1D inversion of multicomponent, multifrequency marine CSEM data: Methodology and synthetic studies for resolving thin resistive layers

Numerical methods for 1D forward modeling and inversion of marine controlled-source electromagnetic (CSEM) data are used to examine the inherent resolution of various acquisition configurations to thin resistive layers simulating offshore hydrocarbon reservoirs. Synthetic data studies indicate that jointly inverting frequencies of 0.1 and 1.0 Hz offers better resolution than inverting either frequency alone. Further increasing the bandwidth or density of frequencies does not produce a commensurate increase in resolution. An inline horizontal electric dipole is found to provide better resolution than either broadside or vertical electric dipoles. The horizontal electric and magnetic fields for any transmitter orientation have better resolution than vertical fields. Separate inversions of electric and magnetic fields perform equally well at recovering the reservoir, and there is no resolution improvement from jointly inverting both fields. Smooth inversion for a multiple resistive layer model detects the presence of all resistive layers, and shallow thin resistive layers do not impact the ability to image deeper resistive layers. The accuracy of the inverted models is improved substantially by including the boundary depths of resistive layers as a priori structure in the inversion. Including the seawater resistivity profile as fixed structure in the inversion is found to be essential for obtaining optimal resolution of subseafloor resistivity.