Special Section — Marine Controlled-Source Electromagnetic Methods Rigorous 3D inversion of marine CSEM data based on the integral equation method

Marine controlled-source electromagneticMCSEMsurveys have become an important part of offshore petroleum exploration. However, due to enormous computational difficultieswithfull3Dinversion,practicalinterpretationofMCSEM data is still a very challenging problem. We present a new approach to 3D inversion of MCSEM data based on rigorous integral-equation IE forward modeling and a new IE representation of the sensitivity Frechet derivative matrix of observed data to variations in sea-bottom conductivity. We develop a new form of the quasi-analytical approximation for models with variable background conductivity QAVBandapplythisformformoreefficientFrechetderivative calculations. This approach requires just one forward modeling on every iteration of the regularized gradient-type inversion algorithm, which speeds up the computations significantly. We also use a regularized focusing inversion method,whichprovidesasharpboundaryimageofthepetroleum reservoir. The methodology is tested on a 3D inversion of the synthetic EM data representing a typical MCSEM surveyconductedforoffshorepetroleumexploration.