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MX SITING INVESTIGATION

GRAVITY SURVEY - COAL VALLEY

NEVADA

Prepared for:

U.S. Department of the Air ForceBallistic Missile Office (BMO)Norton Air Force Base, California 92409

Prepared by:

Fugro National, Inc.3777 Long Beach BoulevardLong Beach, California 90807

30 May 1980

,b b N A W AL. ON O .

1SECURITY CLASSIFICATION OF THiS PAGE (When Data fntered)

READ INSTRUCTIONSREPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM

1. REPORT NUMBER - .e - 3 .c 2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

4. TITLE (and Subtitle) 5. TYPE OF REPORT & PERIOD COVERED

-\L\ .6. PERFORMING ORG. REPORT NUMBER

-N -T -r - C LS- -7. AUTHOR(s) 8. CONTRACT OR GRANT NUMBER(s)

f- C, C1 ' C:4C CCC

9. PERFORMING ORGANIZATION NAME AND ADDRESS ID. PROGRAM ELEMENT. PROJECT, TASK, ,-eL . -' ,/ ., ., N AREA & WORK U.NIT NUMBERS

eC)QV ~~ ~ Co '-1;2 L~ 3 F-II. CONTROLL'ING OFFICE NAME AND ADDRESS 12. REPORT DATE

13. ~ NUMBER qEAGES

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IS.. DECLASSIFICATIONDOWNGRADINGSCHEDULE

16. DISTRIBUTION STATEMENT (of this Report)

17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, If different from Report)

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reverse side if necessary and identify by block number)

20. ABSTRACT (Continue an rev'erse side It necessary and Identity by block number)

Gravity measurements were made in Coal Valley for the purpose of estimatingthe overall shape of the structural basin, the thickness of alluvial fill, and

the location of concealed faults. The estimates will be useful in modelingthe dynamic response of ground motion in the basin and in evaluating groundwater

resources.

DD JN 3 1473 EO1TON OF I NOV 65 IS OBSOLETE

SrCURiTY CLASSIFICATION OF THIS PAGE (W1h4en 'at. Entesred)

47

FN-TR-33-CL

FOREWORD

Methodology and Characterization studies during fiscal years1977 and 1978 included gravity surveys in ten valleys in Arizona(five), Nevada (two), New Mexico (two), and California (one).The gravity data were obtained for the purpose of estimating thegross structure and shape of the basins and the thickness of thevalley fill. There was also the possibility of detectingshallow rock in areas between boring locations. Generalizedinterpretations from these surveys were included in FugroNational's Characterization Reports (FN-TR-26a through e).

During the FY 77 surveys, measurements were made to form anapproximate one-mile grid over the study areas and contour mapsshowing interpreted depth to bedrock were made. In FY 79, thedecision was made to concentrate on verifying and refiningsuitable area boundaries. This decision resulted in a reductionin the gravity program. Instead of obtaining gravity data on agrid, the reduced program consisted of obtaining gravitymeasurements along profiles across the valleys where Verifica-tion Studies were also performed.

The Defense Mapping Agency (DMA), St. Louis was requestedto provide gravity data from their library to supplement thegravity profiles. For Big Smoky, Reveille and Railroad valleys,a sufficient density of library data is available to permitconstruction of interpreted contour maps instead of just two-dimensional cross sections.

In late summer of FY 79, supplementary funds became available tobegin data reduction. At that time inner zone terrain correc-tions were begun on the library data and the profiles from BigSmoky Valley, Nevada, and Butler and La Posa valleys, Arizona.The profile data from Whirlwind, Hamlin, Snake East, WhiteRiver, Garden and Coal valleys, Nevada became available from thefield in early October, 1979.

A continuation of gravity interpretations has been incorporatedinto the FY 80 program and the results are being summarized ina series of valley reports. In reports covering Nevada-Utahgravity studies will be numbered, "FN-TR-33-", followed by theabbreviation for the subject valley. In addition, more detailedreports of the results of FY 77 surveys in Dry Lake and Ralstonvalleys, Nevada are being prepared. Verification studies arecontinuing in FY 80 and gravity studies are included in theprogram. DMA will continue to obtain the field measurements andit is planned to return to the grid pattern. The interpretationof the grid data will allow the production of contour maps whichwill be valuable in the deep basin structural analysis neededfor computer modeling in the water resources program. The

i

NAIIGL IN..

FN-TR-33-CL

gravity interpretations will also be useful in Nuclear Hardnessand Survivability (NH&S) evaluations.

The basic decisions governing the gravity progra, are made byBMO following consultation with TRW Inc., Fugro National and theDMA. Conduct of the gravity studies is a joint effort betweenDMA and Fugro National. The field work, including planniizg,logistics, surveying, and meter operation is done by the DefenseMapping Agency Hydrographic/Topographic Center (DMAHTC), head-quartered in Cheyenne, Wyoming. DMAHTC reduces the data toSimple Bouguer Anomaly (see Section A1.4, Appendix Al.0). TheDefense Mapping Agency Aerospace Center (DMAAC), St. Louis,calculates outer zone terrain corrections.

Fugro National provides DMA with schedules showing the valleyswith the highest priorities. Fugro National also recommendedlocations for the profiles in the FY 79 studies within theconstraints that they should follow existing roads or trails.Any required inner zone terrain corrections are calculated byFugro National prior to making geologic interpretations.

1

ii

"-ram.. NatmemaL. .M |.

FN-TR-3 3-CL

TABLE OF CONTENTS

Page

FOREWORD......................... .. ...

1.0 INTRODUCTION................... . . .. . .. .. .. ....

1.1 Objective.................. . . ... . .. .. .. .. .. 11.2 Location................... . . ...... .. .. .. .. 11.3 Scope of Work............. . .. . .. .. .. .. 1

2.0 GRAVITY DATA REDUCTION .................. 5

3.0 GEOLOGIC SUMMARY .................... 6

4.0 INTERPRETATION................................8I:4.1 Regional - Residual Separation .......... 84.2 Density Selection........................94.3 Modeling.............................10

5.0 CONCLUSIONS..................................12

BIBLIOGRAPHY.....................................13

LIST OF APPENDICES

APPENDIX

A1.0 General Principles of the Gravity Exploration Method

A2.0 Coal Valley, Nevada Gravity Data

LIST OF FIGURES

FigureNumber

1 Location Map - Coal Valley, Nevada. ......... 22 Profile Location Map - Coal Valley, Nevada . . . . 33 Interpreted Gravity Profile WR-5 .......... 154 Interpreted Gravity Profile GC-4 . . . . . . ... 165 Interpreted Gravity Profile GC-5. ......... 176 Interpreted Fault Relationship - Coal Valley,

Nevada . . . . . .. .. .. .. . . . . .* . 18

iii

~jINinfTU.UAL. INC.

FN-TR-33-CL

TABLE OF CONTENTS (Cont.)

Page

LIST OF DRAWINGS

DrawingNumber

1 Gravity Station Location Map In PocketCoal Valley, Nevada

IF

iv

I" NATIINAL, #NO.

FN-TR-33-CL

1.0 INTRODUCTION

1.1 OBJECTIVE

Gravity measurements were made in Coal Valley for the purpose of

estimating the overall shape of the structural basin, the

thickness of alluvial fill, and the location of concealed

faults. The estimates will be useful in modeling the dynamic

response of ground motion in the basin and in evaluating ground-

water resources.

1.2 LOCATION

Coal Valley is located in central Nevada (Figure 1) and includes

parts of Nye and Lincoln counties. The valley is accessible

only by improved and unimproved dirt roads. Caliente, Nevada is

located approximately 55 miles (89 km) east of the valley on

U.S. Highway 93.

Coal Valley is bounded on the west by the Golden Gate Range, on

the south by the North Pahranagat Range and on the east by the

Seaman Range (Figure 2). The Seaman Range and Golden Gate Range

merge together forming the northern boundary.

1.3 SCOPE OF STUDY

The Defense Mapping Agency Hydrographic-Topographic Center/

Geodetic Survey Squadron (DMAHTC/GSS) made the 74 gravity

measurements for the three profiles used in this study (Appendix

A2.0). Data from the DMA gravity library was also used to

establish the regional gravity.

T "00 NATIENAL, INC.

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FN-TR-33-CL 4

Profile positions are shown in Figure 2 and the locations of

the individual stations are shown on Drawing 1. The profile

lengths range between 7 miles (11 km) and 11 miles (18 km),

crossing over the valley fill from rock outcrop to rock outcrop.

The gravity sampling interval is approximately 1-mile (1.6 km)

in the central valley and .25 mile (0.4 km) near the valley

boundaries. The denser sampling was used near the valley flanks

to define any steep gravity gradients associated with boundary

faults, and to resolve anomalies with high spatial frequency

that could be associated with shallow bedrock.

The tolerance for establishing station elevations was 5 feet

(1.5 m). This tolerance for elevation control limits the

gravity precision to 0.3 milligals.

Iou NATIONAL, INC.

FN-TR-33-CL

2.0 GRAVITY DATA REDUCTION

DMAHTC/GSS obtained the basic observations and reduced them to

Simple Bouguer Anomalies (SBA) for each station as dcscribed in

Appendix Al.0. Up to three levels of terrain corrections were

applied to convert the SBA to the Complete Bouguer Anomaly

(CBA). First, the Defense Mapping Agency Aerospace Center

(DMAAC), St. Louis, used its library of digitized terrain data

and a computer program to calculate corrections out to 104 miles

(167 km) from each station. When the program could not calcu-

late the terrain effects near a station, a ring template was

used to estimate the effect of terrain within approximately

3000 feet (914 m) of the station. The third level of terrain

corrections was applied to those stations where 10 feet (3 m) or

more of relief was observed within 130 feet (40 m). In these

cases, the elevation differences were measured in the field at a

distance of 130 feet (40 m) along six directions from the

stations. These data were used to calculate the effect of the

very near relief. The CBA data for the Coal Valley stations are

listed in Appendix A2.0.

T "a NATIENAL. INO.

FN-TR-33-CL

3.0 GEOLOGY SUMMARY

Coal Valley is a typical north-south trending valley of the

Basin and Range physiographic province. The Golden Gate Range

separates Garden and Coal Valleys and consists of westward

dipping Paleozoic limestone and dolomite overlain in the north

r by Tertiary ash-flow tuffs and Quaternary basalt (Howard, 1978).

This block is broken by northeast trending faults. The Seaman

Range, on the east side of the valley, consists of Devonian and

Mississippian limestone, dolomite, and quartzite. These are

overlain by a comparatively thin ash-flow tuff layer followed by

a complex series of mud flows, lava flows, volcanic breccias,

and tuff flows connected with an extinct volcano whose eroded

remnants lie east of the center of Coal Valley (Tschanz and

Pampeyan, 1970). The northern end of the Seaman Range is broken

by numerous north-south and east-west trending faults, most of

which appear to predate the valley sediments (Howard, 1978,

Fugro National, 1979).

The MX fault study revealed that previously mapped faults (for

example Howard, 1978) were lineaments due to vegetation, aeo-

lian, and late Pleistocene-Holocene shoreline features. Gravity

data tends to support such an interpretation.

The valley fill consists of 1) alluvial clay, silt, sand,

and gravel units and 2) Pleistocene lake deposits ranging also

from clayey to gravelly sediments but with clay predominant.

The lake deposits comprise about 40 percent of the surficial

geology. The alluvial fan deposits make up the remaining

WW AINAIC

FN-TR-33-CL

7

60 percent of surficial deposits and generally occur above the

5100 foot level of the old shoreline (Tschanz and Pampeyan,

1970 and Fugro National FY 78 and 79 data). The lake sediments

are probably only a few tens of feet thick, but the valley fill

may be several hundred feet thick (although there is little well

data to substantiate this) (Eakin, 1963).

I

I'I

3 " NNATIOINAL, ID

FN-TR-3 3-CL8

4.0 INTERPRETATION

A valley filled with alluvium which has a low-density relative

to the surrounding bedrock creates a negative gravity anom-

aly. Gravity profiles across such valleys are often U-shaped,

low in the middle of the valley where the fill is thickest and

high on the ends where the fill thins and bedrock emerges.

Interpretation requires removal of regional trends leaving

the gravity reflection of the valley fill. The gravity data

and interpreted geologic models for the three profiles across

Coal Valley are shown in Figures 3 through 5.

4.1 REGIONAL-RESIDUAL SEPARATION

A fundamental step in gravity interpretation is isolation of

the part of the CBA which represents the geologic feature of

interest, in this case the relatively low density valley fill.

The portion of the CBA which corresponds to this alluvial

material is called the "residual anomaly".

The CBA contains long-wavelength components from deep and broad

geologic structures extending far beyond the valley. These

long-wavelength components, called the regional gravity, have

been approximated by linear interpolation between CBA values at

bedrock stations on opposite ends of the profiles. Where only

one end of a profile was on bedrock, the regional value on the

other end was assigned a quantity consistent with the regional

trend of the valley. The regional gravity was subtracted from

the CBA and the resulting residual anomaly profiles were used

to model the valley. This regional separation technique is

T a NATIONAL, INC.

FN-TR-33-CL 29J

only approximate. Some regional effects may still remain after

the subtraction but the error is probably small compared to the

large residual anomaly values of these profiles.

The CBA values and the straight line regional field for each

profile is shown in the top portion of Figures 3 through 5. The

residual gravity anomaly (interpolated at evenly spaced points)

is shown by the crosses (x) in the second block of Figures 3

through 5.

4.2 DENSITY SELECTION

The construction of a geologic model from the residual anomaly,

requires selection of density values representative of the allu-

vial fill and of the underlying rock. Since only very general-

ized density information is available, the geologic interpreta-

tion of the gravity data can be only a coarse approximation.

Average in situ density of the fill material was measured be-

tween depths of 100 to 160 feet (30 to 49 m) in several shallow

borings. The observed density range for the soil was 1.7 to

2.0 g/cm 3 . A larger density of 2.3 g/cm 3 was used in the model-

ing process to approximate the expected overall alluvium density

increase due to compaction with depth (compaction with depth and

age is discussed by Woollard, 1962 and Grant and West, 1965).

The basement material underlying the Coal basin is thought to

be the Paleozoic carbonate rocks which are found in the sur-

rounding mountain ranges. Published values for carbonate rocks

typically range between 2.6 and 2.8 g/cm 3 . The Paleozoic

T -a" NATIENAL, INC.

... . ... .. ... ,.= j _ . .. |. . . , . .. . . _ _ _ ,., .....7

FN-TR-3 3-CL10

carbonate rocks in Nevada are generally reported to be relative-

ly high in density, on the order of 2.8 g/cm 3 . This value was

selected to represent the density of the basement rock.

Relative to a given basement density, the calculated basin depth

is inversely proportional to the density value assigned to the

valley fill materials. A one percent change in the average

alluvial fill density will result in a five percent change in

the calculated fill thickness.

4.3 MODELING

An iterative computer program that calculates the gravitational

field for two-dimensional models was used to approximate the

thickness of alluvium beneath each profile. The cross-sectional

models appear as a set of 0.5-km-wide blocks whose tops are at

surface elevation and whose bottoms represent the alluvium-

bedrock boundary. The elevations at the bottoms of the blocks

were adjusted by iterative computation until the computed grav-

ity anomaly for the valley fill differed by less than one

milligal from the observed residual anomaly.

The gravity anomaly computed from the final model can be com-

pared to the residual anomaly in the second block of Figures 3

through 5. The computed anomaly is shown as the solid line and

the residual anomaly is shown by the crosses (x). The final

calculated basin models are shown in the third block of Figures

3 through 5. The cross sections have a five times vertical ex-

aggeration so that even gentle slopes appear steep. The lowest

block shows a suggested geological interpretation.

-roem NATIONAL. IN.

FN-TR-33-CL11

All three gravity profiles indicate that Coal Valley is asymmet-

rical, with a dominant extremely steep fault between the out-

crops and the deep basin with the largest of these being near

the center of the topographic valley. On the southern profile,

GC-5, a horst is indicated on the eastern side of the basin.

The outcrop pattern (Figure 6) south of the profile can be con-

strued to support this interpretation.

The calculated thickness of alluvium diminishes from approxi-

mately 6900 feet (2103 m) beneath profile WR-5 on the north to

about 2900 feet (884 m) beneath profile GC-5 on the south. The

deep basin floor calculates to be almost flat between profiles

WR-5 and GC-4. The average rate of rise to the south between

these profiles is about 50 feet per mile (9 m/km). This rate

increases approximately ten times to more than 500 feet per mile

(95 m/km) between profiles GC-4 and GC-5. Even this latter rate

is insufficient to explain the proximity of the outcrops to the

south. This may indicate that transverse faulting occurs south

of profile GC-5.

1P AIIaIn

FN-TR-33-CL12

5.0 CONCLUSION

There is a large well defined negative gravity anomaly as-

sociated with Coal Valley. An average density contrast of

0.50 g/cm 3 between the alluvium and bedrock was used to calcu-

late the thickness of the valley fill material.

The gravity interpretation of Coal Valley indicates that the

basin is asymmetrical with a major range bounding normal fault

on the western side of the valley with a series of lesser faults

in the eastern half of the valley. The primary graben block

lies along the Golden Gate Range and is oriented NNE-SSW. Cal-

culated depth ranges from 6900 feet (2103 m) beneath profile

WR-5 and 2900 feet (884 m) beneath profile GC-5 near the south

end.

The calculated bedrock depths can only be approximations since

little is known about the actual density distribution in and

around the valley. Future studies that acquire better density

data or actual depth to bedrock in deep parts of the valley can

be used to refine the gravity interpretation.

IItI

Io

FN-TR-33-CL13

BIBLIOGRAPHY

Cornwall, H. R., 1972, Geology and mineral deposits of southernNye County, Nevada: Nevada Bureau of Mines and GeologyBull. 77, 49 p.

Eakin, T. E., 1963b, Ground-Water Appraisal of Garden and CoalValleys, Lincoln and Nye Counties, Nevada: Ground-WaterResources - Reconnaissance Series, Report 18, 29 p.,1 plate.

,_1963c, Ground-water appraisal of Fahranagat andPahroc valleys, Lincoln and Nye counties, Nevada: NevadaDept. of Conservation and Natural Resources, Ground-WaterResources Reconnaissance Series Report 21, 35 p.

Fugro National, Inc. 1979, MX Siting Investigation GeotechnicalEvaluation, Verification Studies, Volume 1A, 1B, V, FN-TR-27.

1978 and 1979, Verification Study Data of Gardenand Coal Valley, FN-TR-27-VI.

, 1980, Interium Report on Active Faults and Earth-quake Hazards in the FY 79 Verification Sites - Nevada -Utah Siting Region, FN-TR-36.

Grant, F. S. and G. F. West, 1965, Interpretation Theory inApplied Geophysics: McGraw-Hill Book Co., New York.

Hays, W. W., 1976, Interpretation of Gravity Data, U.S. Geol.Survey, Open file report 76-479.

Howard, E. L., 1978, Geologic map of the Eastern Great Basin,Nevada and Utah, Terra Scan Group, Colorado.

Maxey, G. B., and Eaken, T. E., 1949, Ground water in WhiteRiver Valley, White Pine, Nye, and Lincoln Counties,Nevada: Nevada State Engineer, Water Resources Bull. 8.

Stewart, J. H., and Carlson, J. E., 1978, Geologic map ofNevada, U.S. Geol. Survey and Nevada Bur. of Mines andGeol.

Tschanz, C. M. and E. H. Pampeyan, 1970, Geology and mineral de-posits of Lincoln County, Nevada: Nevada Bureau of Mines,Bull. 73, University of Nevada.

West, R. E., 1971, An iterative computer program for calculatingtwo-dimensional models for alluvial basins from gravity andgeologi. data (modified and extended by H. W. Powers Jr.,1974): University of Arizona, Geoscience Department, Geo-physics Laboratory.

1""i NATINAL, INO.

FN-TR-33-CLBIBLIOGRAPHY (Cont.) 1

Woollard, G. P., 1962, The relat-ion of gravity anomalies to sur-face elevation, crustal structure, and geology: Universityof Wisconsin, Dept. of Geology, Geophysical and PolarResearch Center, Madison, Wisconsin, Report 62-9.

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m~~~1N NA I N L IN~0U)0 C C

FN-TR-33-CL

APPENDIX A1.0

GENERAL PRINCIPLES OF THE

GRAVITY EXPLORATION METHOD

FN-TR-33-CLAl-i

Al.0 GENERAL PRINCIPLES OF THE GRAVITYEXPLORATION METHOD

A1.1 GENERAL

A gravity survey involves measurement of differences in the

gravitational field between various points on the earth's

surface. The gravitational field values being measured are the

same as those influencing all objects on the surface of the

earth. They are generally associated with the force which

causes a 1 gm mass to be accelerated at 980 cm/sec 2 . This

force is normally referred to as a 1 g force.

Even though in many applications the gravitational field at the

earth's surface is assumed to be constant, small but distin-

guishable differences in gravity occur from point to point.

In a gravity survey, the variations are measured in terms

of milligals. A milligal is equal to 0.001 cm/second 2 or

0.00000102 g. The differences in gravity are caused by geo-

metrical effects, such as differences in elevation and latitude,

and by lateral variations in density within the earth. The

lateral density variations are a result of changes in geologic

conditions. For measurements at the surface of the earth, the

largest factor influencing the pull of gravity is the density of

all materials between the center of the earth and the point of

measurement.

To detect changes produced by differing geological conditions,

it is necessary to detect differences in the gravitational field

as small as a few milligals. To recognize changes due to

T S NATuAL. $Nm.

FN-TR-33-CLAl-2

geological conditions, the measurements are "corrected" to ac-

count for changes due to differences in elevation and latitude.

Given this background, the basic concept of the gravitational

exploration method, the anomaly, can be introduced. If, instead

of being an oblate spheroid characterized by complex density

variations, the earth were made up of concentric, homogeneous

shells, the y-avitational field would be the same at all points

on the surface of the earth. The complexities in the earth's

shape and material distribution are the reason that the pull of

gravity is not the same from place to place. A difference in

gravity between two points which is not caused by the effects of

known geometrical differences, such as in elevation, latitude,

and surrounding terrain, is referred to as an "anomaly."

An anomaly reflects lateral differences in material densities.

The gravitational attraction is smaller at a place underlain by

relatively low density material than it is at a place underlain

by a relatively high density material. The term "negative

gravity anomaly" describes a situation in which the pull of

gravity within a prescribed area is small compared to the area

surrounding it. Low-density alluvial deposits in basins such as

those in the Nevada-Utah region produce negative gravity anoma-

lies in relation to the gravity values in the surrounding

mountains which are formed by more dense rocks.

The objective of gravity exploration is to deduce the variations

in geologic conditions that produce the gravity anomalies

identified during a gravity survey.

jII NaTI AL. Mn.

FN-TR-33-CLAl-3

Al.2 INSTRUMENTS

The sensing element of a LaCoste and Romberg gravimeter is a

mass suspended by a zero-length spring. Deflections of the

mass from a null position are proportional to changes in gravi-

tational attraction. These instruments are sealed and compen-

sated for atmospheric pressure changes. They are maintained at

a constant temperature by an internal heater element and thermo-

stat. The absolute value of gravity is not measured directly by

a gravimeter. It measures relative values of gravity between

one point and the next. Gravitational differences as small as

0.01 milligal can be measured.

A1.3 FIELD PROCEDURES

The gravimeter readings were calibrated in terms of absolute

gravity by taking readings twice daily at nearby USGS gravity

base stations. Gravimeter readings fluctuate because of small

time-related aeviations due to the effect of earth tides and

instrument drift. Field readings were corrected to account for

these deviations. The magnitude of the tidal correction was

calculated using an equation suggested by Goguel (1954):

C = P + Ncos 0 (cos 6 + sin ) + Scos 0 (cos 0 - sin 0)

where C is the tidal correction factor, P, N, and S are time-

related variables, and 0 is the latitude of the observation

point. Tables giving the values of P, N, ana S are published

annually by the European Association of Exploration Geophysi-

cists.

T U"nS NATIaNAL, INC.

FN-TR-3 3-CLAl-4

The meter drift correction was based o. readings taken at a

designated base station at the start and end of each day. Any

difference between these two readings after they were corrected

for tidal effects was considered to have been the result of

instrumental drift. It was assumed that this drift occurred at

a uniform rate between the two readings. Corrections for drift

were typically only a few hundredths of a milligal. Readings

corrected for tidal effects and instrumental drift represented

the observed gravity at each station. The observed gravity

values represent the total gravitational pull of the entire

earth at the measurement stations.

A1.4 DATA REDUCTION

Several corrections or reductions are made to the observed

gravity to isolate the portion of the gravitational pull which

is due to the crustal and near-surface materials. The gravity

remaining after these reductions is called the "Bouguer

Anomaly." Bouguer Anomaly values are the basis for geologic

interpretation. To obtain the Bouglier Anomaly, the observed

gravity is adjusted to the value it would have had if it had

been measured at the geoid, a theoretically defined surface

which approximates the surface of mean sea level. The dif-

ference between the "adjusted" observed gravity and the gravity

at the geoid calculated for a theoretically homogeneous earth is

the Bouguer Anomaly.

INIS NMATINAL. INC.

FN-TR-33-CLAl-5

Four separate reductions, to account for four geometrical

effects, are made to the observed gravity at each station to

arrive at ihs Bouguer Anomaly value.

a. Free-Air Effect: Gravitational attraction varies inversely

as the square of the distance from the center of the earth.

Thus corrections must be applied for elevation. Observed

gravity levels are corrected for elevation using the normal

vertical gradient of:

FA = -0.09406 mg/ft (-0.3086 milligals/meter)

where FA is the free-air effect (the rate of change of gravity

with distance from the center of the earth). The free-air

correction is positive in sign since the correction is opposite

the effect.

b. Bouguer Effect: Like the free-air effect, the Bouguer

effect is a function of the elevation of the station, but it

considers the influence of a slab of earth materials between

the observation point on the surface of the earth and the

corresponding point on the geoid (sea level). Normal practice,

which is to assume that the density of the slab is 2.67 grams

per cubic centimeter was followed in these studies. The Bouguer

correction (Bc), which is opposite in sign to the free-air

correction, was defined according to the following formula.

Bc = 0.01276 (2.67) hf (milligals per foot)

Bc = 0.04185 (2.67) hm (milligals per meter)

where hf is the height above sea level in feet and hm is the

height in meters.

nUeN NATIONAL, INC.

FN-TR-33-CL

Al-6

c. Latitude Effect: Points at different latitudes will have

different "gravities" for two reasons. The earth (and the

geoid) is spheroidal, or flattened at the poles. Since points

at higher latitudes are closer to the center of the earth than

points near the equator, the gravity at the higher latitudes is

larger. As the earth spins, the centrifugal acceleration

causes a slight decrease in gravity. At the higher latitudes

where the earth's radii are smaller, the centrifugal accel-

eration diminishes. The gravity formula for the Geodetic

Reference System, 1967, gives the theoretical value of gravity

at the geoid as a function of latitude. It is:

g = 978.0381 (1 + 0.0053204 sin 2 € - 0.0000058 sin 2 26) gals

where g is the theoretical acceleration of gravity and 0 is

the latitude in degrees. The positive term accounts for the

spheroidal shape of the earth. The negative term adjusts for

the centrifugal acceleration.

The previous two corrections (free air and Bouguer) have ad-

justed the observed gravity to the value it would have had at

the geoid (sea level). The theoretical value at the geoid for

the latitude of the station is then subtracted from the adjusted

observed gravity. The remainder is called the Simple Bouguer

Anomaly (SBA). Most of this gravity represents the effect of

material beneath the station, but part of it may be due to

irregularities in terrain (upper part of the Bouguer slab) away

j from the station.

I

- mf -WTI-AL. IN .

FN-TR-3 3-CLAl-7

d. Terrain Effect: Topographic relief around the station has

a negative effect on the gravitational force at the station. A

nearby hill has upward gravitational pull and a nearby valley

contributes less downward attraction than a nearby material

would have. Therefore, the corrections are always positive.

Corrections are made to the SBA when the terrain effects were

0.1 milligal or larger. Terrain corrected Bouguer values are

called the Complete Bouguer Anomaly (CBA). When the CBA is

obtained, the reduction of gravity at individual measurement

points (stations) is complete.

Al.5 INTERPRETATION

The first step in interpretation is to separate the portion of

the CBA that might be caused by the lightweight, basin-fill

material overlying the heavier bedrock material which forms the

surrounding mountains and presumably the basin floor. Since the

valley-fill sediments are absent at the stations read in the

mountains, the CBA values at these bedrock stations ar used as

the basis for constructing a regional field over the valley. A

regional field is an estimation of the values the CBA would have

had if the light weight sediments (the anomaly) had not been

there.

The difference between the CBA and the regional field is

called the "residual" field or residual anomaly. The residual

field is the interpreter's estimation of the gravitational

effect of the geologic anomaly. The zero value of the residual

anomaly is not exactly at the rock outcrop line but at some

lh U UL. INC.

FN-TR-33-CLAl-8

distance on the "rock" side of the contact. The reason for this

is found in the explanation of the terrain effect. There is a

component of gravitational attraction from material which is

not directly beneath a point.

If the "regional" is well chosen, the magnitude of the residual

anomaly is a function of the thickness of the anomalous (fill)

material and the density contrast. The density contrast is the

difference in density between the alluvial and bedrock material.

If this contrast were known, an accurate calculation of the

thickness could be made. In most cases, the densities are not

well known and they also vary within the study area. In these

cases, it is necessary to use typical densities for materials

similar to those in the study area.

If the selected average density contrast is smaller than the

actual density contrast, the computed depth to bedrock will be

greater than the actual depth and vice-versa. The computed

depth is inversely proportional to the density contrast. A ten

percent error in density contrast produces a ten percent error

in computed depth. An iterative computer program is used to

calculate a subsurface model which will yield a gravitational

field to match (approximately) the residual gravity anomaly.

-Pn OVATUAL 36O.

E._____,_____

FN-TR-3 3-CL

APPENDIX A2.0

COAL VALLEY, NEVADA

GRAVITY DATA

I!

FN-TR-3 3-CL

COAL VALLEY GRAVITY DATA

PRI IF I I F (;r-'i

STAT TfN L AT. I IJNr. FLFV. TFP-Cflp. frfl4TH FAST 11145V T mt F AA ri,4ATOFNT. rAG MTN DPG MTN *rnrFl TN/HUIT IIT'M ITM (1 rC'AV+

GCO00 375267 115P314* SLP58 0 357'j1Q3PIA 3--I I, 'qljCCO402 3752?6b 115?305 S?ASR 0 4b~lJlq3?7 6.adljll177AiiQRl'7 -t-,l Alt,?.-GCni,103 37S265 1IIS??77 9 17 7R 0 2QFA 1 Q Wt 62S14cO?QAl !L) -I ? ')# 5 It1CCO404 37- ?h3 115?250 SOqOR 0 ?L414~1(3?3 64Qp-14A37?jlF~13? I A A 0.?f);COL.0 37S299 1 15???23 5 n16 8 0 ?1 YsI C; Iih i 3 11 ? 3Q d, q#1 ?7 -?',?;uA ' 7( 1

GCO'J06 379255 11'SPi97 4qr,!B 0 1qh'e1P1AQ 6143701491qhl'-R1? *lZf'7 700Ltu

(;CO'OCI 375291 1 197171 QJ~aaJ o j7'.)iq~n? 6 It17 8 S 1 91 1 -7 "4,) 7(#('A'GCOOOP~ 379,248 1 t 92 1 a LIQLISF 0 1 L11 Q?q h10SOI a?e)6 11( I I uB. 7e)?'lnCCOilfI 379244L 115?115 S Q?7P 0 1411/4 1 C?9 14n 1 4 7c?7IQAI0E (( 7QI ?(CnilO !79228 115 p o tq IApM 0 11d1 qQ?6J 6ah1191/J17UO/J1q990P 1-J f -4 70C'7 UC 7GC(N41t 379212 1151902 4i9l9p 91 ~1 q?!7 6atImfli 7#,l 9A~c;M lQ'Yl 7 O) 0 eGCC0L? 37519q4 11%1707 ejQaP Aa 0 RL1?7 49 S A 7 7 1 QA (O!1 -5'-), 1 .'(,

6 C 0 414 3 7 c1684 1 15 1 4;? is 9 4A F 0 A74 191q7 6 27 a I i I 01 97c9 F-~ eGCO4*15 179147 1191476~ 4qc'4P 0 q 7'1 cq 96r 10 1 4 a 1QCn49(- 1 f tt .i-CCO416I 3751d41 1 151uL11 /19568 n 1011hQ19 6S A cA 2 iQ 5 0 Q7~' Q 1MI Q19GCCoull l7c5136 I1t?409 uo~qP ) I O~jiIIql 1 6C5q1 4oliJ IAlq7Q.J7 1 7 3(

Grnale~ 375132 1 15i'1 07AR n0 1 hil IQ10 n r56bl 49L609979d4 Ih' I t A1 4 E 'rCO/Jlg '75128 I t51 s So IIp 0 1204 1 9OQ7 ;,~oiJ I JQ?()7 1 7.1P IP-Lc7 v ri o ICCO'i2 37S1?3 1 151310 gnlp 0) 131L1.QOPJR 69b/1 I 49?E-.I9 7O?-' liSQ PAIi tj PPr 04?? 175131 1 t ,1? 77 S 17 qH 0 1 62t;1 q I AS7? 31.a8c;R7 I 7Qb 11 -hf)7 PlI AQ

GC04?3 37c;142 1 1~525 SPS 0 22?301 9 1 P 6R75h5IQ01'on c4 QS. 3O M7 oGrO424 1 '7r115? 1 15 1 ?3255R 0?~t1J P?1Z

f n40'J 371;16 3 1110 J7o 0 0flJl166 6 85? 1l'V7? iO197 QP7 7#.

P.Nf) (IF I IST

FN-TR-33-CLi A2 -2

COAL VALLEY GRAVITY DATA

pRI),FILE~ GC 5 ,

STATION LAT, LONG. HFv. TFR-CL[k. NOR1H FAS S 'V TH .C FAA CHAIIDFNT, DFG MIN DFG MIN +¢CC TN/00T tiTto Ult-, GwAV GWA +|000

GCn502 37474o 1IIi 2S3L o 7 1 0 1')341ft3 5 q b Wq71 ,7 37 7 9t 6 2 'f

GCO503 374743 1i ,'"3 z719k o 1734IR3;5 19371 4 /1i Rq197373 931 1 t .?GCAS04 374736 115478 ;85P 0 1 1lilP3142 63Q?414t7a91973b3 b88 51bh2GCo5oS 37413U libPijSl rb64h 0 IS"41833? 601414I9 A71 9 7354 843 81714GCOSob 17723 115?25 56 18P 0 17'183?0 015a 4506l097344 5qu A1580GCOSoT 37L717 1!5PIP9 S578H 0 141,41A369 640911490O719733S 165 812A6GCO508 374711 115PS72 SS366 0 1414JR299 h413014046197327 -3bp 9087GC05o9 37475 115?146 SS016 0 134418288 41AQ14S&755197318 -789 80582GCO5jO 3*Jb97 115A19 54A28 0 131418214 64209144662197306 -lb5 80382GCO511 374672 li5,'21O 5344R 0 120418211 4369q145435197270 -1545 603U9GCO512 574643 1152096 5274H 0 113418180 6053814 632O1 9 72 27 -1225 80683GCnSji 374613 11514Q! 5P77H 0 1124181p7 64bq31p;29q19?l184 -b9Q 81585GCOSA 374606 11bQE4 521 nH 0 109418115 64733147233197173 -90Q 81431GCOS1 37a599 1151938 5?06H 0 108418103 64771147197197163 w975 81379GCnSI6 374593 1151911 52 68 0 1084180q2 6481 114J61197155 -99s o13SoGCO517 374587 1151885 5244k 0 10a18082 k4849141031197145 -1143 81217GCOI8 374580 1151859 52n6B 0 109418070 6a888149q897135 -11&5 81171GC 85 374574 1151833 520RA 0 112.18059 449?6la6r19197121 -115s 81194GCO5?o 374567 n!5180b 5fi n 1184180R47 6,4961470?2197116 -1Ul7 8131"GCO521 3756O 1151780 SPOB 0 12141803S 65004I413219710b -645 81471

EN') OF LIST

'ALN-

FN-TR-3 3-CLA2-3

COAL VALLEY GRAVITY DATA

PfkupIt F *pS

STATTI!N LAT, LUPt-, ELEV. TFp-CUR, NflPTH EAST fl6sv THE.C FAA Cf3AIDFNT, 0FG mlr DFG mN +CmrmE tN/OUT UTM UiTm Gv (;RAv +000

wipnsot 38 148 its 887 59Aiy 0 209'J209q3 &62%1l"S22Mqq4'23 2119 81919'WPO502 38 131 115 915 589S8 C 190420964 A621,t1hSb971994S01 1776 8160NR05nS 38 131 115 943 jS891M 0 18142Oq63 66176146089199401 1359 01726WRO504 38 134 115 972 5744H1 0 169'J2f968 66133146345199405 99P 91571WRO505 38 138 1151003 S6AS8 0 158420975 6608614665119Q411 559 813q4ARHo501 38 154 1t5ijS5 55taM 0 1354~21003 bb01 14Vl5a19q434 -IS ail7WaRnOoA 38 172 1010o6S 95218 0 14542t036 6599614T28%199461 -212 81101ARfl509 38 179 1151086 54A08 o 132421048 iA59A4i~t52aj9Q'471 -371 810h9WR0510 38 17b 1151114 54'42B 0 1254210a2 69?1477599468 -500 81065AR0511 38 178 1151 142 530~h 0 123421045 658831'J7998199470 -658 81049WROS p 38 180 1151170 53 BB 0 1184J21048 6S84214A201199473 -b4S 80997bWiRf511 38 185 1151 198 K3tlH 0 11742105ib6r58011,IA381l99480 11A6b 80)925'PRO514 18 188 1151225 N&MOB C) 115421081 657611 1tu2!199"A4 -1373 80735WROS5IS 38 188 112tP4 524tB C114421080 6S7t91j464(01l9948Q -1756 A04filWP0516 38 18S ll5t.'82 Sin68 rj1094i21054 6567814 84 IS199 48 C0 2U6#6 8028':WRoit? 38 178 1151;99 So0ey 0 9P421017 6S5bn7149ao199470 -216S 7999bWRflSlg 56 187 1151s06 50148 0 94'421051 053501407pat9Q"83 -3b77 79411wR05 q 38 197 1151614 4 9q88 0 83421067 A5142148)1119949 8 -4J347 786M9WROSPO 38 207 1151724 Soj U 7842108?2 65U3j11374n199512 -4b88 103?b'00521 38 216 115t832 50t58 0 774210qb 64872147bAM19952S -'407 78294wRospa 38 ,19 1iis5o,9j8 9 774211n1l 6 483314J77?0199b30 -Q579 783P1wPo5?S 38 222 1151887 %.0?28 (1 78'J21too 6479PI178 35199534 -4433 7851!5'NPOS4 38 e24 1151913 5W8R 784211o9 60541"A001199537 -42?8 78705WRO525 38 226 11Ibl40 50.31k U 8U,4f 1 j l2 *,471 4j'2r%7l4954U -3941 739-41Ok5638 229 l 51o68 c ~ ~ 0 81jq21 ib Aa73j44Sqq15 -35Qj 7931.?

hkn1527 38 231 11IPi95 )':1P 0 83"lJ21i1 b643i1-id53l9qQ? -3251 7963bhROS?S 38 233 IISPM22 %1lzeH 0 8042?1122 1 4Sq 4 149l 6 11 Q9550 -e897 7q977hROSPO 38 236 ItSPnOS SO52P 0 92'J21127 ha5531404Q3194555 -1$19 $0343wRnb3O 38 239 11b~07b s0;78 0 974?11'42 e5 5j4990 6jQ95S9 -2004 s50786'"'0531 38 241 11%?i04 Sn6Sp 0 114'.2t135 64'7alS490t99562 -15305 A1334

END ,F LIST