基于矢量有限元法的三维CSAMT模拟对近场效应影响因素的研究

Affecting Factor Study of 3D Forward CSAMT Numerical Simulation with Vector Finite Element to Near-field Effect

可控源音频大地电磁测深法(CSAMT)的人工可控场源会带来近场效应的问题,如果在实际数据反演时利用了近场数据,将会导致对地质构造的错误推断,从而给实际工作带来麻烦和经济损失。本文采用六面体网格剖分的矢量有限元方法进行三维CSAMT正演数值模拟,通过建立不同的地质模型,研究近场效应的影响因素及规律。层状模型的正演结果与理论解对比验证了算法以及程序的正确性。设计了不同电阻率的均匀半空间、场源下存在异常体、场源与接收点之间存在地形和地表异常薄层等地质模型,分析了视电阻率和阻抗相位的响应特征。得到如下结论:(1)均匀大地电阻率大小、场源与接收点之间存在低阻异常体,场源与接收点之间存在山峰地形以及地表异常薄层均会影响近场效应;(2)场源和接收点之间存在高阻异常体,场源与接收点之间存在山谷地形,场源下方存在异常体虽然影响近区的数据,但是对近区频点基本无影响。研究结果可为实际工作中收发距的选取提供参考,具有一定的理论价值。

The artificial field source of controlled-source audio-fi＇equency magnetotelluric will bring near- field ettect, if near-field data was used in actual data inversion, it will lead wrong inferences of geological structure, bring trouble and economic loss to actual work. To solve this problem, a vector finite-element algorithm based on hexahedral mesh has been applied to simulate CSAMT forward numerical simulation by establishing the dfferent geological models to study the influencing thctors and rules of near source. The algorithm and program was verified by the comparison with the modeling result to theoretical solution. Some models was designed, such as half homogeneous space with ditterent resistivity, abnormal body exists under the field source, 3D topography exists between field source and receiving points and thin layer model of ground surface and so on, The response characteristics of apparent resistivity and impedance phase are analyzed. The conclusions are as follows： （1） Uniformn earth resistivity, low resistivity anomaly and peaks exists between field source and receiving points and thin layer model of ground surface will influence near source. （2） High resistivity anomaly and valley terrain exists between field source and receiving points influence near field data, but it has no ettect on the near source. The results can provide a reterence for selection of send-receive distance in practical work and has a certain theoretical value.