Understanding the effect of Bathymetry on the Marine Controlled Source Electromagnetic Method using Electromagnetic Streamlines

Abstract

Understanding the impact of bathymetry can be a critical in application of the marine controlled source electromagnetic methods. Electromagnetic field strength and direction can be affected by small changes in water column depth. The bathymetry, the air-water interface and sub-surface resistivity variations will all contribute to any single electromagnetic measurement. In such complex geo-electric settings a deeper understanding of how and where each feature is expressed in the electromagnetic response is required. We compute the electromagnetic fields for a model with complex bathymetry and a hydrocarbon target. We compute the response with and without hydrocarbon to investigate the effect of bathymetry. A 2.5D finite element algorithm was used to forward model the MCSEM response. The interaction of the electromagnetic fields with the target and the bathymetry can be appreciated by viewing electric and magnetic streamlines. A key benefit of using streamlines is that they quickly show where the electric or magnetic fields would be most strongly altered by the target or alternatively by bathymetry. This aids both interpretation and survey design.