Mapping the Makushin Reservoir with 3-D E-SCAN Resistivity

Abstract

IN 1983, a high temperature geothermal reservoir was identified at well ST-1 in an area of extreme terrain on the southeast flank of Makushin Volcano. The lateral extent of the reservoir had not been defined, and it was thought that it might extend to the northeast, into an area of road access and more economically viable development costs. The severe terrain had ruled out the use of conventional resistivity methods for reservoir mapping, and the delineation of the reservoir appeared to require the continuation of the successful (but expensive) program of temperature gradient drilling. On learning of E-SCAN's rough terrain resistivity capability, the Alaska Power Authority requested a 3-D E-SCAN of the Unalaska Geothermal Project are in 1984. The 3-D E-SCAN survey result unambiguously defined the boundaries of the reservoir, confining it to the are of extreme terrain in the vicinity of producing well ST-1. The northwestern boundary of the resource is marked by an inclined fault that appears to control the production intersected in well ST-1. Extension of the reservoir into adjacent areas of easier and more economic access was ruled out by the E-SCAN resistivity results.