Inversion of controlled source audio-frequency magnetotellurics data for a horizontally layered earth

We present a technique for inverting controlled source audio-frequency magnetotelluric (CSAMT) data to recover a 1-D conductivity structure. The earth is modeled as a set of horizontal layers with constant conductivity, and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source. The earth model has many layers compared to the number of data points, and therefore the solution is nonunique. Among the possible solutions, we seek a model with desired character by minimizing a particular model objective function. Traditionally, CSAMT data are inverted either by using the far-field data where magnetotelluric (MT) equations are valid or by correcting the near-field data to an equivalent plane-wave approximation. Here, we invert both apparent resistivity and phase data from the near-field transition zone and the far-field regions in the full CSAMT inversion without any correction. Our inversion is compared with that obtained by inverting near-field corrected data using an MT algorithm. Both synthetic and field data examples indicate that a full CSAMT inversion provides improved information about subsurface conductivity.