Geothermal Exploration Using Gravity Gradiometry -- a Salton Sea Example

Abstract

Gravity methods are sensitive to the subsurface distribution of geologic materials of different densities, and have proven value in geothermal exploration. In some geothermal settings, measurements of the earth’s gravity gradient (i.e., gravity gradiometry) may provide advantages over the more traditional measurements of the earth’s scalar gravity field. These include higher resolution of targets less than approximately 10 km deep, and better edge detection for interpreting faults, boundaries of geologic bodies, and other structural features. To determine if the gravity gradient signal from a geothermal exploration target is within the detection limits of commercially available sensor technology, the gravity gradient response was modeled for a simplified 3D geological model of the Salton Sea Geothermal Field in Southern California. This is a waterdominated geothermal field in the Salton Trough with a known 20 mGal residual gravity anomaly. The resulting gradient of vertical gravity in the z direction (Gzz) at the Salton Sea Geothermal Field ranged from -53 to -31 Eötvös (1 Eötvös = 0.1?Gal/m, which is equivalent to 0.1 ppb of the Earth’s gravity field). The local density highs are clearly visible in the calculated gravity gradient response, and are consistent with the known gravity anomaly. Additionally, modeled hypothetical faults associated with the pull apart basin setting are more clearly evident in the gravity gradient data compared with the scalar gravity data. This has significance for exploration of blind geothermal systems, especially where faults do not have surface expression in the cover geology.