Using Electrical Survey Techniques to Identify Drilling Targets in Basin and Range Geothermal Prospects

Abstract

The feasibility of using DC, MT and/or SP electrical geophysical survey techniques to find “hidden” geothermal reservoirs (i.e., those without obvious surface manifestations directly overlying the resource) in the Basin and Range, and of locating promising targets for deep exploratory drilling based on the survey results, is appraised. The approach is purely theoretical. A geothermal reservoir simulator is used to carry out a lengthy calculation (representing 100,000 years of real time) of the evolution of a synthetic but generic Great Basin-type geothermal reservoir to a quasi-steady “natural state”. Once this stable state is reached, “postprocessors” are used to try to estimate what a suite of geophysical surveys of the prospect would see from the surface. Then, based on these results, different survey techniques are compared and evaluated in terms of their ability to identify suitable drilling targets. This process was completed for eight different “reservoir models”, which differ in the boundary conditions imposed at the bottom of the computational domain (and therefore in both the total resource potential for electricity production and the character of the final stable three-dimensional temperature distribution within the region). Of the eight cases considered, four are “hidden” systems so that the survey techniques could be appraised in terms of their ability to detect and characterize such resources and to distinguish them from more conventional geothermal reservoirs. Results suggest that a combination of low-frequency MT (magnetotelluric) and SP (self-potential) surveys will provide the best correlation with the actual underground resource location.