Study Of Three-dimensional Forward Modeling Of CSAMT With Moving Source

Recent developments in geophysical instrumentation, as well as advances in air-borne and borehole technologies, have allowed the acquisition of large electromagnetic (EM) data sets. Three-dimensional (3-D) interpretation of these data sets is very important in mining, in oil and gas exploration. Rigorous 3-D interpretation requires the development of fast and practical 3-D inversion algorithms, at the core of which we need fast forward modeling routines. The integral equation (IE) method is a powerful tool for numerical modeling, especially for models of localized anomalies. This method is based on expressing the electromagnetic fields as a system of integral equations with respect to the anomalous currents within an inhomogeneity. One can transform the integral equations into a system of linear algebraic equations by approximating the anomalous current distribution by piecewise constant functions. The main difficulty with this technique is the size of the matrix of the linear system of equations, which could demand excessive computer memory and calculation time.Several approximations have been introduced to remedy this problem. The first, and most widely known, such approximation is the Born approximation, in which the anomalous currents inside the anomalous domain are neglected. This approximation provides a very fast solution, but in structures with high conductivity contrasts and at high frequencies, this approximation is inaccurate. Another approximation called the quasi-linear (QL) approximation. Within this method the anomalous electric field is assumed to be proportional to the background field through an electrical reflectivity tensor. This tensor is determined by solving a minimization problem based on an integral equation for the scattering currents. The advantage of this approach is that the reflectivity tensor can be assumed, if so desired for sake simplicity and speed, to be simply scalar or diagonal, instead of a full tensor.We compare the accuracy of this approximation with respect to the full IEsolution, the Born approximation, in order to establish the validity of the new approximation.