TAMU-T-72-005 c. 2

A.OAN COI V O~i ~

CaRC~~N Aw":g ~gp'~

SB3 GIBAf

ELECTRICAL RESISTIVITY LOGGING

IN UNCONSOLIDATED SEDIMENTS

Vf'repa ed hy

WILLIAM E. SWEET, JR.

Dopa'tmar>I af Oreasaaraa!>yTexax A*M U ~ i"xrNly

TAMV-SG-72-205 4ugust 1972

TEXAS ASM UNIVERSiTY + SEA~OGRAM

gg gg- T- 7z-Qo5 c 2

ELECTRICAL RESISTIVITY LOGGING

IN UNCONSOLIDATED SEDIMENTS

GIRGULAIt'k;"3 QQP fSea Grant Depository

Partially supported by the NationalSea Grant Progra~, Department of

Commerce, Grant 2-35213

13 Preliminary pages14,2 Text

g9 Figures

William E. Sweet, Jr.

Department of Oceanography

Texas ASM University

August 1.972

TAMU-SG"72-205

NATIONAL 9Q GRANT OKPOStTORYPH.'L LIBRARY BUIl.Dye

URI, HARRAGANSQT BAY CA@PUSHARRAGAiHSHif, R I 02882

FOREWORD

The following report is written by Dr- William E. Sweet, Jr. as a

result of activities carried out during the study of electrical logging

in unconsolidated sediments in aquatic environments.

The application of electrical logging to the field of geological

oceanography is rather new and only a limited number of investigators

worked or are working on this aspect. The Texas ASM study started five

years ago and was initiated by Dr. Frank B. Chmelik. Bamboo poles with

electrodes, borrowed recorders and other equipment provided enough infor-

mation to request the Sea Grant Office for financi,al support to carry out

this study. Dr. Chmelik, with the help of Dr. George L. Huebner, Jr.

and the undersigned developed several editions of hardware required to

carry out the measurements. Dr. Sweet replaced Dr. Chmelik in 1970 and

was involved as pro!set leader until other tasks prevented him from con-

tinuing this assignment.

The report primarily discusses the present state of the art with

regard to electrical resistivity logging in unconsolidated sediments.

The author discusses the successful adaptation of electrical resistivity

logging techniques for use in unconsolidated aquatic sediments, the

results of correlating sedimentary strata by means of resistivity logging,

the correlation of in situ logs with logs obtained from cores collected

from the same station, and the relationship of resistivity to various

geological and geotechnical properties of sediments.

A discussion on the development of electrical logging in industry

is presented first to better emphasize the direction of approach by our

investigating team. The present results could not have been obtained

without the help of many persons and companies, and the availability of

different funda to carry out the fi,eld and laboratory work.

The development of the logging program was carried out under pro-

jects 53571 and 53400 funded by the Sea Grant Program, while most of the

field work was conducted on board the R/V ALAMINOS in conjunction with

programs funded by the Office of Naval Research contract N00014-68-0308-

0002! and the National Science Foundation Grant GA-1296!. Part of the

field work was done on board the R/V ORCA during a cruise supported by

the Sea Grant Office.

Arnold H. Bouma

Project LeaderApril 1972

AaSTBArT

Electrical Resistivity Logging in

Unconsolidated. Sediments

The use of electrical logging techniques to Locate

ore bodies dates back to the 1830's. Hy 1/20 electri-

cal logging techniques were applied to tectonic studies.

The extensive petroleum logging indu. try began in 1927

when. the first well was logged in France. Instruments

and techniques have increased in complexity and sophis-

tication to the present time.

To date, all commercial equipment in use has been

designed to operate in a fluid filled bore-hole from

a stable platform. A few attempts have been. made to

construct and use instruments designed to operate

directly in unconsolidated sediment without the pre-

sencee of a predrilled hole.

Industry, with the exception of' Well Reconnaissance

Inc. of Dallas, Texas, showed no interest in logging

in unconsolidated sedirrrents in aquatic environments and

no ready made instruments were available. The first

probe, developed at Texas A8cM University, consisted of

a stainless steel tube, 300 cm long, with ring electrodes

mounted in a nose cone. The probe was lowered on a

cable and allowed. to free-fall to the bottom. The

sediment was logged during pull-out. The electrical

current was supplied by another cable from the ship.

Indexing was accomplished by a spoked. wheel which slid

up and down the probe and remained essentially at the

water sediment interface while the probe penetrated the

bottom. Magnets in the hub of the wheel tripped magnetic

switches within the barrel of the probe. The switches

were located one foot apart and caused. fiducial marks

to appear on the record when they closed.

The design worked, however, because of the unstable

platform provided by the ship the rate of pullout was

too rapid and irregular. This, coupled with a slow

recorder, resulted in a compressed. and distorted record

section.

A second design eliminated most of the problems

incurred by logging with a moving probe. Individual

electrodes were mounted in a hollow fiberglass pole

in a spiral fashion, each positioned. 90' from the other

and. 2.g cm higher. A large housing mounted on top of

the pole contains electronic stepping switches. Current

is supplied via cable f rom the surface and the signal

is recorded at the surface. The probe free-falls to

the bottom and point resistivity measurements were made

with the probe stationary in the sediment. The probe

can operate in a guarded or unguarded mode.

1'gaea urements made with this probe in the Gulf' of

Mexico indicate that individual sediment,;Layers can

be identified and cnrrpjated.

device was huilt to measure the electrical

resistivity ot cores. Thi: has cvol ved through three

models. Gorcs tal<en in the areas whore the insitu

point rcs.i stivity probe wa- used werc logged and a

high degree of correlation between the logged cores

and the:Lnsitu measurement was demonstrated.

3 brief investigation into the relati onship be-

tween electrical resistivity and the variou" geotechni-

cal properties indicates that resistivity varies directly

with thc percentage of' sand, and the f'ormation factor

and inver..cly with water content, voi d ratio, porosity,

percentage of clay content and the median p s i ze

diameter. ,"specific gravity appears to have no effect

upon resistivity. The relationship ot resistivity to

carbonate content, and to vane shear strength is not, clear.

ACKNOWLZDGHNI' NTS

The author wishes to thank and acknowledge the

assistance of the following persons and organizations

in the conduct of this research and in the preparation

of this manuscript, Dr. Arnold II. Bourns who has been .

associated with the pro,ject since the beginning and

who has served as project leader and co-leader at

variou times. Dr, George H. Huebner,,Tr, provided

invaluable help in the electronics aspect of' the

research. Dr. William R. Bryant and Dr. Richard Rezak

gave both a.ssistance and moral support. A special

thanks to Dr. Karl Z. Koenig and I3r. Melvin C. Schroeder.

Mrs. La Nelda Bullard did most of the final typing

of this manu"cript, and I'mrs. tudy McMillan has taken

care of much of the secretarial wori in the course of

this pro,ject. Mi.,s Vicki Nicholson «ssisted in typin~

preliminary and final draft of this di . crtation.

Mr. Prank 0'liara provided the expert mnchi.nist skill

that was required for the instrumentatin». Mr. Mike

Cooke and Mr. Joe Gray also collaborated in the con-

struction of the in turments, Mr. Milton Looney, and,

Mrs. Karen Weaver performed the tediou. edimentary

lab work required on the project,. Mrs. Gerda Merkel

did the draftin".

Dr. Frank B. Chmelik initiated the program of'

logging in unconsolidated sediments in 1969 and the

present author succeeded. him on the project. Mr. Robert

Mayer, President, of Well Reconnaissance Inc., Dallas

gave both 'technical assistance and. jogging equipment,

to the project. Other equipment was donated by Dresser

Industires Inc., and. Dr. Sylvan J. Pirson, University

of' Texas at Austin. Support was also provided. by

Woodward-Envicon, Inc., Electro''ilm Inc., and the Sun

Oil Company. Fianacial support wa- obtained. from NSF

Grant GA-1296! and QIJR under c ontract N00014-h8-0308-

0002. The primary funding for this research wa.s pro-

vided by the Sea, Grant Program under projects 535 l

and 53400.

also +hank the graduate students and marine

technicians who participated in cruises connected with

this project.

FORWARD

ABSTRACT

ACKNOWI,T',DC,' KNI,NT." V1 >

VAST.1-, OT CONTI;N'I',",

TJIDT OT' T"JGUllI',!'

31

35

Normal Cur rent; Jap~in,~

I aalu.' ed Cuir on<, In~~:I n«

LOGIC> TN ' TN IINCON,'~OT TDAT1,I! HT';DEMENT'>

Pc net; ra ti. an Cant;rol

COAT', LOGGTNG

CORRI,T. ATZOJ'J

SmTMT;NT ANAT.V;.XS I ROCT.'DIIRI.,

70

77

87

Gra.in,-ice D:i.vari.hution

Phy;, 1 r.a.l. Prop nr I;.I c,-, 92

Wa t;er Can I;ant

Specif ie ~ira.vit;y of', olicl..

Va:i 6 Ba, t.i a

93

lNTI. I<01 IICT I ON

VRZ.;I.,NT STATII,", OI TITS @III"."TZON

PBYNC TPT,T;,"; Ol' RH.".I.;lTTVH TY

PRTNCTPTT;,, OI' IlHP T..'iT7VTTY MKP,HIIRI",MENT

xNDII."-TR7AT., AI'I'T,TCAT7oN OI" zrT;C "I'RTCAT, Iver:s7.",T7VzTY

T,nr,u NC;

Porosity

Bulk Density

Vane Shear Strength

Carbonate Content

RELATIONSHIP BETWEEN RESISTIVITY AND SEDIMENT

PROPERTIES

Resistivi,ty vs Scanner Resistance

Resistivity vs Water Content

Resistivity vs Formation. Factor

Resistivity vs Void Ratio

Resistivity vs Porosity

Resistivity vs Percentage Clay Content

Resistivity vs Percentage Sand Content

Resistivity vs Median 5 Diameter

Resistivity vs Carbonate Content

Resistivity vs Specific Gravity

Resistivity vs Vane Shear Strength

Resistivity vs Sound Velocity .

SUGGESTIONS

CONCLUSIONS

BIBLIOGRAPHY

Page

95

95

96

97

99

102

102

107

107

107

114

117

117

122

122

126

129

135

LlST OF FIGURES

Figure Page

resistivity of. NaCl solution vstemperature from Schlumbor LagInterpol'etation :harts, 196/!. 23

Porosity and permeability vs formationfactor after Archie, 1942!.

3 ~

Formation factor vs porosity framSchlumberger Jag Interpretationrharts, lc16g!, 29

Nonoelectrade configuration.

Twa elec trode configuration. 37

Geometric. factor vs distance framel ectrode after Lynch, lc!c>2! .

7 ~

39

41Latera.'1. log arrangement.

Schlumberger multicurve loggingsy tern after Lynch, lcm'~>2!.

9.43

>c.hlumberger automatic; switchincarran«ement after Lynch, 1962!.

10.

Welex frequency modulation system after Lynch, 1952!, 47

Laterolog after Doll. in Lynch, 1952!. 4912.

Schlumberger Lateralop 7' after Dollin Lynch, 19~~2!.

13,

Texas AKN c;antinuous re istivityprobe. Nose conc in in"ert.

Texas MM point, resistivity probe.

Electronic control panel for paintre istivity probe.

Frequency dependence of granodiorite after Keller and Frise hknccht, 1966! . 20

Figure Pa,ge

Water resistance, guard mode.

Water resistance, unguarded. mode.

Comparison of insitu records.

17.

18. 68

l9.

Resistivity curves--values fromrecords in Fig. l9.

20.73

Free-falling probe entering bottomat an angle to the vertical,

21.75

Core logging device adapted fromWidco well logging instrument.

22.78

23 ~ Core logging device developed byDr. Chmelik. 80

24. Core logging device combining loggingsystem and recording system.

Comparison of insitu penetration 4with scanner record of core 3.

25.89

Comparison of insitu penetration 2with resistivity of core 3. 89

Absolute resistivity measuringdevice.

27 ~98

28A. Scanner resistance vs resistivitycore 2. IOO

Scanner resistance vs resistivitycore 3. 101

103

104

Formation factor vs resistivitycore 2.

30A.105

Formation factor vs resistivitycore 3.

303.106

29A. Water content vs resistivity core 2.

298. Water content vs resistivity core 3.

Page

108

109

110

Porosity vs resistivif.y core3213 .

Percentage clay content vs,resistivity core 2.

33A.112

Percentage clay content v.,resistivity core 3. 113

34A. Percnctage sand content. vsresistivity core 2.

34'. Percentage sand content vsresistivity core 3. 116

Median P diameter v; resistivitycore 2.

35A.118

Median g diamef-.er v resistivitycore 3.

35I3 .

119

Ca,rhanatc content vs res,'.",tivityare 2 120

3~!R. ;arhonate cantenf; vs re�isf;ivityc !rc' 121

37A. Speciiic gravity vs r'e.istivitycore 2. 123

Specie'ic gravity vs resi ..f;ivitycore

37B .

Van shear strength vs re..istivitycor'e 4-B. 125

Sound velocity vs re. istivity core2. Part, C,' and part A. .127

Figure

3lA. Void. ratio vs resistivity care 2.

31B. Void ratio vs re,.i..f;ivity core 3.

32A. Porosity v" resistivity core "; .

INTRODUCTION

Electrical resistivity measurements were first

employed over a century ago in mineral exploration.

Various resistivity techniques were used to locate

ore bodies within the crust. of the earth. In time,

interpretation was refined to a point where geological

structures could be defined. The possible application

to petroleum exploration became obvious. Until 3.927

all techniques involving mea.surements were made by

electrodes driven into the ground.

The petroleum logging industry was born in l927

when the first vertical profile wa.s made in a well.

Instrumentation and logging tec.hniques were developed

and improved by trial and error as special tools had

to be designed, built, and tested.

Ideas for this work began to germinate in l967

when Dr. Frank B. C.'hmelik became interested in the

application of electrical logging techniques to the

correlation of recent, near surface sediments. Dr.

Arnold H. Houma soon became involved and served as co-

investigator when the research bec:arne funded by the

Sea Grant program in September l960 anc1 c.ontinued to

Septembc.r l972. Dr. Goorge L. klucbner, Jr. has served

usedAs a consequence, all insitu logging probes

the Texas ARM Geological Oceanography group w~ve

signed and built by project personnel. The

as advisor and consultant on the project since the

early days. The author became co-investigator with

Dr. Bouma when Dr. Chmelik left.

When the present effort began it was assumed that

it would be a relatively simple task to select and

modify equipment from among the sophisticated array

possessed by %he petroleum logging industry, and that

technical help and advice would be readily available.

These assumptions proved to be false. With the lone

exception of Mr. Robert Mayer, Jr. of Well Reconnai sance

Inc. of Dallas, Texas, people in industry seemed in-

capable of comprehending a logging process that did '

not involve a pre-drilled hole. Mr. Mayer was kin 1

enough to discuss the possibilities of the reset

and to loan several valuable instruments to th< ; ro-

ject.

The first marine electrical resistivity pr.~bi= .~.'-

ported in the literature was constructed at the

Atlantic Treaty Organization NATOI laboratories

La Spezia, Italy by Kermabon et al. �969!. Th:: f'4'i'0

probe was too large and bulky for use on boar~i

R/V ALAMINOS and far too expensive for this pro,i.ct.

devices, with the exception of the fi.rst, one which

utilized equipment on loan from Mell I<econnaissance Inc.

were also all in-house products.

The logginp project has developed along two lines

which in a'sense are independent, yet which bear a

necessary and demonstrable relationship to one another.

They are core logging and insitu logging. Bath use

electrical resistivity measurements to identify sed.i-

mentary unit , but obviously the instruments and tech-

niques used are entirely different.. Also the quantities

measured differ in magnitude. IIowever, it, will be

shown in the text tha.t insitu logs can be correlated

with logs of cores obtained from the same locality.

Instruments and techniques have been developed to

provide a, rapid method of correlating and identifying

sedimentary sequences in near surface sediments. It

will also be demonstrated that there is a relationship

between electrical resistivity and various geotechnical

properties of sediments.

Thc main objectives of this st;udy were:

l. To adapt electrical resistivity logging

techniques for use in unconsolidated aquatic sediments,

2. To correlate sedimentary units by means

of resistivity logging.

3. To correlate insitu logs with core logs

from the smne area.

4. To relate resistivity to geological and

geotechnical properties oi' sediments.

PRESENT STATUS OF TUPIK QUESTION

The use of' electrical methods for prospecting or

locating ore bodies began as early as 1830 when Fox

determined that some ore deposits in Cornwall, England

werc a.ssociated with natural electrica.l currents and

pot> ntial... Continental Eur'opean sc'ientist were also

en~aged in research of this nature, but the primitiveness

ol the early equipment, precluded any practical use

".weet, 198>~>! . In lOAO a nonpolarizing el.ectrode was

invented by Barus of the United States C'eological Survey,

a id used to map an extension of the Com tock lode

Rust, 1930!. In 1003, Brown used electrical resisti-

vity method,. to loca,te suspected ore bodies in the

United States Rust, 1936!. The first report of locating

new ore bodies using electrical equipment was given

by Muenster in 1907 Rust, 1938!. By 1912 Charles

Schlumberger had started making resistivity surveys

a.long the earth's urface.

The application of alternating currents and the

use of telephone receivers wa. first proposed by Daft

and William in 1900 Rust, 2.936!. This method was

f'irst, successfu1ly applied by Bergstrom in 1913.

Wenner, in 191 $, published a method for measuring resis-

tivity at, the earth's surface. He developed an electrode

spacing which is known as the Wenner array.

Lundeberg initiated the use of vacuum tube ampli-

fiers in 1918. He also introduced the use of' long,

linear electrodes to replace the point type used by

Schlumberger. Hy 1920 Schlumberger had developed his

techniques to the point of being able to conduct tec--

tonic studies, and that year he published a book ex-

plaining this method. Simultaneously he introduced a.

commercial application and began to apply his techniqu ~

to oil prospecting in 1923. The use of periodically

reversed direct current as introduced by Gish and

Rooney in 1925, increased the type of' measurements that

could be made.

Prior to 1927 electrical resistivity measuren;en':

were made at the sur'face. The major drawback in u;-:.'-iL~.;

surface implanted electrodes is that they;,ive oriden:e

only to shallow depths. There are report:;, however,

that the first bore hole measurements were made by

Ambronn in 1913 Rust, 1938! but. there is no appar,r;i

evidence that down-hole electrical measurements were

further exploited. Consequently, the beginning of the

well logging industry is attributed to C. and P.

Schlumberger who made the first log on. the fifth of

September 1927 in the Pechelbronn field in. France. Isy

July of 1928 the Schlumberger brothers had a, comm:-.rcial

company operating with what they called electrical

coring methods. F~rom then on the expansion of well

logging was very rapid. It wa.s used in the United

States, Venezuela, and Rus sis. in 1929, and in the Par

East by 1930 The Petroleum Times, 1935!.

Electrical logging, as used in the petroel«m in-

dustry, was. es entialh.y qualitativ for the first ten

years. Oil zones could be located because the oil was

more resistive than the water bearing formations. The

electric logs were also excellent correlative devices,

It wa.s recognized early that an apparent resi..tivity,

often quite different from the true resistivity, was

being measured in the bore holes. This wa., caused by

drilling fluid being forced into the pores of the for-

mation to such an extent that, the resistivity of the

drilling fluid was being recorded entirely, or was

strongly influencing -the measured circuit. Martin, et

al. published the first paper on the quantitative use

of well logs in 1938. However, their method. of ob-

taining the true resistivity of rock formations was

too tedious to be practical and economic. A simpler

method of obtaining the true resistivity of the for-

mation was obviously needed, but re. earch along thi

line was greatly curtailed by the war.

Archie {1942! defined. a formation resistivity fac-

tor, denoted as F, as the ratio of the resistivity of

the formation R ! completely saturated with a, brine0

to the resistivity of the brine R !w

F=R/R

He also related formation factor, as it is now called,

to porosity by the following:

F=gf �!

where g is the fractional porosity, and the exponent

m is the slope of the line which defines a log-log plot

of porosity versus formation factor. This now is known

as the Archie equation. Guyod �944! introduced

the term cementation factor for the exponent m noting,

"The higher the degree of cementation, *he larger the

factor m seems to be in general. It is, therefore,

logical to call m the cementation factor." Winsauer

et al. �952! obtained a slightly different value of

F than did Archie. Their work indicated the relation-

ship to be F = 0.62 P ' . Wyllie and Gregory l953!

investigated the influence of particle shape and effect

of cementation upon the formation factor. They showed

that spheres and spherical particles had the lowest

formation factors, and that, blocks and triangular

prisms had higher formation factors. Beach sands, river

sands, and disc-shaped particles had formation factors

that were intermediate to the extremes. Archie's

component,s of' ~,he c'..a;"-slurry system.

Doth Equations 4 and 5 have the same general form

as Equation 3 above. The constant. a in Equation 3 is ~ tl yequivalent to g" ' and P'' ' in Equations 4

and 5. In Equation 4 a i.s greater than unity, and in

5 it is less than unity, although both satisfy the

boundary conditions that F = 1.0 when 8 = 100$. These

equat,ions were developed from a two-shaped system but

more equations can be written for more complex systems.

However, since the clay particles act as conductors

they lead to even more variables in the formulation.

The main contribution of the work by Atkins and Smith

�961! was to give physical meaning to the exponent m.

Patnode and Wyllie �950! pointed out, that, when

conductive clays are present, the apparent formation

factor is less Chan the true forrration. factor. Keller

�951! decided that the clay content effects conductivity;

increasing the conductivity of the connate water by ion

exchanges and conduction along the clay particles. Per-

kins et al. �954! decided that the abnormal resistivity

shown. by shaly reservoir materials is due to absorption

of ions. Wyllie and Southwick �954! concluded "That

the true formation factor of a dirty sand...is not a

quantity capable of simple measurement." However, it.

may 'be derived from a series of measurements made by

11

a1

b «!m�!

Papadakis in an unpublished manuscript written comm., 1970!

performed the mathematical work defining

saturating solutions of varying conductivities. This approxi-

mation may be close but must always be somewhat less than the

true formation factor.

The Russians conducted marine electrical prospecting sur-

veys in the Caspian Sea between 1931 and 1937. Their surveys

consisted of towed current and receiver e1ectrodes, a method

similar to surface resistivity surveys. Royce �967!, invest-

igated the e1ectrical resistivity of sediments from the HeringSea, In general, he found that resistivity varied directly

with wet bulk density, apparently inversely with the percentage

of clay size material present, and inverse1y with porosity. He

also compared formation factor with the physical properties and

found that it was an inverse function of the porosity, and that

it varied directly with wet bulk density and inversely withmedian phi diameters.

Kermabon et al. �968! made a study of the porosity, density

and electrical resistivity of marine sediments from 21 cores

taken from the Tyhrrenian Sea. They found that the following

form of the Archie equation gave a better fit to their data:

the parameters a and b. He found that a third degree

polynomial curve gave a good f1< to their data. It

is possible to calculate porosity when the formation

factor has been calculated. Papadakis' equations is:

n = -5.9021 F + 40.0416 F � 105.3899 F + 171.2~g04 !3 2

where n = porosity. lf the porosity is known the wet

density can be calculated from:

d =- 2.60 � 0.01/8 n!

Kermabon et al. l969! showed correlation between

resistivity-curves obtained with an insitu probing de-

vice and curves obtained from cores taken in the vicinity.

The results indicated the practicality of this type of

investigation.

Several other authors have reported the results

of investigations into the techniques of' loggin;; in

unconsolidated marine sediments. Keller l95$!

developed a nuclear probe for measuring the bu1 k den.,ity

insitu.. Pautot �967! was able to correlate specific

horizons between cores on the basis of electrical

resistivity. Hutt and Berg �968! developed mathematical

relationships between electrical and thermal conducti-

vity. Correlation with experimental data cast some

doubt upon the validi,ty of the calculations. Chmelik

and Bouma l970! demonstrated a qualitative use of

13

resistivity logs in identifying zones of interest in

cores.

15

PRINCIPLES OF RESISTIVITY

Resistance may be defined as the property of an

electric circuit that, opposes movement of charge and

dissipates power. The most common Unit is the ohm-

meter which is the resistance offered by a cube ope

meter on a side. Resistance may also be defined. by

rearranging Ohm's law to give the mathematical

relation'.

R = E/ I

where R is the resistance in ohms, Z is the potential

in volts, and I is the current in amperes. Conductance

is the reciprocal of resistance, and is expressed in

units called mhos per ~eter.

Resistivity is a property of the material through

which the electric current is moving. It may be related

to resistance by the equation:

p = R A/L

where P is resistivity in ohm-meters or ohm centimeters,

A is the cross sectional area, and L is the length of the

conductor.

The passage of en electrical current through a

mineral occurs by an ionic or electronic process. Solid

conductors fall into three groups as defined by the

manner of electrical conduction. The three groups are

metals, electron semi-conductors and solid electra-

lytes.

The conductance of metals is rather high. In a

primitive sense metallic bonding between atoms may be

thought of as an orderly arrangement of metallic ions

surroUnded by a cloud of valence electrons. The high

conductivity of metals is due to two facts'. 1! very

little energy is required. to move a valence electron

from one atom to the next, and 2! the large number of

movable electrons available. The percentage of native

metals occurring in the marine environment is extremely

low, and they are relatively unimportant as conductor"

in the marine environment.

Electron semi-conductors are minerals, most conmonly

the sulfides, arsenides, tellurides and oxides of the

metallic elements. Conductance is by electron motion

but is less than in metals. This is due to the lesser

number of available electrons. Also, the energy lovel

of the electrons must be raised consider ably beL'ore they

are free to wander through the crystal lattice. This

energy boost is usually pr ovided by heat. The incumber

conducting electrons increases with temperature accor.1-

ing to the following mathematical relationship Kelle~

and Frischknect, 1966!:

where ne is the number of electrons, B is the energy

required to make the electrons mobile, k is Boltzman's

constant 8.62 x 10 eV/C or 1.38 x 10 lerg/C ! T

is the absolute temperature, e is the natural logarithm,

and B is a trait of the material. If B is low the

material is a good conductor. lf 8 is high the material

may be an excellent insulator. Silicate minerals for

example require large activation energies and under con-

ditions found .in surfs.ce and near surface environments

they act, as insulators to the passage of an electric

current.

In an ionic bonded compound such as sodium chloride,

the sodium atom gives up a sole valence electron to com-

plete the outer electron shell of the chlorine atom.

The compound is then held together by coulomb forces

because the ions are oppositely charged. This bonding

force is extremely strong compared to that of an

applied electric field and thus ionic conduction

should not occur. However, imperfections in the

cnystal lattice known as ~Schottk defects end impsc-

fections induced by heat known as Frenkel defects allow

a current to flow when an electric field is applied.

n cc e U~kT �2!

where n~ is the number of jumps per unit time and Uis the height of the potential energy barrier through

which the jump occurs.

Experience has shown that high temperature con-

ductivity is an intrinsic property of the material and

a product of thermally displaced ions from the lattice.

The conductivity in the lower temperature ranges is

structure-sensitive and. may be due to impurities or

defects in the crystal structure. Conductivity may

be approximated by Keller and Frischknecht, 1966!:

All crystals conte. in some kind of imperfection.

These are usually impurity ions of the wrong valence

substituted into tho crystal lattice. All ions vibrate

about their positi on in the lattice, and heat will

increase the amplitute of vibration. Occasionally

the ion will jump into the next valence position.

These jumps are purely random. When an electr ic

field is applied, jumps tend to move in the direction

of the field and. a current is initiated. The fre-.

quency of the jumps is temperature dependent and

mathematically predictable, according to the expression

Keller and Prischknecht�, 1966!:

-U /kT -U jkTC=Ae 1 +Ae

1 2 »!

Al and A2 are decided by the number of available iona

and their mobilities and Ul and U2 are the activation

energies needed to free the iona.

is electrolytic. The conducting fluid ia contained

in the pores and fractures within the rock. It is

thus seen that the resist'ivity of most of the near-

surface rocks ia directly related to the por osity,

permeability and the amount and salinity of the inter-

stitial water and is not an intrinsic property of the

rock.

Conduct;ors snd semiconductors are not frequency

dependent whereas solid electrolytes are frequency

dependent to a certain d.egree. Figure 1 illustrates

the frequency dependence of a sample of granod,ior'ite.

It; should be noted that at high temperatures resistiv~

i,ty is nearly constant and, not frequency dependent,

while at low temperatures the resistivity is nearly

inversely proportions.l to t;he frequency. For all

pr act;ical purposes extrinsic conductivity is fre-

quency dependent for frequencies above a few cycles

per second. Intrinsic conductivity is not frequency

dependent at least up t;o the megahertz range.

Electrical conduction in most sedimentary rocks

to8

IO7

E ~OsO

i02Io

IO6

E

+ IO"

V!COUJ

l0~

~O' iO' IO' Io'

FREQUENCY CYCLES PER SECOND

Fig. 1. Fr equency dependence ofgranodiorite %after Keller andF ri s hkne cht, 1 <366 j,

AF C V + C V + .. C V ! ] JI!

where F is the Faraday number which i., 9~~ 500 couLomb"

When a salt is dissolved in water, the ions dis-

sociate ond are free to move about independently in

the solution. Under the impetus o~ an electric field

the ion' migrate toward the polos of opposite charge,

The accele'ra tin« ions czeote o viscous drag which limits

their velocity. The terminal velocity reached. is callecj

the mobility of the ion. Specifically mobility may be

defined a" the velocity in meters per second produced

by a potential gradient of one volt pez meter.

Two other. factors affecting the mobility of ions

are concentration and temperature. At high concentra-

tions the iona tend to interfere with one anothez and

retard the terminal velocity, Increasing temperature

reduces viscosity and allows an increase in terminal

velocity.

The current which flows in a sokut,ion can be

determined. by multiplying the concentration of the tons

times their velocity. The current. flowing throu~« on

electrolyte under a. potential of one voLt can be detez>

mined by the formula given by Keller and Fzischknecht,

A is the cross section area and G and V are the con-

centrations and mobilities of the ions present. The

resistivity may be determined by the following equa-

tion Keller and Frischknecht, 1966!:

�5!

The above formula holds for flow through a cross

sectional area. of one meter at a potential of one

volt. N indicates the mobilities.

The formation water normally contains a variety

of different salts. To avoid a number of chemical

This may bc defined as e. solution of sodium chloride

tl at would have the identical re istivity as the

particular olution for which it is substituting

Keller and Frischknecht, 1966, p. 17!.

Figure 2, abts,ined from the Schlumberger Log

Interpretation Charts, gives the resistivity in ohm-

me ers at various temperatures for. various NaC1

solut ions.

The conductivity of sedimentary rocks and un-

consolidatedd sediment s is a direc t, function of the

interstitial water distribution and its salinity.

lt is beyond the scope of this research to determine

23

OOOOo~Po~

OO~oOp

Oog

OOO

OOI

OQ I

OOZ

ID4

M Cha5 LC4 ~Q W

Q Q ca4

CO ~0 o

OOR

00&

009

V!

0 UJ

os UJ~~ Xoo

Io

ZQ

0

OCC

000I

OOQ I

000 2

0

O

000 R

000 0'

~O,Ooppp

OpOpg0Ppppp

000 G

V!V!IJJK

000 OI

000 R I

0

o E

Z0

IX00 Zz le ~ ~0

00

3o~-

0 0 0 ~ g~0 hJQ 6J

0 0 0 0 0 0 0cn 0

0 O 0

3 tl A J. U 8 3 d lhl 3 l

ih 0RE 30

Opp~POO~

QppOOOOppOO o6.

ppoOOc'Poo

POO~OOOO,Opp

Og

oOO

Oop

IA Alo g J. iD lD5/ U ID J 6

C C0 O

QI

o ara 4

0 QR G

0 b0

~B4<U

rn QIIJ

e [jj

8~ 0

bo 4

the ultimate causes which control the salinity of the

water. What have been studied are the factors con-

trolling the distribution of the water within the

sed.iment. It is therefore primar ily the porosity and

the permeability that determine the resistivity of a

particular rock or sediment. They do not define re-

sistivity in its totality as other factors influence

Resistivity may be influenced by seven factors

expressed in the general formula Dakhnov, 1959!:

Rt fl b! f2 ~ f3 Sw fg t! f5 D! f~ R f7 w !

Rt = resistivity' of the sediment

b = clay/silt ratio

gl = po. osity of the sed.iment

S = the partial saturation of the sedimentw

t = temperature of the sediment

D = the cation exchange capacity of the sediment

R = the resistivity of the sediment minerals

R = the r esistivity of the interstitial water

In deeply buried consolidated sediment., the degree

of formation saturation is an important factor in re-

sistivity calculations. However, most recent ma~ ine

sediments are completely saturated; therefore, tllis

The temper s�factor may h."-. taken as on<. an< i�n ~r - d.

tur<' is easily me a,"!urcd arid i '.; '.';"U,-. 1 1y tt>e sar!>e as the

bottom water f or the f'irst, fcw mct i!rs into the so< i�

ment. Thc cf'f<.ct of the < ation e~<.ha<>pe capa«ity is

nearly net~lip .ble «i>@pare<t to the <:-]'f oct; < f saline

water and th<.r..f ore can be i,n':"ei.. The bulk of the

mineral= pr <>s<>nt in m.>st sedim<>nt�s ar e non-conductinp

Thus, Pauation l6 can beand can also he i,-. nnr e<l .

reduce«i to thrco parameters w'hen dealin< with the upper

few meter s of' un<:onsolidated. marine sediment;., i.e.,

the rc-iativity of marine =e<1iment= ma" be det,ermined

from the re.,i t ivity of the inter.",tit i, 1 water, the

porosity, rn<l th<> «mo!!nt <-f «1;. r ! n th«-..�.'.d! Y11<'I>.t

any spec if i« t<->mT>eratur<..

Numerou" saudi<-;s have been ma<ie > el atin<; resistiv-

ity to poros ity. I«;mbers of the pot~ oleum in<lustry

have been the pr im<, invest igatn .; in the:,' ' stu<lics.

Figurc ', i;: redrawn from A< «hie �9112.! an'i re 1.at;cs

both poros ity en<i permeability to f'oz ma tion f act >r ..

Tho dependence nf formation factor upon permeability

is sc small that all e ff orts have been concentrate.'.

upon thc F-9 relation,hip. This metho<l of detcrminin~

formation factor ia somewhat cumbersome as a laz ~c

number of measurements must, be made and tho avora~<-...,

of the e measuremcnts then plotte<l.

CCo I�CJ 500

CA A

IX

I OQ

50

10zO

RO I

O.i

POROSll Y

D

4

I��V7LJ0'

OI-

Z

O

100

50

10

5 II 5 IQ 50 IOO 500 f000 5000 O.I 0.3 1.0

PERM E A BI L I T Y, MILL I OA RC YS PORQSI T Y

0. 5 II,O 5 10 50 IOO O.l 0.3

P ERMEA 8 I L I T Y, M I L L I OA RC YS

Fig. 3. Porosity end permeability v; �;formation factor after Archie, 1<3'I2'I,

27

The exponent m which increases with increasing

cementation is called the cementation factor. It

has been shown to vary between 1.3 and 2.2 with the

lower figure reported for unconsolidated sands. A

few authors have reported higher and lower values.

Atkins and Smith �961! demonstrated that m

varied with particle shape see page 6! and called it

the shape factor. Although they did not carry their

work into consolidated formations they did suggest

that the process of cementation would alter particle

shape by increasing angularity. Thus m would increase

formulas will give satisfactory results:

F = .81 8 in sands

F = 8 in compacted formations

�7 !

ge!

with increasing cementation. They also showed that g

would be equal to unity when gl = 100/o in a single

component system. However multi-component systems,

as found in nature, would give values higher or lower

than 1 depending upon the ratios. It is not always

practical nor possible to determine a and m. Nany

investigators have found that values of 1 and 2 fop a

and m respectively will give nearly the same answers

as the more precise equations for normal porosity ranges

of 10 to 30fo.

The Schlumberger Company' states that the following

F P 62 g 2el5 �9!

Zquation 19 is the so-called Humble formula which

Schlumberger uses in preparing their charts. An example

of one of their charts is given in Figure

31

PRINCIPLES OF RESISTIVITY NFASURFNENT

Neasurement of the resistivity of geological

formations to electr ical currents is called electrical

logging and the resultant records are called electr ical

logs. In the l~g years since the first electrical log

was run in a well, logging .equipment and devices as

well a.s techniques and interpretation procedures

have advanced. considerably in sophistication and

precision.

The equipment in u"e has developed from single

electrode to multi-electrode devices of various

spacing... Mhile these .,o-called conventional type

resistivity logs are still in u e they have been

largely superceded by focused current type of devices,

electro-magnetic induction device,"., and micro-resistivity

and pressure pad type of devices.

The single electrode resistivity log consists of

two electrodes one of which A! is lowered into t' he

well and the other H! which is grounded near the well

Figure 5!. The distance between electrodes is far

enough to be con"idered infinite. A constant current

is sent down the cable to the in hole electrode A!

which spreads out concentrically if the surrounding

medium is electr ically homogeneous. The total

GENERATOR

Fig. 5. Monoelectrode configuration.

resistance between the two elecrodes is measured, The

resistance can be calculated by dividing the earth

medium into a series of concentric shells and, summing

the resistance.=, of all the shells from the electrode

to infinity.

The total resistance is:

R P �O!

where P is the bulk resistivity of the surrounding

medium and g is the radius of the electrode.

From the above equation a geometric factor or

constant may be derived. As indicated by its name,

it is a function of the geometry of the electrode or

electrode array being used. It is a factor which con-

verts total resistance to resistivity.

In the case of a spherical electrode this geometric

factor is:

Zl!

The re si" tance, as measur ed by a single electrode,

is controlled by the material closest to the electrode.

This may be shown to be true by examining Equation 20.

The variable g is the rad.ius of a sphere and in this

case is the radius of the electrode. If g is allowed

RP

�2!

where P is the resistivity of the surrounding medium,

R the grounding resistance at the electrode, and r e

the radius of the button. It can be seen that the

resistance is proportional to the resistivity of the

surrounding medium and inversely proportional to the

diameter of the measuring electrode.

to increase in size it can be seen that at any distance

gl from g, the resistance Rl at that distance is less

than 2g and will decrease witt increasing distance

from g.

The geometric factor for a flat button type of

electrode with an insulated backing, as used in the

Texas ANN probe, is.'

INDUSTRIAL APPLICATION OF ELECTRICAL

RESISTIVITY LOGGING

The well logging industry was founded upon the

principles di.cussed in the previous chapter. Some

of the fundamental principles were known prior to the

first log run in 1927; many more were di covered and

elaborated upon empirically and experimentally down

through the years.

Logging equipment, methods, and interpretation

techniques- have made significant advances in sophisti-

cation and accuracy. The first well log was a three

electrode point resistivity log. Today multiple

electrode arrays of various spacings are used.. There

are also other methods of logging besides resistivity

logging that have been developed. To enumerate, they

are sonic logging, electro-magnetic induction logging

and caliper logging, neutron logging, gamma ray logging,

formation density logging, and thermal decay time

logging. The last four are all forms of radioactivity

measurements, active and passive.

The common factor in all form" of industrial

logging is an open fluid.-filled hole and a stable operat-

ing platform which allows control of logging speed.

A major problem encountered by commercial ele.tri-

cal loggers is the presence of a hole filled with a.

fluid having a different conductivity than the forma-

tion fluids. For this reason most of the applied

current will tend to diffuse directly up the hole

rather than spread laterally into the formation.

Several methods have been developed to compensate for

this.

Normal Current Logging

The use of monoelectrode devices give little in-

formation concerning the true resistivity of the

formation surrounding the bore hole. The monoelectrode

system, therefore, is rarely if ever used. A two

electrode system, known as a normal system, is applied.

to improve the depth of penetration of the current and

thereby increase the depth of investigation from the

bore hole and decrease the influence of the drilling

fluid.

For this arrangement four electrodes are used;

two current electrodes called A and B and two measuring

electrodes called M and N Figure 6!. Current electrode

A and measuring electrode M are lowered into the hole

while electrodes B and N are grounded at the sur.'a'.e.

METER GENERATOR

Fig. h. Two electrode configuration,

The distance between electrodes A and N is called the

AN spacing. The midpoint of the spacing is used. as

a reference:point for plotting the resistivity data.

The depth of penetration of any device incz eases as

the AN spacing increases. However, because of the

--- response of the system there are practical limitations

upon the sire of the spacing. In practice two spacings

are used: 16 inches and 6g inches. Figure 7 indicates

the depth within the formation away from the electrodes

that contribute the most to the measured signal. The

figure has been calculated for electrodes buried in an

isotropic, homogeneous medium of irfinite extent.

Departures from these values may be expected because

none of these conditions are entizely met, in practice.

In fact, whole series of departure curves have been

constzucted to correct for departures from true z e-

sistivity caused by the size of the bore hole, the

drilling. fluid, and the formation thickness.

One of the problems encountered in the use of

the normal system is that, as the bed thickness

approaches the AN spacing, the apparent resistivity

recoz'ded is greatly reduced. If the bed thickness

is equal to or less than the AN spacing then the recorded

value reverses and indicates that the resistive bed is

39

I,O

O I-C3

0.8

30

Fig. 7. Geometric factor vs distance fromelectrode after Lynch, 1962!.

C30.6

I�ILJ

O

�0.4

C9

0,2

CC 9

4JI�

5 IO l5 20 25

DISTANCE FROM POWER ELECTRODE FT.!

actually more conductive than the beds above and

below. It is seen that the depth of penetration

improves as the electrode spacing is increased.

However, it is not feasible to use very widely spaced

devices as the resistive beds of interest would be

missed. In point of fact, a 6g inch spacing is the

largest used for normal devices.

A three electrode system places both measuring

electrodes M and N in the hole along with current

electrode A {Figure 8!. This arrangement is referred

to as a lateral lop. The point of measurement lies

half way between the M and N electrodes. As with the

normal device, this point is referred to as "0" and it

is the AO spacing that determines the characteristics

of the log. In practice, a spacing of 18 feet 8 inches

is used. That is, the A electrode is located 18' 8"

above midpoint 0. Since the electrode arrangement is

unsymmetrical, an unsymmetrical response occurs

opposite resistive beds. Because the measuring

electrode is located at the bottom of the instrument,

good information is obtained for the bottom of a

resistive formation but there is little information

about the top of the bed. This information can be

obtained by reversing the electrode positions. The

GENERATORMETER

L..

Fig. 8. Lateral log arrangement

Another parameter called the SP or ~s ontaneous

potential, which is a measure of a naturally occurring

electrical. potential, is recorded simultaneously with

the resistivity logs. Since it is impractical to main-

tain the theoretical electrode arrangement previously

described for the various curves, use is made of the

principle of reciprocity to log several curves simul-

taneously. The principle states that in any system of

four electrodes, distributed in any configuration,

the voltage potential between I and 2, resulting from

a current between 3 and g, is exactly equal to the

voltage difference between 3 and 4. if an identical

current is passed between 1 and 2.

The Schlumberger method of multicurve logging is

shown in Figure 9. Both current electrodes are in the

hole and measuring electrode N is placed at an infinite

d.istance. This group of electrode arrangements gives

rise to other problems. The M electrode not only

measures resistivity but also SP. The B current

electrode must be on the sonde for measuring the

lateral log, unlike the normal, will always show a

positive deflection opposite a resistive bed of any

thickness. In actual logging practice several electrode

spacings are run simultaneously.

M

18 8

64 8

MiSP a-- 16 II

A

32"M,9SP

16

NORMAL

64

NORMAL

18 8

LA7E RA L

SP

LOGG IN GSONDE

systemFig. 9. Schlumberger multicurve logging ai'ter Lynch, 1962!.

lateral curve but must be at an infinite distance to

record the normal curves. Schlumb.rger has solved

these problems by using a pulsed current and an auto-

matic switching arrangement Figure 10!. The genera-

tor puts out a pulsed square wave alternating current

of very low frequency �5 CPS!, thus circumventing the

inductive and capactive reactances produced by sinus-

oidal alternating currents of higher frequencies. The

current has alternating polarity and a constant voltage.

The switching arrangement in Figure 10 also illustrates

how the direct SP current and the alternating pulsed

current are separated after being received at the N

electrode. The resistivity galvanometer, having its

terminal connections alternated in synchronization

with the current generator, sees the pulsed current as

a direct current and the direct SP current as a 15

cycle per second alternating currents The SP gal-

vanometer sees the SP as a direct current and the

pulsed current as it is. Since both galvanometers

are direct current instruments they will only record.

that current which they see as direct. As stated

before, the normal electrode spacings require the B

current electrode to be at infinity while the lateral

devices require that it be on the sonde. This is

RESI ST I V I T Y

GALVANOMETER

VANOMETJR

Fig. 10, Schlumberger automatic switchingarrangement a fter Tynch, 1962 I .

accomplished by use of a similar switching device

which alternates the B electrode between the sonde

and ground. The galvanometers are gang switched so

that they record only the appropriate current.

telex; a Halliburton Company, has solved the

problem differently. They' use a frequency modulated

system which requires only a single conductor cable.

Figure 11 is a schematic drawing of their system. A

$00-cycle current generator supplies all the current

necessary to operate the entire system. The formation

current converter supplies a 200 CPS current to the A

electrode. The M electrodes � labeled B, C and D�

measure the potentials and send the signals into fre-

quency modulated current transmitters T , T and TI

They are centered at 8000, 10,500 and 14,000 CPS

respectively. They are modulated by the signals from

the M electrodes and the modulated currents are

simultaneously sent up the cable to the receivers.

Each receiver is tuned to a single 'carrier frequency

and will discriminate against the others. The infor-

mation is removed from the carrier, converted into

direct current and sent to the recording galvanometers.

This company usually runs an 18 inch normal and a

and a 16 foot lateral.

RECORDING

GALVANOMfTER5

R 400 � CYCLECURRENT

GENERATOR

200- CYCLE FORMATION

CURRENT CONVERTER

8

C --- Tp�

D 0� Fig. 11. Wel ex frequencymodulate.on system

a.f ter Lynch, 1962! .

Focused Current Logging

When highly resistive formations are encountered,

or when a very saline drilling mud is used, a very

high ratio occurs between the true resistivity of the

formation and the resistivity of the mud; i.e.,

Rz/Rm is very high. Mhen this occurs the long lateral

log gives readings so far from the true resistivity

that large corrections are necessary. The invasion of

the drilling fluid can be so deep into the formation

that the shorter spaced, normal configurations measure

only the resistivity of the invaded zone. Where a

high Rz/Rm ratio occurs most, of the current tends to

flow in the bore hole.

To overcome this difficulty, focused current de-

vices have been developed. These systems are designed,

to force the current into the formation d.irectly

opposite the current electrode by preventing the

current from spreading out spherically from the source.

These systems are called guard logs or laterolo~s.

In its simplest form the guard system consists of

three electrodes Figure 12!. A central current

electrode is bracketed above and below by two auxilliary

power electrodes called guards. The measuring current

is applied to the central electrode. Enough auxilliary

METER

CON

CUP

Fjg. 12. Iat,erolog a.fter Doll j.n L~~ch, i~36? }.

current is applied to the guard electrodes to keep

them at the same potential as the center electrode.

The main current, unable to flow up or down the hole,

must flow laterally into the formation. Only at

some distance into the formation can the current begin

to spread spherically and return to the grounded

electrode. The resistance measured is that of the

material in the current path from the in-hole electrode

to the surface. As with the normal and lateral de-

vices the material nearest the electrode exerts the

greatest influence upon the total resistance.

There are several systems available commercially

and used by Schlumberger, Welex and Dresser-Atlas

three of the major logging companies. The Welex Guard

Log has a central cylindrical electrode that is three

inches long and. guards above and below that are five

feet long. The very narrow current electrode gives

very fine definition.

Schlumberger's Laterolog 3 is very similar to the

Welex Guard Log ~ However, their Laterolog 7 is quite

different Figure 13!. In the Laterolog 7 arrangement

seven point electrodes are used: a center current

electrode Ao, two guard electrodes A> above and A> be-

low, a pair of sensing electrodes between Al and A

Fig. 13. SchlumberI er Laterolog 7 after D oil in Lynch, 1962!.

labeled Nl' and hl, and another pair between A and A2labeled N2 and N2'. The midpoint between Nl' and Nlis called Ol and the midpoint between N2 and N2' is

02. A constant current is sent through electrode A

A bucking curr ent, is sent through electrodes Al and

A2. This current is adjusted so that the two pairsof sensing electrodes MlN2 and Nl'N2' are brought tothe same potential, that is so that there is no

potential drop between the M-N' electrodes. Since the

A current is constant any potential measured between

a sensing electrode and one at the surface is a measure

of the resistivity of the formation opposite the

center electrode .. The Ol-02 spacing is 32 inches andthis is the width of the focused current sheet.

Their Laterolog 3 has a current sheet six feet to one

foot thick, allowing for finer resolution.

The electrical logging industry is primarily

geared to aid in the exploration for petroleum hydro-

carbons. A small portion of industry participates in

the exploration for water. Electrical resistivity

methods are also used in mineral exploration and in-

clude bore hole and surface techniques.

Electrical logging, as used in the petroleum in-

dustry, has three major uses. First, it is used for

correlative purposes to trace key beds from well to

well and assist in structural and stratigraphic

studies. The second ma]or use is in the determina-

tion of porosity and water saturation. A third use

is to coordinate and assist in evaluating other

formation evaluation techniques.

55

LOGGING IN UNCONSOLIDATED SEDIMENTS

When this research began it was assumed that

present industrial logging equipment and techniquesI

could be applied to recent marine sediments with some

modifications. It was soon determined that not only

was this impractical, but that industry seemed unable

to envision logging operations where no open hole

existed.

A resistivity logging probe designed to operate

in unconsolidated marine sediments was built by

Kermabon et al., 1969. The basic principle of the

probe was relatively simple ~ It consisted of electrode.",

mounted on the end of a non-conducting probe which

could be inserted or dropped into the sediments. As

the probe was withdrawn the resistivity of the sedi-

ments would be continuously measured. There is a

major advantage that this operation has over industrial

logging in that there is no drilling fluid pre"ent

possessing a conductivity different than the formation

fluid conductivity. Therefore, the resistivity measured

is the true formation resistivity. There is, however,

a disturbed zone around the probe caused by the forced

entry of the probe into the sediment. This disturbed

zone will have a resistivity different than that of the

undisturbed sediments and thus effect the resistivity

reading. Kermabon et al. �969! solved this problem

using a mechanical arrangement to project the electrodes

into the undisturbed sediments. The Texas A8cM probe

used a guard electrode arrangement to focus the current,

deep into the sediment.

The first Texas A8cM probe Figure 14! consisted

of a barrel made of heavy wall, stainless steel tubing

380 cm long. The tubing had an outer diameter of 5 cm

and an inner diameter of 2 1/2 cm. A nose cone con-

taining five brass ring electrodes was attached to the

end of the probe. A solid stainless steel tip was

formed to the end of the nose cone. The shank of

this cone consisted of a 23 cm length of the stainless

steel tubing which has been turned. down to an OD of

3 cm. Over this shank were placed polyvinylchloride

rings which were partially turned down at one end.

Fitted over the turned down portion of the PUC rings

were five evenly spaced brass ring electrodes. The

entire cone was precision machined so that the outer

diameters of the PVC rings and the brass electrodes

were the same as the outside diameter of the probe

barrel, or 5 cm. Insulated wires lead back from the

electrodes into the hollow cone and up the barrel to

Pig. 14. Texas A&M continuous resistivity probe.Nose cone in insert.

water tight MN connectors at the top of the probe.

Seals between the PVC rings made the nose cone water

tight.

A stainless steel housing of slightly larger

diameter was welded, to the top of the probe. A water

tight cap containing water proof connectors fitted

to the top of the housing. The entire probe was then

watertight. Pour large stabilizing fins were welded

to the top of the probe, and a bail welded to these for

raising and lowering purposes.

An indexing device was developed to record. the

depth of penetration of the probe into the sediment

and to indicate /he depth within the sediment of goneq

of interest at Which the resistivity was measured.

The indexer consisted of a spoked wheel IIO 1/2 cm in

diameter. The wheel and spokes were made out of an

octagonal steel, rocL 9mm i,n diameter. The hub was

made to a sufficient inner diameter to easily slide

up and down the stainless steel rod. Small stops

attached just above the electrode ring assembly pre-

vented the wheel from falling off. Nounted within

the hub and held in place by epoxy resin were l8

permanent magnets. Strung a3�ong at one foot intervals

within the stainless steel rod were a series of mag-

netic proximity switches.

When the probe hit bottom after a free fall or

fast real out the spoked wheel remained. essentially on

the water-sediment interface. As the probe passed

through the magnetic field of the hub each magnetic

switch was closed in turn causing a fiducial mark to

be registered on the record.ing chart ~ The same process

happened in reverse as the probe was being withdrawn.

Depth of penetration could be determined by counting

the number of fiducial marks. Since this was synchro-

nized with the resistivity recording, the d,epth of any

zone of interest could be determined. Power to the

probe and information from the probe were conduct|;d

via a seven conductor, waterproof cable. The probe

must be lowered and raised via a steel cable,

The probe was first tested in the Gulf of Mexico

in September 1970 during cruise 90-A-12. Numerous

insitu measurements were made on this cruise and the

equipment worked wells The recording clearly indicated

when the probe entered the bottom and when it was

withdrawn. Resistivity variations could be seen within

the sediments.

However, the limitations of the equipment became

obvious at once. The first limitation was directly

related to the speed of the recording paper chart drive

which was too slow. The winching capabilities aboard

ship, which was constantly in motion, did not allow a

slow, constant rate of pull out. The net result was

that 8 to l0 feet of section was recorded within 2-g

centimeters on the chart paper. This was far too com-

pressed to register much detail. Also at this time

it was determined that the magnetic field in the hub

was too strong. Switches were closing premature'ely

and confusing the depth readings.

Research vessels are not very stable platforms,

so that even if more accurate winch controls were avail-

able and faster recording speeds obtained logging

with a moving d.evice would still be erratic. The in-

dexing device could be improved upon but there would

still be a moving part subject to all the frailties

of moving parts. During the course of the probings

the nose cone attachment bent out of line from the

barrel. At the time this caused no leakage, but con-

tinued usage certainly would have. However, this type

of measuring system will be used in very shallow

water by attaching the ring electrode assembly to a

long rod, and raising and lowering by hand.

To overcome the difficulties inherent in making

measurements from a moving platform a different type of

measuring device was required. A probe was designed

that is in use at present �971}. Nith this design,

point resistivity measurements are made while the

probe remains stationary in the bottom.

The probe was made of a fiberglas tubular pole

265 cm long and averaging 6 cm in diameter. A stain-�

less steel tip was fastened to the bottom of the pole.

A heavy walled steel housing 72.5 cm long and 26 cm

in diameter was mounted. on top of the pole. The mount-

ing was made watertight and secure by inserting the

fiberglas pole into a cylindircal steel jacket that

was welded to the bottom of the housing. A metal cover

was bolted to the top of the housing and made water-

tight with an 0-ring seal. A chain link bail was

attached to the top of the housing. Contained within

the housing were two electronic stepping switches.

The probe barrel contained 102 cylindrical brass

electrodes 1 cm in diameter. They were counter sunk

so that they were nearly flush with the outer surface

of the barrel Figure 15!. The electrodes were arranged

in a spiral fashion up the pole so that each electrode

was positioned 90' from the other and 2.5 cm highe~.

Insulated wires lead back from each electrode through

the hollow pole to the stepping switches. The tube

Fig. 15. Texas A&M point resistivity probe

63

was filled with an epoxy resin that hardens to a

flexible, rubber-like consistency to further ensure

against leakage.

The stepping switches, which can be operated

automatically or manually, serve to switch the sensing

current from one electrode to the other. The system

is designed so that it may be operated in a normal

mode or a guard mode. This is a mono-electrode system

as discussed on page 29 ' The probe is usually operated

in a guard mode. In this mode three electrodes are

used simultaneously to make a measurement. A sensing

current is sent through the middle electrode and a buck-

ing current, is sent through the electrodes above and

below the sensing electrode on the same side of the pole;

that is, the three electrodes are in a vertical line.

The bucking current is of such magnitude that a zero

potential exists between the sensing electrode and the

two guard electrodes thus forcing the sensing current

to flow some distance into the undisturbed sediment be-

fore returning to ship's ground and completing the

circuit. In stepping through successive measurements

three electrodes are switched simultaneously and each

succeeding measurement is made 90' from the previous

measurement and 2.g cm higher. Because of the geometry

of the system electrode number 5 is the first sensing

electrode and number 97 is the last sensing electrod.e.

The stepping sequence was' from the bottom to the top

of this probe.

The power supply and. control unit was located

aboard ship and current supplied to the probe via an

insulated, multi-conductor cable. The cable plugs

into a waterproof connector unit at the top of the probe

housing. An alternating current of 36 ma is used

to prevent polarity of the electrodes.

The point resistivity probe was first tested in-

side and immediately outside Galveston Bay during

cruise 71-0-8 aboard the R/V ORCA in August 1971.

Because the conducting cable did not have a strength

member the probe had to be lowered via a, steel cable.

The conducting cable was payed out by hand and taped

to the support cable at 8-10 foot intervals.

The procedure was to lower the probe into the

water and to make one or more scans of the electrodes

to determine a resistivity base line. The probe was

then allowed to free fall into bottom sediment where

two or more sequences of measurements were stepped

through to determine sediment resistivity. The probe

was then raised into the water column and the water

resistivity was checked again. These water scans werq

made with sediment adhering to the probe and with the

sediment washed off to determine if the sediment had

any influence upon the recorded resistivity. A dif-

ference was found. The total average difference

between both runs was 2.09 ohms. As was expected,

the higher value was recorded with the mud sticking

to the probe. Individual electrodes varied by greater

or lesser amounts.

There are two probable explanations for the vari-

ation in the amount of resistivity increase Bt each

electrode. First, there is a difference in the re-

sistivity of the sediment adhering to the individual

electrodes. Second., there is variation in the amount

of sediment that adhers to different electrodes. This

was obviously the case as it could be seen that the

probe was not uniformly coated when taken from the

water. Sediment build-up around some electrodes was

greater than others because all electrodes were not

completely flush with the surface of the probe.

At the time of construction the only fiberglas

pole available was one constructed of wrapped fiber-

glas matting. The material was bulky and inhomogeneous

and resulted in a pole with an uneven outer diameter.

It was too time consuming to try and set each electrode

completely flush with the surface. Since the magni-

tude of electrode response is influenced by the area

of electrode exposure to the medium being measured,

this obviously accounts for some variation in electrode

response.

A second disadvantage in this particular pole was

that it was relatively weak and eventually snapped as

the ship swung widely on its anchor. However, sufficient

data were obtained prior to breaking to indicate the

workability of the design.

A Hewlett-Packard model 680 strip-chart recorder

was used to record the return signal. The electronic

control system which supplied power to the probe and

monitored the return signal also controled the electrode

stepping mechanism Figure 16!. A control is provided

whereby the system steps automatically or may be

stepped manually. The scanning rate of the automatic

stepping control may be varied.

Figure 17 is a typical strip-chart recording made

in the water. As the current is switched from one

electrode to the next a current surge occurs in the

line. This is the cause of the tail-like spikes which

occur between each electrode measurement. The width

Fig 16. Electronic control pane]. for pointresistivity probe

230230

JIO 20

L . I'30 IO 20

RESISTANCE ohms!

Fig. 17. Water resistance, Fig. l8. Water resj stance,guard mode. unguarded mode.

of the recorded measurement is a function both of the

electrode scanning rate and the speed of the chart

paper moving through the recorder . A finite amount of

time is required for each electrode measurement to

stabilize.- The value read. for each electrode re-

sistance is that value occurring just before the

switch steps to the next electrode.

The resistivity of sea water, though it varies

with salinity and temperature, averages about 1/10

ohm/m . The probe is measuring total resistance and.

therefore a different value will be recorded which can

be normalized.. The resistance averages 16.41 ohms.

The measurements have been made using the guard

electrode configuration.

Figure 18 is a strip chart recording made at the

same location using the normal mode. The average re«

sistance of all the electrodes is 15.96 ohms. This is

only about one half ohm less than the average value

achieved with the guard system.

It is assumed that a column of sea water equal

to the length of the probe is nearly homogeneous and

isotropic, and any variation in electrical resistivity

is too small to be detected by the equipment. The

obvious differences in resistance recorded may therefore

be attributed to electrode response characteristics.

In this case the large variation in exposed surface

area among the electrodes' is primarily responsible

for the measured differences.

For correlative purposes the recorded values can

be used directly. However, to obtain the true re-

sistance of the sediments an electrode correction

factor must be applied. The electrode correction

factor was determined to be the difference between

each electrode reading and the water resistance as

determined by averaging all electrode readings made

in the water. The correction factor should then beI"

added algebraically to each electrode reading made

in the sediment. The internal circuit was deter-

mined to be f ive ohms and this should be subtr acted

fr om all readings.

Penetration Control

The insitu probe is designed primarily as a

correlative tool. The records themselves may be com-

pared for similarity of character and degree of re-

sistance, Figure l9. To aid visual inspection it is

often expedient to plot the resistivity values of each

71

ZI-CL4JO

4JKO0

230

P5

230I I I

20 30 40 50 20 30 40 20 30 40 20 30 40 50

RESISTANCE ohms!

Fig. 19. Comparison of insitu records

electrode on square ruled graph paper and to connect

the points. Figure 20 is a plot of the values ob-

tained from the records in Figure 19.

The measurements were made while the ship swung

on anchor, and each penetration is close -to the

other. There are close similarities between certain

portions of the curves. In many cases similar de-

flections can be traced across all four records.

There is some evidence of thickening and thinning

of individual units.

One of the significant features of the curves

Figure 19! is that super imposed upon some sharp

variations in resistivity is the large drop in average

resistivity following electrode 75 in P3, Pg, and P5

and electrode 80 in P2. The great similarity in the

lower two-thirds of the curves indicates that the

probe penetrated to approximately the same depth on

each drop.

If the upper parts of curves 3 and. g are examined

above electrode 70 it can be seen that the large'jumps

in resistivity occur in groups of four. That is, the

very low reading occurs at electrodes which are on the

same side of the probe. This suggests the following

explanation.' the probe which has no rigid directional

guides is allowed to free-fall into the sediment,

O

0

O CV

O W0! Hl.d30 3808d

Fig. 20. Resistivity curves � values from recordsin Figure 19

0O '4

C4

while the ship swings about the anchor. As a conse-

quence, the barrel can be at some angle to the verti-

cal, not necessarily large Figure 21!. At impact,

and for some small time following, motion of the probe

is not parallel to the axis of the barrel. At impact

the axis of the probe barrel makes a small angle with

the vertical. Since the axis of the barrel is not

parallel with the direction of movement a slightly

elongate hole will be inade near the surface. However,

since the angle is very small, the confining pressure

of the sediments will finally deflect the motion of

the probe into a direction parallel to the axis of

the barrel. From the point within the sediment at

which the probe begins moving parallel to its own axis,

to the maximum depth of penetration, the disturbed

zone around the barrel should extend an equal distance

in all directions away from the probe. Above that

point, on the side of the probe toward the vertical

there would be a more extensively disturbed zone than

anywhere else around the circumference of the barrel.

This disturbed zone will contain more water than the

sediments on the other three sides of the probe and

will have less resistance than the sediment in any

other direction. For that reason there will be large

jumps in the resistivity as the scan moves up the pole.

Dl

Fig. 21. Free-falling probe enteringbottom at an angle to the vertical.

Penetration 5 is interesting in that the lower

half of the curve is quite similar to the other three.

However, the disturbed zone begins lower and the upper

15 electrodes, with one exception, show very low re-

sistance. It is apparent that the upper $0 cm of the

barrel is projecting above the sediment-water inter-

face. Since the lower half of all curves compare

well, all penetrations were to approximately the same

horizon. Therefore, the variations i.n the upper half

may be attributed to a depression in the bottom

topography.

CORE LOGGING

The insitu probe can be used directly for corre-

lation, but very little can be determined dizectly

about the nature of the sediments being measured,

Consequently cores must be taken pez iodically an/

analyzed.

Concomitantly with the development of the insitu

probe has been the development of a core logging device.

The primazy purpose of the coze logger, or scanner as

it has been variously called, is to determine zones

of interest within cores that are not visibly apparent

and which might be missed by serial sampling.

Kermabon et al. �969! constructed a device to

measure the electrical resistance along the length of

a coze. Subsequently several core logging devices,

each an improvement over the previous model, have been.

developed at Texas ANN University.

The first ANN logging device was adapted from a

portable Widco water-well logging instrument on loan

from Well Reconnaissance, Inc. of Dallas, Texas,

Figure 22. The instrument package consisted of a

sonde which was normally lowered into the well, and

a supply-recordez' unit.

Fig. 22. Core logging device adapted from Widco welllogging instrument

The sonde was mounted on a wooden sled. Two bronze

pins, which served as electrodes, were physically and

electronically attached to the sonde. The sled straddled

a wooden tray which held the core. To operate, the

electrodes -were inserted into the core, the current

switched on and the sled. hand cranked the length of the

core. The .variations in resistance were recorded as

tiny spots on a narrow str ip of paper. This equipment

was successfully used aboard the USNS MANE during

the summer of 196$ in the Gulf of Mexico. The use of

pins as electrodes proved to be quite destructive

to the cores.

A second and much improved version of the core

logger was constructed by Dr. Chmelik utilizing a thin

blade as an electrode, Figure 23. Two stainless steel

traveling bars 91 centimeters long were mounted in

rigid end braces. A box-like carriage was designed. to

ride along the two bars. Attached to the carriage

was an adjustable arm which held the electrode. An

electric motor was mounted upon the carriage and through

a linkage of cables and pulleys was able to drive it

in either direction. A rheostat controlled the speed.

The entire system including the core was connected

to a common ground. A constant alternating current of

A 4IC4

0 4I'0

4I

bO600

4l

O

m 44

28 milliamperes was applied. Therefore any changes

in potential recorded had to be caused by variations

in resistance within the core. The variations in

resistivity were recorded. on a Mestronix four channel

recorder.

A Selsen interlock system was used. to synchronize

the logging speed with the recording drive speed. One

Selsen motor was mounted upon the carriage and the

other within the recorder. The system provided. a

constant ratio between the core length and. the length

of the recording.

This system worked reasonably well; however cer-

tain inadequacies in equipment, and design gave rise

to a new core scanner, Figure 24. With this new in-

strument the logging system and the recording system

are combined. The recording stylus travels along

with the measuring electrode. This provides a record

that has a one-to-one ratio with the length of the

core. The advantages of the system are two-fold.

First, the entire system can be transported as a

single unit. Second, any changes in resistance can

be immediately related to the exact spot in the core.

One disadvantage wifh the system is that small changes

which may be significant, and which would appear as

Fig, 24. Core Togging device combining logging systemand recording system

abrupt sharp peaks on a compressed section, are smoothed

out on the one-to-one scale and are less readily

apparent. This does not, however, present any insur�

mountable problems.

There- are several major problems encountered in

core logging which have not been entirely overcome.

First, the. extruded cores and those removed from

plastic liners consist of saturated fine-grained sedi-

ments in various degrees of consolidation. As a

result they do not have a constant diameter through-

out their lengths. For this reason it is nearly im-

possible to maintain the blade electrode at a constant

depth within the sediment. It has been determined

empirically that among other things the measured re-

sistance is inversely proportional to the size of the

electrode area exposed to the medium being measured.

Therefore, low spots and high spots encountered by the

horizontally traveling electrode record as increases

and. decreases in resistance as the blade is alter-

nately exposed and buried. One method used to allevi-

ate this problem is to adjust the slowly moving

electrode by hand as it travels along. By this method

it is possible to maintain the blade at a nearly con-

stant depth. Another partially successful method is to

coat all but the tip end of the blade with a non-

conducting epoxy resin thus limiting the amount of

electrode to a constant factor.

A second problem encountered is that despite

the fact that the electrode is a very thin blade,

a certain amount of sediment tends to build up ahead

of it. Thi- has the effect of incroasing electrode

area reducing somewhat the recorded resistance.

Even with a coated blade the sediment may reach up

to the electrode holder having the same effect.

A third problem involves passage of the electric

current through the sediment. Nost of the Texas ARM

cores are extruded into PVG half round trays onto a

metal strip which runs the length of the tray. During

logging this strip is connected to ground. Brass discs,

which fit against each end of the core, are also con,�

nected to ground. For purposes of interpretation it

is assumed that the current passes directly through

the core to a ground point di.- ectly opposite the

electrode, and that the recorded resistance repre-

sents this narrow zone. In fact the current will

spread out in all directions from the electrode and

tend to follow the path of least resistance to ground.

This may or may not be the shortest physical distance

to ground. The total current will actually take many

paths and the recorded resistance will be a summation

of these individual paths. Very small zones of sig-

nificant resistivity changes will not be recorded

directly but will be integrated into the total re-

sistance of a surrounding zone. For this reason aI

very small, zone with a sharp resistivity change may

not show up on the record as prominently as a much

larger zone which only has a slightly higher re-

sistivity than the surrounding material.

It was also discovered that by placing a metal

strip or wire beneath the core a light shadow appeared

on X-ray radiographs. Even if the strip was removed

before radiographing the resulting damage to the

under surface of the core caused by removing the

strip affected the radiographs in the same manner as

the strip. These shadows tended to obscure subtle

changes within the core which could normally be de-

tected by X-rays. The present practice is to place

the grounding rod alongside the core and by rotating

the PVC tray inserting the electrode as far away as

possible from the rod. The bulk of the current then

travels through the upper half of the core. There

is no evidence that, zones of resistance are missed

by this method.

87

CORREI AT ION

Figure 20 clearly demonstrates the possibility

oi' identifying and correlating sedimentary units be-

tween successive penetrations of the insitu probe.

There are rapid changes in sediment ty'pes or microfacies

in both vertical and horizontal directions in the upper

few meters of recent marine sediments. Because of the

time and economics involved, cores are normally taken

at such horizontal spacings that only gross lithologic

units can be traced. There are both academic and

economic reasons for wanting to determine the lateral

extent of certain microfacies. The insitu probe can

provide a rapid and economical means of doing this'

In its present state of development the probe is

not yet a quantitative instrument. It is necessary to

obtain some cores in order to tie the resistivity units

to specific lithological units. The insitu records

must be used in conjunction with records obtained

from the core scanner. A measure of reliability of

the probe is how well it compares with resistivity

measurements made upon the cores.

Several cores were taken in the area of the probings,

but at best core recover y was no more than three quarters

of the depth of probe penetration. Core number 71-0-8-3

was used to compare with the upper three quarters of

the probe data.

Figure 25 is a graph of probe penetration 4,

run 2, overlain by a plot of the scanner record of

core 3. The absolute values of the two curves are

different; however if the fluctuations due to electrode

response characteristics are smoothed out the two

'curves will be nearly syrrznetrical.

Figure 26 is a graph of probe penetration 2,

run 2, overlain by a plot of resistivity values of

samples taken from selected locations in core 3. As

in Figure 25 there is a degree of symmetry between

the two curves that is more than fortuitious.

23020

230

3040

I-CLLJjCl

RESISTANCE ohms!

Fig. 25. Comparison of insitupenetration 4 with scannerrecord of core 3. Not to scale.

L ~ ..I50 40 30 20

RESISTIVITY ohm/cm3!

Fig. 26. Comparison of insitupenetration 2 with resistivityof core 3. Not to scale.

SZDIMFNT ANALYSIS PROCEDURE

The procedures used on this pro ject to determine

the grain size distribution and their physical properties

is identical to those used by Cernock �967 and 1970!,

and the following descriptions will be quoted nearly

verbatim from Cernock �970!.

Grain Size Distribution

The grain size distribution was determined by

pipette analysi.s following the method described by

Krumbein and Pettijohn �938, p. 166-172!. The method

employs Stokes~ law for calculating the settling

velocities of the limiting sizes for particle grades

finer than 4 phi .64 mm!. From the relation V = h/t-�

a time schedule for the complete pipette analysis was

prepared for specific sampling depths h! and the

velocities V! calculated for a constant temperature

of 20'C and a. sediment specific gravity of 2.65. The

time schedule and complete laboratory procedure was

identical to that described by Cernock �967!. The

samples containing sandy material were wet sieved to

determine the weight of the particle grades coarser

than g phi. The weights of the residue for each grade

were added to give the total sample weight. Each weight

was then divided by the total sample weight to obtain

percentages for construction of the cumulative curves.

The grain median diameter was obtained from the

50/0 line of the cumulative curve. The percentage

sand-, silt-, and clay-size particles was taken as

the percentage by weight for the following limiting

sizes: sand, les than g phi .6g mm!; silt, 4-9 phi

.6$-.002 mm!; and clay, greater than 9 phi less

than .002 mm!.

Physical Properties

The physical properties which were compared with

electrical resistivity were bulk density, water content,

specific gravity of solids, void ratio, and porosity.

The procedures for the determination of these physical

properties were similar to those outlined by Lamb �951!

and Cernock �967!.

The wet samples were weighed to an accuracy of

one ten-thousandth �/10,000! of a gram,.and re-weighed

after drying for 24 hours in an oven at 105-110 C.

The dried samples were allowed to cool to r oom tempera-

ture in a dessicator, reweighed, crushed into a fine

powder, and redried in the oven. After cooling in a

air comparison pycnometer utilizing helium gas. The

weights and volumes of wet and dry sediment and. the

amount of water originally present in the sample

were then calculated. No corrections were calculated

for the weight and volume of salts present in the

sediment upon drying. All samples were assumed to

be one hundred percent saturated., an assumption that

is consistent with measured degrees of saturation

for similar sediments Cernock �967!.

Water Content

Water content or moisture content w is defined as

the ratio in percent of the weight of water W to the

weight of the oven-dried solids W in a given sediment� s

mass; or

WW = 100

Ws

�3!

Specific Gravity of Solids

The specific gravity of solids 7 of the sediments

mass is defined as the ratio of the weight in air of

dessicator, the samples were weighed and. their volumes

accurate to + 0.02 cm ! were determined with a Beckman

W

~SVs~w

�4.!

where V is the volume of the oven-dried sediment1.

mass, and p is the density of distilled water at g Cw

which is 1.000 gn/cm3. The density of solids p iss

obtained by dropping the p from Equation 2$.

Void Ratio

The void r atio e is def ined as the ratio of the

volume of voids to the volume of solid particles in

a given sediment mass. Provided all the voids are

filled with water, i.e., the sediment is one hundred

percent saturated, the void ratio is equal to

�5!e = M100 ~s

where w is the water content and y is the specific

gravity of solids.

a given volume of solid particles to the weight in air

of an equal volume of distilled water at a temperature

of Q C or

Porosity

The porosity n is defined as the ratio in percent

of the volume of voids to the total volume of the

sediment mass. The porosity can be calculated. from

the void ratio as

�6!n = e100

1 +e

Bulk Density

Bulk density or wet un'.t weight pd is defined asthe ratio of the total weight of the sediment mass to

the total volume of the sediment mass irrespective of

calculated from values of the specific gravity of

solids y and the void ratio e ass

+ ePd � Ys

1 + e�7!

Vane Shear Strength

Vane shear tests were made on the cores at the

same locations where laboratory and electrical r e-

sistivity measurements were made. These locations

the degree of saturation. If all the voids are filled

with water, the bulk density in gm/cm can then be

were picked at points of change of resistance as re-

corded on the core scanner. The undisturbed shear

strength of the samples was measured by a motor driven

Farnell miniature vane shear apparatus following the

methods described by Richards �961! and Cernock �967!.

Since the sediment, was sheared in an undrained and

saturated state, the shear strength was equal to the

cohesion of the sediment Moore, 1964!.

Carbonate content

Carbonate content was determined from a 0.5-1.0

gram portion of the powdered, dried sediment samples

used in the determination of specific gravity of solids.

The percentage by weight of carbonate in these samples

was determined by the Scheibler method. The apparatus

used in this method and the step by step procedure are

described and discussed by Bouma et al. �969, p. $1-56!.

97

RELATIONSHIP BETWEEN RESISTIVITY AND SEDIMENT PROPERTIES

Samples were collected from the logged cores at

points indicating changes in r esistance. Routine size

analyses were performed on these samples and the

carbonate content and water content were determined.

Specific density, void ratio and porosity were calcu-

lated. Plastic 35 cc monoject syringes were modified

for use as miniature piston coring devices by cutting

off the lower end of the tube. Samples were then

abstracted from the original core in a relatively un-

disturbed condition.

A carbon block was shaped to fit snugly down into

the top of the tube pressing against the sediment, and

serving as one electrode Figure 27!. The open end

of the tube was held in contact with a flat carbon

block which served as the other electrode. Excess

sample could be squeezed out by the upper electrode

so that each sample was measured at a constant volume.

A constant, alternating current of 36 milliamps

was supplied by the same control unit that powered

the probe. The specific resistivity was calculated

from Equation 10.

By the use of graphs the sediment resistivities

were compared with the other sediment properties and

PLASTIC TUBE

J

Fig. 27. Absolute resistivitymeasuring device.

I

i II I

I

I I

I I I I II I I II I II I

I I I

CALI NDRICAL CA RBON BLOCK

ECTAN GULAR CARBON

BLOCK

the total resistance as measured on the core scanner

also see Chmelik et al. 1969!.

Resistivity vs Scanner Resistance

Figures 28A and 28B are graphical comparisons be-

tween the resistivity of the individual sample plugs,-

and the resistances measured at the same points in the

core by the core scanner. The absolute magnitudes of

the two measurements cannot be compared except that

the values of the specific resistivities are higher

than those measured on the core scanner. This is as

expected. The important factor to be considered is that

the pattern of change is the same for nearly all points

being compared. Considering core 2 Figure 28A! the

only break in the pattern occurs approximately l/3 of

the way down from the top. However, the very high

reading seen on the scanner was actually caused by the

electrode striking and being deflected by a hard object.

This value is not a true reading of the resistance at

that point.

Core 3 Figure 28B! shows a high resistivity

value near the top. The lower resistance value is

probably caused by a spreading of the current around

the zone of high resistivity. There is a small

0

200200

L I I ! L ! I

Fig. 28A. Scanner resistance vsresistivity core 2.

40 30 20

RESISTANCE ohms!

70 60 50 4O

RESIST I Vll 7 ohm/cm>!

0 0

I 8~~ I05

L i I L L I i I i I J40 50 20 Io 80 70 60 50

RESISTANCE ohms! RESIST I V IT Y ohm <<m !

Fig. 28B. Scanner resistance vsresistivity core 3.

discrepancy at the end of the core, but this does not

invalidate the reliability of the scanner system.

Resistivity vs Mater Content

Conductivity in saturated marine sediments is

primarily ionic, and is obviously partially a function

of the water content. Figures 29A and 29B are graph-

ical comparisons between the resistivity and the water

content of the samples. The inverse relationship be-

tween the two is very clearly demonstrated. Since

the ionic concentration of sea water may be equated

with salinity, and salinity is quite constant within

any localized area, this factor probably does not

account for any significant changes in resistivity

within any one core or within any one limited sedi-

mentary province.

Resistivity vs Formation Factor

Resistivity is directly proportional to the

Formation Resistivity Factor as indicated by Equation l.

This is also clearly and graphically indicated by

Figures 30A and 30B.

200

80

Fig. 29A. Water content vs resistivity core 2.

4JCCO O

70 60 50WATER CONTENT %!

DRY M/EI GI IT

L .i J .L70 60 50

frESI ST I V IT Y ohm/cm >!

0

l83I83

L ! I I J70 60 80 70 60

RESISTIVI 7 Y ohm 'cm3!

80 5090

WATER CONTENT %!DRY WEIGHT

Fig. 29B, Water content vs resistivity core 3.

105

I I=1I I

4JCLO

200 200

Fig. 30A. Formation factor vs resistivity core 2.

xl-e&Z4J

8 7 5

FORMATION FACTOR

J .i.. i .70 60 90

RESISTIYITY ohmt'cm>!

I 83l83

I50

Fig. 30B. Formation factor vs resistivity core 3.

7 6

FORMATION FACTOR

L i I I I80 70 60

RESISTIVITY ohm/cm>!

107

IResistivity vs Void Ratio

Void ratio is the ratio of the void volume to tQe

volume of the solid particles in a given sediment

mass. It would be expected that there is an inverse

relationship between resistivity and void ratio in a

fully saturated sediment. Figures 31A and 31B

clearly illustrate this inverse relationship. The

relationship can only hold true if the sediment is

completely saturated with a conducting fluid. If

the voids -were filled with air or a non-conduct!.ng

fluid, the resistivity would be very high.

Resistivity vs Porosity

Porosity, which is a percentage ratio between.

void volume and the total volume of the sediments,

bears the same inverse relationship to resistivity

as does the void ratio. Figures 32A and 32B illus-

trate this relationship. This relationship al o re-

quires that the sediments be fully aturated with a

conducting fluid.

Resistivity vs Percentage Clay Content

Figures 33A and 33B clearly illustrate an in-

verse relationship between resistivity and the percentage

0 0

E

200200

I I I l2 I 70 60 SO

RESISTIVITY ohm/cm !VOID RATIO

Fig. 3lA. Void ratio vs resistivity core 2.

109

0

XC9W

CLO

l83 83

5070 60

RESIST I V IT Y ohm/cm3!VOID RATIO

Fig. 31B. Void ratio vs resistivity core 3.

200

80I i. L

70 60 50

PORGS IT Y /o !

L I70 60 50

RESISTIVITY ohm/cm ~!

Fi.g. 32A. Porosity vs resistivity core 2.

I83 I83

I i....l=~ J80 70 60

RE S I S T I V I T V � hm ~c m -!

L. J70 60 50

POROSITY /c!

Fig. 32B. Porosity vs resistivity core 3.

0

200 200

L I L I..L

Fig. 33A. Percentage clay content vsresistivity core 2.

! .J. z. I.. ...!60 SO 40 30

CLAY '/o!

70 60 50RES I STIV IT Y Ohm/cm>!

113

0

I83I83

L I I j IL ! l J l .I

Fig. $3B. Percentage clay content vsresistivity core 3.

LIJKO

60 50 40

CLAY { /o!

80 70 60 50RESISTIVITY ohm/cm>!

of clay in the sample. There is some conductivity

associated with the clay particles. However, the

higher conductivity associated with the clay probably

owes as much to the high porosity in the very fine

fraction.

Resistivity vs. Percentage Sand Content

Figures 34A and 34B illustrate a direct relation-

ship between resistivity and the amount of sand in

the sample. This relationship exists because of the

non-conducting nature of the sand grains plus the

fact that there is a decrease in porosity as the sand

content increases. In figure 28A at a point 1/3 the

way down from the top an anomalous situation occurs

which shows a higher sand content coupled. with a lower

resistivity. This may be explained in part by the sam-

pling procedure. As previously noted a hard object,

probably a shell or shell fragment, occurred at this point

in the core. Two samples were taken from each location

in the core for resistivity determination and for size

analysis, If the shell fragment was included in the

size analysis fraction and not in the other, an abnormally

high sand fraction would be indicated which would have

no effect upon the resistivity measurement.

0

200 200L l J

Fig. 34A. Percentage sand content vresistivity core 2.

. l....l I I. I I....J70 60 50 40 30 20 IO 0

SAND %!70 60 .">0

RESISTIVITY ohml m3!

I83 183

S0 20 l0

Fig. 348. Percentage sand content vsresistivity core 3.

30

SANP t/ !70 60 50

RESISTIVITY ohm/cm3!

117

Resistivity vs Nedian 8 Diameter

Figures 35A and 35B show an inverse relationship

between resistivity and the median 8 size of the

sample. The average 8 size for the two cores is 7.2.

The sediment can therefore be classified. as a very

silty clay with varying amounts of sand. The 8 size

classification system is such that the larger the 8

number, the finer the sediment fraction. Silt ranges

fr om 4-9 0, with the clay fraction being greater than

9 9 and the sand fraction being less than 4 gl. Figure

35 serves to corroborate the evidence presented in

Figures 33 and 3Q.

Resistivity vs Carbonate Content

The relationship between resistivity and carbon-

ate content is not clear also see Chmelik et al. 1969!.

The upper end of Figure 36A shows no apparent relation-

ship except for points 8, 9, and 10 which vary directly.

The lower segment of the curve varies inversely. In

Figure 36B the right hand end of the curve varies

inversely whereas the left hand end varies directly.

Core 2 shows large fluctuations in carbonate content

from top to bottom with no definite trend. Core 3

shows a steady decline in carbonate content from top

200200

J. IIO 9 8 7 6 5 4 3

h~iEDIUM

L ~ l I70 60 50

RESISTIVITY ohm/cm>!

Fig. 35A. Median P diameter vs resistivity core 2.

119

00

I83183

6 80

Fig. 35B. Median P diameter vs resistivity core 3.

4JO

8 7MEDIUhl $

70 ' 60 50

RESISTIVITY ohm/cm>!

0

200

Fig. 36A. Carbonate content vs resistivity core 2.

5 2CAR f30NATE % !

0 60 50RESIST I V I T Y ohm/cm>!

121

I ' I1 1

EO

!83 I83

J I

CARBONATE %!

Fig. 36B. Carbonate content vs resistivity core 3.

.XI-C9

UJ

4J

O C3

I ~ l i .i80 . 70 60 50

RESISTIVITY ohm/cm>!

to bottom with a few fluctuations. In a general sense

the resistivity of core 3 also decreases toward the

bottom. Both figures present conflicting evidence,

and many more measurements are needed to determine

whether in fact a relationship does exist. The amount

of carbonate in each sample is very small, and it is

doubtful whether it does have any influence upon theI

resistivity.

Resistivity vs Specific Gravity

The specific gravity of the sediment samples varies

very little. Figures 3 A and 37B indicate that there

is no apparent relationship between resistivity and

specific density. An indirect relationship might exist

in the case of a metallic placer deposit.

Resistivity vs Vane Shear Strength

Vane shear measurements were made upon a piston

core obtained from a different location than Site 1

where the insitu probings were made. Samples were ob-

tained from the same locations in the core where the

vane shear measurements were made. Figure 38 is a

graph comparing resistivity with vane shear strength.

200 200

l3 2

SPECI F IC GRAVITY

L70 60 50 40

RESI ST I V IT Y ohm/cm>!

Fig. 37A. Specific gravity vs resistivity core 2.

EO

I83I83

402.7

SPECIFIC GRAVITY

I�CgzEU

4JIZO D

L i L i L80 70 60 50

RE Sl ST I V IT Y Ohm/cm >!

Fig. 37B. Specific gravity vsresistivity core

125

470470

C3

LEJ

565565

LLIKOD

l~ I z 3 1 1 i J L200 160 I 50 I40 I 20 60 50

VANF SHEAR STRENGTH PSF! RESISTIVITY ohm/cm~!

Fig. 38. Vane shear strength vsresistivity core 4-B.

The number of measurements is too small to determine

absolutely whether a relationship exists between the

two parameters. Theoretically some relationship should

exist. The sediment was sheared in a saturated and

undrained condition. Recording to Moore �964! the

shear strength should be equal to the cohesion of the

sediment in this state. Since a more cohesive sedi-

ment would in general contain less water than a less

cohesive sediment, resistivity could be expected to

vary directly with vane shear strength. It is doubtful

whether any mathematical ratio exists between the two.

Resistivity vs Sound. Velocity

Figure 39 parts C and A represent graphical compari-

sons between resistivity and sound velocity. The core

was taken in a plast,ic liner and the sound velocity was

run on the core while it was still in the liner. The

resistivity samples were obtained from the same locations

at which the sound velocities were run. There is a

slight symmetry to the graphs which suggest that there

may be a relationship between sound velocity and re-

sistivity of the sediments. Obviously many more measure-

ments will have to be made before a definite pattern can

be established. A relationship would be expected since

both parameters are a function of void ratio.

127

I I I I I I I ~ I417

4I7

E O

548548 PART C

1 I I I J L I.1520 I 500 I 480 70 60 50

7IB7IB

E O

797797

I 540 1520 1500 l4 80

VELOCITY SOUND m/sec!

Fig. 39. Bound velocity vs resistivitycore 2. Part C and part A.

Z I-

IJJ

4JO

X zW

4J

O <3

il 540

70 60 50

RESISTIVITY ohm/cm>!

129

SUGGE ST I ONS

Instrumentation played an extremely significant

role in this study. The project was fortunate in having

the services of a very capable machinist. However,

since only a portion of his time was available, and

funds were too limited to farm the work out, constru=-

tion absorbed large portions of the project time.

It has been. said that anything in production i" al-

ready made obsolete by something else on the drawing

board. In the case of the insitu probe several sug-

gestions can be made to improve the efficiency of the

instrument. Because of the nature of the interface

it is not always possible to determine how deep the

probe has penetrated. A small transducer mounted on

the underside of the housing could monitor the distance

from this point to the bottom and indicate depth of

penetration. If a new probe is constructed ring elec-

trodes should be used. This would more effectively

utilize the guard system, and would allow narrower

sheets of current, to be focused thus increasing defin-

ition of the measurement. During cruise 72-A-5

March 3-10, 19'72! a single conductor STD cable was

successfully used eliminating the need for an extra

cable.

Future probes will probably have a tape recording

unit built into the housing to eliminate the necessity

for a conducting cable and thus increasing the depth

of operation.

This study work carried out in this study

firmly establishes that relati.onships exist between

electrical resistivity and various geological and

geotechnical properties of unconsolidated sediments.

Much more work must be done to determine the quantita-

tive aspect of these relationships.

CONCLUSIONS

Research into electrical resistivity logging in

unconsolidated marine sediments was undertaken because

of the necessity of developing a rapid means of cor-

relating thin sedimentary layers in a horizontal

direction. Recent marine sediments are very hetero-

geneous and changes in characteristics are frequently

encountered. within short distances.

Increasingly more and. more structures are being

erected upon the ocean floor and an increasing number

of submar ine pipelines are being laid beneath the

sea floor. All of these activities require extensive

foundation studies. The rapid change in sediment

type in a horizontal direction require s that numerous

cores be taken to make these comprehensive studies.

These operations are expensive and very time consuming.

It has been demonstrated that it is possible to

correlate individual sedimentary units by means of a

series of insitu resistivity measurements. It has

also been demonstrated that by taking and logging a

minimum number of cores that the individual layers

can be identified and correlated. At the present

stage of development neither the insitu probe nor the

core scanner are completely quantitative instruments.

132

In the course of this investigatio,. it was demonstrated

that relationships exist between the electrical resi

tivity of unconsolidated marine sediment and certain

of their geotechnical prop rties and some sedimentary

parameters. It was shown that resistivity bears an

inverse relationship to water content, void ratio,

porosity, percentage of clay and median diameter, and

that it bears a direct relationship to the formation

factor and the percentage of sand. There is a sugges-

tion that some relationship may exist between the per-

centage of- carbonate and. the resistivity. There is

also the possibility that a relationship exists between

resistivity and vane shear strength, as well as

resistivity and sound velocity. Specific gravity

appears to have no effect upon the resistivity of the

sediment. Nany more analyses will be required to

positively establish or refute these relationships.

Despite the lack of large amounts of statistical

data concerning these relationships it has been demon-

strated that electrical resistivity can be applied. as

a rapid technique for correlation of sedimentary layers

and that a much greater number of insitu readings can

be made in a given unit of time when compared to the

number of cores that can be taken.

133

In summary the objectives of this study have

been achieved.

l. Electrical resistivity logging techniques

have been developed for use in unconsolidated. aquatic

sediments:

2. Sedimentary units have been correlated by

means of resistivity logging methods.

3. Insitu logs have been correlated with core

logs from the same area.

4. Electrical resistivity ha.s been related to

geological and geotechnical properties of sediments.

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