Mapping thin resistors and hydrocarbons with marine EM methods, part II - Modeling and analysis in 3D

The electromagnetic fields surrounding a thin, subseabed resistive disk in response to a deep-towed, time-harmonic electric dipole antenna are investigated using a newly developed 3D Cartesian, staggered-grid modeling algorithm. We demonstrate that finite-difference and finite-volume methods for solving the governing curl-curl equation yield identical, complex-symmetric coefficient matrices for the resulting N X N linear system of equations. However, the finite-volume approach-has an advantage in that it naturally admits quadrature integration methods for accurate representation of highly compact or exponentially varying source terms constituting the right side of the resulting linear system of equations. This linear system is solved using a coupled two-term recurrence, quasi-minimal residual algorithm that does not require explicit storage of the coefficient matrix, thus reducing storage costs from 22N to ION complex, double-precision words with no decrease in computational performance. The disk model serves as a generalized representation of any number of resistive targets in the marine environment, including basaltic sills, carbonates, and stratigraphic hydrocarbon traps. We show that spatial variations in electromagnetic phase computed over the target are sensitive to the disk boundaries and depth, thus providing a useful complement to the usual amplitude-versus-offset analysis. Furthermore, we estimate through the calculation of Frechet sensitivity kernels those regions of the 3D model which have the greatest effect on seafloor electric fields for a given source/receiver configuration. The results show that conductivity variations within the resistive disk have a stronger influence on the observed signal than do variations in the surrounding sediment conductivity at depth.