Exploring for Hidden Geothermal Systems

Abstract

By definition, hidden (or “blind”) geothermal systems lack obvious surface manifestations such as hot springs, fumaroles, or hydrothermally altered ground. Most of the now-documented but previously-hidden geothermal systems (e.g., the geothermal reservoirs in the Imperial Valley of southern California, U.S.A.) were discovered accidentally, as a result of exploration drilling for oil and gas and/or mining activities. Although the occurrence frequency of hidden geothermal systems is unknown, a methodology is needed to identify promising targets for subsequent deep confirmation drilling. Based on theoretical calculations, Pritchett concluded that a combination of earth-surface electrical surveys (self-potential, magnetotelluric, and DC resistivity) in concert with other relevant information may be used to locate blind or hidden geothermal systems. Subsequently, Garg and coworkers used available geophysical and reservoir data from the Beowawe geothermal field (Nevada, U.S.A.) to demonstrate that electrical surveys may be employed to infer favorable subsurface geothermal reservoir characteristics. In this paper, we review the historical experience and the results of the theoretical studies. We suggest that a suite of electrical surveys together with shallow heat flow surveys and relevant local geological observations offers a promising approach to the identification and preliminary characterization of hidden geothermal systems.