Gravity and ground magnetic data from selected traverses in the Amargosa desert and vicinity, Nevada and California

Gravity data were obtained using a LaCoste and Romberg meter (USGS meter G8) and observed gravity values were referenced to the International Gravity Standardization Net 1971 (ISGN 71) gravity datum (Morelli, 1974). Gravity observations were tied to two ISGN 71 base stations (Harris and others, 1989); the original Beatty post office (BPO), now the Death Valley visitor station, and DUNE, west of the junction of U.S. Highway 95 and Nevada State Highway 29 in Amargosa Valley. The observed gravity value given for BPO in Harris and others (1989) has been revised to 979,566.45 mGal (D.A. Ponce, USGS, written communication, 1997).
Observed gravity at each station was adjusted by assuming a time-dependent linear drift between readings of a base station at the start and finish of each daily survey. This adjustment compensates for drift in the instruments spring. All gravity data and their associated parameters are given in Table 1. Observed gravity values are considered accurate to about 0.05 mGal based on repeat measurements over several mountain calibration loops (Barnes and others, 1969; Ponce and Oliver, 1981). Gravity data were reduced using standard gravity corrections (Harris and others, 1989) and a reduction density of 2670 kg/m3. Inner-zone terrain corrections (zones A, B, C, and D of Hayford and Bowie, 1912), which are necessary to account for variations in topography near a gravity station, were obtained using a program developed by Spielman and Ponce (1984). The calculations use digitized topography in a digital elevation model (DEM) and a procedure by Plouff (1977). A regional isostatic field was calculated using an Airy-Heiskanen (Heiskanen and Vening Meinesz, 1958) model for local compensation of topographic loads (Simpson and others, 1983, 1986). This model assumes a crustal thickness of 25 km, a crustal density of 2670 kg/m3, and a 400 kg/m3 density contrast between the crust and mantle. This regional isostatic field was subtracted from the complete Bouguer anomaly, thus removing longwavelength variations in the gravity field that are inversely related to topography. The resulting isostatic residual gravity anomaly, therefore, is a reflection of local density distributions within middle to upper crustal levels. The isostatic residual gravity anomaly for the Amargosa Valley study area, incorporating the 152 new gravity observations from Table 1, is shown in figure 4. The new gravity stations were established along the same traverses as the ground magnetic profiles described in the following section and were intended to complement those data.