Controlled‐source audiomagnetotellurics in geothermal exploration

Abstract

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.