Study of the dependence of electromagnetic field penetration depth on the parameters of the subsurface model in forward mapping for transient electromagnetic method using approximation algorithm of Christensen

摘要

Approximation for forward mapping of transient electromagnetic method (EM) data proposed by Christensen is much faster, but at the same time, less accurate than the conventional methods for getting solution. For the each time of the observed EM response, Christensen uses the solution for the equivalent homogeneous half-space, whose lateral conductivity is equal to the total lateral conductivity of the stack of subsurface model layers that are higher than the electromagnetic-field penetration depth at the moment. In most cases, this approach allows us to obtain sufficiently accurate solutions when considering the low-contrast patterns, consisting of a small number of layers. However, Christensen's method does not allow having satisfactory convergence of field and simulated data for complex, conductivity contrasting models of sedimentary cover. This paper presents the analysis of the impact of electromagnetic field velocity on the value of discrepancy between calculated and observed EM response as one of the determining factors of matching errors. Finally, to achieve the most accurate results of simulation, it introduces the dependence of electromagnetic field propagation velocity on the parameters of subsurface model. Consequently, there is a better match of simulation results obtained by this method and conventional methods of forward mapping.