Deep dc resistivity surveying has been performed at the Ahuachapan Geothermal Field in western El Salvador. The field lies on the north base if the volcanic range that runs the length of the country and is the site of a significant active geothermal powerplant. The technical purpose of the surveying was to assess the geophysical properties of the geothermal regime around the existing field. The survey was carried out as a colinear dipole-dipole survey using dipole spacings of 500m and 1000m, and dipole separations of up to fourteen dipole lengths. Results included (1) the observation that the geothermal reservoir was not readily detectable due to surface lithologic changes and extensive cultural effects, (2) the hydrothermally altered areas are readily recognized by their low resistivities, (3) local lithologies appear to be mappable based on their in situ resistivities, (4) the greater field logistical convenience of the shorter 500m dipole spacings is negated by the increase in topographic effects, and (5) if appropriate dipole separations are used, there is no need to run surveys of both 500 and 1000 meter dipole spacings to gather shallow and deep information. 7 figs
