Calculation of all-time apparent resistivity of large loop transient electromagnetic method with very fast simulated annealing

Abstract

In large loop transient electromagnetic method (TEM), the late time apparent resistivity formula cannot truly reflect the geoelectric model, thus it needs to define the all-time apparent resistivity with the position information of measuring point. Utilizing very fast simulated annealing (VFSA) to fit the theoretical electromagnetic force (EMF) and measured EMF could obtain the all-time apparent resistivity of the measuring points in rectangular transmitting loop. The selective cope of initial model of VFSA could be confirmed by taking the late time apparent resistivity of transient electromagnetic method as the prior information. For verifying the correctness, the all-time apparent resistivities of the geoelectric models were calculated by VFSA and dichotomy, respectively. The results indicate that the relative differences of apparent resistivities calculated by these two methods are within 3%. The change of measuring point position has little influence on the tracing pattern of all-time apparent resistivity. The first branch of the curve of all-time apparent resistivity is close to the resistivity of the first layer medium and the last branch is close to the resistivity of the last layer medium, which proves the correctness of the arithmetics proposed.

Keywords

very fast simulated annealing (VFSA) / large loop transient electromagnetic method / rectangular loop / all-time apparent resistivity