Numerical analysis of eddy current effect for low frequency electromagnetic method in cross-well

摘要

In order to analyze the eddy current caused by the excitation signal and its influence on the secondary field response, the general analytical expressions of eddy current density for low frequency electromagnetic in cross-well is deduced. Based on the axially symmetric time harmonic electromagnetic theory, this expression is established by Helmholtz equation of electric field in radial non-uniform formation. Eddy current density distribution of infinite homogeneous conductive medium and layered media in cross-well show that the metal casing has a strong absorption to electromagnetic signal and the frequency of the emission signal has important influence on its capability of penetrating the casing. The character of EMF (electromotive force)-frequency curves for the non magnetic high resistance formation demonstrates that the low frequency electromagnetic signal can penetrate the casing perfectly, and it also shows that response signal has the highest resolution to the formation resistivity while excited by the frequency about 200Hz.