3D Modeling of Novel Focused Source EM Survey versus the Standard CSEM

摘要

We present a novel Focused Source Electromagnetic (FSEM) method that exploits an idea of focusing the EM field in the vertical direction to provide deep-reading resistivity data. The focusing technique increases the spatial resolution and the depth of investigation of land and marine EM survey, as compared to the conventional Controlled Source EM (CSEM) method. We demonstrate the high efficiency of the focusing principle on challenging 3D models of the geological formation simulating seafloor bathymetry and other complicated shallow effects, which can mask the deeper reservoir response. The focusing reduces the distorting effect of shallow structures dramatically. The idea of the vertical focusing of the EM field have been developed from Laterolog resistivity well logging principle. Laterolog is known to eliminate the axial borehole current at the receiver, reducing the effects of the conductive borehole and of the shoulder beds. However, Laterolog requires an automatic feedback loop to cancel the borehole axial current. It would be hardly feasible to apply such a feedback system on the earth's surface. We suggest a focusing procedure, which is rather based on software, than on hardware solution; still, it is an active focusing system giving results practically equivalent to the physical feedback-loop focusing. Combining together the power of our focusing technique and the power of our 3D numerical modeling method, we handle exceptionally challenging test cases and show that FSEM allows simple visual interpretation of deep reservoir responses and automatic cancelling unwanted shallow effects in cases when the standard CSEM requires full 3D inversion for wide band of frequencies or measurement times. We show that FSEM has an advantage as compared to the standard CSEM in anisotropic formations as well.