Three-dimensional Modelling Of Controlled Source Audio Magnetotelluric Including Topography

Controlled-source audio frequency magnetotelluric(CSAMT)survey is a kind of electromagnetic sounding technology which was put forward by Goldstein and Strangway(1975),.Zonge et al formed detailed measurement method in 1991,and has been widely applied to mineral exploration,hydrocarbon exploration,geothermal exploration and engineering exploration.Most CSAMT field data,as well as the other electric prospecting methods' field data,are collected in the presence of topography and greatly affected by it.At present,the method of mature 3D research results are few,3D with topography research results is almost no.So most of CSAMT field data are interpreted assumed a two-dimensional geologic structure and topography.However,the actual geological structure and topography in the field are usually 3D,which leads to the interpretation of the results may be wrong.In order to obtain a more reliable interpretation of the results,the urgent need to develop a terrain controlled-source audio frequency magnetotelluric 3D and inversion algorithm for data processing and interpretation.This paper from the basic Maxwell equations of the CSAMT,a staggered mesh finite-difference(FD)method is applied to discrete the integral form of Maxwell equations,and put forward a pseudo-delta function instead of the source so as to solve the source singularity in the direct simulation field forward ideas.In numerical simulation,we often divide the area into the far enough,and the influence of the anomalous body on the boundary is very small.Therefore,we consider that the field on the boundary is generated by a dipole in a uniform half space,and using Fortran language to achieve the simulation calculationIn this paper,the results of this algorithm are compared with those of the previous horizontally topographic results,and the correctness of the horizontal topography of the algorithm is verified.At the same time,the 3D topography simulation technique is added to the forward modeling,and the results of this algorithm are compared with the Comsol software results and the MT response using the finite element method(FEM).The conclusion is drawn that the algorithm is accurate and reliable under the 3D topography.In order to discuss the influence of topography,this algorithm is used to simulate the electric field,magnetic field,apparent resistivity and phase response of the 3D hill or valley topographic in survey area model at different frequency,slope angle,underground resistivity and the derection of source,and then analyze the influence law;Then,the electric field,the magnetic field,the apparent resistivity and the phase response of the source is located at the top of the 3D hill which is calculated at different frequencies,skin depth,slope angle,underground resistivity and the derection of source,and also analyzed the law;Finally,the CSAMT response of the 3D hill or valley topography between the source and the survey area is simulated.