interpretation of geophysical well-log measurements in ...open-file report 81-389 open-file report...

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Open-File Report 81-389 Open-File Report 81-389

UNITED STATESDEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

Interpretation of Geophysical Well-Log Measurements

in Drill Holes UE25a-4, -5, -6, and -7,

Yucca Mountain, Nevada Test Site

by

J. J. Daniels, J. H. Scott, and J. T. Hagstrum

Open-File Report 81-389

1981

Prepared by the U.S. Geological Surveyfor

Nevada Operations OfficeU.S. Department of Energy

(Memorandum of Understanding DE-AI08-78ET44802)

This report is preliminary and has not been reviewed for conformity with U.S.Geological Survey editorial standards. Use of trade names is for descriptivepurposes and does not constitute endorsement by the U.S. Geological Survey.

TABLE OF CONTENTS

Page

Abstract . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

Introduction .... . . . . . . . . . .. 1

Geologic Considerations ... . . . . . . . . . . . . . . . . . . .. 2

Lithologic Interpretation from the Response Characteristics of

Geophysical Well Logs in Ash Flow Tuffs . .. . . . . . . . . . . . 4

Resistivity .... . . . . . . . . . . . . . . . . . . . . . .. 6

Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Neutron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Gamma Ray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Induced Polarization (IP) ........... .... .... . . 23

Magnetic Susceptibility .. . . . . . . . . . . . . . . . . . . . 26

Conclusions ............................. . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

(under the auspices of the U.S. Department of Energy) at NTS. Exploratory

drill holes UE25a-4, UE25a-5, UE25a-6, and UE25a-7 were drilled and cored to

depths of 138 m, 133 m, 128 m, and 143 m, respectively, on the northeastern

flank of Yucca Mountain to investigate the geologic characteristics of the

(listed in order of increasing age): Tiva Canyon, Yucca Mountain, Pah Canyon,

and Topapah Spring Members of the Miocene Paintbrush Tuff. Holes UE25a-4, -5,

and -6 were vertical core holes, while hole UE25a-7 was drilled at an angle

of

260 from vertical. The location of these drill holes is shown in Figure 1.

This study discusses the physical properties of the tuff units as measured

with U.S. Geological Survey borehole geophysical research equipment.

Geologic Considerations

The sequences of ash-flow and bedded tuffs at NTS have been classified

as

stratigraphic units primarily on the basis of genetic relationships and cool-

ing histories. The cooling histories of the tuff units determine the degree

to which they become welded (i.e., non- to partially welded, moderately weld-

ed, densely welded) and are due largely to the temperature of emplacement and

the thickness of cooling units (Smith, 1960).

Zones of crystallization and alteration are superimposed on the variously

welded portions of the vitric tuffs, although their presence may be dependent

on the degree of welding. Devitrification of the pyroclastic flows has oc-

curred throughout almost all of the densely welded portions of the tuffs.

Associated with the thickest densely welded zones are inner cores character-

ized by lithophysal cavities. These are nearly spherical, mainly unconnected

voids that are commonly lined with secondary minerals. Vapor-phase minerals

that crystallize from the hot volatiles released by the cooling tuff units are

2

Abstract

Exploratory holes UE25a-4, UE25a-5, UE25a-6, and UE25a-7 were drilled at

the Nevada Test Site (NTS) to determine the suitability of pyroclastic depos-

its as storage sites for radioactive waste. Studies have been conducted to

investigate the stratigraphy, structure, mineralogy, petrology, and physical

properties of the tuff units encountered in the drill hole. Ash-flow and

bedded tuff sequences at NTS comprise complex lithologies of variously welded

tuffs with superimposed crystallization and altered zones. Resistivity,

density, neutron, gamma-ray, induced-polarization, and magnetic-susceptibility

geophysical well-log measurements were made to determine the physical proper-

ties of these units. The interpretation of the well-log measurements was

facilitated by using a computer program designed to interpret well logs. The

broad features of the welded tuff units are readily distinguished by the

geophysical well-log measurements. Some mineralogic features in the drill

holes can be identified on the gamma ray, induced polarization, and magnetic

susceptibility well logs.

Introduction

As much as 3000 m of rhyolitic tuffs that were erupted from the Timber

Mountain-Oasis Valley caldera complex (late Tertiary time (16-9 m.y.)) at NTS

mantle an eastern portion of the Basin and Range province. The tuff units and

their associated calderas have been the subject of mapping and detailed study

by the U.S. Geological Survey (Byers, et al, 1976; Christiansen, et al, 1977;

and Lipman, et al, 1966).

To study the suitability of pyroclastic deposits as storage sites for

radioactive waste, four exploratory holes were drilled in the summer of 1979

I

found mostly as linings or fillings of lenticular vugs. Alteration of the

tuffs by ground water has resulted in zones of zeolitization, silicification,

and calcitization. The ground water level is well below the bottom depth of

each of the drill holes considered in this study. The lithologic intervals

and the distribution of crystallization and alteration zones as determined by

Spengler (1980) are given in Figure 2.

Lithologic interpretation from the Response Characteristics

of Geophysical Well Logs in Ash Flow Tuffs

Each geophysical well-log measurement is affected by the physical proper-

ties of the rock, the interstitial fluid of the formation, the conditions in

the borehole (fluidity and rugosity), the volume of rock investigated by the

probe, the vertical resolution of the probe, and the design characteristics of

each probe; and so should be considered an apparent rather than a true physi-

cal property value.

The initially unsaturated condition of the rocks surrounding these shal-

low drill holes makes interpretation of the lithologies particularly diffi-

cult. Drilling artificially introduces fluid into the formation, causing the

rocks surrounding the drill hole to become partially saturated. Large closed

voids in the rocks can remain dry throughout the period of time that geophysi-

cal well-log measurements are made. The resistivity and neutron-neutron

measurements are sensitive to the degree of saturation of the rocks with

fluid. The fluid level could not be maintained to the surface in any of the

drill holes in this study. Therefore, the resistivity and neutron well logs

are shown for that portion of the drill holes for which a "standing" water

level could be maintained.

4

Figure l.--Location of drill holes U925a-4, UE25a-5, UE25a-6, and UE25a-7Yucca Mountain, Nevada Test Site.

'-"0 BEATTY WASH - H .. i "Ir - - I

f.6 c

7omc

25A5

/

+2MG

25A7/

SCALE (meters)I I

0 40 80

.3

Interpretation of individual geophysical well logs is made by assigning

symbols to different value ranges on the geophysical well logs. The value

ranges are chosen to best correspond with the lithologies given in Figure 2.

However, the lithologies in Figure 2 were not chosen on the basis of physical

properties and are often different from the computer-assigned symbols for the

geophysical well logs.

Resistivity

Resistivity is a measure of the ease with which electric current passes

through a material. Borehole resistivity depends upon the porosity, the fluid

resistivity and the grain resistivity of the rock. The resistivity of ash-

flow tuffs should be a function of (a) welding, (b) devitrification, and

(c) void space in the rocks. Resistivity in saturated welded tuffs should

increase with the degree of welding and decrease with the degree of devitrifi-

cation and the amount of void space (including fractures) in the rocks. When

the lithophysal zones are unsaturated, the void space may cause an increase in

the measured resistivity.

The resistivity well logs (16" normal) for drill holes UE25a-4, UE25a-5,

UE25a-6, and UE25a-7 are shown in Figures 3, 4, 5, and 6, respectively.

Symbol-logs, obtained by assigning lithologic symbols to resistivity value

ranges, are also shown in these figures. There is a general correspondence

between the lithologic log and the symbol log interpreted from the resistivity

values. The high resistivity values for the Topopah Spring Member can be

fairly consistently interpreted as welded tuffs. However, in each of the

drill holes there is a zone of high resistivity at the top of the Topopah

Spring and a zone of low resistivity at the base of the drilled Topopah Spring

6

'I

UE258-4 LITHOLOGY_ _ _ _ _ _ _ _ _ _ _ _ _ _

UE25a-5 LITHOLOGY_______________

UE26s-6 LITHOLOGY_______________

UE27a-7 LITHOLOGY_______________

EXPLANAT ION_______________

0-Qac

jjPC...

TOYJ

U,a:

z

0-i

0

z,i:tL

I-a-0

CAa:

z

ia-.

0

Tpp(Aa:LUJH

z

LQ

13

DENSELYWELDED

MODERATELYWELDED

NON- PARTIALYWELDED

VITROPHYRE

BEDDED

VITRIC

DEVITRIFIEDVAPOR- PHASE

LITHOPHYSAL

ARGILLIZED

ALLUVIUM

Tpt

:'S.

Oac QUATERNARY ALLUVIUM & COLLUVIUMMIOCENE PAINTBRUSH TUFFTpc TIVA CANYON MEMBERTpy YUCCA MOUNTAIN MEMBERTpp PAH CANYON MEMBERTpt TOPOPAH SPRING MEMBER

ilgure 2.--Lithulogic wcil-logs (interpreted from core) for drill holesUE25a-4, UE25a-5, UE25a-1, and UE25a-7, (msodified after Spengler,in press).

UE25a-4 RESISTIVITYohm-m

0 3000 6000

(b)UE25a-4 RESISTIVITYASSIGNED SYMBOLS

UE25a-4 LITHOLOGY_ _ _ _ _ _ _ _ _ _ _ _

LITHOLOGYEXPLANATION

RESISTIVITY SYMBOLSEXPLANATION

0

10

20

30

40

50tn

W 60awE 70

80

- 90

0 100

0 -

10-

20-

30-

40-

50-

W 60-W' 70-

80-

0 I 00

110-

120-

130-

140

150

E...

_

'

=&r=gm=

HUM

me

r _-

C,)

Vi 60

1 70

80

i- 900-

0 100.

110~

120

130

140

150

DENSELYWEL DED

MODERATELYWELDED

NON TO PARTIALYWELDED

VITROPHYRE

BEDDED

VITRIC

DEVITRIFIEDVAPOR-PHASE

M

0x

-iiU,

:.,. LITHOPHYSAL

ARGILLIZED

ALLUVIUM

Figure 3.--Resistivity well-logs and interpretation for drill hole UE25a-4:(a) resistivity well-log, (b) computer assigned symbols, and(c) lithologic well-logs (after Spongler, in press).

(3)UE25a-5 RESISTIVITY

ohm-m0 3000 6000

(b)UE25a-5 RESISTIVITY

ASSIGNEO SYMBOLSUE25a-5 LITHOLOGY

_ _ _ _ _ _ _ _



0

10

20

30

40

50(I)n:W 60w' 70

80I- 90

wo 100

110

120

0 ~10~

20

30

40

50-(n

W 60-wE 70-z

80-

a- 90-

o 1 00-

110 -

120 -

130-

140

1501

QOC

Tpp

....

I

... .

....

DENSELYWELDED

MODERATELYWELDED


VITROPHYRE

BEDDED

VITRIC


Me 500 - ..

I 500 -_; 2000 - Rt| -

- 2000 -_z 2500

2 3000

3 5005" 000

50 500

6000

LITHOPHYSAL

ARGILLIZED

ALLUVIUM

Figure 4--Resistivity well-logs and interpretation for drill hole UE25a-5:(a) resistivity well-log, (b) computer assigned symbols, and(c) lithologic well-logs (after Spengler, in press).

UE26a-6 RESISTIVITYohm-m

0 3000 6000

UE25a-6 RESISTIVITYASSIGNED SYMBOLS

LUE26a-6 LITHOLOGY LITHOLOGY

EXPLANATION

50 -(I,

t 60-

' 70-z

80

I- 90

0 100.

120

130

140

150

20 -

30-

40-

50-UA

W 60-H

Z 70-

80-

:E 90

0 100

110 -

120

130

140*

150

IiU)

Ix

z

a.wjC3

DENSELYWELDED

MODERATELYWELDED


VITROPHYRE

BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM


500

4000 0

-4

2 25000

3000

3500

x 4000m 4500(f 5t000

Figure 5.--Resistivity well-logs and interpretation for drill hole UE25a-6:(a) resistivity well-log, (b) computer assigned symbols, and(c) lithologic well-logs (after Spengler, in press).

(a)UE27a-7 RESISTIVITY

ohm-m0 3000 6000

OUE25a-7 RESISTIVITYASSIGNEO SYMBOLS

UE27a-7 LITHOLOGY_ _ _ _ _ _ _ _


0-

10-

20 -

30-

40

50(n

W 60I-w' 70

80

1 90

100

110

120

130

140

150

(I,W

z

y

0.

0 -

10

20-

30-

40

50

60

70

80

90-

1 00-

110 -

120-

130-

140-

150-

107

20-

30-

40-

50-

U 60-

x 70-

80-

- 90-tL

o 100 -

110-

120-

130-

140

150-

QOC I

44±H

1'i

DENSELYWEL DED

MODERATELYWELDED


VITROPHYRE

BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

IALLUVIUM

RES ISTI V ITY SYMBOLSEXPLANATION

fi500- .(n,-i 1000-:: 1500-W

2 25004° 30003F 3500

M 4000

Q 4500

5000

5500

66000

;1 ..'.I.

Figure 6.--Resistivity well-logs and interpretation for drill hole UE25a-7:(a) resistivity well-log, (b) computer assigned symbols, and(c) lithologic well-logs (after Spengler, in press).

interval that cannot be explained solely by variations in the degree of weld-

ing. It is possible that a vitrophyre causes the high-resistivity zone at the

top of the Topopah Spring and a lithophysal zone causes the low resistivities

at the bottom of drill holes UE25a-5 and UE25a-6. However, it is also likely

that there are variations in the degree of welding which cannot be seen in the

drill core but do affect the resistivity. A comparison of the four resistivi-

ty well logs for the Topopah Spring Member indicates the following: (a) a

high degree of welding (or low alteration and fracturing) for drill hole

UE25a-5, (b) an intermediate degree of welding for drill hole UE25a-4, and

(c) a low degree of welding (or high alteration and fracturing) for drill

holes UE25a-6, and UE25a-7. The stratigraphic members above the Topopah

Spring are less densely welded, resulting in low resistivity values. If the

degree of welding in the Topopah Spring is approximately the same for each

drill hole, then near-surface fracture zones (with increased seasonal ground

water percolation) are most likely to occur near drill holes UE25a-6 and

UE25a-7 and least likely to be present near drill hole UE25a-5.

Density

The density measurement probe consists of a gamma ray source (Cs137) and

one, or more, gamma ray detectors. Gamma rays emitted by the source are

scattered by the rock, and the gamma radiation measured at the detector de-

creases as the electron density of the rock increases. By using two detec-

tors, the adverse effects of fluid invasion, mudcake and rugosity can be

minimized, resulting in a computed compensated-density that is approximately

equal to the bulk density of the rock. The computed bulk density may be too

low when there are large (sharp) variations in the diameter of the borehole.

1 1

Therefore, the density well log should always be interpreted in conjunction

with the caliper well log (Figure 11). Non-welded and highly altered units

have low bulk densities, while densely welded units have high bulk densi-

ties. Devitrified and lithophysal zones should have relatively low densities.

The density well logs for drill holes UE25a-4, UE25a-5, UE25a-6, and

UE25a-7 are shown in Figures 7, 8, 9, and 10, respectively. Density symbol-

logs, obtained by assigning lithologic symbols to density value ranges are

also shown in these figures. The high density values in the Topopah Spring

Member can be consistently interpreted as being caused by the presence of

welded tuffs. In each of the drill holes there is an increase in the density

near the top of the Topopah Spring which is probably caused by the vitrophyre

indicated on the lithologic log. The lowest densities for the logged interval

in each drill hole occur in the non- to partially welded zones in the Yucca

Mountain and Pah Canyon Members, while intermediate density values were mea-

sured in the Tiva Canyon. However, the Tiva Canyon is classified as a densely

welded unit by the lithologic core description. The average bulk density

values, computed from the geophysical well logs, for the logged interval below

the upper Topopah Spring vitrophyre are as follows: (a) UE25a-4 has an

average bulk density of 2.09, (b) UE25a-5 has an average bulk density of

2.16, (c) UE25a-6 has an average bulk density of 2.13, and (d) UE25a-7 has

an average bulk density of 2.11. Therefore, the Topopah Spring in UE25a-5 has

the highest bulk density, while UE25a-4 has the lowest bulk density. The high

bulk density values in drill hole UE25a-5 are consistent with the high resis-

tivity values (and the least amount of fracturing) noted previously in this

paper.

12

(a)

UE25a-4 DENSITYg/cm

1.0 2.0 3.0

F 0

100 -

20 _

30

40

50U,

LW 60-

2: 70Q

80-

C3 500

II0~

120

130

140-

150-

(b)

UE25a-4 DENSITYASSIGNEO SYMBOLS

(c)

U)w

z

i

0

0

10

20

30

40

50

60-

70-

80-

90 -

100-

110 -

120-

130

140-

150

..........

..........

UE25a-4 LITHOLOGY

20 g Tp

40 .

50 - Tp

w60 .;K:

70 .... Tpp

80- .9 0 .

a]10g..

RH]

=ff

i....


DENSELYWEL DED

MODERATELYWELDED


VITROPHYRE

DENSITY SYMBOLSEXPLANATION

* 0R 0.5

- 1.0-

2 1.5-c)

20-W 25-

3.0

I I I I. . . . .I I I II'll -I'll.....I'll.......I ...

...... :'. �

I

. .4

BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM

Figure 7.--Density well-logs and interpretation for drill hole UE25a-4:(a) cle:1sity well-log, (b) computer assigned synbols, and(c) lithologic well-logs (after Spengler, in press).

(a) (b)

UE25a-5 DENSITYASSIGNED SYMBOLS

(C)

UE25a-5 DENSITY9/cm2.0 3.0

I-U-

Li

0

1.00-_

10

20

30

40

50

60

70

80

90

100

11 0

120

130

140

150

0 O

10

20-

30-

40-

50-

W60Q-

x 70 -

80

, 90.0-

o too.

110

120-

130-

140-

150-

..........

UE25a-5 LITHOLOGY_______________

0-

1 0* Q0C

20 -

30- PC

40-::

50 T-u)

W 60

E 70z ..

80

F- 90-

tnn- .


i....

DENSELYWELDED

MODERATELYWELDED


VITROPHYRE

mz(n-I

0-

0.5-

1.0-

1.5.

S.-.%%

-A...

%-A-.I 4.1

od 2.50Z5-

3.0-


BEDDED

VITRIC

DEVITRI FIEDVAPOR-PHASE

LITHOPHYSAL

ARGILLIZED

ALLUVIUM

-1

Figure 8.--Dcnsity well-logs and interpretation for drill hole UE25a-5:(a) density well-log, (b) computer assigned symbols. and(c) lithologic well-logs (after Spengler, in press).

(a) (b)


(C)UE26a-6 DENSITY

9/cm2.0 3.01.0

O--

10

20

30

40

50Un

W 60Ill: 70

x 890

a 100

110

120

130

140

150

0-

10-

20-

30-

40-

50-

W 60-wc 70-

80-

. 90-

0 t oo

110-

120-

130-

140

150 -

MOE

Kg

UE26a-6 LITHOLOGY

0 Q0C

20-

30 gt~ Tpc

40-

50 -.,, k

w 60 - ' p

1: 70 - :s

s,90-

120

150- 2Z

MFHEEEEEEaHa

....

i ...


DENSELYWELDED

MODERATELYWELDED



0 *

1.0

0C-)Z(A

1.ti.

20

25

30'

U....'

-...-...

VITROPHYRE

BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

2 ALLUVIUM

Figure 9.--Dcnsity well-logs and interpretation for drill hole UE25a-6:(a) density well-log, (b) computer assigned symbols, and(c) litrlo(1oic well-logs (after Spengler, in press).

(a)

UE27a-7 DENSITY9/cm

1.0 2.0 3.C

(b)


(C)

UE27a-7 LITHOLOGY LITHOLOGYEXPLANATION

DENSITY SYMBOLSEXPLANAT ION

. DENSELY

f WELDED

MODERAT,WELDED

... NON TO I:: WELDED

VITROPHY

BEDDED

ELY

PARTIALY

'RE(I)

u 60-w

2 70-

80-

L 90-

0 100 -

1 1 0

120-1 20 -

I 30

140

150

U)

a:u:

uiH

0

Un

w2:

2:z

I

w03

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM

H .--H J1Y! .. , II-I wd iflt ,r I -tat ion Ior drilII hldel UE2$ d- _:(,I) ,- itv ,- I- lo,~. ( h) c( dl as signed svmbois , an d

1 ~i~ I !i - .t1 ,I .~~ ,: .-r Spe md.1o.r, i n ress) .

(a) (b)


(C)

UE21'a-6 DENSITY9/cm2.0 3.0

UE26a-6 LITHOLOGY LITHOLOGYEXPLANATION


1.0O_

10 -

20-

30-

40

50cn

60-w

-_ 70~,, z80-

° 100-

110

120-

130

140

150

0

10

20

30

40

50

W 60

I-9 70

80

(L 90~

1 1 00110 -

120-

130-

140-

150-

-xx)

TPC

TOp

DENSELYWELDED

MODERATELYWELDED


VITROPHYRE

R 0-

aQ5,-I

-< 1.0 -

_ 1.5-

Z) 20-

25-

30-

-"I-....

BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM

liguro 9.--Dcnsity well-logs and interpretation for drill hole UE25a-6:(a) density well-log, (b) computer assigned symbols, and(c) lithologic well-logs (after Spengler, in press).

(a) (b)


(C)

UE27a-7 DENSITY9/cm

1.0 2.0 3.0

UE27a-7 LITHOLOGY_ _ _ _ _ _ _ _



U)O_

w 60-

1- 70-

90-

90 -

100-

I 1 0

120

'30

140

150

0

I 0 .........

20

30

40

50

so

14. 60 .....

E 70

80 * ::.

a. 90 ''''CL

0100

110

120

13 0 TM

140E

150

r

U)

I:

a-

0

0 -

20-

30-

40

50

60

70

80

90

1 00

11 0

120

130

140

150

: Q0C

TPr,

TOP

Tpp

ICI

DENSELYWELDED

MODERATELYWELDED


VITROPHYRE

I.

Q 0m .z 0

-< 1.0

Z 15 .5.G) :::::

2D.

25 1 9

30BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM

> ! :ro ~ .- r ~ v I .:, ~i , - I - loos .a:n i n~r rpretat ion for dr ;II hoto UFEQ3a- :(,;) !,-I : i t ".4 I- 1 o . ( b ) co2~-)uter ass igned s\-bo Is , and

1. ~~ ~~ ( ~ ,~.,! 1,~ ,t~r .Zpcen-~icr, i~ prcss) .

UE25a-4 CALIPERcm

0 10 20 30 40

UE25a-5 CALIPERcm

0 10 20 30 40

UE26e-6 CALIPERcm

0 10 20 30 40

UE27a-7 CALIPERcm

0 10 20 30 40

CAa:wwx

z

a-0

W 60wx 70-z

80-

- 90-CLw

1 00 -

11 0

t20-

130

140

150

V)

w

z

2:

w0

()xww

z

a-

w_

0

Figure ll.--Caliper logs for UE25a-4, UC25a-5, UE25a-6, and UE25a-7.

Neutron

The neutron well-logging probe consists of a neutron source and detector

separated by approximately 50 cm. The number of neutrons counted by the

detector is an inverse function of the hydrogen content of the rock surround-

ing the borehole. In saturated material the neutron count rate is approxi-

mately proportional to the degree of welding. However, this is not necessari-

ly true in unsaturated rocks. In fact, the neutron well logs for drill holes

UE25a-4, UE25a-5, UE25a-6, and UE25a-7 (Figures 12, 13, 14, and 15, respec-

tively) show an inverse relationship between the degree of welding and the

neutron count rate! The assigned symbols for the neutron well logs correspond

closely to the core-interpreted lithology, when the neutron well logs inter-

pretation is based on this paradox. A constant value for fluid saturation in

each of the formations must be assumed in order for this interpretation to be

valid. The lithophysal zone located near the bottom of drill hole UE25a-6

(Figure 14) shows a very low neutron count, rate which indicates that the

cavities in this zone probably are interconnected.

Interpretation of well logs that are indicative of mineralogy

The measured responses of magnetic susceptibility, induced polarization,

and gamma ray well logs are primarily a function of changes in the mineralogy

and chemistry of the rocks rather than changes in physical properties. Inter-

pretation of these logs is as follows:

Gamma Ray

The gamma ray probe measures the natural gamma radiation emitted by the

rocks surrounding the borehole. The principle natural gamma ray-emitting

18

a1

UE25a-4 NEUTRONcps

(b)UE25a-4 NEUTRONASSIGNED SYMBOLS

(c)UE25a-4 LITHOLOGY

_ _ _ _ _ _ _ _


NEUTRON SYMBOLSEXPLANATION

12000

10

20

30

40

50()

tw 60

x 70z

80M

a 90

o3 100

11 0

120

130

140

150

30000

(oOfwLI-w

z

a-w0l

0

10

20

30

40

50

60

70

80

J90

1 00

110

120

130-

140-

150-

0 -

10-

20-

30

40-

50a:W 60I-

x 70-

80-

i 90-a-

i100-

1100

120

130

140

150

!set

Qoc

T PC

, TOY

0- ....................

DENSELYWEL DED

MODERATELYWELDED


VITROPHYRE

BEDDED

-4iMT

0-

5000 -

10000 -

15000 -

20000-

25000-

30000

a.a..-L-L-L

-L

a.~iia. R xa.s .::

Tpt

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM

Figure 12.--Neutron well-logs and interpretation for drill hole UE25a-4:(a) neutron well-log, (b) computer assigned symbols, and(c) lithologic well-logs (after Spengler, in press).

(a)UE25s-5 NEUTRON

cps


OUE25a-5 LITHOLOGY LITHOLOGY

EXPLANATIONNEUTRON SYMBOLS

EXPLANATION1200 t

00 30000

10

20

30

40

50W0,

hi 60I-

S .70

80C Z

I- 90010.0 1 00

1.10-

120-

130-

140-

1 50-

. . . . I O -

10 -

- 20-

30-

50 ........

- Si 60- ...

X 70

- 80- eeI

* 90

C3 0-00I -

- 120-

-130 -

- 140-

- 150 -__

(ntY

z

0-

0

0*

10.

20-

30

40

50-

60

70

80

90.

100-

1 0-

120

130

140

150

0

Qac

Tpc

* Tpy

Tpp

-*Tpt

II....

, ....

DENSELYWELDED

MODERATELYWELDED


VITROPHYRE

C

--I

-1

mC)-uC,

0 -

5000 -

1 0000

15000-

20000

25000

30000

BEDDED

4.::'

:!L

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM


Wa:

,W

IJ

0

UE25a-6 NEUTRONcps

12000 3000O - I....I,

10-

20- -

30-

40-

50

60

70

80-

90-

100-

110-

120-

130

140

150


(c)UE26a-6 LITHOLOGY

_______________


0

Irw

1

a-

0

0-

10-

20-

30-

40-

50-

60-

70-

80

90'

100

110 i

120

130

140

150

TOM4��

UM=Ppp.........umaIm

me

iffHl�i

.L

0-

fiiS

....

1:.

DENSELYWEL DED

MODERATELYWELDED


VITROPHYRE

NEUTRON SYMBOLSEXPLANATION

o 0 -F --Z

s 50001 -2;j 10000 -|-

m 15000 .4-

z 200000@ 25000 2 ':A

30000 ....(A-I-

z

I-a_w03

I

iI

80-

90 -

100.

110.

120-

130-

140-

150-

BEDDED

VITRIC


LITHOPHYSAL

ARGILLIZED

ALLUVIUM


a)UE259-7 NEUTRON

cps

(C)

UE27a-7 LITHOLOGYUE25a-7 NEUTRONASSIGNED SYMBOLS


NEUTRON SYMBOLSEXPLANAT ION

Li,Xd

z

a

(I

1:

I

Id

Ca

lo0-

30 -

40 -

60 .. .....

*70 :::::

12 -

I30 ' l

140 R

150 _

IrI-Id

a-IdM

zo4020

30

40

50-

60-

70-

80-

90-

100-

110 -

120-

130-

140 -

150-

.1�

'. t

.- 4'

...

.....

- :7

...

)001

t 0

Qo0c

1TPC

0- RRRRfffiHH49

Z==

I

DENSELYWELDED

MODERATELYWELDED

NON- PART IALYWELDED

--I

-4m

C)

0-

5000 -

10000 -

15000 -

20000 -

25000 -

30000-

ff HEM- M.T;.;.i......I'll.........

VITROPHYRE

BEDDED

VITRIC


LITHOPHYSAL

ARGILUZED

ALLUVIUM


minerals are uranium-series isotopes and potassium-40. Since potassium bear-

ing minerals are common in both primary and secondary crystallization regimes

in welded tuffs, the gamma ray well log measurements are principally a measure

of relative abundance of potassium.

The gamma ray well logs for drill hole UE25a-4, UE25a-5, and UE25a-7 are

shown in Figure 16. The Topopah Spring Member has a fairly consistent gamma

signature that is correlatable between each of the four drill holes. However,

the units above the Topopah Spring show that there is a great deal of varia-

tion in the potassium content of the four drill holes. The intensity and

character of the gamma ray response above the Topopah Spring is approximately

the same in drill holes UE25a-6 and UE25a-7 (Figures 16), but is lower than

the response in UE25a-4. The intensity of the gamma ray measurements in drill

hole UE25a-5 (Figure 17) is the lowest of any of the gamma ray well logs. If

the amount of potassium is related to post-emplacement chemical alteration,

then these logs indicate that there could be a fairly large differences in

alteration between the three drill holes.

Induced Polarization (IP)

The IP measurement is made by recording the decay voltage at a potential

electrode from a time-domain current source. The potential electrode is

located on the probe at a spacing of 10 cm from the current source. The rate

of decay of the potential during the current-off time period is inversely

proportional to the electrical polarizability of the rock. A high IP response

in volcanics may be caused by the presence of cation-rich clays, zeolites, or

pyrite and other sulfides. However, in some cases iron oxide minerals can.

contribute to the IP response.

23

UE25a-4 GAMMA RAYcps

0 100 200 300


0 100 200 300


0 100 200 300

[1:

I

a-wj0

0-

10

20

30-

40 -

50-

60 -

70 -

80-

90

100'

11 0

120

1 30

1 40

150

LiJ

z

2:

0

0-

10

20

30

40

50-

60-

70 -

80

90-

100-

11 0.

120

130

1 40

150

U)w

7-

z

2:

a-wiO

0-

10

20

30

40-

50-

60-

70-

80-

90-

100-

1100

120

130

140

150

W

z

a

0


0 100 200 300

60- i I

10 - -

20-

30

40-

50-

60-

70

80

90

100

110

120-

130-

1401

150 _

Figure 16.--Gammaa ray well-logs for UE25a-4, UE25a-5., UE25a-6, and UE25a-7.

UE25a-4 IPpercent

0 25

UE25a-5 IPpercent

25

UE26a-6 IPpercent

25

UE27a-7 IPpercent

0 250 00

10

20

30

40

0Oi I I a I . I. 0 0

(,W

2:

U,0

wLnC

50

60

70

80

90

100

110

120

130

140

150

I-fWz

2:

0

10 -1

20 -

30 -

40-

50-

60-

70 -

80-

90-

100 -

110-

120-

130-

140-

150-

C')ww

z

a-

20-

30-

40'

50-

60-

70

80

90

100

110

120

130

140

150

(/,w

'-4

a_1-0Cl

0-

10-

20-

30-

40

50

60-

70

80-

90

100

110

120

130

140

150

r

K

Figure 17.--Induced polarization well-logs for UE25a-4, UE25a-5, UE25a-6,and UE25a-7.

The IP well logs for the drill holes considered in this study are shown

in Figure 17. Unfortunately, these values are unreasonably high and the value

of these particular well logs is questionable. Hagstrum and others (1980a)

has shown that IP well-log measurements in the fluid-saturated zone of ash

flow tuffs have values that are normally in the 0-to-4 percent range. The IP

log is apparently strongly affected by the fluid invasion in the undersaturat-

ed volcanic rocks.

Magnetic Susceptibility

Magnetic susceptibility is the measure of the intensity of magnetization

of a magnetizable substance in the presence of a known magnetic field. The

magnetic susceptibility of a rock depends largely on the amount of ferrimag-

netic minerals that it contains. Magnetite is the most important ferrimagnet-

ic mineral affecting the magnetic susceptibility measurements. Magnetic

susceptibility measurements in welded tuffs have been assumed to be primarily

a function of the amount of magnetite contained in a rock. However, Hagstrum

and others (1980b) have found that the size of the magnetite grains is also an

important factor affecting the magnetic susceptibility of welded tuffs. The

magnetic susceptibility well logs for drill holes UE25a-4, UE25a-5, UE25a-6,

and UE25a-7 are shown in Figure 18. These well logs will be discussed in

detail in another paper (Hagstrum and others (1980b).

Conclusions

The broad features of the welded tuff sequence encountered in drill holes

UE25a-4, UE25a-5, UE25a-6, and UE25a-7 are readily characterized by their

physical properties measured by the geophysical well logs. Welded tuffs,

26

I I

UE25a-4 MAC SUSCSI units

0 15000


0 15000


0 15000


0 15000

(nwHw

z'-4

I

H

0

0i

10

20

30

40

50

60

70

80-

90 -

100-

1 10

120

1 30

1 40

150

CA,en

z

4a-0

0-

10-

20-

30

40

50-

60 -

70-

80 -

90 -

100

110

120

130

140

150

(ACZH

z'-4

I

0

0

10-

20-

30

40

50 -

60-

70-

80-

90

100

110

120

130

140

150

(ncr_wH

z

He0._

0

0

10-

20-

30

40

50-

60-

70 -

80 -

90-

100

110

120

130

140

150

Figure 18.--Magnetic susceptibility well-logs for UE25a-4, UE25a-5, UE25a-6,and UE25a-7.

however, are extremely complex lithologic units involving the superimposition

of several factors and processes, and a detailed interpretation requires

equally detailed lithologic information. Interpretation of geophysical well

logs for drill holes UE25a-4, UE25a-S, UE25a-6, and UE25a-7 was complicated by

the lack of fluid saturation. Partial fluid saturation caused direct correla-

tion between neutron response and porosity, rather than the usual inverse

relationship. The partially fluid-saturated rocks also caused abnormally high

IP values. The density, and resistivity logs indicate that near-surface

fracture zones are least likely to be present near drill hole UE25a-5.

More mineralogic and petrologic work is needed to clarify the causal

elements of well-log response in welded tuffs and to shed more light on the

unexpected response values of the well-log measurements. Future studies must

also include laboratory physical-properties measurements to link the mineral-

ogic and petrologic work to the geophysical well-log measurements.

28

References

Byers, F. M., Jr., Carr, W. J., Orkild, P. P., Quinlivan, W. D., and Sargent,

K. A., 1976, Volcanic suites and related caldrons of Timber Mountain-

Oasis Valley caldera complex, southern Nevada: U.S. Geological Survey

Professional Paper 919, 70 p.

Christiansen, R. L., Lipman, P. W., Carr W. J., Byers, F. M., Jr., Orkild, P.

P., and Sargent, K. A., 1977, Timber Mountain-Oasis Valley caldera com-

plex of southern Nevada: Geological Society American Bulletin 88, p.

943-959.

Hagstrum, J. T., Daniels, J. J., and Scott, J. H., 1980, Interpretation of

geophysical well-log measurements in drill hole UE25a-1, Nevada Test

Site, Radioactive Waste Program: U.S. Geological Survey Open-File Report

80-941, 32 p.

1980, Analysis of the Magnetic Susceptibility well log from drill hole

UE25a-5, Yucca Mountain, Nevada Test Site: U.S. Geological Survey Open-

File Report 80-1263, 33 p..

Lipman, P. W., Christiansen, R. L., and O'Connor, J. T., 1966, A composi-

tionally zoned ash-flow sheet in southern Nevada: U.S. Geological Survey

Professional Paper 524-F, 47 p.

Smith, R. L., 1960, Ash flows: Geological Society America Bulletin 71, p.

795-842.

Spengler, R. W., 1980, Preliminary Interpretations of geologic results obtain-

ed from boreholes UE25a-4, UE25a-5, UE25a-6, and UE25a-7, Yucca Mountain,

Nevada Test Site: U.S. Geological Survey Open-File Report 80-929,. 33 p.

Spengler, R. W., Muller, D. C., Livermore, R. B., 1979, Preliminary report on

the geology and geophysics of drill hole UE25a-1, Yucca Mountain, Nevada

Test Site: U.S. Geological Survey Open-File Report 79-1244, 43 p.

GPO 830-378

29

interpretation of geophysical well-log measurements in ...open-file report 81-389 open-file report...

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