On the Air–Sea Boundary in Transient Marine CSEM Detection Modeling of Subseafloor Hydrocarbon Reservoirs

The published scientific literature provides extensive results and discussion of approximate finite-difference time-domain (FDTD) computational modeling of marine controlled-source electromagnetics (CSEM) detection of hydrocarbon reservoirs buried under the seafloor. “Approximate” here refers to the neglection of displacement currents in the calculation to reduce the computational burden of the simulation. This leads to the widely used approximate continuation boundary conditions at the ocean-air interface to avoid the free-space region in the simulation where inclusion of displacement currents is required. However, an analysis of when the use of such continuation boundary conditions sufficiently reduces the accuracy of the calculated results is lacking in the published literature. This letter addresses this issue and reports the application of the complete and standard, three-dimensional full-vector Maxwell's equations FDTD method to modeling CSEM hydrocarbon detection. We provide accurate results for shallow and deep-water CSEM problems and determine that the continuation boundary condition is inadequate at large (>; 4.5 km) source-to-receiver distances in deep-water detection problems and at all distances in shallow-water problems.