隐伏地质构造的大地电磁有限单元法正演模拟

Magnetotelluric forward modelling for underground geologic structures by finite element method

对常见的隐伏地质构造体进行大地电磁场正演模拟,并研究和总结了构造体对大地电磁场的响应特征.由麦克斯韦方程组导出大地电磁场的边界条件,应用有限单元法求解变分问题并得出刚度矩阵,根据线性方程组计算出相应的场值,再由改进的视电阻率定义式求出视电阻率.模拟结果显示:改进的视电阻率定义式相对于传统的Cagniard定义式能有效地提高视电阻率曲线的收敛速度和逼近程度;隐伏地质构造体对大地电磁场的响应具有一定的规律性,通过分析两种极化模式下的正演结果,可以有效地识别地质构造体的存在和分布情况.通过实例分析,得出隐伏地质构造正演模拟与实际观测资料的互相验证关系,为实践应用中的反演工作提供了重要的指导意义.

Magnetotelluric method of familiar underground geologic structures were forward modeled under the mode of TE and TM polarizations, and the responding characteristics of MT were studied in this paper. According to the boundary conditions of MT which derived by Maxwell equations, the authors solved the variational problem and worked out the stiffness matrix by finite element method, and the apparent resistivity was obtained by a modified apparent resistivity formula. Simulation results indicate that the modified apparent resistivity formula can effectively improves the calculation convergence rate and modeling precision of apparent resistivities. MT responses of underground geological structures have their discipline. By analyzing the results of two polarization modes, geological structures can be effectively identified. The studied results have instructional effects on the inversion of practical applications.