Telluric, self-potential, and surface temperature profiles on los humeros caldera

Abstract

Two geophysical lines of 16.5 and 17 Km in lerigth have been surveyed in Los Humeros Caldera, in central-eastern Mexico, in order to delineate preliminary areas of geothermal interest. The E-field ratio telluric method, which consist of collinear dipoles and measurements at frequencies of .05 and 8 Hz, was used as a reconnaissance tool to get an idea of the relative changes in electrical conductivity across the caldera; self potential and surface temperature data were also collected along the lines. Tellurics suggests sharp conductivity contrast on the west side of both lines and at both frequencies; in addition, the northern line appears to define quite well the collapsed portion of the. c.aldera along the profile, the topography pf which is oblitarated by pyroclastics. Telluric and gravity data agree quite well in delineating the major structural features. Self-potential responses show that.the western caldera rim is approximately 800 mV above the electric potential of the eastern rim. The average suiface temperatures (i.e., 0.5 to 1 m depth) ori the western caldera portion.are from 3° to 6°C above the temperatures on the eastern portion. On the basis of this evidence the western flank of the caldera is proposed as the primary exploration target for geotermal purposes.