Influence Of Resistivity Anisotropy On Marine Controlled-source Electromagnetic Responses

Marine CSEM is one kind of geophysical methods used to probe geologicalstructure below the seabed by measuring the distribution law ofelectromagnetic field emitted artificially on the sea or the seabed.As the increased emphasis on marine resources, offshore exploration hascontinued to increase. The marine CSEM also gradually become a very usefuldetection technology of probe offshore gas hydrate layer.As we all know, the underground conductivity of the medium oftenengender anisotropy. The observation and studies from laboratory haveshown that gneiss and other types of rock showed obvious conductivityanisotropy. In the fold and thrust areas hydrocarbon reservoirs arefrequently over-lain by thick, dipping sedimentary sequences includingshales and thinly interbedded sandstones. The sedimentary sequences andthe thin bedded sand-shale sequences can exhibit macroscopic electricalanisotropy. It is estimated that about30%of the world’s oil and gas existin lithology fractured strata and thin siltstone interbedded, while thesedimentary sequences and the thin bedded sand-shale sequences canexhibit macroscopic electrical anisotropy. When lithology fracturedstrata fissures part is poured by the seawater, the conductivity on thevertical direction of the fracture surfaces will be much higher than inthe direction of the fracture surfaces.When interpreting the marine CSEM data, the seafloor earth model isusually assumed to be electrically isotropic. While sedimentary sequencesand shales and thinly interbedded sandstones showed significant conductivity anisotropy. The sedimentary sequences and the thin beddedsand-shale sequences can exhibit macroscopic electrical anisotropy. Wheninterpreting the marine CSEM data, conductivity anisotropy should not beoverlooked. Otherwise it might get the wrong submarine geoelectric model.And the numerical simulation is an effective means and methods to studythe anisotropic medium electromagnetic field characteristics.This paper extends the scope of numerical simulations of marinecontrolled-source electromagnetic (CSEM) fields in a particular case ofanisotropy (dipping anisotropy) to the general case of anisotropy by usingan adaptive finite element approach, and implement the man-machineinterface by using MATLAB visualization module, it would be more directand convenient to implement the numerical simulation with different modeland different parameters. In comparison to a dipping anisotropy case, thefirst order spatial derivatives of the strike-parallel components arisein the partial differential equations for generally anisotropic media,which cause a non-symmetric linear system of equations for finite elementmodeling. The adaptive finite element method is employed to obtainnumerical solutions on a sequence of refined unstructured triangularmeshes, which allows for arbitrary model geometries including bathymetryand dipping layers.Furthermore, we also compared with the two-dimensional dippinganisotropic code results with the results of this paper, the final resultswere completely consistent. And numerical results of a2D anisotropicmodel show both anisotropy strike and dipping angles have great influenceon the marine CSEM responses.