2D forward modeling for line sources frequency domain electromagnetic method using quadtree grid
The forward modeling of control source electromagnetic methodspresents challenges arising from source singularities and difficulties in handling boundary conditions.This paper addresses the issue of low simulation accuracy for singular sources by transforming the simulation accuracy of the source into that of the gird using surface integration and locally refining the quadtree grid arbitrarily.The study reveals that the electromagnetic field in the subsurface is induced by the electromagnetic field in the air at the lateral boundary far from the source,rather than being directly transmitted from the source through the earth.Consequently,utilizing 1D plane-wave side boundary conditions is considered more reasonable and accurate than employing first-order absorbing side boundary conditions.Through case analysis,we also found that the field at the air side boundary is significantly altered by the secondary field induced by the earth medium.Unreasonable first-order absorbing air boundary conditions,based on primary fields,are identified as the main cause of low modeling accuracy at the boundary.However,expanding the grid outward to cover a larger boundary region can ensure computational accuracy in the core region.Additionally,the study finds that the full-zone apparent resistivity of the line source can effectively reflect the underground electrical structure and can be utilized to obtain approximate models of underground electrical structures by substituting Cagniard apparent resistivity in magnetotelluric inversion techniques.Particularly,the full-zone apparent resistivity of the tipper can also reflect the variation of underground structures,providing possibilities for underground structure detection without electric field observations.However,it emphasizes a clear distinction between the two,highlighting that full-zone apparent resistivity also possesses certain geometrical sounding capabilities.The article concludes by emphasizing the urgent need for the development of full-zone apparent resistivity 2D/3D inversion techniques with sources.
Line source frequency domain electromagnetic methodForward modelingQuadtree gridFinite difference methodFull-zone apparent resistivity
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