Electromagnetic and Schlumberger resistivity sounding in the Roosevelt Hot Springs KGRA

Abstract

One‐ and two‐dimensional modeling of the Schlumberger soundings at the Roosevelt Hot Springs KGRA have indicated a low‐resistivity zone of approximately 5 Ω-m paralleling the dome fault. The low resistivity of this zone is probably due to intensely fractured and altered water‐saturated rock. A zone of resistivity 12 Ω-m extending to the west of the dome fault is probably due to leakage of brine away from the geothermal system through alluvium or moderately altered rock. A resistive basement underlies the conductive zones and is believed to be essentially nonporous and unaltered rock. A major problem in the application of one‐dimensional (1-D) modeling of Schlumberger data in the Roosevelt Hot Springs KGRA is poor resolution of the 1-D parameters. The joint inversion of Schlumberger and electromagnetic sounding data gives a least‐squares 1-D conductivity model in which parameters are much better resolved than are the model parameters estimated by the inversion of Schlumberger data alone. One‐dimensional modeling of Schlumberger soundings along a traverse does indicate the presence of a 2-D inhomegeneity but it gives no hint of the possible complexity of that inhomogeneity even though the parameters of the models fitting each sounding have acceptable standard deviations when constrained by electromagnetic sounding data. Since the model parameter standard deviations are model dependent, good resolution of 1-D model parameters does not indicate that the assumption of a 1-D model is valid. On the other hand, the possible complexity of structure is brought out by 2-D modeling of the same data, but since the degrees of freedom for complex 2-D models is large, a thorough study of the resolution of such models is prohibited costly at present. In these circumstances, we must constrain the 2-D models with independent geologic or geophysical data and then accept the subsequent best‐fit model as semiquantitative.