Research On Shielding Of Low-Resistivity Layer In Prospecting By Transient Electromagnetic Method

Recently Transient Electromagnetic Method (TEM), as a kind of time-domain electromagnetic prospecting approaches, has experienced a period of rapid development and been widely applied. It has promising perspective of application and development potential in areas such as mineral resource prospecting, tectonic exploration, geological survey concerning hydrology, engineering and environmental protection, as well as the best choice in hydro-geological survey of coalfield due to its extraordinary sensitivity to low resistivity, its high efficiency and convenient setup. However, this method may have negative effect for prospecting depth and resolution especially when low-resistivity abnormality or low-resistivity layer lies above the target object just due to its sensitivity to low resistivity, which is increasingly notable with expansion of its application field and scope. Actually, the layer with low-resistivity has shielding effect on TEM field when it lies on prospecting object especially when the earth surface is a giant-thick covering the low-resistivity layer, the prospecting depth achieved by TEM method may be greatly implicated, resulting in prolongation of prospecting time and decrease of single-to-noise ratio. Therefore under this condition the method cannot achieve the correct prospecting depth and favorable result. Because of above stated reasons the impact of low resistivity on prospecting depth through agency of TEM extracts more and more attention to research on the shielding effect of low resistivity and its impact on prospecting by TEM, which is of great significance both for theory and practical application.Focuses of this dissertation are concerning: (1) Inducing analytic solution of TEM response of horizontal layered medium using central coincident loop device applied widely in TEM middle to deep depth prospecting; computing the TEM field decay curve of typical geo-electric model by G-S method; researching effects on TEM prospecting when middle layer is characterized by its low resistivity. (2) Realizing numerical simulation of TEM field response with 2-dimensional finite-difference time-domain method (FDTD); analyzing in detail the response characteristic and shielding effect of low-resistivity object or layer in surface or water prospecting on TEM field using numerical simulation through agency of FDTD and computation result through G-S method; discovering the conglomeration function of low-resistivity objects on induction eddy-current, and deceleration function and deferral effect of low-resistivity layers on diffusion of TEM field; (3) studying conglomeration and loss of energy of TEM field in the low-resistivity layer using time-frequency representation or time-frequency distribution; analyzing shielding effect of low-resistivity layer on TEM field from aspect of energy; (4) analyzing in detail the impact of covering low-resistivity layer on TEM prospecting using examples, and putting forward issues that should be paid attention to in practical TEM prospecting.Creative achievements made through research on shielding effect of low-resistivity in TEM prospecting are as follows:(1) Realizing FDTD numerical simulation of TEM field in slowly changing time-domain electromagnetic field. TEM field is slowly changing in middle to deep depth prospecting, which is characterized by unperceivable wave and dominant dissemination, thus it can be assumed to be quasi-static state approximate and the wave equation of electromagnetic wave radiation is retrogressive and replaced by diffusion equation. The solution through agency of FDTD employs directly the curl...