Controlled-source audiomagnetotellurics in geothermal exploration

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 trans-mitter 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 geo-thermal area in Utah produced apparent resistivity contou