Three-dimensional regularized inversion of marine controlled-source EM data based on unstructured tetrahedral meshes

In order to better interpret controlled-source electromagnetic (EM) data over a topographic seafloor, we propose a 3D regularized inversion method for frequency-domain marine controlled-source electromagnetic (MCSEM) survey based on unstructured grids. We first discretize the model domain by unstructured tetrahedral elements and calculate the MCSEM responses and sensitivity using the edge-based finite-element method (EB-FEM). Then, we use the Gaussian-Newton method to solve the inverse problem. To improve the stability, we use a logarithmic transformation method to achieve the upper and lower constraints for model parameters in the inversion process. We test the effectiveness of our algorithm by inverting synthetic data both for a flat seafloor with a single survey line and an undulating seafloor with multiple survey lines. The inversion results show that our 3D inversion method can well reconstruct the location and resistivity information of the resistive oil reservoirs under the seafloor with high efficiency.