【作者】 雷旭友；

【导师】 李正文；

【作者基本信息】 成都理工大学 ， 地球探测与信息技术， 2010， 博士

【摘要】 根据电磁波层析成像技术、方法的现状和发展方向,结合工程勘探、环境保护、文物调查、防灾减灾等地质工程勘察的实际需要,开展多源、多孔对的高分辨率跨孔电磁波层析成像方法研究。主要研究内容分为五个部分:第一,建立电磁波在含水岩溶区、采空区、破碎带、以及含水层等不良地质条件下传播的理论与方法,通过大量的、与实际地质情况相符合的数值模型演算,获取不同地质模型的电磁场的数值特征,尤其是辐射电场的分布特征。同时,通过固定发射和接收位置,观测发射天线馈电压和接收观测值随时间变化规律,研究获得天然场对观测值的影响规律,将天然场的影响归类于辐射电场强度的变化,最大限度地消除天然场的变化导致的反演误差;第二,建立介质对电磁波衰减因子和吸收系数图像重建的成像反演方法。对正演模拟所得到的电场值加入5%的随机扰动后,视为观测资料,并以此进行电磁场衰减的成像反演计算,检验反演方法的可行性及分辨率;第三,针对实际工程地质勘察不同源多孔对特性,建立多源、多孔对联合反演方法,从而更好地约束钻孔附近区域的物性特征;第四,将多源、多孔对联合反演方法用于实际工程地质勘察中,以渝怀铁路K419+710～K419+850路基岩溶塌陷病害地质调查和二连浩特至河口国道主干线桥址岩溶勘探等为例,说明本文提出的多源、多孔对联合反演方法的应用效果;第五,利用VC++集成电磁波正演、反演试验数据,集成正、反演成像方法和实际资料处理数据,建立跨孔电磁波层析成像软件系统。正演模型的建立是进行电磁波层析成像的基础。在电场衰减的正演模型中进行源近似,忽略了源区介质对电磁波传播的影响,从而将近场区的影响纳入到衰减成像中,大大降低了成像的准确性。考虑源区(近场区)介质对电样波传播的影响,研究电场衰减特征,建立适用于高分辨率电磁波层析成像的正演模型。首先引入电场“衰减因子”的概念,即介质吸收系数的倒数。建立关于衰减因子的逐次线性化层析成像方法。然后引入点源性质的源激励,取代近场处理,代入成像方程与衰减因子同时反演。这样可以有效地消除近场岩石导电性对源的影响,克服了传统的电磁波成像方法中对源激励进行简单近似计算的不足。建立介质吸收系数与辐射点源强度联合反演的成像方法。线性方程组求解采用最小二乘奇异值分解法(LSQR),能够使方程快速收敛。根据岩溶地区各类岩石、土层、含水层及空洞等地质结构的介电常数与导磁性,合成电磁波的吸收系数值,建立电磁层析成像数值实验的正演模型。其中激励源是根据输入电流强度及进行源区假定后近似计算得到的。通过模型检测,寻找不同类型地质体的电场强度观测值特征。通过大量的实际观测数据,检测基于电场衰减的层析成像技术的可靠性。将多源、多孔对联合反演方法研究中的数值实验、工程实例进行集成,建立即查即用的数据库。利用VC++可视化程序设计语言建立实性较强的多源、多孔对联合反演的电磁波层析成像软件系统。更多还原

【Abstract】 It is based on both the status and the developing direction of the electromagnetic tomography technique, and the geological engineering investigations for the engineering exploration, the environmental protection, the cultural relics survey, the disaster prevention to carry out the study on the high-resolution cross-hole electromagnetic tomography based on the multi-source, multi-crosshole data. The studied contents mainly include five parts: the first one, to develop the theory and methods for the electromagnetic propagation in the complex mediums such as the Karst regions, the goave, the crushed zone, the water layers, and the coal layers, etc. The numerical characteristics of the electromagnetic field based on the different geological models are investigated by using a large number of numerical calculations, where the models are designed in accordance with the actual geological conditions. Secondly, the tomographic inversion has been developed in this study to reconstruct the images for either absorption coefficient or attenuation factor of the electromagnetic waves. The electric field value, the result of the numeric modeling, was added 5% random perturbations to be regarded as the observations, which were then input into the tomographic system to check the method and resolution. The third one, a joint inversion was developed based on multi-source, multi-crosshole data, in an attempt to better constraint the properties of matter near the drilling holes. The fourth, the multi-source multi-crosshole joint inversion has been applied to the practical engineering geological survey. In this study, three examples are illustrated to display the excellent behavior of the tomographic inversion. The three applications are the geological investigation for the railway subgrade in Yuhuai railway (K419+710～K419+850), the Karst investigation for a bridge in national main line between Erlianhaote and Hekou, the exploration for the Tianshengxia bridge site basis on the Guiyang-Kunming railway, and the soil cave exploration for Zhengzhou-Xi’an passenger dedicated line. Finally, data from the forward modeling and inversion, the calculated programs for nodeling and inversion, and the actual data processing are collected by a software system for the electromagnetic tomography which is developed by using VC++ language in this study.Forward model is the basis of electromagnetic tomography. In the previous forward model of the electric field attenuation, an approximate is used to neglect the medium effect of the source region on the electromagnetic wave propagation. Thus the effects from the near field have to be added into the attenuation imaging. As a result, the accuracy of imaging is greatly degraded. Considering the effect of the source (near-field region) medium on the electrical wave propagation, this study investigate the attenuation characteristics of the electric field, and try to construct a forward model for the application of high-resolution electromagnetic tomography. First of all, an electric field attenuation factor is introduced as the reciprocal of the electric absorption coefficient. A successive linear tomography method is established for the attenuation factor. Then, an excitation with point-source nature is introduced to replace the near-field processing, which is substituted into the imaging equation. Both the excitations and the attenuation factor are obtained by solving the equation. The joint inversion would effectively eliminate the near-field effects from the electrical conductivity of rocks in source regions, so as to overcome the lack of a simple source approximate calculation in the traditional methods of electromagnetic imaging. In the joint inversion for both attenuation factor and source function, the sparse linear equations are solved by using the least square QR decomposition method (LSQR), which let the equation quickly convergence. It is based on both the conductivity and permeability of the actual geological structures, such as the different rocks, soil layers, water layers, hollow caves, etc, to compute the absorption coefficient distribution of the electromagnetic wave, and to further construct the forward models for the numeric test. In the forward calculations, the source excitations are assumed by considering the input current intensity and source regional medium. Through model testing, the characteristics of the electric field strength for the different types of geological bodies are obtained. A large number of the actual observational data was used to check the reliability of the electric field based on the attenuation tomography presented in this study.A database has been built by integrating the numeric test data, the engineering examples, the forward method and inversion. Using VC++ programming language, I establish a visually tomography software which is based on the multi-source, multi-crosshole electromagnetic data.更多还原