Explanation of the gravity method and depth to basement computation.
Gravity data
Gravity
measurements were made at more than 1200 locations by the authors and their
assistants. Combined with older data this gives more than 2300 data points
used for the gravity map of the 30 by 30 minute quadrangle.
We wish to thank Roberts Voorhies, MCAGCC 29 Palms for assistance with
access to do the survey, especially in the Range areas and Dennis Clark,
USGS San Diego, for providing the well information that was vital for
the depth to basement computation.
All gravity data were reduced using standard gravity corrections, including:
(a) the earth tide correction, which corrects for tidal effects of the
moon and sun; (b) instrument drift correction, which compensates for drift
in the instrument's spring; (c) the latitude correction, which incorporates
the variation of the Earth's gravity with latitude; (d) the free-air correction,
which accounts for the variation in gravity due to elevation relative
to sea level; (e) the Bouguer correction, which corrects for the attraction
of material between the station and sea level; (f) the curvature correction,
which corrects the Bouguer correction for the effect of the Earth's curvature;
(g) the terrain correction, which removes the effect of topography to
a radial distance of 166.7 km; and (h) the isostatic correction, which
removes long-wavelength variations in the gravity field inversely related
to topography.
Conversion of LaCoste and Romberg gravity meter readings to milligals
was made using factory supplied calibration constants and a secondary
calibration factor determined from multiple gravity readings over the
high-precision Mt. Hamilton calibration loop east of San Jose, Calif.
(Barnes and others, 1969). Observed gravity values were based on a time-dependant
linear drift between successive base readings and were referenced to the
International Gravity Standardization Net 1971 (IGSN 71) gravity datum
(Morelli, 1974, p. 18). The primary base station was PBB8 (Roberts and
Jachens 1986) south of Twentynine Palms where the observed gravity is
979,396.28 mGal (9.7939628 m/sec2). Free-air
gravity anomalies were calculated using the Geodetic Reference System
1967 formula for theoretical gravity on the ellipsoid (International Union
of Geodesy and Geophysics, 1971, p. 60) and Swick's formula (1942, p.
65) for the free-air correction. Bouguer, curvature, and terrain corrections
were added to the free-air correction to determine the complete Bouguer
anomaly at a standard reduction density of 2.67 g/cm3.
Locations for most of the new stations released here for the first time were obtained either with conventional surveying
equipment or a differential GPS system with most positions determined
in real time.
Terrain corrections, which account for the variation of topography near
a gravity station, were computed using a three-part process: the innermost
or field terrain correction, inner-zone terrain correction, and outer-zone
terrain correction. The innermost or field terrain correction was estimated
in the field, using a system of tables and charts, and typically extends
to a radial distance of 53 or 68 m, Hammer (1939) zone C or Hayford and
Bowie (1912) zone B, respectively.
Inner-zone terrain corrections were styled after the Hayford and Bowie
(1912) system that divide the terrain surrounding a gravity station into
zones and equal area compartments. For all the newer stations average
elevations for each compartment were computed from a detailed digital
elevation model (DEM) derived from USGS 7.5 minute DEMs with a resolution
of 30 m. Inner-zone terrain corrections typically extended to a radial
distance of 0.59 km, Hayford and Bowie (1912) zone D. Terrain corrections
were then calculated based on the average estimated elevation of each
compartment (Spielman and Ponce, 1984).
Outer-zone terrain corrections, to a radial distance of 166.7 km, were
computed using a DEM derived from USGS 1:250,000-scale topographic maps
and an automated procedure (Plouff, 1966; Plouff, 1977; Godson and Plouff,
1988). Digital terrain corrections are calculated by computing the gravity
effect of each grid cell using the distance and difference in elevation
of each grid cell from the gravity station.
Finally, a regional isostatic gravity field was removed from the Bouguer
field assuming an Airy-Heiskanen model for isostatic compensation of topographic
loads (Jachens and Roberts, 1981). An assumed crustal thickness of 25
km, a crustal density of 2.67 g/cm3,
and a density contrast across the base of the model of 0.4 g/cm3
was used and the computation was carried out for topography averaged over
3 by 3 minute compartments to a distance of 166.7km from each station.
Isostatic and terrain corrections beyond that distance were interpolated
from a grid generated from Karki et al. (1961).
Principal facts of the gravity data are available on the Get
Data page. Gravity values are expressed in milligals (mGal), a unit
of acceleration where 1 Gal equals 1 cm/sec2.
With a gravity data set and the surface distribution of basement rocks it becomes possible to compute possible basin configurations.
The procedure
begins with a computer program that attempts to separate the residual gravity
field into two components, a basement component and a basin component. All
gravity measurements located on pre-Cenozoic rocks are considered to be
on basement. Wells that penetrate basement are used as depth control points.
An isostatic residual gravity grid and several files of gravity stations
are input along with a mask grid that shows areas considered basement.
We have made sure that there is at least 100 meters from any basement
gravity station to the nearest area in the grid designated as basin since the program sometimes has trouble when a basement station is on the edge of a basin. The
mask grid was made using geolgy from Dibblee (1967, 1968) and Moyle (1984).
A density contrast versus depth model based on any available density
information is used by the program. For the Surprise Spring area numerous
wells that penetrated basement rocks were available to constrain the computed
depths. In the Deadman Lake Basin and the Joshua Tree Basin the only wells
penetrating the basement rocks are near the margins so the total depth
is uncertain although a variety of different models give similar
configurations and relative depths.
The method used in this study to estimate the thickness of Cenozoic rocks
is an updated version of the iterative method developed by Jachens and
Moring (1990) that incorporates drill hole and other geophysical data
(B. Chuchel, U.S. Geological Survey, written communication, 1996). The
method requires knowledge of the residual gravity field, of the exposed
geology, and of the vertical density variation within the Cenozoic basin
deposits. Data from drill holes that penetrate the basement surface (defined
to be pre-Cenozoic in age) and any other geophysical data that provide
constraints on the thickness of the basin fill are also used.
The method separates the gravity field into two components,
that which is caused by variations of density within the pre-Cenozoic
bedrock and that which is caused by variations of thickness of the Cenozoic
basin fill. To accomplish this process, the gravity data are separated
into observations made on bedrock outcrops and observations made on Cenozoic
deposits. The second set of observations is inverted, using a
density-depth function estimated to characterize the Cenozoic deposits, to yield
the thickness of Cenozoic deposits. The inversion
is complicated by two factors: (1) bedrock gravity stations are influenced
by the gravity anomaly caused by low-density deposits in nearby basins,
and (2) the bedrock gravity field varies laterally because of density
variations within the bedrock. The inversion presented here does not
take into account lateral variations in the density distribution of the
Cenozoic deposits.
To overcome these difficulties, a first approximation of the bedrock gravity
field is determined by interpolating a smooth surface through all gravity
values measured on bedrock outcrops. Bedrock gravity values are also calculated
at locations where drill holes penetrated bedrock using the Cenozoic density-depth function.
The basin gravity is then the difference between the observed
gravity field on the original map and the first approximation of the bedrock
gravity field and is used to calculate the first approximation of the
thickness of Cenozoic deposits. The thickness is forced to zero where
bedrock is exposed.
This first approximation of the bedrock gravity is too low near the basin
edges because of the effects of the nearby low-density deposits on the
bedrock stations. The bedrock gravity station values are "corrected"
for the effects of the low-density deposits (the effects are calculated
directly from the first approximation of the thickness of the Cenozoic
deposits) and a second approximation of the bedrock gravity field is made
by interpolating a smooth surface through the corrected bedrock gravity
observations. This iteration leads to an improved estimate of the basin
gravity field, an improved depth to bedrock, and a new correction to the
bedrock gravity values. This procedure is repeated until successive iterations
produce no significant changes in the bedrock gravity field.
Use of this method at Yucca Flat Nevada where there are an extremely large
number of wells (Phelps et al. 1999)
showed that the procedure operating with some of the wells can produce a
reasonable approximation of the true basement surface as determined by all the wells.
These results in a basin with
exceptional well control suggest that the method is very good at predicting
the shape of the basin but may not produce precisely the magnitude of
basin thickness.
Lateral variations in basin fill density, unless
abrupt, would not change the overall general shape of the basin as predicted
by the method.
The method has several inherent limitations:
In some areas the distribution of basement gravity stations is not
ideal due to the large areas covered by later deposits or the difficulty
of reaching the basement rocks to obtain measurements. The distribution, depths and density or litholigic information from
wells is usually less than ideal. In deeper basins wells seldom penetrate
basement rocks. Well logs may be unreliable. Some wells that claim to
penetrate basement rocks may not do so. Several logs in the Joshua Tree
Basin show alternating basement and sedimentary rocks. One well that
claimed to bottom in "granite" was later deepened and passed
through another zone of "sandstone." The process assumes it is dealing with a basin and must satisfy the gravity
field between the ground surface and the basement surface. It can not
model any density variations below the bedrock surface. While different density-depth functions can be used in different area,
a sharp step appears where the change occurs. The results from the program
may also be affected slightly by the grid spacing.
Biehler, Shawn, Tang, R.R., Ponce, D.A., and Oliver, H.W., 1988, Bouguer
gravity map of the San Bernardino Quadrangle, California: California Division
of Mines and Geology regional Geophysical Map Series, Map 3B (Gravity),
scale 1:250,000, 1 oversize sheet.
Dibblee, T.W., Jr, 1967, Geologic Map of the Joshua Tree Quadrangle,
San Bernardino and Riverside Counties, California: U.S. Geological Survey
Miscellaneous Investigations Report I-0516, 1 over-size sheet, 3 p.
Dibblee, T.W., Jr, 1968, Geologic Map of the Twentynine Palms Quadrangle,
San Bernardino and Riverside Counties, California: U.S. Geological Survey
Miscellaneous Investigations Report I-0561, 1 over-size sheet, 3 p.
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.
Hammer, Sigmund, 1939, Terrain corrections for gravimeter stations:
Geophysics, v. 4, p. 184-194.
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 Union of Geodesy and Geophysics, 1971, Geodetic Reference
System 1967: International Association of Geodesy Special Publication
no. 3, 116 p.
Jachens, R.C., Langenheim, V.E., and Matti, J.C., Relationship of the
1999 Hector Mine and 1992 Landers fault ruptures to offsets on neogene
faults and distribution of late Cenezoic basins in eastern California:
Bulletin of the Seismological Society of America, in press.
Jachens, R.C., and Moring, B.C., 1990, Maps of the thickness of Cenozoic
deposits and the isostatic residual gravity over basement for Nevada:
U.S. Geological Survey Open-File Report 90-404, 15 p.
Jachens, R.C., and Roberts, C.W., 1981, Documentation of a FORTRAN program,
'isocomp', for computing isostatic residual gravity: U.S. Geological Open-File
Report 81-574, 26 p.
Karki, P., Kivioja, L., and Heiskanen, W. A., 1961, Topographic-isostatic
reduction maps for the world for Hayford Zones 18-1, Airy-Heiskanen system,
T=30 km: International Association of Geodesy, Isostatic Institute, Helsinke,
no. 35, 25 p.
Morelli, C., ed., 1974, the International gravity standardization net
1971: International Association of Geodesy Special Publication no. 4,
194 p.
Moyle, W.R., Jr., 1984, Bouguer gravity anomaly map of the Twentynine
Palms Marine Corps Base and vicinity, showing thickness of sedimentary
deposits, deep wells and geology: U.S. Geological Survey Water-Resources
Investigations Report 84-4005, 1 over-size sheet.
Phelps, G.A., Langenheim, V.A., and Jachens, R.C., 1999, Thickness of
Cenozoic Deposits of Yucca Flat Inferred from Gravity Data, Nevada Test Site, Nevada:
U.S. Geological Survey Open-File Report 99-310.
Plouff, Donald, 1966, Digital terrain corrections based on geographic
coordinates [abs.]: Geophysics, v. 31, no. 6, p. 1208.
Plouff, D., 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.
Roberts, C.W., and Jachens, R.C., 1986, High-precision gravity stations
for monitoring vertical crustal motion in southern California: U.S. Geological
Survey Open-File Report 86-44, 78 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.A., 1942, Pendulum gravity measurements and isostatic reductions:
U.S. Coast and Geodetic Survey Special Publication 232, 82 p.
Gravity
methods
Depth
to basement computation method
References
cited
Barnes, D.F., Oliver, H.W., and Robbins, S.L., 1969, Standardization of
gravimeter calibrations in the Geological Survey: Eos, Transactions, American
Geophysical Union, v. 50, no. 10, p. 626-627.
