A hybrid finite difference and integral equation method for modeling and inversion of marine CSEM data

Summary One of the major problems in the modeling and inversion of marine controlled source electromagnetic (MCSEM) data is related to the need for accurate representation of very complex geoelectrical models typical for marine environment. At the same time, the corresponding forward modeling algorithms should be powerful and fast enough to be suitable for repeated use in hundreds of iterations of the inversion and for multiple transmitter/receiver positions. To this end, we have developed a novel 3D modeling and inversion approach, which combines the advantages of the finite difference (FD) and integral equation (IE) methods. In the framework of this approach, we solve the Maxwell's equations for anomalous electric fields using the FD approximation on a staggered grid. Once the unknown electric fields in the computation domain of the FD method are computed, the electric and magnetic fields at the receivers are calculated using the IE method with the corresponding Green's tensor for the background conductivity model. This approach makes it possible to compute the fields at the receivers accurately without the need of very fine FD discretization in the vicinity of the receivers and without the need for numerical differentiation and interpolation. We have also developed an algorithm for 3D inversion of MCSEM data based on the hybrid FD-IE method. A model study for the 3D inversion of MCSEM data is presented to demonstrate the effectiveness of the developed hybrid method.