3-D joint inversion of CSAMT and magnetic data based on cross-gradient theory

Joint inversion that simultaneously inverts multiple geophysical data sets to recover the underground model is increasingly being applied in the field of exploration. We present a 3-D joint inversion algorithm for the controlled-source audio-frequency magnetotelluric (CSAMT) and magnetic data. First, a 3-D forward and inversion algorithm for the CSAMT method is developed. The forward problem has been solved by the way of the staggered-grid finite difference method based on the control equation concerning the electric field and we use the Limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) inversion method to invert the CSAMT data. Second, a regularization factor is introduced in the 3-D magnetic inversion, and the magnetic susceptibility is updated in logarithmic form so that the inversion magnetic susceptibility is always positive. Third, a 3-D joint inversion of the CSAMT and magnetic data is completed by cross-gradient coupling where a structural similarity is enforced through a term in the objective function. Testing the algorithm on synthetic data sets shows that the joint inversion is superior to individual inversions.

Keywords: CSAMT, magnetic method, LBFGS, cross-gradient, joint inversion