Study Of CSAMT 2.5-dimensional Forward Modeling With Topography

CSAMT (Controlled Source Audio-frequency Magnetotelluric) method is an efficient electromagnetic method; it is widely used in many domain of today's geophysical exploration.2.5-D forward problem is to compute the response of a two-dimensional conductivity structure excited by a three dimensional current source or a finite source. It is important because many geological targets are approximately 2-D. In those cases a fully 3-D medium is not needed and the computation cost can be greatly reduced by solving just the 2.5-D problem.Here I attempt to use a pseudo-delta function to distribute the response of the dipole source current. Using this function we do not need to separate the total response field into primary field and secondary field, which is very difficult for complex structure with topography response. Using the pseudo-delta function, we can get the total field directly, and do not needs to consider the error come from field separation .To deal with the topography response conveniently, I employ an isoparametric finite-element technique, which can partition the target area into elements with various shapes according to the topography.In the process of 2.5-D forward modeling, I first employ Fourier transform to the field in the invariant conductivity direction, transforming the filed form frequency domain to wave number domain. Then at each of a set of discrete spatial wave numbers, an isoparametric finite-element method is used to obtain the total filed directly. Through space derivation The EM field components of the other two directions can be obtained. At last, the field is transformed back to space domain by inverse Fourier transform.Also in this paper some simple examples are presented to validate the code.