Basin Geometry Estimates using Terrestrial Gravimetry; Study of Bridgeport Valley, CA

Abstract

We use basin thickness estimates derived from terrestrial gravimetry to define the geometry of basin-fill aquifers in Bridgeport, California. Existing gravity data coverage for the Bridgeport basin is sparse, and does not depict a basin shape or anomaly that is consistent with models for nearby Eastern Sierra basins when rendered under inverse distance weighted (IDW) interpolation and infinite-slab basin thickness approximation. We combine existing data from the Pan American Center for Earth & Environmental Sciences (PACES) gravity database with original observations to construct a more complete gravity record. Land-access permissions restrict placement of new observations; however, we gain access to make and incorporate new observations on previously un-surveyed ranchland in the central extent of the basin. New observations are accurate to 0.1 mGal with relative elevation control by geodetic GPS to 0.3 m. We collect data over sparse areas in lines of 5 – 10 points with 200 m spacing for use in gravity gradient interpolation. We also increase observation density along major roads, with new road point spacing of roughly 500 m. New estimates show maximum basin thickness of approximately 670 m, and average range-front fault dips of 24.25° and 12.55° (northwest boundary and east, respectively) near the central basin. The basin gravity residual identifies three prominent linear trends, suggesting a triangular shape for the basin in plan view.