Electrical Resistivity 

Measurements, 

Midway and Kure Atolls

GEOLOGICAL SURVEY PROFESSIONAL PAPER 680-D
Electrical Resistivity 

Measurements, 

Midway and Kure Atolls

By GEORGE V. KELLER

GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 680-D

Electrical resistivity measurements are used 

to estimate porosity of reef sediments at 

various depths

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 25 cents (paper cover)
CONTENTS

	Page 

Abstract--.-.-.---_________________________________________________________ D1

Introduction and acknowledgments___________________._______________________ 1

Sample measurements_______________________________________________________ 1

Well-logging measurements in place.__________________________________________ 4

Resistivity soundings______________________________________________________ 6

Summary. _________________________________________________________________ 7

References cited.___________________________________________________________ 8

ILLUSTRATIONS

Page

FIGURE 1. Lithologic, apparent porosity, and formation-resistivity-factor logs from the Sand Island hole cores and a resistivity log from the same drill hole___._---_-_--____________-_-___-___________-___--_--_-_--____---_- D2

2. Lithologic, apparent porosity, and formation-resistivity-factor logs from the Reef hole cores.___________________ 3

3. Scatter-plot showing correlation between formation resistivity factor and apparent porosity for carbonate samples

from Reef and Sand Island holes..________________________________________________________________ 4

4. Histograms showing distributions of computed values for apparent porosity, wet-bulk density, and apparent grain

density for carbonate samples resaturated with brine-_----____-__-______-_-___--_---_-_-_------------- 5

5. Graph showing average relations between porosity and electrical resistivity_________________________________ 5

6. Graph showing resistivity sounding data from Sand, Eastern, and Green Islands___-_______---_-_-__---_---- 6

in

GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

ELECTRICAL RESISTIVITY MEASUREMENTS, MIDWAY AND KURE ATOLLS

By GEORGE V.

ABSTRACT

Electrical resistivity measurements made on recovered core 

samples, by drill-hole logging, and by surface sounding have 

been used to estimate the porosity of carbonate sediments composing Midway and Kure atolls in the western part of the 

Hawaiian Islands. The porosities within the first few tens of 

feet of the surface generally exceed 50 percent by volume but 

decrease by a factor of two or three at a depth of 500-1,000 feet. 

The use of resistivity measurements seems to provide a better 

estimate of porosity than porosity measurements made directly 

on recovered core samples because of difficulties in collecting 

an unbiased sample of high-porosity materials.

INTRODUCTION AND ACKNOWLEDGMENTS

The drilling of two holes through reef sediments into 

underlying basalt on Midway atoll, Hawaii, has been 

described by Ladd, Tracey, and Gross (1967). In addition to the other scientific studies carried out in conjunction with this drilling project, electrical resistivity 

measurements were made of the carbonate sediments. 

Some measurements were made on the sediments in 

place using surface-based and drill-hole measurement 

techniques, and other measurements were made oil the 

samples recovered from the drill holes. The purpose was 

to provide information on the possible applicability of 

electrical resistivity surveys to the study of reef 

sedimentation.

The electrical properties of limestones, and in particular of reef limestones, have been the subject of several 

studies related to petroleum exploration. Archie (1952) 

indicated that the relation between electrical resistivity 

and water-filled porosity in carbonate rocks is essentially the same as that in sandstones, except that the 

complex pore structures in carbonates cause a greater 

variability in the resistivity. McCrossan (1961) studied 

resistivities from electric logs made in calcareous interreef deposits in Alberta and found that resistivity values seemed to be related to the proximity to reef structures. Pirson (1963) pointed out that the variety of 

pore structures in carbonate rocks should have some 

effect on the behavior of electrical properties. He indi1 Colorado School of Mines, Golden, Colo.

cated that it is important to distinguish between matrix 

porosity and structural porosity. All the work reported 

in the literature, however, has dealt with ancient carbonates, where it is reasonable to expect that recrystallization and diastrophism may have modified the original 

pore structures markedly. The drilling at Midway atoll 

offered the opportunity to study electrical properties 

of reef rocks having a minimum of such modifications. 

I am indebted to Harry S. Ladd, U.S. Geological Survey, for making these studies possible, and to the 

Hawaii Institute of Geophysics, University of Hawaii, 

under whose auspices the program in the Midway area 

was carried out. The work was supported by the National Science Foundation (grant No. GP4728) in cooperation with the U.S. Geological Survey and the U.S. 

Office of Naval Research.

SAMPLE MEASUREMENTS

Two holes were drilled on Midway atoll: one on Sand 

Island, which reached basalt at a depth of 516 feet, and 

the other just inside the reef on the north side of the 

lagoon, which reached basalt at a depth of 1,261 feet 

(Ladd and others, 1967). Cores were cut with a diamond 

drill with a rubber-sleeve core barrel, which preserved 

even the friable material for recovery at the surface. 

Most cores were 3 inches in diameter. A sea-water base 

drilling mud was used.

Approximately 160 small samples were selected from 

the 1,700 feet of carbonate core material for studies 

of electrical properties. The samples were air dried and 

cut to regular shapes for electrical properties studies. 

Samples were resaturated with a 0.5 normal solution of 

sodium chloride by vacuum boiling; that is, the samples 

were immersed in the saturating solution and placed in 

a vacuum for 12 hours. The vacuum chamber was then 

opened to atmospheric pressure so that the saturating 

solution would enter the pore structures of the samples.

After saturation, the samples were stored in the 0.5 

normal solution of sodium chloride for 2 months to 

permit the extra salts remaining in the pore structure 

from the original water content to redistribute by diffu351-472 6

Dl
D2 GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

sion. The wet samples were then weighed, and their 

volumes were determined by their loss in weight on suspension in water. The electrical resistance between opposite faces of the samples was measured by using a 

two-terminal method: the roughly cylindrical samples 

were placed between two metal plates which provided 

the current. The contact between the plates and the rock 

was made through felt saturated with salt water. Kesistance measurements were made at a frequency of 1,000 

cycles per second to avoid electrode polarization 

problems.

After the resistance measurements were made, the 

water content of the samples was determined by measuring the loss in weight on drying at a temperature of

Electrical resistivity was determined from the resistance measurements and from the dimensions of the 

samples by using the usual formula :

(1)

where R is the measured resistance, A is the cross-sectional area through which current flows, L is the thickness of the core sample in the direction of current flow,

Ww is the weight of the sample when water saturated, 

and Ws is the weight of the same sample suspended in 

water. The ratio of volume to length was used as the 

cross-sectional area of a sample because the samples 

were irregular in outline and the cross-sectional area 

could not be determined directly.

The results of these measurements are given in figures 

1 and 2. The resistivity measurements are expressed in 

terms of a formation resistivity factor, which is the ratio 

of measured rock resistivity to the resistivity of the 

saturating water. Lithologic logs are from Ladd, 

Tracey, and Gross (1967). As can be seen, there is no 

obvious correlation between values for the formation 

resistivity factor and the lithology.

Both the formation resistivity factor and the apparent porosity (the volume fraction of water absorbed 

by the samples on resaturation) vary widely. Calculated 

apparent porosities range from about 4 percent to about 

60 percent, and the formation resistivity factor correspondingly ranges from 1.7 to 150. The correlation 

between the formation resistivity factor and the 

apparent porosity (water content) for carbonate samples is given in figure 3.

LITH- DEPTH, APPARENT POROSITY. 

OLOGY IN FEET IN PERCENT 

 0 20 40

IIIIIIIIIJ

,,^I ni u

^i--1

_. . _

1L 100

200

1

300

- 400

Knn

1 ' 1 ' 

o

0

o

o °

_n

0 

0

° 

° 0 

0 

° 0

-

°° 0

o

° o o 

o 0 ° °0

_ o

g ° o

0 

0 0

° ' 0 

0 -.

\° 8°°

<* 0

 °o 

o

0

, 0 1 , 1 ,

FORMATION RESISTIVITY FACTOR

10 100 1000 0

RESISTIVITY. 

IN OHM-METERS 

2468

FIGURE 1. Lithologic, apparent porosity, formation-resistivity-factor logs, and a resistivity log from the Sand Island hole, Midway 

atoll. Lithologic log from Ladd, Tracey, and Gross (1967). See figure 2 for explanation of lithologic symbols;
ELECTRICAL RESISTIVITY MEASUREMENTS, MIDWAY AND KTJRE ATOLLS D3

LITH- DEPTH. APPARENT POROSITY, 

OLOGY IN FEET IN PERCENT FORMATION RESISTIVITY FACTOR

0 20 40 1 10 100 1000

lllllllll

i

rtT rfTT

/

/

' /

/

T^-

-V

Ziir

 i 

100

IE

200

T 

300

i

400

T

j_

z_

7~

500

1

600

700

800

900

_

~ innr>

1 ' 1

o 

o 

o

- O 0

o

° 8

o 

o

0

o

0

o

: o 

o 

o

o o

o 

o 

o

o

_ o

0

0 

0

o

0 

0

o 

o

o

0

o 

o

o

o

0 0

o 

o

o 

o

o

0

o

o

o 

o 

o

1 , 1

o 

o

o

_ 0 _

o

0 0 

°» 0 "

0 

° 0 

0 °

o 

o

0 °

°°

o 

o

0 °

o 

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 o 

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0

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1

EXPLANATION

Sediment, unl thified

Sediment, lithified

1X1

Limestone, leached 

and recrystallized

/ i

1 I

i /

Dolomite

_ L. __ .

Clay or silt, calcareous

FIGUEK 2. Lithologic, apparent porosity, and formation-resistivity-factor logs from the Reef hole, Midway atoll, 

lithologic log from Ladd, Tracey, and Gross (1967).
D4 GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

i.o

o o

00 °

§0.1

I-" 

Z 

Ld hZ 

O

o

CC 

Ld 

ho oo

OO O o O

° O

. ^°o° o

0.01 10 100

FORMATION RESISTIVITY FACTOR

FIGURE 3.- -Scatter-plot showing correlation between formation resistivity factor and apparent porosity (water content) for 

carbonate samples from, the Sand Island and Reef holes.

The low porosities and associated high resistivities 

measured on samples are probably not representative 

of rocks in place. This can be seen by considering the 

average density of the sample volumes not saturated by 

water. An apparent grain density, o-G , for the samples 

was determined from the formula

Wd-Ws

I Ww-Wd 

WW -WS

(2)

where Wa is the weight of the dried sample. The computed values for apparent grain density, wet-bulk density, and apparent porosity are given as histograms in 

figure 4. Most of the grain densities are significantly 

less than the density for calcite (« 2.7), indicating that

the non-water-saturated volumes of the samples must 

contain appreciable void space. In some samples, the 

dry void space may amount to one-third of the nonsaturated volume. Presumably, this void space is in the 

form of micropores which were not permeable to resaturation with the technique used.

WELL-LOGGING MEASUREMENTS IN PLACE

Electrical-resistivity logging surveys were run four 

times in the Sand Island hole. In all surveys meaurements were made with a standard 16-inch normal electrode array lowered in the drill hole. Mud resistivities 

were measured each time borehole surveys were run; the 

mud resistivity ranged from 0.25 to 0.31 ohm-meter. A 

composite electric log is shown in figure 1, along with 

the sample measurement logs.
ELECTRICAL RESISTIVITY MEASUREMENTS, MIDWAY AND KURE ATOLLS D5

30

20

10

01 

1.4 1.8 2.2 2.6 

APPARENT GRAIN DENSITY

J 

3.0

30 r 

I

u

LJ 20 

en

10

1.4

30 f 

1.8 2.2 2.6 

WET-BULK DENSITY

3.0

20

10

20 40 60 

APPARENT POROSITY. IN PERCENT

80

FIGURE 4. Distributions of computed values for apparent porosity, wet-bulk density, and apparent grain (framework) density for carbonate samples resaturated with brine.

Several features of the log deserve consideration. The 

resistivity is approximately 2.0 ohm-meters in the upper 

part of the hole and decreases abruptly to 1.1 ohmmeters at a depth of 76 feet. This upper zone of high 

resistivity may represent a lense in which sea water 

has been diluted by rain water. In the rest of the section, 

the electric log shows marked variations in resistivity 

over short depth intervals, superimposed on a trend 

of increasing resistivity with increasing depth. The log 

can be divided into three sections within which the 

resistivity is nearly uniform:

Depth (feet) Ohm-meters 

90-180 __________ - 0. 7 

180-330 __________ I- 2 

330-505 _______ 3. 8

The rocks at these depths are probably saturated with 

sea water of the same salinity throughout, and, thus, the 

variations hi resistivity reflect changes in the water content (or porosity) and texture of the rock. If average 

resistivity for a depth interval is divided by the 

resistivity of sea water (0.25 ohm-meter), the average 

resistivity-porosity relations shown hi figure 5 may be 

used to estimate the porosity:

Depth (feet)

90-180. 

180-330. 

330-505.

Porosity 

derived from 

log (percent)

51

36

17

Average 

porosity for 

cores (percent)

36

23

16

1.0

0.1

0.01 I I I I I I

10

I I I 

100

FORMATION RESISTIVITY FACTOR

FIGURE 5._Average relations between porosity and electrical 

resistivity, based on the measurements summarized in figure 

3. Number of measurements in parentheses.
D6 GEOLOGY OF THE MIDWAY AKEA, HAWAIIAN ISLANDS

Note that the porosity value derived from the electric 

log comes progressively closer to the average porosity 

value determined from core samples at greater depths, 

where porosities are lower. This reflects the fact that it 

is more difficult to obtain representative samples of 

high-porosity material than of low-porosity material. 

It may also reflect the possibility that the unsaturable 

porosity noted in the measurements on core samples is 

the porosity which disappears first on reduction of 

porosity.

RESISTIVITY SOUNDINGS

Resistivity soundings were made at four localities 

two on Sand Island, one on Eastern Island of Midway atoll, and one on Green Island of Kure atoll using 

surface-based techniques. A modified single-pole electrode array was used in making these soundings; current 

to the ground was provided at two fixed electrodes, about 

1,000 feet apart, and the electric field was measured with 

a second pair of electrodes, which was moved out along 

a traverse extending from one end of the current pair.

In all soundings, the current electrodes were placed on 

the beach, a few feet seaward from the low-water line. 

This provided a contact resistance low enough that a 

single automotive storage battery was sufficient to provide a current of 10 amperes to the ground. Most of the 

electric-field measurements were also made along the 

beach but about 20 feet inshore from the wave line to 

avoid electrical noise generated by wave action on the 

beach. Attempts to measure voltages farther away from 

the beach were not successful. With the equipment used 

in these surveys, the high contact resistances met in the 

dry sand soil inland led to unreasonably high noise 

interference.

It might be expected that the presence of sea water 

laterally away from the sounding lines would have 

some effect on the results. The depth of sea water within 

a distance equal to the maximum electrode separations 

('as much as 2,000 ft), however, was no more than several tens of feet, and the resistivity (0.25-0.30 ohmmeter) was lower than that of the rocks forming 

the islands by a factor of no more than three or four.

1 10 100 1000

ELECTRODE SPACING. JN FEET

FIGUEE 6. Resistivity sounding data from Sand, Eastern, and Green Islands. A» north shore of Sand Island, 

near the Sand Island drill hole; X, northeast peninsula of Sand Island, Midway; , northwest shore of 

Eastern Island, Midway; O, north shore of Green Island, Kure.
ELECTRICAL RESISTIVITY MEASUREMENTS, MIDWAY AND KURE ATOLLS D7

As a result, one might expect the sea water to have some 

effect on the measurements made at spacings of less 

than 100 feet.

The resistivity sounding data are presented in figure 6; apparent resistivity values computed from the 

measurements are plotted as a function of the distance 

from the end of the current electrode pair to the midpoint of the receiving electrode pair using a bilogarithmic graph. Because the short-spacing parts of the 

sounding curves may be affected by conduction through 

the sea, and because very few theoretical curves are 

available for interpreting data obtained with the nonstandard array used in this study, the sounding curves 

cannot be interpreted in as much detail as is usual for 

resistivity surveys.

The first sounding (A in fig. 6) was made along the 

north shore of Sand Island about one-fourth mile from 

the Sand Island drill hole. The data indicate a surface 

layer 24 feet thick which has a resistivity of 0.71 ohmmeter. This layer overlies a second layer extending to 

a depth of approximately 660 feet which has a resistivity of 0.97-1.15 ohm-meters. The resistivity for the zone 

below 660 feet is at least 9 ohm-meters. The more resistant rock below 660 feet may be volcanic rock like 

that found at a somewhat shallower depth in the Sand 

Island drill hole.

The second sounding (X in fig. 6) was made along 

the northeast extension of Sand Island which was built 

up from fill material. The near-surface interpretation 

is very nearly the same as that at the first location; the 

surface layer is 27 feet thick and has a resistivity of 

0.75 ohm-meter. This is underlain by a second layer 

which has a resistivity of 1.30 ohm-meters. The marked 

decrease in apparent resistivity at larger spacings is 

probably caused by a metal sea wall crossed by the 

traverse line, rather than by changes in resistivity at 

depth.

The third sounding ( in fig. 6) was made along the 

northwest shore of Eastern Island. Here, the surface 

layer is 42 feet thick and has a resistivity of 0.90 ohmmeter, which is slightly higher than that of the surface 

layer at the Sand Island survey locations. This surface 

layer is underlain by a layer which has a resistivity of 

1.6 ohm-meters and extends to a depth of 320 feet. The 

third layer has a resistivity of at least 4 ohm-meters. 

This third layer may be denser coral rock like that in 

Sand Island drill hole or it may be volcanic rock.

The fourth sounding (Q in fig. 6) was made along 

the lagoonside beach at Green Island, Kure. The data

are considerably different from those obtained at Midway. The surface layer has a very low resistivity, 0.43 

ohm-meter, and extends to a depth of 42 feet. The surface layer is underlain by moderately resistant rock 

which has a resistivity of 2.3 ohm-meters.

These resistivity values combined with the correlation between resistivity and porosity shown in figure 5 

may provide the best estimate of the actual porosity, 

averaged on a large scale, for the rocks forming the 

islands, if it can be assumed that all the rocks are saturated with normal sea water. On this basis, the porosities inferred from the resistivity sounding data are:

Depth Resistivity Inferred

Sounding locality

Sand Island _ _ _ ___.

North shore ___ _.

Northeast peninsula__ __ _ ___-

Eastern Island. ___ _ .

Green Island. __ ___.

(ft)

0-24

24-660

0-27

27-?

0-42

42-320

>320

0-42

>42

(ohm-meters)

0.71

. 97-1. 15

.75

1. 30

.90

1.6

4

.43

2.3

porosity

0.51

. 37-. 40

.49

.34

.44

.30

.17

.65

.24

At the sounding locality near the Sand Island drill 

hole, the porosities inferred from the resistivity soundings are in excellent agreement with the values inferred 

from the electric log measurements. This agreement 

supports the theory that the averages determined from 

core studies are not significant.

SUMMARY

Resistivity studies on samples and on rocks in place 

using conventional well-logging and surface techniques 

may be effective in determining the porosity of the 

rocks forming coral atolls. Porosities measured on samples are not as reliable as those determined in place 

through resistivity measurements, because of (1) the 

difficulties involved in sampling high-porosity materials without bias toward the low-porosity components, 

and (2) the blind porosity contained in some of the 

coral samples.

The porosity of the material in the first few tens of 

feet is surprisingly high, greater than 50 percent of the 

rock volume in three of the four localities considered. 

This porosity is almost certainly of two forms intergranular and intragranular. The near-surface material 

in the survey areas is largely detrital and is composed
D8 GEOLOGY OF THE MIDWAY AREA, HAWAIIAN ISLANDS

of highly angular fragments, so that considerable intergranular porosity exists. Intragranular porosity also 

exists in the clastic material forming these sediments, 

because the material is not old enough for this porosity 

to have been significantly reduced. As a result of the 

combination of the two porosities, a rock with some 

competence may have a porosity in excess of 50 percent.

The porosity decreases with depth by a factor of two 

or three within the first 500-1,000 feet.

Porosity differs among the areas where these measurements were made, but no information is available 

about the possible meaning of these differences.

REFERENCES CITED

Archie, G. E., 1952, Classification of carbonate reservoir rocks 

and petrophysical considerations: Am. Assoc. Petroleum 

Geologists Bull., v. 36, no. 2, p. 278-298.

Ladd, H. S., Tracey, J. I., Jr., and Gross, M. G., 1967, Drilling 

on Midway atoll, Hawaii: Science, v. 156, no. 3778, p. 

1088-1094.

McCrossan, R. G., 1961, Resistivity mapping and petrophysical 

study of Upper Devonian inter-reef calcareous shales of 

central Alberta, Canada: Am. Assoc. Petroleum Geologists 

Bull., v. 45, no. 4, p. 441-470.

Pirson, S. J., 1963, Handbook of well log analysis for oil and 

gas formation evaluation: Englewood Cliffs, N. J., PrenticeHall, Inc., 326 p.

U.S. GOVERNMENT PRINTING OFFICE: 1969