An Accelerated Algorithm for 3-D Multifrequency CSEM Imaging With Undulating Topography

Three-dimensional frequency-domain controlled-source electromagnetic (CSEM) inversion is an essential technology for subsurface conductivity imaging. In this letter, we develop an efficient 3D inversion scheme for multi-frequency CSEM (MFCSEM) data measured on a topographic earth. Firstly, the model is discretized using the unstructured mesh, which has the ability to simulate the undulating topography. Then, we divide the frequency range into two independent frequency intervals and use the rational Krylov (RK) subspace algorithm with the OpenMP/MPI hybrid parallelization scheme to accelerate the calculations of MFCSEM forward and adjoint forward. Finally, the nonlinear conjugate gradient (NLCG) method is utilized to solve this optimization problem. We invert a synthetic data set to verify the computational performance, and the results show that our algorithm is efficient and can obtain reliable inversion results. Furthermore, it also indicates that ignoring the effect of topography can cause severe distortion to the inversion results.