Abstract
TDEM method is a popular used method in exploration geophysics. The method detects the underground resistivity structures by analyzing the transient phenomena of the EM field occurred when the source current is rapidly shut out. A problem that we often embarrassed is that the above surface measurements failed to detect the structures in the depth in case that the near surface layer were conductive. To solve this problem EM tomography measuring system is suggested, which, by putting the sensor through borehole into the subsurface, extended our measuring domain from two to three dimension. This is a leading edge in this field under its developing way.
Before we can make the EM tomography system a practical one in exploration, many questions have to be answered, such as where could we expect the most efficient measuring and in what span of the measuring time should we take to achieve that. These are all the basic matter for us in designing the system and should be clarified by numerical simulation. In this study, whole set of equations is derived by directly solving Maxwell's equation under correspondent boundary conditions. According to which the propagating behaviors of the EM field in layered earth are simulated. By summarizing the results in a series figures, the essences of equivalent-layer problems are discussed. The superiority of TDEM tomography system is clarified. To evaluate the measuring system's efficiency, a new concept called detectability is introduced. All these provided a convenient criterion in designing our new EM system.
