INTERPRETATION OF GRAVITY SURVEYS IN GRASS AND BUENA VISTA VALLEYS, NEVADA

Continuing the LBL study of geothermal exploration techniques and the assessment of potential reservoirs begun in 1974, detailed gravity surveys were made in the vicinity of Leach Hot Springs, Grass Valley, and Kyle Hot Springs, Buena Vista Valley, in northern Nevada. The terrain-corrected Bouguer gravity values were gridded, corrected for regional gradients (Grass Valley only) bya least squares polynomial fit to the data, and then processed by means of an inversion scheme to yield a contour map of the basement configuration. The three-dimensional inversion algorithm assumes a two-layer earth of variable first layer thickness but constant density contrast between layers. Although a fit to observed data can be obtained for a range of density contrasts, a contrast of 0.06 g/cm{sup 3} gave interpreted results most consistent with results from other geophysical data: (a) agreement between the minimum thickness contours and the margins of exposed Paleozoic rocks and Cretaceous intrusives; (b) agreement between the calculated basement depth and report information from a deep drill hole in Grass Valley; and (c) agreement between estimated basement depths and interpretation of seismic reflection-refraction data. To illustrate the information value of the depth-to-basement calculations, we show comparisons of gravity, electrical resistivity, and seismic interpretations along selected profile lines. Gravity and seismology gave nearly identical results for the location and dip of the Hot Springs fault in Grass Valley. The gravity interpretation agreed reasonably well with that of the electrical basement, although the electrical basement is consistently 200-400 meters deeper than the density or velocity interface