Imaging CSEM data in the presence of electrical anisotropy

TLDR: In this paper, a 3D CSEM imaging algorithm that treats transverse anisotropy is developed and used to produce a consistent resistivity model for overflight and broadside electric field measurements.

Abstract: Formation anisotropy should be incorporated into the analysis of controlled-source electromagnetic CSEM data becausefailuretodosocanproduceseriousartifactsintheresulting resistivity images for certain data configurations of interest.Thisfindingisdemonstratedinmodelandcasestudies. Sensitivity to horizontal resistivity will be strongest in the broadside electric field data where detectors are offset from the tow line. Sensitivity to vertical resistivity is strongestforoverflightdatawherethetransmittingantennapasses directlyoverthedetectingantenna.Consequently,consistent treatment of overflight and broadside electric field measurements requires an anisotropic modeling assumption. To produce a consistent resistivity model for such data, we develop and use a 3D CSEM imaging algorithm that treats transverse anisotropy. The algorithm is based on nonlinear conjugate gradients and full wave-equation modeling. It exploits parallel computing systems to effectively treat 3D imaging problems and CSEM data volumes of industrial size. We use it to demonstrate the anisotropic imaging process on model and field data sets from the North Sea and offshore Brazil. We alsoverifythatisotropicimagingofoverflightdataaloneproduces an image generally consistent with vertical resistivity. However,superiordatafitsareobtainedwhenthesameoverflight data are analyzed assuming an anisotropic resistivity model.