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 transmitter overcomes the main limitation of conventional AMT: variable and unreliable natural source fields. Reliable CSAMT measurements can be made with a simple scalar receiver. 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 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. Comparsions between 2D and 3D model results show that a 2D MT program can be used to interpret CSAMT data. A CSAMT survey produced apparent resistivity contour maps at four frequencies: 32, 98, 977, and 5208 Hz. These maps show the same features as a first-separation dipole-dipole resistivity map.