Data Preprocessing and Starting Model Preparation for 3D Inversion of Marine CSEM Surveys

The marine controlled-source electromagnetic (MCSEM) method has been evolving into a geophysical imaging tool for increasingly complex geological settings. At the same time, 3D inversion algorithms for arbitrary survey layouts demand improved data quality compared to standard processing. Using a state-of-the-art survey acquired in the fall of 2007, we present a processing sequence starting from time-domain electromagnetic data acquired by seabed receivers to providing frequency domain data input and data weights for advanced processing. This includes determination and/or quality control of receiver orientation and time synchronization, and we show the quality of azimuthal receiver data to be adequate to be included in future inversions. Further, navigation data are adapted to a discretized grid upon determining the seafloor bathymetry. For missing or inadequate coverage from seismic surveys, the bathymetry can be extrapolated from navigation data with a spline-based algorithm, which is also described. It has proven beneficial, both in computational time and for recovering a meaningful model, to obtain a starting model for a full 3D inversion scheme by inverting reference receivers using a global, simulated annealing scheme, the result of which is imprinted onto the seafloor.