3D Inversion of ZTEM data

ZTEM data are airborne electromagnetic data which record the vertical magnetic field that results from natural sources. The data are transfer functions that relate the local vertical field to orthogonal horizontal fields measured at a reference station on the ground. The transfer functions depend on frequency and provide information about the 3D conductivity structure of the Earth. The practical frequency range is 30-720 Hz and hence it is possible to see structures at depths of a kilometer or more if the earth is of moderate conductivity. This depth of penetration is significantly greater than that obtained with controlled source EM techniques and, when coupled with rapid spatial acquisition with an airborne system, means that ZTEM data can be used to map large-scale structures that are difficult to survey with ground based surveys. We develop a Gauss-Newton algorithm to invert ZTEM data. The algorithm is applied to synthetic examples and to a field data set from the Mt. Milligan Cu-Au porphyry deposit in British Columbia.