Focused-source electromagnetic survey versus standard CSEM: 3D modeling in complex geometries

Abstract

A novel focused-source electromagnetic (FSEM) method focuses the EM field in the vertical direction to provide deep-reading resistivity data. FSEM offers better spatial resolution and greater depth of investigation than the conventional controlled-source electromagnetic (CSEM) method for land and marine EM surveys. We have proven the high efficiency of FSEM by analyzing 3D models of various complex geologic formations in the presence of seafloor bathymetry, shallow resistive gas-hydrate overburdens, and secondary gas reservoirs formed above deeper oil reservoirs. Combining the power of our focusing technique with the power of our 3D numerical modeling method, we have developed exceptionally challenging test cases to conclude that FSEM automatically cancels unwanted shallow effects and allows simple visual interpretation of deep reservoir responses. In addition, FSEM is insensitive to imperfections in the setup geometry. We achieve these advantages using a proper combination of measurements acquired in the receiver excited by transmitters situated at different space points. The method is promising in anisotropic formations as well.