Numerical modeling analysis of short-offset electric-field measurements with a vertical electric dipole source in complex offshore environments

We simulated and analyzed short-offset transient electric-field measurements excited by a vertical electric dipole (VED) source over complex 3D offshore models. A finite-element time-domain modeling algorithm was used to efficiently model complex offshore structures. Using a series of cross-sectional snapshots of transient electric fields in the complex offshore models, we examined the characteristics of the short-offset seafloor electric-field measurements. The numerical modeling analysis indicated that the short-offset horizontal electric-field (E-x) measurements are very sensitive to subtle multidimensional seafloor topography near a VED source and can show a sign reversal at late times. The sign reversal occurs because the VED source is no longer normal to the seafloor. The occurrence of the sign reversal limits the application of the 1D inversion to the E-x measurements, even at a short source-receiver offset. In contrast, the short-offset vertical electric-field (E-z) measurements are robust to subtle seafloor topography around the source, and can be interpreted using the 1D inversion. The 1D inversion of the short-offset E-z measurements over the complex 3D offshore models shows that the measurements lack the resolution of the thickness and the resistivity of a hydrocarbon reservoir and a salt dome, but can provide useful insights into their lateral extent.