Theoretical and field tests indicate that the controlled-source audiomagnetotelluric (CSAMT) method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system. Utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics—variable and unreliable natural source fields. Reliable CSAMT measurements can be made with a simple scalar receiver. Our calculations for a half-space show that the plane-wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration. Three-dimensional (3-D) numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane-wave source, showing that the method is valid.

A CSAMT survey at the Roosevelt Hot Springs geothermal area in Utah produced apparent resistivity contour maps at four frequencies: 32, 98, 977, and 5208 Hz. These maps show the same features as those of a dipole-dipole resistivity map. We also collected detailed CSAMT data at 10 frequencies on two profiles. Two-dimensional (2-D) plane-wave modeling (transverse magnetic mode) of the resulting pseudo-sections yields models similar to those derived by modeling the dipole-dipole resistivity data. However, CSAMT resolved details not shown by the resistivity modeling. Thus, high resolution along with an efficient field procedure make CSAMT an attractive tool for geothermal exploration.

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