Geothermal Exploration Activities, Including Intermediate Depth Temperature Gradient Drilling, at the Naval Air Facility, El Centro, California

Abstract

The US Navy Geothermal Program Office (GPO) is conducting geothermal energy exploration adjacent to Superstition Mountain in western Imperial Valley, California, within the Target 101 bombing range of the Naval Air Facility, EI Centro (NAFEC). Shallow temperature gradient hole (TGH) completed by Chevron in the early ‘80’s indicated higher than background gradients along the northeast side of Superstition Mountain. Exploration activities conducted by the Navy over the past few years indicates that this thermal anomaly appears to coincide with an active transtensional right stepover occurring between the parallel northwest trending Superstition Mountain fault and the Superstition Hills fault to the north. Critically stressed fractures appear to serve as conduits for upwelling geothermal fluids. Based on earlier shallow drilling, detailed geologic mapping, seismogenic fault mapping, stress analysis, and a variety of geophysical surveys, including gravity and magnetic, self-potential electrical and magneto-telluric, the Navy sited two moderate depth test holes on this target. These were completed in December 2008 confirming the existence of a moderate temperature anomaly with temperatures reaching approximately 186°F (85.6°C) at less than 2300 feet. This thermal anomaly coincides with hydrothermal alteration in both Pliocene sediments and Miocene volcanic rocks intersected by the test holes. A third slim test hole is planned for 2010 and is designed to explore the heart of the thermal and SP anomalies, as well as the down-dip extension of the northwest-trending range-front fault and a conjugate structure extending northeastward from Superstition Mountain.