Directional Resistivity 

Measurements in Exploration 

For Uranium Deposits on the 

Colorado Plateau

GEOLOGICAL SURVEY BULLETIN 1083-B

This report concerns work done on behalf 

of the U.S. Atomic Energy Commission 

and is published^ with the permission of 

the Commission

Directional Resistivity 

Measurements in Exploration 

For Uranium Deposits on the 

Colorado Plateau

By GEORGE V. KELLER

EXPERIMENTAL AND THEORETICAL GEOPHYSICS

GEOLOGICAL SURVEY BULLETIN 1083-B

This report concerns work done on behalf 

of the U.S. Atomic Energy Commission 

and is published with the permission of 

the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE^ WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C. - Price 20 cents (paper cover)
CONTENTS

Page*

Abstract..___-----------_---------------------------------------- 37

Introduction...-.___________-_________-___-_-__-----_------------- 37

Acknowledgments. _____________-_________----__----__------------- 39-

Measurements in the Spud Patch area__-____-_--------_------------- 40>

Location and geologic setting,.,________________________________ 40<

Previous work__________________________________ ____________ 40>

Methods of measurement____--____----_--..------_-------------- 40-

Measurements in the White Canyon district---.------.--------------- 59

Location and geologic setting,____________-.__-____------------- 59

Conclusions____-______________________________--___--_---------- 65

Literature cited.__________-_-_____-__-___---____------------------ 72

ILLUSTKATIONS

Page

FIGURE 9. Index map of southwestern Colorado and southeastern Utah. 39

10. Single-pole method of measuring directional variations in

resistivity._________________________________________ 42

11. Curves showing ratio of apparent to true resistivity and

normal-potential-drop ratio____________________________ 43

12. Numerical favorability versus resistivity-thickness product-.- 46

13. Contour map showing favorability for the occurrence of uranium ore in the Spud Patch area._____________________ 47

14. Lee partitioning method of measuring directional variations

in resistivity..-__________________________________ 48

15. Example of the treatment of data obtained with the Lee

partitioning method________________-_________-______ 51

16. Patterns of directional resistivity and favorability for typical

drill holes._________________________.___ 52

17. Frequency distribution of favorability and resistivity____- 54

18. Correlation coefficients between resistivity and favorability-- 55

19. Frequency distribution of the angle between direction of

excess resistivity and favorability____________________ 56

20. Hypothetical favorability distribution about an average drill

hole________________________________ 58

21. Typical electric log of a section of the Chinle formation. ____ 61

22. Effect of a rim on the standard Lee arrangement.__________ 62

23. Effect of a rim on the inverted Lee arrangement_________ 64

24. Examples of resistivity patterns corrected for rim effects. _ _ _ 65

25. Resistivity patterns obtained with various spacings about

the same drill hole__________________________________ 66

26. Directional resistivity patterns for a channel in the B-claim

area, White Canyon district_-_________-_-_-___________ 67

27. Equipotential traces under different assumed conditions in

a uniform ground_______-_--_________-__-_--_-____-_ 69

28. Proposed method of determining directional-resistivity variations______________________________________________ 71
IV CONTENTS

TABLES

Page 

TABLE 1. Summary of resistivities determined from electric logs in the

Spud Patch area_________________________ _ 41

2. Resistivity trends determined from single-pole data._ ___ 48

3. Reliability of predictions by single-pole measurements. _____ 49

4. Summary of a statistical study of directional resistivity data

obtained with the Lee partitioning method ______________ 53

j>. The angles between the directions of greatest favorability 

and greatest resistivity measured with the partitioning 

system. ______ _______________________________________ 57
EXPERIMENTAL AND THEORETICAL GEOPHYSICS

DIRECTIONAL RESISTIVITY MEASUREMENTS IN

EXPLORATION FOR URANIUM DEPOSITS

ON THE COLORADO PLATEAU

By GEORGE V. KELLEB

ABSTRACT f

A study of the electrical properties of the Morrison formation in the Uravan 

mineral belt of the Colorado Plateaus province indicated that there is a significant 

correlation between electrical resistivity and the relative favorability for occurrence of ore. The differences in resistivity were not large enough to provide a 

recognizable target for standard resistivity field methods, especially where the 

ore-bearing sandstone member is more than a few hundred feet deep. Measurement of resistivity trends by placing one electrode in a drill hole and spreading 

the others out radially on the surface seemed to offer a means of exploiting the 

resistivity-favorability correlation.

Field tests of such directional-resistivity measurements were made in the Spud 

Patch area in San Miguel County, Colo., and the White Canyon district, San Juan 

County, Utah. In the Spud Patch area two methods were tried; in one a current 

electrode was placed in the drill hole, and in the other a potential electrode. 

The second method was the more tedious but provided the more readily interpretable results. A comparison of the resistivity trends thus determined with 

the favorability estimated from geologic indexes indicated that directionalresistivity methods could predict the location of favorable areas at distances of 

600-1,000 feet with a high degiee of success.

In the White Canyon district directional-resistivity measurements were made, 

on the assumption that the conglomerate which is found in many channels filled 

with the Shinarump member of the Chinle formation has a high resistivity. 

The measurements were successful in tracing the channel conglomerate where 

surface conditions were favorable.

INTRODUCTION

The U.S. Geological Survey has carried on a research program to 

develop practical and economical methods of exploration for the 

uranium ores of the Colorado Plateau. Although ore occurs in many 

formations on the Colorado Plateau, most of the important deposits 

are in the Morrison formation of Late Jurassic age and in the Chinle 

formation of Late Triassic age. In the Morrison formation in the 

Uravan mineral belt, the ore bodies form irregular tabular masses 

within the Salt Wash sandstone member. In the Chinle formation,

37
38 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

in the White Canyon district, the ore is localized in the Shinarump 

member in channel scours that have been cut into the underlying 

Moenkopi formation of Early and Middle (?) Triassic age.

There are many guides to exploration, but the only positive method 

is drilling: first, at wide spacing to classify areas according to relative 

favorability for occurrence of ore on the basis of geologic information, 

followed by drilling at smaller spacing to locate and outline ore 

deposits.

In hopes of delineating favorable areas and locating ore more 

rapidly and at less expense, various geophysical exploration methods 

have been tested. Electrical-resistivity surveys over known ore deposits indicated that although ore cannot be located directly, thick 

sections of the Salt Wash sandstone member, which have been found 

to be the most favorable areas for ore occurrence (Weir, 1952), can 

be traced at depths of a few hundred feet (Davis, 1951). These 

results were confirmed by electric-logging studies (Keller, 1959) which 

showed a direct correlation between favorability and the product of 

sandstone thickness and electrical resistivity. It does not seem likely, 

however, that standard electrical-resistivity surveys can be of much 

assistance in exploration for deposits at depths of more than 200 feet. 

However, the existence of a small but significant resistivity anomaly 

associated with the favorable areas made it desirable to investigate 

the use of less conventional resistivity-exploration methods.

As a result of his work hi southern Ohio, F. W. Lee (written communication, 1948) suggested that resistivity anomalies of the size of 

those associated with favorable ground could be detected to depths 

of 4,000 feet by tracing the direction of resistivity variations from 

measurements made with electrodes in a drill hole and on the surface 

around a drill hole. According to Lee, "there often is a decided 

advantage in making inhole potential observations where there is an 

underground condition which greatly modifies the electrical-potential 

distribution. It will be seen that such inhole measurements will 

assist in determining the location of the geologic body in question, 

whereas surface observations will produce an entirely different picture."

The use of directional-resistivity surveys in conjunction with widespaced drilling to delineate areas favorable for uranium and vanadium, 

if shown to be reliable, could reduce the number of drill holes necessary 

and thus reduce the cost and time involved.

Field tests of the method were made in the Spud Patch area, San 

Miguel County, Colo. (fig. 9), where the Morrison formation is widely 

exposed and also in the White Canyon district, San Juan County, 

Utah, where there are ancient channels. The Spud Patch area was 

chosen for the first tests because there is considerable geophysical 

information about the Spud Patch area available from earlier surveys
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 39

t

i

>*J\ )

« <$% YiL, \j*«. s^r*-^ 5* (^x |Xf

^ J l

\^

\Z* V^tH.^><"v"

VV p -^VV( A

// 1

s-J/ I

)rr^

f ^^^^~rite

l / /

^^r

/ X SPUD PATCH

DWhite Canyon /) ' A ^7 

^^^ Monticello^ ^ 1 ® f

\. _ /^ ^T*^ Dove Creek \ j^^jL*. : \ /

x FREY CANYON |

V-x

\ Is \_ J ^

~^^L ^^^Bluff

JVAN .a \ ^^"^^^^-^ "- x 

^^-N /^ /^-^ ^N

^Mexican Hat

/r

UTAH sjT

ARIZONA ^ 

.f 10 0 10 20 80 Miles 

& iii i i i

1

^ O \C\rt f > 

, u Corte^M@^

©

I COLORADO

//NEW MEXICO

FIGUEE 9. Index map of Spud Patch and Frey Canyon areas of southwestern Colorado and southeastern 

Utah showing where field measurements were made.

(Davis, 1951; Keller, 1959); the terrain is flat, soil covered, and open; 

and there are many drill holes in the area ranging in depth from 100 

to 300 feet.

ACKNOWLEDGMENTS

The work described in this report was part of a program carried 

on by the U.S. Geological Survey on behalf of the Division of Raw 

Materials, U.S. Atomic Energy Commission. The assistance of Leo 3. 

Miller and other members of the Atomic Energy Commission at the 

Frey Canyon camp is gratefully acknowledged.
40 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

MEASUREMENTS IN THE SPUD PATCH AREA 

LOCATION AND GEOLOGIC SETTING

The Spud Patch area is in the southernmost part of the Uravan 

mineral belt (Fischer and Hilpert, 1952) on the Egnar Plain about 

5 miles north of Egnar, Colo. The Morrison formation is widely 

exposed in this region and dips about 10° northwestward into the 

Dolores and Disappointment Valleys. In the Spud Patch area the 

Salt Wash sandstone member is overlain by mudstones and conglomerates of the Brushy Basin shale member ranging in thickness from 

almost zero to several hundred feet.

PREVIOUS WORK

There are a number of worked-out mines along the exposed rim of 

the Salt Wash. In 1949, 137 exploratory holes were drilled in 

an attempt to trace the ore-bearing zones away from these mines. 

In 1950 and 1951 in a more detailed program of drilling, more than 

400 holes were drilled in an area approximately 3 by 5 miles.

An electrical resistivity survey was made in part of the area in 

1950 (Davis, 1951). The results indicated that the favorable areas 

could be determined fairly well by empirical methods of interpretation. 

In 1952, electrical well logs were made in 100 drill holes in the same 

area (Keller, 1959). Of these, 44 electric logs showed a complete 

thickness of the ore-bearing sandstone of the Salt Wash. These logs 

are summarized in Table 1.

On these logs the area under the resistivity curve through the sandstone member, which ordinarily is ore bearing, was planimetered to 

find the product of resistivity and thickness. The average resistivity 

was determined by dividing this area by the thickness of the sandstone. 

The classification according to favorability was made by geologists 

on the overall appearance of the cores taken from the drill holes. 

The semifavorable class, however, is not intermediate to the favorable 

and unfavorable classes necessarily but includes those drill holes which 

did not fit in with the geologic guides that were used in determining 

favorability. On the basis of these figures, it was believed that 

directional-resistivity variations could be used to predict favorability 

trends, at least in the Spud Patch area.

METHODS OF MEASUREMENT

A standard electric-logging unit was modified for use in measuring 

directional-resistivity variations. At first a single-pole electrode array 

(fig. 10) was used. This consisted of an inhole current electrode, 

Ci, and a current-return electrode, (72 , placed on the surface at a 

considerable distance from the hole. Potential-pickup electrodes, 

PI, P2f and P3, were then placed along a radial line from the drill
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 41

TABLE 1. Summary of resistivities determined from electric logs in the 

Spud Patch area

Drillhole

Depth

interval of

Salt Wash

sandstone

member

(feet)

~*"r*fl5lF

Resistivitythickness

product

(ohm-m-feet

Average

resistivity

(ohm-m)

Favorable drill holes

SP-142. _____ T ____________ . __________ . .....

151- __________________ . _ - __ - ____ - __

210. _______________ ... ____________ .. ....

357- .. . . - ___ ......

293 _______ . ____ ____ . _____ . __ .... .....

245 _________ - _________ . _ - ___ - ___ ... _

254 

68 . . . .... .-.... ...-.... ..

153

123 

251 _______ . ____________ - _______ .... 

348 

131 ... __ .... ___ - ____ . __

33. 

1 _______ . _____________ . __________ ...

262 

60 

284 _________ . ________ . __________ . .....

323 

306. _ ... ___ .... ____ . _ __ ... ..

42. ______ __________________ . _ . _ ....

117 _______ - ____________ . _____ - __ - __

80. ____ - _ _________ __ ..... ..........

38 

87-160

84-184

64-172

83-138

71-119

14-74

41-109

25-104

54-131

60-124

14-58

142-195

64-210

90-164

26-64

21-69

56-148

110-186

32-60

12-50

76-132

89-149

54-113

32-80

29,600

25,400

22,700

21,100

20,100

19,700

19,200

18,200

17,400

16,800

14,900

14,900

14,400

14,400

14,300

14,200

13,700

13,600

13,300

13,200

13,200

10,400

10,200

7,620

16,400

405

254

210

293

303

270

283

179

285

220

229

207

108

232

223

214

167

179

221

330

236

173

172

136

230

Semi-favorable drill holes

SP-5- ~ -

48.. ________ . _____ ....... _____ ..... ........

114 . __ - _ - _______ . __ . ____ ..... .........

125 _________ - ___ - __ - ___ - _ 

282

145 - ____ - _ ___________________ - ...

124. ___ ..... _________ . _______ . ___ . _ . _ 

294 . .............

317 ____ ... __ - _________ . . 

147 _______ . __________ . ____ . .............

220 _______ __ . ____ . _____ . ___ ... .....

82 _______________________ . _____ ...

47-134

53-145

65-137

38-133

45-116

88-155

77-110

61-115

40-93

81-126

69-89

19-fiQ

27,400

19,300

18,600

15,500

14, 100

13,500

11,500

10,900

10,300

10,100

4,270

3,800

13,300

206

241

273

218

199

224

201

203

165

207

213

82

203

Unfavorable drill holes

SP-10 --...- - ..... .....

8. ... ..... .... .... ... ... .... ... .... ... ... ...

77. _______________ - _ ... ___ _ ...... __

12.. .._.... ........ ..............................

201 

28. __ - __ . __________ . ___________ . _ .

118. __ - __ - __________________ . _ .

37-44

20-53

32-58

24-128

35-104

36-57

18,700

9,330

8,520

7,460

4,770

4,600

4,260

4,030

7,710

283

283

196

287

46

75

203

176

180

hole at distances of 50, 100, and 200 feet, as the ore-bearing sandstone 

in the Spud Patch area is at a depth of 50 to 150 feet. The electrodes 

were lead hemispheres, 5 niches in diameter, placed in shallow holes 

filled with a solution of sodium chloride. A constant current, com503213 59 2
42 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

 RECORDER - rRECORDER

Current is passed between the inhole 

electrode, Ci, and the surface electrode 

Cz, which is some distance away. The 

potential drops between electrode pairs 

Pi-Pz and PZ-PS are recorded

21 CYCLES PER SECOND 

COMMUTATED CURRENT

C2

O

FIGUBE 10. Single-pole method of measuring directional variations in resistivity.

mutated at 21 cycles per second, was passed from the inhole electrode 

to the distant current-return electrode as the inhole electrode was 

raised through the drill hole. The potential drop between pairs of 

the surface electrodes was automatically recorded as this happened, 

first between the inner pair of electrodes (at 50 and 100 feet) and then 

between the outer pair of electrodes (at 100 and 200 feet). The 

measurements were repeated in 8 positions about the drill hole on 

lines 45° apart. These potential measurements closely resembled the 

curves ordinarily obtained in electric logging. The recorded potentials 

were large when the inhole electrode was opposite a sandstone of high 

resistivity and low when it was in a mudstone of low resistivity.

The relation between the recorded potential differences and ground 

resistivity varies as the electrode, Ci, is moved through the drill hole 

even where there is a uniform earth around the drill hole. If the 

inhole electrode were at the surface, the resistivity, p, and recorded 

potential difference, E, would be expressed by

where E2i is the potential difference from PI to P2, I is the current, 

p is the electrical resistivity of the ground, and a is the spacing between 

the electrodes. When the electrode is lowered into the drill hole, 

the relative distances to the several surface electrodes vary, and the 

equation becomes

. p

2ira 1/2 I

(2)

where d is the depth of the inhole electrode below the surface. This 

means that as the inhole electrode is lowered in the drill hole, the
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 43

potential difference generated at the pickup electrodes decreases, as 

shown in figure 11. The solid curve in figure 11 shows the relation 

between the apparent resistivity calculated by use of equation 1 and 

the true resistivity, as a function of the ratio of the depth of the 

inhole electrode to the distance between the drill hole and the inner 

potential electrode.

Because of the inverse relation between the potential about a 

single-pole current source and distance, the voltage drop between the 

2 potential electrodes at 50 and 100 feet, and those at 100 and 200

o

I

o

S 0.5 

o

cc

LU 

_J

<

1.0

- 1.5

t

LU

o

Q 

O

o:

55

O

X

o 

o

< 

o:

2.0

2.5

3.0

'$>

\m

0.2 0.4 0.6 0.8 

RATIO OF APPARENT TO TRUE RESISTIVITY

1234 

NORMAL POTENTIAL-DROP RATIO

1.0

FIGUBE 11. Curves showing ratio of apparent to true resistivity and normal-potential-drop ratio.
44 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

feet would be the same in uniform ground when the current electrode, 

Ci, is at the surface. The ratio of the voltages between the outer 

and the inner pairs of pickup electrodes will ordinarily be somewhat 

greater than unity if resistivity increases with depth. As the current 

electrode is lowered in the drill hole, the voltage between the outer 

pair of electrodes should decrease more slowly than the voltage between 

the inner pair, so that the ratio of the outer to inner voltages should 

increase as indicated by the dashed curve in figure 11. The field 

data did not conform to these predictions even though resistivity 

increased with depth. In order to apply theoretical techniques to the 

interpretation of these data, it would be necessary to calculate curves 

for" 2 or 3 layers rather than a uniform earth, and this would be a 

complicated procedure.

; As there was not a theoretical basis for the interpretation of the 

single-pole data, an empirical approach was tried. The recorded data 

were plotted on polar graphs, and the results were compared with the 

cfistribution of f avorability in the Spud Patch area. On any particular 

sjet of logs, a depth within the ore-bearing sandstone was chosen, and 

corresponding voltages were read for each pair of pickup electrodes 

and for each orientation. These voltages, and the ratio of the outer 

voltage to the inner voltage, were plotted as a function of direction 

on polar graphs. On many of these graphs a maximum direction can 

tye inferred from the distance-resistivity patterns, but in general the 

results were discouraging. The directions shown by the graphs of 

tjhe voltage between the outer electrodes, the voltage between the 

ikmer electrodes, and the ratio of the two voltages were not always 

donsistent.

| In spite of that fact, an attempt was made to correlate each set 

<^f data with favorability. Drill logs were available from each of the 

drill holes for comparison with the geophysical measurements, but 

Only the qualitative indications of favorability such as "favorable," 

fsemifavorable," and "unfavorable" had been assigned to these logs. 

j)oris Weir (1952) has pointed out the advantages of a favorability 

scale consisting of weighted numerical values for each of the geologic 

factors used as ore guides, and I have followed her suggestion in 

determining favorability indexes for the drill holes in which directional 

Resistivity measurements were made and for adjacent drill holes. 

; In the favorability scale as originally set up, quantitative measurements of sandstone thickness, thickness of the gray-green mudstone 

at the base of the sandstone, the ratio of red to green mudstone within 

the sandstone, and qualitative estimates of the relative amount of 

crossbedding and the carbon and iron oxide spotting the sandstone 

were used. For the present work only the summaries of the geologic 

logs were available, and the only factor known quantitatively was
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 45

the sandstone thickness. For this reason the favorability indexes are 

subject to errors resulting from a shift in emphasis on the features 

recorded in the log summaries for each of the 3 drilling yeafs.^ In 

assigning numerical values to the different factors, the following 

weights were used:

1. Sandstone thickness: Zero points for thickness of less than 30 feet 

to 8 points for thicknesses of more than 200 feet.

2. Thickness of gray-green mudstone at the base of the sandstone: 

Zero for none to 8 points for a "very thick" unit.

3. Color of the mudstone splits within the sandstone: Zero points for 

all red to 8 points for all green.

4. Kadiation anomalies: Zero points for none, 4 points for a trace, 

6 points for a trace to 0.1 percent eU, and 8 points for more 

than 0.1 percent eU.

5. Appearance of the sandstone: I point for "poor," 3 points fo£ 

"fair," and 6 points for "good."

6. Presence of carbonaceous material: Zero points for none, 1 point 

for "scarce," 2 points for "some," and 4 points for "abundant."

7. Presence of iron-oxide spotting: 2 points, if mentioned.

Numerical indexes were determined in this way for 57 drill holes 

in which electric logs had been run. These indexes do not always* 

agree with the geologist's qualitative estimate of favorability, but 

the correlation with electric log data is excellent (fig. 12).

In order to compare the favorability with the directional-resistivity 

patterns, a contour map of favorability was prepared from the numerical indexes (fig. 13), then a circle with a radius of 600 feet was drawn 

about each drill hole in which measurements had been made, and the 

highest value of favorability intersected by this circle was used to 

define the direction (or trend) toward maximum favorability (column 

4, table 2).

It might be expected that the best correlation between favorability 

and resistivity would be obtained by using the favorability at about 

200 feet, as 200 feet is the maximum electrode spread. However, 

the favorability contours cannot be determined on such a fine scale. 

The distance of 600 feet was selected because it is the average spacing 

of the drill holes considered in preparing the favorability map. As 

favorable areas probably have dimensions of several thousand feet, 

it is reasonable to expect that favorability trends controlling resistivity variations over distances of 200 feet will be reflected on the 

favorability map at 600 feet.

To estimate the reliability of the directions predicted by the resist 

tivity data, the angle between the resistivity trend and the direction 

toward greatest favorability was measured for each set of data. II 

these two directions were randomly oriented with respect to each
46 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

RESISTIVITY THICKNESS PRODUCT 

(ohm-meters X feet)

10,000 20,000

10

^

-J 

CD

I

JJ 20

5

30

\\

* 1P\ \

Average for 11 > 

unfavorable holes \ Average for 14 

\ semifavorable holes

\

\

Average for 24 v 

favorable holes \

\

\

\

\

V

Average for 8 " 

mineralized holes

\

\

FIGURE 12. -Numerical favorability versus resistivity-thickness product.

other, then for large numbers of measurements the absolute value of 

the average angle of error would be 90°. An angle of less than 90° 

would indicate some correlation between the resistivity trend and 

direction of favorability. The results of such a study for the singlepole data are given in table 3.

The results of the single-pole group of measurements were negative. 

There is but 1 chance in 11 that any of the resistivity parameters
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 47

MAGNETIC 

NORTH

EXPLANATION

Drill hole and number

 25 

Favorability contour, approximately located. 

Contour interval 5 units of favorable 

geologic factors

Most favorable areas

1000 1000 2000 Feet

FIGUBE 13. Contour map showing favorability for the occurrence of uranium ore based on geologic

factors in the Spud Patch area.

studied give any better than a random estimate of the direction 

toward maximum favorability.

Because of the discouraging results, the single-pole method was 

discontinued in favor of the Lee partitioning system. In this system, 

four equally spaced surface electrodes are placed on a line centered
48 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

TABLE 2. Resistivity trends determined from single-pole data 

[All trends are expressed as angles measured counterclockwise from magnetic north]

Drill hole

8P-1 ............... ................ .........

8 _ . 

9.... _ . _ 

10 

19 _

31 

37 

38.. _ ... 

41- _ - _ __ .

42 

48. ____ - 

60. _ .

111 

114...... _ ....... _ ..... _ ....

116 

118 

125 .

131 . 

143 

148 

243 

244. 

260

Maximum 

trend of

En

55

130 or 340

130

75

70

225

225

215

120

195

45

45

100

225

335

120

60

120

20

30

265 or 15

100 or 270

Maximum 

trend of

EM

10

70

25

25

110

45

It 100

7 315

105

70 or 250

185

200

220

220

30

300

45

130

15

35

120

250

Maximum 

trend of 

ratio EM, En

345

60

330

20

250

185

45

90

330

45

240

170

195

135

45

40

260

45

40

270

45

310

240

Maximum 

trend of 

favorability

80

240

225

280

60

272

330

350

320

355

98 or 250

140

280

348

302

0

235

275

285

315

275

300

88

about a drill hole (fig. 14). A fifth electrode is then placed in the 

drill hole opposite the formation being studied. Current is passed 

between two of the surface electrodes, one on either side of the drill 

hole; and the potential drop is recorded between the inhole electrode 

and the remaining surface electrodes, first on one side of the drill 

hole and then on the other. Measurements are made at 6 electrode 

orientations, or on lines 30° apart around the drill hole, so that 

resistivities are obtained hi 12 directions.

A group of 38 directional-resistivity measurements were made with 

this system in the Spud Patch area. The inner surface electrodes

21 CYCLES PER SECOND 

COMMUTATED CURRENT

r- RECORDER RECORDER

Cz

O

This method differs from the single-pole 

procedure, fig 10, in that four equally 

spaced surface electrodes are placed 

on a line centered about a drill hole. 

The potential drops between Po-P\ 

and Po-P2 are recorded

FIGURE 14. Lee partitioning method of measuring directional variations in resistivity.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 49

TABLE 3. Reliability of predictions by single-pole data

Voltage measured between inner electrodes _ . 

Voltage measured between outer electrodes.....

Mean error

angle

95 

94 

87

Standard

deviation of

error angle

110 

108 

103

Standard

deviation of

mean error

(degrees)

23 

23 

22

Probability

of significant

prediction

0.173 

.138 

. . 107

were placed at distances of 67 feet on either side of the drill hole, and 

the outer electrodes were placed at 200 feet. The same electrode 

spacings were used about all 38 drill holes because the Salt Wash was 

at about the same depth throughout the area. During the measurements a constant current was passed first between the outer two 

surface electrodes Ci and C2, and the potential drop was measured 

from the inhole electrode P0 to the inside surface electrodes Px or P2. 

In many places a repeat set of measurements was made with the 

positions of the surface current and potential electrodes being reversed 

 that is, a constant current was passed between the inner two surface 

electrodes, and the potential drop was measured between the inhole 

electrode and the outermost surface electrodes. In all these logs it 

was found that the recorded voltage varied as the inhole electrode 

was raised through the drill hole, being high in zones of low resistivity 

and low in zones of high resistivity. As 2 measurements, oriented at 

180° to each other, were made with 1 electrode spread, it would be 

expected that the sum of these 2 measurements would be constant, 

as it would be the potential drop between a pair of surface electrodes 

PI and P2. This is not so; the sum is greater when the inhole electrode 

is in a zone of low resistivity than when it is in a zone of high resistivity. Considerable effort was spent during the fieldwork to determine the cause of this discrepancy in the data, but no instrumental 

cause, such as current leakage, could be found. Subsequent laboratory work (Keller and Licastro, 1959) has indicated that the cause 

lies in the high dielectric constant and very low conductivity of 

sandstone rocks in the Morrison. A commutator is used with the 

power supply to provide a 21 cycles-per-second square-wave current 

to the ground. The voltage between the pickup electrodes is rectified 

by a second set of commutator rings coupled mechanically to the 

current supply rings, before the signal is recorded. In this way the 

polarity of the pickup is reversed at the same instant the current 

polarity reverses. If there were no phase shift in the ground, as in 

ohmic conduction, the rectification by this procedure would be 100 

percent efficient. : However, if there is a phase shift in the ground, as 

there is when conductivity is low and the dielectric constant is high, 

then the commutator will reverse the pickup signal during a current

503213 69 3
50 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

surge. The average voltage after rectification will be less than it 

should be. This will be particularly noticeable if one of the electrodes 

is in sandstone, because sandstone rocks cause a larger phase shift 

than mudstone rocks.

Because of the variations in voltage caused by this phase shift, the 

average drop in potential over a 20-foot interval was used for interpretation rather than values at a single depth. The average drop 

was determined by planimetering the chosen area under the recorded 

curves. The manner in which these data were handled is shown in 

figure 15. The resistivities determined by planimetering were plotted 

against direction, as shown in the upper left hand diagram. Two 

curves are presented, one for each arrangement of surface current 

electrodes. The upper center plot shows the same data after averaging; that is, first, the 2 values for a given direction are averaged; 

second, these values are divided by the average for all 12 directions; 

and third, an average is formed for every set of 3 adjacent values. 

The first procedure is designed to eliminate baseline errors and reduce 

the errors caused by contact resistance at the surface electrodes. The 

second step is designed to reduce all the data to a comparable scale. 

The moving average used in the third step is intended to reduce the 

effect of one-point anomalies, which are probably due to instrumental 

errors. The upper right-hand plot shows the method which was used 

to determine the direction of maximum resistivity trend. As previous 

work had indicated that directions of high resistivity would be the 

ones most likely to be associated with favorability, only the amounts 

of resistivity in excess of the average for any one drill hole were 

plotted. These excess resistivities were used to define a direction of 

maximum resistivity.

For comparison of these data with favorability, the map shown in 

figure 13 was used. About each drill hole in which measurements 

were made, 4 circles with radii of 400, 800, and 1,200 feet were drawn. 

Then 12 radii were drawn in each of these circles to correspond to 

each of the directions for which resistivities had been measured. The 

numerical favorability was taken from the map at the intersection 

of each of the 12 radii with each of the 4 circles. These data were 

handled in the same manner as the resistivity data, as shown in the 

3 lower diagrams of figure 15. The individual favorabilities were 

plotted as a function of direction, as shown in the lower left hand 

plot. Then, the data for the 1,200-foot ring were averaged in the 

same manner as the resistivity data, as shown in the lower center 

plot. Finally, the favorability in excess of the average was plotted 

separately, as shown in the lower right hand plot of figure 15. The 

graphs of excess-favorability plots and the excess resistivity for 20 

drill holes are shown in figure 16.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 51
52 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

PRINCIPAL DIRECTION OF EXCESS FAVORABILITY 

,EXCESS RESISTIVITY 

XCESS FAVORABILITY

DRILL-HOLE NUMBER

SPf

PRINCIPAL DIRECTION OF EXCESS RESISTIVITY

SP10

SP15

SP14

SP17

SP16 SP18

SP19 SP24 SP29 SP31

SP37 SP38 SP42

SP44 SP48 SP52 SPSS 

FIGURE 16. Patterns of directional resistivity and favorability for typical drill boles.

The first step in the comparison of the resistivities with the favorabilities consisted of a statistical analysis of the correlation between 

individual values of both factors for each of the four rings on which 

favorability had been determined. The resistivity data were divided 

into seven groups, according to increasing magnitude. Then the 

average relative favorability corresponding to the groups of resistivity 

data was determined. The results are summarized in table 4.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 53

TABLE 4. Summary of a statistical study of directional-resistivity data obtained 

with the Lee partitioning method

Range of resistivity 

(ratio to average)

<0.80 . - ... 

0,80-0.89. ........... _ ...

0.90-0.95.....................

0.96-0.99...... ._._..-_..-.-

1.00-1.04............. ___ ..

1.05-1.09..... ...............

1.10-1.19.--..........--....-.

>1.19 .... ...... ...........

Comparative probabilities '..

400-foot ring

Oases

51

48 

58 

59 

49 

59 

52 

47

Average favorability

1.009 

.964 

.980 

.958 

1.065 

1.009 

1.014 

.992

0.9999

600-foot ring

Cases

50 

64 

51 

M 

33 

52 

56 

56

Average favorability

0.970 

.957 

.994 

.994 

1.040 

.986 

1.031 

1.024

0.9996

800-foot ring

Cases

41 

67 

53 

50 

40 

58 

58 

51

Average 

favorability

0.954 

.987 

.961 

1.019 

1.009 

1.031 

1.005 

1.035

0.9995

1,200-fopt ring

Cases

59 

56 

54 

44 

48 

49 

49 

59

Average favorability

0.968 

1.014 

.975 

.969 

.977 

1.035 

1.013 

1.031

0.9992

i Probability that the favorability corresponding to resistivity groups less than 1.00 is significantly less 

than that for resistivity groups greater than 1.00.

The last line of this table indicates that there is a highly significant 

increase in favorability in those directions which show higher-thanaverage resistivities. There is but one chance in several thousand 

that these results could have been obtained from a random set of data.

If the individual groups in the table are considered, the results are 

not so convincing. Only those groups of data with a resistivity 10 

percent greater than, or less than, average correspond to significant 

variations in favorability. In other words, small increases or decreases 

in resistivity can be correlated with small increases or decreases in 

favorability, but large variations in resistivity cannot be correlated 

well with large variations in favorability.

The lower degree of correlation between the very large deviations 

from the average may be due to the fact that these large deviations 

are more likely to be errors, or that the errors involved may tend to 

be all in one direction. The frequency distributions of resistivities 

and favorabilities are shown in figure 17.

The difficulty of our problem is apparent when it is realized tiiat 

the average variations in favorability or resistivity being considered 

are only 10 percent. As the favorability distribution is groupedEso 

closely about unity, it is highly probable that if a resistivity c6nsiderably larger or smaller than the average is obtained because of a 

random error, the favorability associated with it will be close to unity. 

For this reason, errors will all tend to be in one direction~when the 

end classes of the two distributions are compared. This may in part 

explain why there is a better correlation of small variations in resistivity and favorability than large ones.

In order to find at what distance from a drill hole that resistivity 

data best predict favorability trends, the data of table 4 were used 

to compute correlation coefficients between resistivities and favor-
54 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

120-

o 100- i i 

o

DC. m sn

< 60-

u_ O

QJ uj 40-

QQ 

5

1 20-

0

1 1 

1 1

1 1

^6 o

ti ' b it

tO I I \ ' §

°"l<4 ° » 1 °"

1 t

'' 1 ft

? 1 l\

II 1 \

1 1 b

1 1 

1 1

1 1

i 1 

1 1

\

\

^

 d -O--D

0.4 0.8 1.2

NORMALIZED FAVORABILITY

1.6 2.0

.o

o

IX

o

o: UJ 

tO

140

120

100

80

60

40

20

0' 

/

V

1 A 1" 

. 1 1 !

1 / I 1

ral | i 1 «

Z' 1 6 CT

""If »!""

jo ld

/! I

Q 1 *

' *

| \

 i \

1

1 

1

\ \

\^

0.4 0.8 1.2

NORMALIZED RESISTIVITY

1.6 2.0

FIOUBE 17. Frequency distribution of favorability and resistivity.
RESISTIVITY MEASUREMENTS, URANIUM, COLORAIX) PLATEAU 55

abilities for each of the four rings. The results are presented graphically in figure 18. The correlation coefficients are less than 0.2 in 

four cases, indicating a very poor correlation. However, because 

there is a large amount of data involved (about 500 sets of values for 

each computation), these correlations are significantly better than zero.

The computations indicate that the best prediction from resistivity 

data is obtained at distances of 600 to 800 feet from the drill hole 

under study. A prediction cannot be tested very close to a drill hole 

and is very poor at distances of more than 1,500 feet. It might be 

expected that the prediction would be best at distances of a few; 

hundred feet, the maximum electrode spacing that was used. Figure1 

18 merely illustrates that no fair estimate of correlation can be 

obtained on a scale finer than the grid used in contouring the favorability map of figure 13. The success of prediction of the resistivity, 

data may actually be better at shorter distances, but the information; 

to check this is not available.

As a more realistic measure of the ability to predict direction of 

favorability trends from the resistivity data, the direction of greatest 

excess resistivity, as defined in figure 16, was compared with the5

200 400 600 800 1000 

DISTANCE AT WHICH FAVORABILITY IS MEASURED, IN FEET 

FIGTTRE 18. Correlation coefflcients^between resistivity and f j volubility.

1200
56 EXPERIMENTAL AND THEORETICAL GEOPHYSICS 

10

a:

UJ

ur

QC

86

9

DC

S

V)

S 4

O 

U. 

O

ce LU

OD

i*

X o\

^

\

\

\

\

^ 

\\ 

o \o\

o

X<.

i 0 40 80 120 160 185 

ABSOLUTE VALUE OF ERRORS, IN DEGREES ;

FIGURE 19. Frequency distribution of the angle between direction of excess resistivity and favorability.

direction of greatest excess favorability for each of the 38 drill holes,. 

Data from the 600-foot rings were used. Figure 19 shows a frequency^ 

distribution graph for the angle between these two directions. These 

data are summarized in table 5.

; The standard deviation of this distribution is 75°, and the mean 

sibsolute angle of error is 58°. If there were a uniform distribution 

of angles from 180° to 180°, as there would be if there were no 

correspondence between the directions of maximum resistivity and 

favorability, the average angle between the two would be near 90° for 

a large enough number of cases. The most probable standard deviation for the average angle was calculated to be 11.7°, so that the 

difference between 58° and 90° is 2.82 times as great as the standard 

deviation of the mean. Probability tables show that the chances are
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 57

TABLE 5. The angles, in degrees, between the directions of greatest favorability and 

greatest resistivity measured with the partitioning system

Drill hole

SP-1 

5. -- - 

10 

14.. __ .. _ . __ _ ____ . _

15.. . _ .. _ . _ _____ .. ___ .

16. _ ... ___ . _ .. _ . -

17 

18.. _ . _ ... _ .. ___ . _ ..

19.... 

24. 

29 

31 

33 - . 

37. _ .. __ . _ - _ . _ 

38 _ ... __ . __ .. _ 

42. _ .. __ . . _ . _ _ . __ . _

44 . __ . _ - _ _ - __ _ 

48. _________________________________

52 ___ ___ _ _

53. 

60

111 _ . _ _ .

114 _ _ .. _ ___ . _ 

116 

123. _ ... . __ .. _ . ___

139 __ . _ _

140. __ . _ __ . 

141 _ _

145

152... __ __ .. _ .

153 . __ _ . _

211 .. _ .-.-- _

230 . _ . __ .. .- _ . . ......

243 _ ... 

260 .

267 __ __ _

344 .. __ .. _ . . . . ____ .

Direction of 

excess 

resistivity

85

5

215

193

22

235

110

175

320

72

165

206

252

125

252

60

22

72 and 285

150

15

215

65

90

255

355

250

70

260

288

267

40

308

78

130

130

72 and 260

38

262

Direction of 

excess 

favorability

80

35

280

195

200

195

18

190

60

20

264

272

252

330

350

355

340

98 and 250

230

55

140

280

348

302

100

260

28

285

320

257

267

265

30

275

88

75 and 280

45

238

Angular 

error

5

-30

-65

t\

-178

4fr

92 -15 -ioa

62 -99

-66

a

165 -98

65

42

-80

-40

76

145

102 -47

-105 -10-

42 -25

-32

10

133

42

48 -145

42 3 and -20-

-7

24

-1.4

200 to 1 that this difference is caused by a significant correlation 

between resistivity and favorability rather than by chance.

An average error of 57° is large, but this may in part be owing to 

the errors involved in the determination of the directions of excess 

resistivity and favorability. The largest errors are probably those 

which enter into the determination of the direction of excess favorability. Favorabilities were estimated from qualitative geologic log 

summaries rather than from quantitative measurements; and as several 

of the factors involved were recorded in only a most general manner, 

errors could enter into the numerical values assigned to these factors. 

The magnitudes of the possible errors, given as estimated standard 

deviations, were probably as follows:

Thickness of sandstone.__________

Thickness of basal mudstone_____.

Color of mudstone splits________

Radiation anomaly______________

Appearance of sandstone_________

Presence of carbonaceous material. 

Presence of iron oxide spotting_._.

PoMt 

0 

3 

2 

0 

2 

1 

1
58 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

If it is assumed that these errors are not interrelated, the resultant 

numerical favorability should have an error with a standard deviation 

of 4J£ points. From this factor the average error in the direction of 

excess favorability can be estimated. Ordinarily, the favorability 

pattern about a drill hole is determined by the 4 neighboring drill 

holes, even though 12 values of favorability were taken from the 

contour map of figure 13 in each case. From the frequency distribution of favorability values (fig. 17), it can be assumed that in the 

average case, one of these 4 drill holes will have a favorability of 32, 

two of them will have a favorability of 24, and the fourth will have a 

favorability of 16. This hypothetical case is illustrated in figure 20.

32 

28

15 24 -O -O- O 

20

16 -oPRINCIPAL DIRECTION OF 

EXCESS FAVORABILITY -

FAVORABILITY

DIRECTIONAL-FAVORABILITY 

PATTERN

EXCESS-FAVORABILITY 

PATTERN

FIGURE 20. Hypothetical favorability distribution about an average drill hole.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 59

It is then assumed that these are the actual favorabilities pertaining 

to each of the drill holes; but that in the process of evaluating the core 

descriptions, an error with a standard deviation of 4% points is introduced. This means that the difference between any 2 favorabilities 

will have a standard deviation of 6 points (the square root of the sum 

of the squares of the 2 deviations). The excess favorability for the 

hypothetical case is 8 points, only slightly greater than the errors 

involved. The excess favorability equals the standard deviation at 

an angle of 73° from the direction of greatest favorability. This 

means that 68 percent of the time the favorability direction determined from the core logs differs by 73° or less from the actual direction 

of greatest favorability. This agrees closely with the standard deviation of 74° found between the experimentally determined resistivity 

and favorability trends.

If it could be said that the above figures are precisely correct, then 

the standard deviation of the angle between resistivity trends and the 

true direction of favorabiJity increase would be only 12°. However, 

the estimation of the errors in favorability is not precise, and the 

figure "12°" has very little significance. Rather, it can only be said 

that the errors in prediction are on the average less than 57°, and 

possibly much less.

MEASUREMENTS IN THE WHITE CANYON DISTRICT

In addition to the fieldwork in the Morrison formation at the Spud 

Patch, 22 sets of directional-resistivity measurements using the Lee 

configuration were made in the Frey Canyon area of the White 

Canyon district in southern Utah.

LOCATION AND GEOLOGIC SETTING

The Frey Canyon area is 60 miles west of Blanding and 30 miles 

south of Kite, Utah (fig. 9). In this area the rocks are nearly flat 

lying, with a dip of a few degrees to the southwest, away from Monument uplift. The area is dissected by many canyons, and there are 

numerous mesas. The base of White Canyon, at an elevation of 

about 4,800 feet, is formed from the Cedar Mesa sandstone member 

of the Cutler formation. Above this lies a series of mesas known as 

the Moss Back, with the Moenkopi formation, the Chinle formation 

(including the Shinarump member), and the Wingate sandstone 

exposed on rims. The sandstones of the Chinle and Moenkopi 

formations are good cliff formers, so in many places there are ledges 

on the mesa rims a few hundred to some thousands of feet wide.

Uranium ore is found in the Shinarump in the ancient channels 

that have been cut into the old erosion surface of the Moenkopi 

formation. These channels range from a few to several tens of feet 

in depth and are several hundred feet wide. Individual channels
60 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

may be traced for several miles. In exploration drilling, the holes 

are generally spaced at 200-foot intervals along a channel after its 

course has been predicted by drilling near an outcrop. If uranium 

minerals are found, the channel is outlined by close-spaced drilling 

about the discovery hole. Generally, from 20 to 50 drill holes are 

necessary to explore a channel, and these drill holes range in depth 

from about 20 feet near the outcrop of the Shinarump to 600 feet on 

the talus slopes of the overlying Chinle.

Electrical-resistivity and natural-potential surveys were carried out 

in the White Canyon district during 1953, but the results were difficult 

to interpret because of the complexity of the anomalies in the Chinle 

formation (W. H. Jackson, written communication). However, it 

seemed desirable to attempt directional-resistivity measurements in 

the Shinarump because although no electrical anomaly was known to 

be associated with the ore, it might be expected that the sandstone in 

the channel fillings would have a low water content and high resistivity, and it thus could be traced with directional-resistivity measurements. During part of August and September, 1953, measurements 

were made in 4 channel sections, 3 in the Ears claim drilling area, and 

1 in the Bee claim drilling area of Frey Canyon.

A typical electric log through the channel filling of the Shinarump 

member and overlying Chinle is shown in figure 21. The crosshatched area shows the channel filled with Shinarump. The resistivity of the channel is so great (about 1250 ohm-meters) that it would 

probably present an easy target to trace with directional-resistivity 

measurements. The channel would be easy to find if it were overlain 

only by mudstones of the Chinle, in which the resistivity is approximately 8 ohm-meters. However, drilling in much of the area was 

being carried out through the Moss Back member of the Chinle formation, which is about 80 feet thick and in which the resistivity is more 

than 1000 ohm-meters. The presence of this high-resistivity sandstone makes the interpretation of resistivity measurements uncertain.

In addition to the difficulties caused by the presence of the Moss 

Back member, the terrain was generally unfavorable for precise 

resistivity measurements. As the benches on which the drilling was 

being carried out are relatively narrow, many of the drill holes are 

close to runs. These rims would be expected to distort the directionalresistivity patterns. In much of the area the benches are steeply 

sloping, rather than flat, and covered by high-resistivity float. Not 

only did these factors make the field procedure difficult, but they also 

reduced the reliability of the measurements. Inasmuch as these 

conditions are typical of the areas in which the Shinarump is found, 

the utility of directional-resistivity measurements had to be evaluated 

from two points of view. First, it had to be established that there was
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 61

NATURAL DEPTH, RESISTIVITY, 

POTENTIAL IN FEET IN OHM-METERS

Shale unit

Moss Back 

member

Shale unit

Shinarump 

member

"H MV r~

40

140

160

220

500 1500 2500 (Solid line) 

50 150 250 (Dashed line)

Moenkopi 

formation

FIGTJEB 21. Typical electric log of a section of the Chinle formation showing natural-potential and resistivity 

measurements. Cross-hatched pattern indicates part of resistivity curve corresponding to the Shinarump 

channel.
62 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

bC2'

Correction factors for the half-spread 

nearest the rim

14- 2P

10

5.0

2.0

1.0

0.5

0.2

0.1

Correction factors for the half-spread 

farthest from the rim

0.4 0.5 0.6 0.7 

1

0.8 0.9

1+2R

FIGURE 22. Effect of a rim on the standard Lee arrangement.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 63

a sufficiently large resistivity anomaly associated with Shinarumpfilled channels to serve as a tracer; and second, it had to be shown 

that the disturbing terrain effects could be corrected or neglected.

The effect of rims can be studied analytically (fig. 22). The case 

of an infinite linear rim making an angle, 6, with the electrode arrangement was considered for both a regular Lee array of electrodes and a 

Lee array with the current and potential electrodes interchanged. 

In the first case the earth resistivity would be given by

WPiCi'P* CYPi C, n

(3>

0 _ 0 i . R_L_ 4._J__i__L_ p~pl 1+2R' K ~C2'Po~l~C1 'P2 d'P0 C2'P2

where PI is the apparent resistivity calculated from the conventional 

formula for the half of the configuration closest to the rim; p2 is the 

apparent resistivity for the half of the configuration farthest from the 

rim; and the quantities such as C2'Pi are the distances between the 

potential electrodes PQ, PI, and P%, the current electrodes Ci, and <72, 

and the current electrode images CV and C2 r.

These equations are expressed so that the effect of the rim can be 

considered as a multiplying factor to the resistivity calculated from 

the observations under the assumption that no rims are present. 

These multiplying factors were calculated for four angles between the 

direction of the electrode lines and orientation of the rim. The results 

are presented in the graphs of figure 22. Calculations were carried 

out also for the inverted Lee arrangement; the results are shown 

in figure 23.

These correction curves were applied to the field observations where 

the distance between a drill hole and the rim was within several 

times the electrode spacing a. An example of these corrections is 

shown in figure 24.

The effect of a high-resistivity surface layer cannot be so readily 

evaluated. If there are lateral variations in resistivity in a surface 

layer, the effects of these variations may far outweigh the effect of 

variations in a layer at depth. In such a case the effectiveness of the 

Lee configuration of electrodes is doubtful. In order to check whether 

or not directional resistivity patterns were being controlled by the 

surface layer, patterns were determined about several drill holes with 

different electrode separations in areas where the 80-foot thick Moss 

Back member crops out. The results of these experiments are shown 

in figure 25. Somewhat different patterns are obtained with different 

electrode spacings; hence there must be some doubt about those 

patterns determined in areas with high-resistivity surface layers.
EXPERIMENTAL AND THEORETICAL GEOPHYSICS

Inverted electrode array near a rim

10

5.0

2.0

0.5

0.2

0.1

Correction factors for the half-spread 

nearest the rim

0.4 0.6 0.8 1.0 1.7 2.4

1+2P

Correction factors for the half-spread 

farthest from the rim

1+2/? 

FIGTJBE 23. Effect of a rim on the inverted Lee arrangement.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 65

JE P 

fETI

*

DIRECTIONAL-RESISTIVITY PATTERNS

UNCORRECTEO

RESISTIVITY WITH NORMAL ARRAY 

RESISTIVITY WITH INVERTED ARRAY

UNCORItECTED

CORRECTED 

PRINCIPAL CIMECmON

FIGTJBE 24. Examples of resistivity patterns corrected for rim effects.

The field data obtained in the Frey Canyon area should be con 

sidered in the light of these various disturbing factors. Excess 

resistivity patterns over a channel with favorable surface conditions 

are shown in figure 26. Here the channel sediments are overlain by 

25 to 125 feet of low-resistivity mudstone of the Chinle formation. 

The channel could be outlined by following the resistivity trends.

In other areas there is a much poorer correlation between direction 

of the channel and resistivity trends. In some places the discrepancy 

may be the result of irregular surface conditions, as there was very 

poor correspondence between the resistivities measured with the 

normal and the inverted Lee arrays. In other places the surface 

layer is the Moss Back member, and the resistivity trends are probably 

controlled by channels within the Moss Back. These channels in 

general overlie channels in the Shinarump, and thus, even though 

there is some correlation between the resistivity patterns, the correlation must be viewed as inconclusive.

CONCLUSIONS

The success of the initial experiments using resistivity trends to 

trace favorability patterns in the Morrison, and the channel sediments 

filled with the Shinarump member of the Chinle, indicates that further 

work could profitably be carried out, particularly on the development 

of methods of measuring directional-resistivity trends. The goal of 

such work should be the development of a reliable method of locating
66 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

I

03 

.1

U3H 310H W3H SIGH Tiiaa
EXPLANATION

, 5874 

Structure contour, approximately located. 

Drawn on base of Shinarump member 

of Chinks formation. Contour interval 

2 feet; datum is sea level

Area with excess resistivity 

Principal direction of excess resistivity

Excess-resistivity graph, in 

percent, for drill hole

Channel sediments

MAGNETIC 

NORTH

//%%&&* ////''~ f

/ S.<. <4mm: yf.-.-.yf-.::::::

,...Piii!i!l!

;::iJ!:iliJ:!::i::i:i:::i

 x-.: :: iin-j^n::ii;i:;;:;n; 

.^;ii^H;;:;;i:;;:;;ii

V ^

7 /

w / / / // /

p!j;;!!!jj;j!!!!jpp/ / / 

/

::::::::::::::/::7:::/ / ,

/ 

/

/

FIGURE 26. Directional-resistivity patterns for a channel in the B-
68 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

drill holes most judiciously during an exploration drilling program. 

The use of directional-resistivity measurements during the primary 

wide-spaced phase of exploration drilling could conceivably reduce by 

75 percent the number of drill holes necessary to locate favorable areas. 

The saving in drill holes would be accomplished by eliminating generally unfavorable areas and by permitting drill holes to be spaced 

farther apart than is now the custom, without missing favorable areas. 

The ability of the method to predict favorability trends in the Spud 

Patch area at distances of from 600 to 800 feet would permit the 

spacing of drill holes to 1,500 feet without risking missed favorable 

areas. This is double the spacing used in the original drilling program 

at Spud Patch.

The results of the present work indicate the method could be used 

in areas of the Morrison formation where a suitable correlation has 

been established between resistivity and favorability by electric 

logging. The method could also be used in areas of the Shinarump 

member of the Chinle where the channel sediments are not overlain 

by the Moss Back member or an equivalent high-resistivity sandstone.

The present equipment is limited to use at a maximum electrode 

spacing of from 300 to 500 feet by its low sensitivity. These considerations indicate that further development should be directed 

towards increasing the sensitivity of the present equipment and to 

devising methods of minunizing the disturbing effects of surface 

irregularities. The first problem is primarily one of instrument 

design, while the second deals with field technique and methods of 

interpretation.

The nature of the second problem may best be seen by considering 

the resistivity patterns that would be associated with several hypothetical conditions of the ground. Figure 27 shows the potential 

distribution in a uniform ground. The equipotential surfaces are 

symmetric about the center of the electrode spread. The potential 

difference between P0, the inhole electrode, and either PI or P2 is the 

same, and there is no directional pattern. This illustrates also why 

it is desirable to have the current electrodes equidistant from the drill 

holes. In a uniform earth there is no variation in potential as the 

inhole electrode is moved through the drill hole. This simplifies 

interpretation, as any deviation from this condition must be caused 

by directional variations in resistivity.

Figure 27 also illustrates the conditions being sought an area of 

high resistivity, such as a favorable sandstone lens, to one side of a 

drill hole. Here the equipotential traces are warped in the vicinity 

of the resistant zone, and the field is no longer symmetric about the 

drill hole. The voltage recorded in the direction of the resistant zone 

will be greater than normal, and the voltage in the opposite direction
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 69

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FiouEE_27. Equipotential traces under'difEerent assumedjconditions in a uniformj^round.
70 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

will be less than normal. This diagram also illustrates how the use 

of an inhole electrode can detect anomalies too small to be noted 

with surface electrodes alone.

Figure 27 shows how surface discontinuities in resistivity can 

adversely affect directional-resistivity measurements. A small resistant body near one of the surface potential electrodes can alter the 

potential distribution enough to provide a distinct directional-resistivity effect. A resistant body near the surface electrode causes a 

larger effect than the same body near the inhole electrode as the 

current density is much higher near the surface electrode. Because 

of the difference in current densities in the 2 places, a typical range of 

variation that might be expected at the 2 electrodes would be 20 

millivolts per ampere at the inhole electrode and 200 millivolts per 

ampere at the surface electrode. Thus, surface discontinuities in 

resistivity are more effective in establishing resistivity trends than 

subsurface variations.

To overcome this effect the surface electrodes must be placed at 

positions that always have the same potential. Various methods of 

doing this have been considered, but all involve an impractical amount 

of labor in the choice of a spot with the correct potential each tune 

the electrode spread is rotated. Rather, it seems that the solution to 

the difficulty may lie in comparing the potential of the inhole electrode 

with an arbitrary external potential not related to the flow of current 

through the ground. Such a circuit is illustrated in figure 28.

The purpose of this circuit is to obtain a reference potential exactly 

equal to the potential of the partitioning plane in a homogeneous 

earth. This is done by shunting the ground circuit between the two 

current electrodes with a pair of series resistors exactly equal in size. 

The midpoint of these two resistors will have the same potential as 

the partitioning plane, and any variation in potential between this 

point and the inhole electrode must be caused by warping of the 

potential field near the inhole electrode.

This method will be subject to errors if the contact resistances at 

the two current electrodes are not approximately equal. The equalizing of these resistances presents no problem in field operations. In 

the work described here, they were equalized by pouring salt water 

about the current electrodes.

Better results might also be obtained if directional variations in 

electrical properties other than volume resistivity were studied. As 

was pointed out in the first section of this report, the resistivities 

associated with favorable ground are only one-third greater than those 

associated with unfavorable ground. It is possible that anomalies 

in other electrical properties such as dielectric constant or capacity 

for induced polarization may be of larger relative magnitude.
RESISTIVITY MEASUREMENTS, URANIUM, COLORADO PLATEAU 71

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72 EXPERIMENTAL AND THEORETICAL GEOPHYSICS

LITERATURE CITED

Davis, W. E., 1951, Electrical resistivity investigations of carnotite deposits in 

the Colorado Plateau: U.S. Geol. Survey TEM-232, issued by U.S. Atomic 

Energy Comm. Tech. Inf. Service, Oak Ridge, Tenn.

Fischer, R. A., and Hilpert, L. S., 1952, Geology of the Uravan mineral belt: 

U.S. Geol. Survey Bull. 988-A, p. 1-13.

Keller, G. V., 1959, Electrical properties of sandstones of the Morrison formation: 

U.S. Geol. Survey Bull. 1052-J.

Keller, G. V., and Licastro, P. H., 1959, Dielectric constant and electrical resistivity of natural-state cores: U.S. Geol. Survey Bull. 1052-H.

Weir, Doris B., 1952, Geologic guides to prospecting for carnotite deposits on 

the Colorado Plateau: U.S. Geol. Survey BuU. 988-B, p. 15-27.

O