Isostatic gravity map of the Death Valley ground-water model area, 

Nevada and California 

By D.A. Ponce, R.J. Blakely, R.L. Morin, and E.A. Mankinen 


Prepared in cooperation with the 


Nevada Operations Office 


National Nuclear Security Administration 


U.S. Department of Energy 


(Interagency Agreement DE-A108-96NV11967)


Pamphlet to accompany 


Miscellaneous Field Studies MF-2381-C


2001 
U.S. Department of the Interior 

U.S. Geological Survey 

1

INTRODUCTION 

Gravity investigations of the Death Valley ground-water model area are part of an interagency 

effort by the U.S. Geological Survey (USGS) and the Department of Energy (Interagency 

Agreement DE-AI08-96NV11967) to help characterize the geology and hydrology of 

southwestern Nevada and parts of California. The Death Valley ground-water model is located 

between lat 35°00’ and 38°15’N., and long 115° and 118° W. 

An isostatic gravity map of the Death Valley ground-water model was prepared from over 40,000 

gravity stations, most of which are publicly available on a CD-ROM of gravity data of Nevada 

(Ponce, 1997). The map also includes gravity data recently collected by the U.S. Geological 

Survey (Mankinen and others, 1998; Morin and Blakely, 1999). A subset of these gravity data in 

the Nevada Test Site and vicinity were described in detail by Harris and others (1989) who 

included information on gravity meters used, dates of collection, sources, descriptions of base 

stations, plots of data, and digital and paper lists of principal facts. For display purposes only, 

gravity data within Yucca Flat were thinned by a factor of 10. The digital gravity data set was 

gridded at an interval of 400 m using a computer program (Webring, 1981) based on a minimum 

curvature algorithm by Briggs (1974). The resulting grid was then interpolated to a 200-m grid 

to minimize pixel size, and then it was color contoured. 

GRAVITY METHODS 

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. 

Observed gravity values were referenced to the International Gravity Standardization Net 1971 

(IGSN 71) gravity datum (Morelli, 1974, p. 18). 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 2,670 

kg/m3

. 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) with an assumed crustal thickness of 25 km, a crustal density of 2,670 kg/m3

, and a density 

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contrast across the base of the model of 400 kg/m3

. Gravity values are expressed in milligals 

(mGal), a unit of acceleration or gravitational force per mass equal to 10-5 m/s2

. 

Terrain corrections, which account for the variation of topography near a gravity station, were 

computed using a combination of manual and digital methods. Terrain corrections consist of a 

three-part process: the innermost or field terrain correction (estimated in the field and typically 

extending to a radial distance of 53 to 68 m), inner-zone terrain correction, and outer-zone terrain 

correction. 

Inner-zone terrain corrections were made using either Hayford and Bowie (1912) or Hammer 

(1939) systems that divide the terrain surrounding a gravity station into zones and equal area 

compartments. Average elevations for each compartment were manually estimated from the 

largest scale topographic maps available, usually USGS 1:24,000-scale maps. The terrain 

corrections were then calculated based on the average estimated elevation of each compartment. 

Inner-zone terrain corrections typically extend to a radial distance of 0.59 to 2.29 km. With the 

advent of computer processing and the availability of detailed digital elevation models (DEMs), 

modern day inner-zone terrain corrections were computed using USGS 7.5’ DEMs with a 

resolution of 30 m derived from USGS 1:24,000-scale topographic maps. 

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; 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. 

DISCUSSION 

In general, isostatic gravity anomalies reflect lateral (horizontal) density variations in the middle 

to upper crust. Thus, gravity anomalies can be used to infer the subsurface structure of known or 

unknown geologic features. Gravity anomalies within the Death Valley ground-water model 

reflect carbonate rocks, calderas, deep sedimentary basins, and linear geologic features such as 

faults. Many of these features play an important role as aquifers or confining units in the region 

and their distribution is important to the understanding of the hydrogeologic framework of the 

area. Pre-Cenozoic carbonate and crystalline rocks underlie most of the region and their 

subsurface distribution is especially important in evaluating the hydrogeology of the area. 

Tertiary volcanic rocks also play a significant role in the extensional history of the area and in the 

formation of large collapse calderas. Thick accumulations of these volcanic rocks are present in 

the central and northern part of the Death Valley ground-water model (for example, Timber 

Mountain and vicinity). Quaternary alluvial deposits exist throughout the study area and are 

composed of non-marine sedimentary and volcanic rocks, generally Oligocene and younger in 

age. These deposits play an important role in the saturated-zone hydrology of deep alluvial 

basins within the southwestern Nevada volcanic field (Grauch and others, 1999) and probably in 

other deep basins as well. Linear isostatic gravity anomalies may reflect faults such as the basin 

and range bounding Death Valley-Furnace Creek fault on the western margin of the Grapevine 

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Mountains, in the central and western part of the study area, or the Thirsty Canyon fault zone 

along the western margin of the Silent Canyon caldera complex at Pahute Mesa (Mankinen and 

others, 1998), in the central part of the study area. 

As expected, gravity highs occur over pre-Cenozoic carbonate and crystalline rocks that typically 

have densities on the order of 2,700 kg/m3

. Prominent gravity highs occur over the Grapevine 

Mountains in the central part of the ground-water basin and in the Spring Mountains in the 

southeast quadrant of the study area. Gravity lows occur throughout the study area and may 

reflect sedimentary basins filled with low-density alluvial deposits with densities ranging from 

1,600 to 2,200 kg/m3 or thick accumulations of volcanic rocks with densities of about 2,200 to 

2,500 kg/m3

. Gravity lows over Owens and Saline Valleys in the western part of the groundwater basin and Las Vegas Valley in the southeast part of the study area reflect thick sedimentfilled basins. One of the largest gravity lows in Nevada is associated with a thick volcanic 

sequence associated with the Silent Canyon caldera complex in the central part of the groundwater basin just north of Timber Mountain. 

A number of geophysical studies were undertaken to aid in characterizing the geologic and 

hydrogeologic setting of the region. A summary of geophysical investigations at Yucca 

Mountain that includes gravity, magnetics, electrical methods, seismic methods, heat-flow, and 

stress data was provided by Oliver and others (1995). More recent studies characterized the 

geophysical framework of the southwestern Nevada volcanic field (Grauch and others, 1999; 

Mankinen and others, 1999) identifying a number of subsurface features that might control or 

influence ground-water flow. These features include resurgent calderas and prominent 

geophysical lineaments that probably reflect faults. Blakely and others (2000) summarized 

geophysical investigations of the Death Valley regional ground-water basin. 

The diverse physical properties of lithologic types that underlie this region are well suited to 

geophysical investigations. The contrast in density between pre-Cenozoic basement and 

overlying unconsolidated alluvium, for example, produces a distinctive pattern of gravity 

anomalies that can be used to predict the depth of pre-Cenozoic basement in three dimensions. 

In most parts of the study area, this surface corresponds to the top of the carbonate aquifer, an 

important element of the hydrologic framework. A companion map, which shows the depth to 

basement, discusses a model of this surface based on the inversion of gravity data (Blakely and 

Ponce, 2001). 

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