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

We present a technique for inverting controlled source audio-frequency magn etotelluric (CSAMT) data to recover a 1-D conductivity structure. The earth is modeled as a set of horizontal layers with constant conductivity, and t he data are apparent resistivities and phases computed from orthogonal elec tric 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 solut ion is nonunique. Among the possible solutions, we seek a model with desire d character by minimizing a particular model objective function. Traditiona lly, CSAMT data are inverted either by using the far-field data where magne totelluric (MT) equations are valid or by correcting the near-field data to an equivalent plane-wave approximation. Here, we invert both apparent resi stivity and phase data from the near-field transition zone and the far-fiel d regions in the full CSAMT inversion without any correction. Our inversion is compared with that obtained by inverting near-field corrected data usin g an MT algorithm. Both synthetic and field data examples indicate that a f ull CSAMT inversion provides improved information about subsurface conducti vity.