Inversion of controlled-source tensor magnetotelluric data for a layered earth with azimuthal anisotropy

Fractures in the upper part of the crust are pervasive in crystalline areas . They often are aligned and nearly vertical. Magnetotelluric studies carri ed out in different parts of the world often indicate that the upper brittl e part of the crust is electrically anisotropic. One model is that deep flu id-filled fractures, when observed from the surface, mimic an anisotropic m edium. The transfer functions of the controlled-source tensor magnetotellur ic (CSTMT) method contain information from current systems running both par allel and perpendicular to the principal horizontal axes of anisotropy, so that the CSTMT method is capable of detecting azimuthal anisotropy. For the CSTMT method, electric dipoles provide the source field, and hence fields are localized. Thus, the distortion generated by a major conductive anomaly lying outside the induction volume defined by the transmitter and receiver positions will be small. Therefore, one-dimensional models often may be va lid. We have developed a nonlinear least-squares inversion approach to inve rt CSTMT data for azimuthal anisotropy in an one-dimensional layered earth. Nearsurface distortion effects on the impedance tensor are parameterized a s a real distortion matrix. The elements of the distortion matrix are incor porated into the model parameters. By using tipper functions which are less distorted by near-surface structures, near-surface distortion effects can be removed adequately One striking feature of our inversion algorithm is th at the partial derivatives of the response functions, with respect Co model parameters, are given in analytical forms, which results in an efficient c omputation of the Jacobian matrix. Theoretical studies show good convergenc e and good resolution of the model parameters. We have applied the inversio n scheme to a set of controlled-source data from the Siljan impact structur e in Sweden. The derived models give much better data fits than a correspon ding isotropic model.