| Abstract: | Ten magnetotelluric soundings were made near the production zone at Cerro Prieto, bringing to 17 the number of stations occupied in and around the geothermal field during 1978 and 1979. Results from the first seven soundings were reported elsewhere (Gamble et al., 1980). Data were analyzed in the field using a DEC LSI-11 microcomputer installed in a recording truck. The new soundings provide new information on the geometry of the geothermal system. We find evidence for a narrow resistive zone plunging southeastward, at a shallow angle, from a concealed apex a few hundred meters north of the power plant. This zone comes within about 500 m of the surface and can be traced roughly 5 km to the south. This zone correlates very well with a region of hydrothermal metamorphism, which has been identified by means of detailed studies of cores and cuttings from wells. The three dimensionality of this feature, combined with the influence of the large resistivity contrast between valley sediments and Cucapa Range granites 10 km to the west, makes a rigorous quantitative interpretation impractical. Although such an interpretation awaits further advancements in the techniques of calculating electromagnetic scattering by complex geological structures, the general picture seems clear. A comparison of the subsurface resistivity model with the position of the production zone and with subsurface geology suggests that the heat source lies at depth, roughly 4 km SSE of the present power plant.North of the power plant a two-dimensional interpretation of the magnetotelluric data is possible. A good fit between observed and calculated parameters is obtained for a subsurface model that is consistent with the model derived from dipole - dipole de resistivity measurements. |
