3-D approximate inversion for transient electromagnetic soundings

In this paper, based on the theories of smoke ring, current lines in the Earth, induced by transmitting source on the Earths surface, are regarded as downward moving and outward expanding current filaments which are called as images of transmitting source. By comparing the magnetic field of the current filaments with that measured near the source, we can determine the image depth which can be used for estimating the downward vertical velocity of the image by using a cubic spline interpolation method. Then the resistivities varying with depth can be approximately estimated through a comparison between the estimated velocity of the image and the downward diffusing velocity in a homogeneous half space. We discussed the case of a circular transmitting loop, and developed a corresponding approximate inversion program which has been tested for 1-D layered-Earth models and real data.