Study Of Three Dimensional Staggered-grid Finite Difference Modeling And Source Effect Of CSAMT

Controlled source audio-frequency magnetotelluric(CSAMT) method is a frequency domain sounding method using an artificial source which is developped from magnetotelluric(MT) method and audio-frequency magnetotelluric method.Because of the magnitude of the artificial source is stronger than the natural source, the time used to observe the EM field can be significantly reduced. So the productivity of this method is very high. However, the numerical simulation of CSAMT especially in 3D is very difficult in comparison with the MT method since the importing of the source. Besides, the importing of the source also bring some problems, such as near field effect, shadow effect and copying effect.To improve the application effect of CSAMT method and overcome the interpretation error that source effect brought are the hotspots in the field, and also the research difficulties. So developing three dimensional electromagnetic numerical method for CSAMT is necessary.In this work, we completed the theories and formulas of three dimensional staggered grid finite difference method from the Maxwell equations. In order to eliminate the singularity of the point source, we divide the electric field into the primary and the secondary field before calculation. The primary field is calculated by analytical method, the second order partial differential equations of the secondary field is discreted by finite difference method using Yee’s staggered grid method. The compressed row storage format(CSR) is adopted to store the coefficient matrix in order to save the storage space. The Symmetric quasi minimum residual method(SQMR) is adopted to solve large complex coefficient equations.We implemented a three dimensional forward program using C++ programming language and verified the correctness of our code. Finally, We calculated the response of some typical models and analyzed near field effect, shadow effect and copying effect.