Tipper data forward modeling and inversion of three-dimensional tensor CSAMT

Abstract

The tensor controlled-source audio-frequency magnetotellurics (CSAMT) method has multiple sources, and its resolution is higher than scalar CSAMT. The two observed sets of orthogonal horizontal electric and magnetic fields within the tensor CSAMT are transformed into apparent resistivity, phase or impedance which are the same as magnetotellurics (MT). However, the common measurement of tensor CSAMT does not further imitate MT in acquiring the vertical magnetic field and calculating tipper data, which has been widely proven to improve resolution in MT. In some special areas (such as deserts), the electric field is not easy to collect, so the use of tensor CSAMT tipper data is expected to increase measuring points and improve inversion in these areas. Based on this, we used the staggered grid finite difference method to simulate and study the two types of tipper data from the tensor CSAMT. The forward simulation results showed that the tipper characteristics of tensor CSAMT were similar to those of MT to a certain extent. However, the tipper data showed serious asymmetric properties at low frequencies or near offset. The simulation results indicated that it was necessary to consider the influence of the transmitting source when using tensor CSAMT full-band tipper data. For this reason, the limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) method was used to implement the tensor CSAMT three-dimensional (3D) tipper inversion. The inversion tests of the theoretical model proved that the tipper data could provide useful information and improve the 3D inversion resolution.