26

2.23

1.49

2.29

998.1

4.43

2.51

1.39

2.37

1.62

2666

4.22

2.17

1.54

1.35

1.43

5783

3.28

2.12

1.59

2.9

1.85

695.9

2.691.82408.52.04

1.6

2.9

45

1.991.47413.42.03

1.91

2.14

4397

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300-400

-350

-300

-250

-200

-150

-100

-50

0

resistivity[ohm*m]


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tdem useful tol for identifying and monitoring fresh-saltwater interface

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