Initial Results of Magnetotelluric Array Surveying at the Dixie Valley Geothermal Area, with Implications for Structural Controls and Hydrothermal Alteration

摘要

A new-generation MT array measurement system was applied in a contiguous bipole deployment at the Dixie Valley thermal area. Basic goals of the survey are 1), resolve a fundamental structural ambiguity at the Dixie Valley thermal area (single rangefront fault versus shallower, stepped pediment; 2), delineate fault zones which have experienced fluid flux as indicated by low resistivity; 3), image the disposition of resistive, possible reservoir formations in the subsurface; and 4), from a generic standpoint, investigate the capability of fully sampled electrical data for resolving subsurface structure. MT surveying using a remote reference was carried out on three, 4-6 mile long profiles totalling 14 line miles, each crossing the Stillwater fault zone approximately at right angles, over the frequency range of 10 kHz to 0.03 Hz. These lines cross the Senator Fumeroles area, the Cottonwood Creek and main producing area, and the low-permeability region through the section 10-15 area. Initial 2-D inversion suggests that shallow pediment basement rocks extend for a considerable distance (～1-1/2 km) southeastward from the topographic scarp of the Stillwater Range. A particularly low resistivity zone flanks the interpreted main offsetting fault and may be due to alteration from geothermal fluid outflow and upflow.