【作者】 石显新；

【导师】 陈明生；

【作者基本信息】 煤炭科学研究总院 ， 矿产普查与勘探， 2005， 博士

【摘要】 瞬变电磁法(简称TEM法)是近年来发展很快并得到广泛应用的一种时间域电磁勘探方法。由于这种方法对低阻体特别灵敏，且工效高、装备轻便，故在矿产勘探、构造探测、水文地质与工程地质调查、环境地质调查等众多领域都有着广阔的应用前景和巨大的发展潜力，是目前煤田水文地质勘探的首选方法。然而，正是因为TEM法对低阻体敏感，当低阻体或低阻层位于欲探测地质目标体上面时，将会对这种方法的探测深度及分辨率造成不利影响。随着TEM法应用领域与范围的不断扩大，低阻层对其探测的影响日益突出。实际上，当低阻层覆盖在勘探目的层之上时，低阻层对TEM场具有屏蔽作用。特别是当地表为巨厚低阻覆盖层时，将会对TEM法的探测深度有很大影响，此时，TEM法勘探所需的探测时间加长，导致晚期资料的信噪比下降，以致达不到预想的勘探深度，不能取得理想的探测效果。因此，低阻层对TEM勘探的影响应当引起高度重视，开展低阻屏蔽层对TEM法勘探的影响研究，对于TEM法勘探的理论和实际应用都具有重要意义。 本文所完成的工作主要有：(1)针对目前在中、深度TEM法勘探中应用最多的中心回线装置，推导出水平层状介质TEM响应的解析解，用G-S方法计算了典型地电模型的TEM场衰减曲线，研究了当中间层为低阻层时对TEM法探测的影响；(2)实现了二维时域有限差分法(简称FDTD法)对TEM场响应的数值模拟，通过FDTD法数值模拟和G-S方法计算结果详细分析了地面勘探或水域勘探中低阻体或低阻层对TEM场的响应特征和屏蔽效应；揭示了低阻异常体对感应涡流的聚集作用、低阻覆盖层对TEM场扩散的减速作用及延时效应；(3)应用时—频表示或时—频分布研究了TEM场能量在低阻层中的聚集和损耗，从能量的角度分析了低阻层对TEM场的屏蔽作用；(4)通过应用实例详细分析了低阻覆盖层对TEM法探测的影响，并指出在实际TEM法勘探中应注意的问题。 本文通过对TEM勘探中低阻层屏蔽效应的研究，取得的创新性成果为： (1)在缓变的时域电磁场中实现了TEM场的FDTD数值模拟。在中、深层勘探情况下，TEM场是缓变的，其波动性已经很不明显，扩散特征起支配作用，波动方程退化成了扩散方程。由于FDTD法直接从时域Maxwell方程的旋度方程求解，在应用准静态近似后磁场的旋度方程不能构成显式的时间递推迭代公式，因此，需引入虚拟更多还原

【Abstract】 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更多还原