Spectral analysis of marine CSEM data symmetry

We demonstrate, using synthetic and real data, how simple symmetry properties of marine controlled source electromagnetic data can provide useful information about the main resistivity discontinuities below the seafloor. Our method can be used directly for detecting resistive bodies, such as mineralized reservoirs or geological discontinuities, and for improving inversion performance. We define an asymmetry attribute for the magnitude or phase of the inline electric field responses for source positions on either side of fixed receiver stations. The asymmetry attribute quickly provides a preliminary model of resistivity discontinuities that can be refined by modelling and inversion. Moreover, symmetry properties provide a constraint for speeding up the inversion itself. This integrated approach of spectral analysis of symmetry, using different frequencies and offsets, combined with modelling and inversion, is an effective methodology for localization of resistivity bodies, such as mineralized reservoirs. In contrast, the real data case demonstrates that direct inversion of CSEM data without a proper initial model can lead to an incorrect solution that is inconsistent with seismic and well information.