U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Basement Structure Beneath Langford Well Lake Basin, Fort Irwin, California,

Based on Inversion of Gravity data.

By Robert L. Morin1

Open–File Report 00–518

2000

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards

or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes

only and does not imply endorsement by the U.S. Government.

1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
TABLE OF CONTENTS

INTRODUCTION......................................................................................................................................................1

ACKNOWLEDGMENTS............................................................................................................................................1

GRAVITY REDUCTION.............................................................................................................................................1

PROCEDURE...............................................................................................................................................5

DENSITY–DEPTH FUNCTION......................................................................................................................6

CONCLUSIONS.....................................................................................................................................................10

REFERENCES CITED...............................................................................................................................................13

ILLUSTRATIONS

FIGURE 1. TOPOGRAPHIC MAP OF STUDY AREA. ................................................................................................................2

 2. LOCATION OF GRAVITY STATIONS.....................................................................................................................3

 3. ISOSTATIC GRAVITY MAP....................................................................................................................................4

 4. DEPTH–TO–BASEMENT MAP. .............................................................................................................................9

 5. STRUCTURAL CONTOUR MAP...........................................................................................................................11

 6. FAULTS INFERRED FROM GRAVITY GRADIENTS...............................................................................................12

TABLES

TABLE 1. DENSITY–DEPTH FUNCTIONS. ..............................................................................................................................7
1

INTRODUCTION

Gravity data were used to study the basement structure of Langford Well Lake basin at the U.S.

Army National Training Center, Fort Irwin, California. Figure 1 shows the location of the study area.

During 1996 and 1999, 290 new gravity stations were measured. These data were merged with existing

data to produce a depth–to–basement map, which, in turn was converted to a structure map of the

basement surface below alluvial fill. This information can be used to help interpret water flow and

reservoir capacity of the basin. In addition, gravity gradients were used to suggest locations of faults

through or below alluvial fill. These gradients may be evidence for repositioning or extending mapped

faults.

The locations of gravity stations are shown in figure 2 plotted on a colored grid of topographic

elevations generated from 30 m DEM’s (Digital Elevation Models). As shown by figure 3, gravity data

used in this study are sufficiently accurate to permit 1–mGal contour intervals. Much of the older regional

data in this study area are of lesser quality although they were included because they sufficiently

represent regional gravity.

ACKNOWLEDGMENTS

Greg Smith provided much assistance in collecting and processing the gravity data.

GRAVITY REDUCTION

Conversion from meter readings to milligals was made using factory calibration constants. The

meter used in the 1996 survey has an additional calibration factor determined by multiple gravity

readings over the Mt. Hamilton calibration loop east of San Jose, Calif. (Barnes and others, 1969).

Observed gravity values were based on an assumed linear drift between successive base readings.

Vertical and horizontal controls were made by two techniques. The data collected in 1996 were

established with laser surveying equipment, which has a vertical accuracy of about 0.1 m. The

horizontal and vertical controls of the data collected in 1999 were made with a precision GPS system

which also has a vertical accuracy of about 0.1 m.

Theoretical gravity at sea level is based on the Geodetic Reference System 1967 (GRS 67)

(International Association of Geodesy, 1971, p. 58) for the shape of the spheroid. The datum for the

observed gravity is the International Gravity Standardization Net 1971 (IGSN 71) (Morelli, 1974. p. 18).

Observed gravity was calculated by adding meter drift and earth–tide corrections to the milligal

equivalent meter readings. Free–air anomalies were calculated by subtracting the theoretical gravity

from the observed gravity and adding the free–air correction as defined by Swick (1942, p. 65). Simple

Bouguer anomalies were calculated by subtracting the Bouguer correction from the free–air anomaly.

The Bouguer correction accounts for the attraction of rocks between the station and sea level, using a

rock density of 2.67 g/cm3

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Langford 

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Coyote

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Alvord Mountain

Langford

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Lake

Basin

Figure 1. Topographic map of study area. Solid lines, selected roads; dashed lines, edges of lake beds.

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Figure 2. Location of gravity stations and outline of basement exposure shown on topographic map of study

area. Black dots, pre–existing stations; red dots, stations collected in 1996; blue dots, stations colleced in 1999.

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Figure 3. Isostatic gravity map of study area. Contour interval 1 mGal. Black dots, gravity stations.

4
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Complete Bouguer anomalies were calculated by adding the terrain correction and the

curvature correction to the simple Bouguer anomaly. The gravitational effects of terrain out to 166.7 km

from each station were removed from observed gravity in three steps: field corrections for local terrain

out to 68 m (Hayford and Bowie 1912); inner–zone corrections, for terrain between 68 m and 0.59 km

(Hayford and Bowie 1912; Spielman and Ponce, 1984) using 30 m DEM’s; and computer–based

corrections (Plouff, 1966; Plouff, 1977; Godson and Plouff, 1988) for terrain between 0.59 km and 166.7

km.

These data were processed through an isostatic reduction program (Jachens and Roberts, 1981)

in order to suppress the effects of deep density distributions that buoyantly support the topography.

The isostatic reduction assumes an Airy–Heiskanen model with the following parameters from the

station to 166.7 km: density of topography above sea level, 2.67 g/cm3

; crustal thickness at sea level, 25

km; density contrast across the base of the model crust, 0.4 g/cm3

. From 166.7 km to a point on the

opposite side of the Earth, isostatic and terrain corrections were taken off maps by Karki (1961). These

corrections were added to the output of the isostatic program of Jachens and Roberts (1981) to produce

the isostatic correction. Isostatic anomalies are calculated by adding the isostatic correction to the

complete Bouguer anomaly. The isostatic gravity map is shown in figure 3.

DEPTH–TO–BASEMENT

The depth to pre–Cenozoic basement beneath Langford Well Lake basin was calculated using

gravity data, bedrock geology, and variable density of sediments. For this study area, basement consists

mostly of Cretaceous and Jurassic plutons. Alvord Mountain, in the southern part of the study area,

contains Tertiary volcanic basalts, which are considered basement for this exercise. Digitized polygons of

basement outcrop were made using up–to–date geologic maps (David Miller, U.S. Geological Survey,

written commun., 1998). These polygons are shown on figures 2, 4, and 5. The sediments are Quaternary

and Tertiary in age.

PROCEDURE

Depth–to–basement was calculated using a method developed by Jachens and Moring (1990) and

since updated to use constraining depth data such as wells that penetrate basement or seismic estimates

of basin fill (Bruce Chuchel, U.S. Geological Survey, written commun., 1996). The gravity data are sorted

into three files: measurements that lie on bedrock in the study area, measurements that lie on fill in the

study area, and measurements that are outside the study area. Data outside the study area are used to

avoid edge effects in this method. The procedure first calculates a grid based just on bedrock stations. In

areas of basin fill, values derived from the grid are subtracted from actual measurements, and the

difference is used to estimate the thickness of basin fill assuming a specified density–depth function.

There are a couple of difficulties with this initial estimate: 1) Gravity observed at bedrock stations near

the edge of the outcrops are lowered by the effect of the nearby low–density fill. 2) The lateral densities of

the bedrock are not taken into account, which effects the basement gravity field. The lateral variations of

the density of Cenozoic deposits are not taken into account in this inversion method.
6

To get around these problems, a smooth surface is fit through the basement gravity grid by

interpolation, including locations where the basin thickness is known and using the density–depth

function. The first estimate of basin thickness is made and used to correct the basement gravity at all

gravity station locations. The basin thickness is forced to zero within the bedrock polygons. This gives a

first approximation of the basement gravity. This process is repeated until the basement gravity shows no

appreciable change.

DENSITY–DEPTH FUNCTION

The density–depth function allows for changes in density of Cenozoic deposits as a function of

depth. Various density–depth functions were tried in order to match known or suspected physical

properties of Cenozoic deposits in the Langford Well Lake basin. The density–depth functions list the

density of each layer and the depth from the surface of the contact between each layer. The gravity data

were reduced with a density of 2.67 g/cm3

. The depths used are in kilometers.

Unfortunately, there are very few absolute constraints in this basin (table 1). There are eight wells

in the basin that were used, but only one, LW1 reaches basement and its depth is very shallow (32 m, 105

ft). There are also two seismic refraction lines with preliminary depth estimates. The location of the wells

used and the seismic lines (line 1 and line 2) are shown on figure 4. The rest of the wells did not reach

basement. Several density–depth functions were tried in order to determine a basin at least as deep as the

wells that did not reach basement and consistent with seismic depths.

The Quaternary/Tertiary (Q/T) boundary is estimated to be at a depth of 0.13 km (Jill

Densmore, U.S. Geological Survey, written commun., 1996). Friable Tertiary sandstone from a depth of

about 240 m (800 ft) in LL1 well has a measured a dry bulk density of about 2.4 g/cm3

. The estimated

depths to basement from seismic reflection are 171 m (560 ft) for line 1 and 287 m (940 ft) for line 2 (David

Berger, U.S. Geological Survey, written commun., 1996).

Table 1 shows the results of the various density–depth functions. The table lists the well

identification, depth drilled, the calculated depth based on the density–depth function, and the difference

between the well depth and the calculated basin depth at the well location. Seismic information is also

shown. Because most of the wells do not reach basement, the assumption is that the basin thickness at

those locations is deeper than the bottom of the wells. Therefore, the density–depth function should

produce a negative number in the difference column. The density–depth function should also produce

basin depths at the locations of the seismic profiles that approximate those depths estimated from the

seismic interpretation. The values in the difference column should be close to zero for the seismic

locations.

The first density–depth function used in this study was originally developed for the Basin and

Range (Jachens and Moring, 1990). This yielded a basin thickness much too shallow. Increasing sediment

density increases basin thickness. The density–depth function developed from seismic lines 1 and 2, were
7

Table 1. Density–depth functions.

[Depths and differences in feet.]

Standard Density–Depth Function: 2.02, 2.12, 2.32, 2.42 g/cm3 0.2, 0.6, 1.2 km 

Note Well ID Well

Depth

Seismic

Depth

Calculated

Depth

Difference

LX–1 586 259 327

LL1 800 484 316

basement LW1 105 2 6 7 9

LP1 703 627 7 6

LP2 702 457 245

LP3 601 383 218

TH–8 374 617 -243

TH–10 301 440 -139

Line 1 560 357 203

Line 2 on lake 940 649 291

Based on Seismic Line 2 

Density–Depth Function: 1.33, 2.04, 2.32, 2.42 g/cm3 .02, 0.29, 1.2 km 

Note Well ID Well

Depth

Seismic

Depth

Calculated

Depth

Difference

LX–1 586 183 403

LL1 800 410 390

basement LW1 105 1 3 9 2

LP1 703 547 156

LP2 702 384 318

LP3 601 308 293

TH–8 374 538 -164

TH–10 301 364 -63

Line 1 560 289 271

Line 2 on lake 940 564 376

Based on Seismic Line 1 

Density–Depth Function: 1.64, 1.80, 2.19, 2.35, 2.42 g/cm3 .02, .06, 0.17, 1.2 km 

Note Well ID Well

Depth

Seismic

Depth

Calculated

Depth

Difference

LX–1 586 177 409

LL1 800 452 348

basement LW1 105 1 6 8 9

LP1 703 692 1 1

LP2 702 416 286

LP3 601 321 280

TH–8 374 669 -295

TH–10 301 390 -89

Line 1 560 292 268

Line 2 on lake 940 735 205
8

Table 1. Density–depth functions. —Continued

Based on Rock Sample and Q/T Boundary; no constraints 

Density–Depth Function: 2.17, 2.27, 2.37, 2.42 g/cm3 0.13, 0.24, 0.6 km 

Note Well ID Well

Depth

Seismic

Depth

Calculated

Depth

Difference

LX–1 586 341 245

LL1 800 689 111

basement LW1 105 3 4 7 1

LP1 703 1053 -350

LP2 702 643 5 9

LP3 601 525 7 6

TH–8 374 1027 -653

TH–10 301 617 -316

Line 1 560 479 8 1

Line 2 on lake 940 1152 -212

Based on Rock Sample and Q/T Boundary; using well 

Density–Depth Function: 2.17, 2.27, 2.37, 2.42 g/cm3 0.13, 0.24, 0.6 km

Note Well ID Well

Depth

Seismic

Depth

Calculated

Depth

Difference

LX–1 586 364 222

LL1 800 702 98

basement force LW1 105 103 2

LP1 703 1056 -353

LP2 702 653 4 9

LP3 601 541 6 0

TH–8 374 1040 -666

TH–10 301 620 -319

Line 1 560 489 7 1

Line 2 on lake 940 1105 -165

Based on Rock Sample and Q/T Boundary; using well and forcing 

Density–Depth Function: 2.17, 2.27, 2.37, 2.42 g/cm3 0.13, 0.24, 0.6 km

Note Well ID Well

Depth

Seismic

Depth

Calculated

Depth

Difference

 force LX–1 586 570 1 6

 force LL1 800 790 10

 Basement force LW1 105 104 1

LP1 703 1112 -409

 force LP2 702 702 0

 force LP3 601 603 -2

TH–8 374 1158 -784

TH–10 301 692 -391

Line 1 force 560 558 2

Line 2 on lake force 940 964 -24
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Figure 4. Depth–to–basement map of study area. Contour interval 0.1 km. Lines indicate basement 

geology. Red dots, basement gravity stations; black dots, basin gravity stations; yellow squares, well 

locations; yellow triangles, seismic lines.

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based on velocity–depth cross sections (David Burger, U.S. Geological Survey, written commun., 1996).

Although these velocities are unreasonably slow for basin fill sediments, and yielded depths much too

shallow, the estimated depth of the basin, based on seismic refraction, seem realistic. Conversion of

velocity to density was made by an equation of Gardener and others (1974).

Using the Q/T boundary estimate of 0.13 km provided the best fit. Quaternary sediments above

this boundary were assigned a density of 2.17 g/cm3

 and Tertiary sediments below this boundary were

assigned a density of 2.27 g/cm3

. The Tertiary sediments below 0.24 km were assigned a density of 2.37

g/cm3

, which reflects the measured density of a core sample at the bottom of the LL1 well. These results

placed the basement near or below the depths of the wells and the seismic depths. By forcing the LW1

well to basement, all of the differences became smaller except seismic line 2, which also is an

improvement. The final step was to force all of the wells whose differences were not negative to the drill

depths. Because these wells did not reach basement they could have been forced deeper, but a more

conservative approach was not to force the depths deeper than is known. The seismic depths were forced

to fit.

CONCLUSIONS

The resulting basement thickness grid is shown in figure 4. A structure map is shown in figure 5,

calculated by subtracting the depth–to–basement from topography. The resultant map is a structural

contour map of the basement surface. Figure 5 essentially is a topographic map of the study area with

sediments removed.

Langford Well Lake basin, based on current data, contains three sub–basins. A small sub–basin

lies under the lakebed, as expected. A much broader but shallower sub–basin lies southwest of the

lakebed. Although not as deep, it is of much larger area. An arc–shaped basin about the same size as the

middle basin lies 4.5 km south of the lakebed. The entire Langford Well Lake basin is closed at about the

500–m level and spills to the Coyote Lake basin to the southwest.

Wells LP1, LP2, LL1, LP3, and LX–1 are aligned in a row on what appears to be a ridge between

the north basin and the middle basin. Although well TH–8 is not very deep, the calculated depth of the

basin is over 335 m (1,100 f) deep at that well site, which is located just north of the middle basin. The

southern basin has very poor control. It appears to be at least as deep as the middle basin, but additional

gravity stations are needed to better delineate its size and depth.

Locations of maxima in maximum horizontal gravity gradients are shown in figure 6. The

steepest horizontal gradients of gravity anomalies caused by a tabular body tend to lie directly over the

edge of the bodies if the edges of the bodies are vertical and far enough away from other sources. A

procedure of Cordell (1979) uses the lateral variation of gravity anomalies to extract gradients of density

directly from gravity measurements. Juxtaposed rocks of contrasting densities cause these gradients.

These boundaries can be caused by geologic contacts or faulting. Mapped faults are also shown on this

map. There is good correlation between some of the mapped faults and some of the maxima of gradients.

Dashed lines emphasize linear trends inferred from these gradient maxima which could be extensions of

faults not visible at the surface. The gradient maxima shown in figure 6 may be useful as evidence of

additional faulting in this study area.
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Figure 5. Structural contour map of study area. Contour interval 60 m relative to sea level. Black dots, 

gravity stations; dashed lines, lake beds; solid lines, basement geology. 

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Figure 6. Location of maxima in maximum horizontal gravity gradient shown on topographic base. 

Black lines, mapped faults; small open circles, gradient maxima between 1 and 3 mGal/km; large open 

circles, gradient maxima between 3 and 8 mGal/km; dashed lines, possible locations of faults inferred 

from gradients.

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REFERENCES CITED

Barnes, D. F., Oliver, H.W., and Robbins, S.L., 1969, Standardization of gravimeter calibrations in the

Geological Survey: Eos (American Geophysical Union Transactions), v. 50, no. 10, p. 626–527.

Cordell, L., 1979, Gravimetric expression of graben faulting in Santa Fe country and the Española Basin,

New Mexico, in Ingersoll, R.V. (ed.) Guidebook to Santa Fe Country, 30th Field Conference:

New Mexico Geological Society, 59–64.

Gardner, G.H.F., Gardner L.W., and Gregory A.R., 1974, Formation velocity and density; the diagnostic

basics for stratigraphic traps: Geophysics, 39, 770–780.

Godson, R.H., and Plouff, Donald, 1988, BOUGUER version 1.0, a microcomputer

gravity–terrain–correction program: U.S. Geological Survey Open–File Report 88–644–A,

Documentation, 22 p.; 88–644–B, Tables, 61 p., 88–644–C, 5 1/4–inch diskette.

Hayford, J.F., and Bowie, William, 1912, The effect of topography and isostatic compensation upon the

intensity of gravity: U.S. Coast and Geodetic Survey Special Publication no. 10, 132 p.

International Association of Geodesy, 1971, Geodetic reference system 1967: International Association

of Geodesy Special Publication No. 3, 116p.

Jachens, R.C., and Roberts, C.W., 1981, Documentation of a FORTRAN program, ‘isocomp’, for

computing isostatic residual gravity: U.S. Geological Survey Open–File Report 81–574, 26 p.

Jachens, R.C., and Moring, B.C., 1990, Maps of thickness of Cenozoic deposits and the isostatic residual

gravity over basement for Nevada: U.S. Geological Survey Open–File Report 90–404.

Karki, Pentti, Kivioja, Lassi, and Heiskanen, W.A., 1961, Topographic isostatic reduction maps for the

world for the Hayford Zones 18–1, Airy–Heiskanen System, T=30 km: Publication of the

Isostatic Institute of the International Association of Geodesy, no. 35, 5p., 20 pl.

Morelli, C., ed, 1974, the International gravity standardization net 1971: International Association of

Geodesy Special Publication no. 4, 194 p.

Plouff, Donald, 1966, Digital terrain corrections based on geographic coordinates [abs.]: Geophysics, v.

31, no. 6, p. 1208.

Plouff, Donald, 1977, Preliminary documentation for a FORTRAN program to compute gravity terrain

corrections based on topography digitized on a geographic grid: U.S. Geological Survey

Open–File Report 77–535, 45 p.

Spielman, J.B., and Ponce, D.A., 1984, HANDTC, a FORTRAN program to calculate inner–zone terrain

corrections: U.S. Geological Survey Open–file Report 84–777, 20 p.

Swick, C.H., 1942, Pendulum gravity measurements and isostatic reductions: U.S. Coast and Geodetic

Survey Special Publication no. 232, 82 p.