高密度电阻率成像

介绍了高密度电阻率法的概念及该方法的工作特点，重点介绍了对阵列数据进行电阻率成像的佐迪法和2D积分法．并利用数值模拟结果及研究实例证实，两种成像方法均为可行的高密度电阻率成像方法．     

基于边界积分方程的2维电阻率层析成像

提出了一种新的二维电阻率层析成像方案。与国内外已有成像方法相比,该方法无需计算雅可比矩阵,不必要作线性化近似,这不仅节省了计算时间,而且使成像迭代过程,稳定,可以对实际上广泛存在的大一对比度介质获得满意的成像效果,因而大大提高了电阻率层析成像技术的这用性。中给出的数值模拟结果显示了该方法的有效性。     

小回线源TEM法全域视电阻率成像在某金矿水害中的应用

在煤矿水害问题中,利用瞬变电磁法研究煤矿的底板水、顶板水、断层或陷落柱特征,已经有很多成功的案例;但是在金矿水害中的应用,尚未发现成功的案例.目前,在生产实践中,瞬变电磁法在煤矿水害资料解释还停留在利用晚期视电阻率近似处理、解释的水平.金矿水害数据采集位于深部采场,需要全空间瞬变电磁资料解释技术.基于以上背景,介绍利用反函数原理计算的全空间全域视电阻率成像,对金矿水害探测的瞬变电磁资料进行处理解释.研究发现:该解释方法不仅计算速度快,且精度满足要求,方便实测数据的快速处理.同时能够去除晚期视电阻率成像在早期、过渡期出现的假异常;通过钻孔验证和开挖,证明该成像解释方法对金矿水害防治,效果非常明显.     

高密度电阻率法成像

介绍在地质大学电法科研组“高密度电阻率法正、反演软件”的基础上,研制高密度电阻率法2.5维电阻率成像系统;讨论不平地形条件下高密度电阻率法2.5维电阻率成像的算法和软件,可用于九种常用电极装置观测结果的电阻率成像。对理论和实测数据的成像结果表明,该成像系统程序运行稳定,成像效果很好。     

等位线追踪法在井间电阻率成像中的应用初探

井间电阻率成像是近年来发展起来的在工程应用很广泛的方法,其关键技术之一是反演方法,目前国内外主要是基于最小二乘法.综合国内外各种资料,本文借鉴医学CT引进等电位线的研究角度来进行电阻率反演追踪成像,根据测得的电流、电位推导出反演视电阻率的解析解,并在野外相对复杂的场地进行反演成像实践及解释,相对传统算法等位线追踪法计算简单快捷有效,反演成像精度比较高.该法可以结合传统的有限元等数值方法进一步研究以提高精度.     

摄动求解Fréchet导数的电阻率层析成像方法

针对电阻率层析成像 ,比较了求解Fréchet导数的 4种方法 ,并提出摄动方法计算该导数 ,使得每一次迭代过程只计算一次正演。井间电阻率成像的数值模拟结果表明了该方法的稳定性和高效性 ,实际资料的处理证明了方法的有效性     

坑（井）-地电阻率成像技术数据处理方法研究

开发坑（井）一地电阻率成像技术数据处理软件包,对分布式地面一坑道电磁信号接收仪记录的时间序列进行分析处理,最终得到相应的视电阻率和相位参数。实际资料处理结果表明,在远区情况下,需考虑信噪比水平,选择合适的计算方法。当信噪比很低或较高时,可选择根据卡尼亚电阻率定义式直接比值的方法,该方法计算简便、效率高;当信噪比一般时,采用最小二乘或Robust等估算方法,能在一定程度上改善数据处理质量。     

摄动求解Fréchet导数的电阻率层析成像方法

针对电阻率层析成像 ,比较了求解Fréchet导数的 4种方法 ,并提出摄动方法计算该导数 ,使得每一次迭代过程只计算一次正演。井间电阻率成像的数值模拟结果表明了该方法的稳定性和高效性 ,实际资料的处理证明了方法的有效性。     

电阻率反演及其应用

针对大部分沉积岩具有层理结构,从其电性上来看,它们是由各种不同电阻率的地层组成的,这样的地层其电阻率呈现出各向异性。在对电阻率断面进行观测时,要在层状介质背景剖面的基础上,利用二维电阻率扰动反演方法,将背景场环境中的电性层结构进行掌控,并对敏感矩阵进行计算,通过实验得到良好的效果。      

考虑关断时间的瞬变电磁视电阻率计算及不同波形浅层分辨特征分析

发射电流波形与关断时间会影响瞬变电磁早期响应,导致瞬变电磁浅部信息缺失形成探测盲区．为了分析发射波形对瞬变电磁探测盲区的影响,文中实现了考虑发射波形影响的瞬变电磁全域视电阻率定义方法,并基于该算法对常用的发射波形瞬变响应对浅层电性介质的分辨特征进行分析．首先,采用电流波形与阶跃响应的褶积实现了任意发射电流的全波形瞬变电磁响应计算,将地下电阻率作为全波形响应的函数,利用反函数原理实现了全波形响应的视电阻率计算．然后,以航空瞬变电磁装置为例,利用文中视电阻率计算方法分析了层状介质不同发射波形瞬变电磁视电阻率曲线特征．根据不同盖层厚度视电阻率曲线可知,当覆盖层较厚时,不同发射波形的视电阻率成像结果与模型电性特征基本相近;当盖层厚度较小时,半正弦波、三角波和梯形波等off-time较长的发射波形瞬变电磁响应不能反映浅层的电阻率信息,早期道视电阻率受到深部电性影响,形成了一定范围的探测盲区．最后,利用文中视电阻率计算方法对不同发射波形的复杂三维模型正演的数据进行分析,半正弦波、三角波和梯形波的瞬变电磁响应都能反映异常的地表位置,但会丢失浅部地层信息,深部异常出现时间提前且纵向拉伸．这是由于关断时...     

ON THE COMPLETENESS OF DATA SETS WITH MULTIELECTRODE SYSTEMS FOR ELECTRICAL RESISTIVITY SURVEY

This paper describes how, using a surface linear array of equally spaced electrodes, potential data can be obtained for use in electrical resistivity imaging. The aim is to collect a complete data set which contains all linearly independent measurements of apparent resistivity on such an array using two-, three- or four-electrode configurations. From this primary data set, it is shown that any other value for apparent resistivity on the array can be synthesized through a process of superposition. Numerical tests show that such transformations are exact within the machine error for calculated data but that their use with real field data may lead to noise amplification.     

Time‐lapse electrical resistivity imaging of solute transport in a karst conduit

The use of electrical resistivity surveys to locate karst conduits has shown mixed success. However, time‐lapse electrical resistivity imaging combined with salt injection improves conduit detection and can yield valuable insight into solute transport behaviour. We present a proof of concept above a known karst conduit in the Kentucky Horse Park (Lexington, Kentucky). A salt tracer solution was injected into a karst window over a 45‐min interval, and repeat resistivity surveys were collected every 20 min along a 125‐m transect near a monitoring well approximately 750 m downgradient from the injection site. In situ fluid conductivity measurements in the well peaked at approximately 25% of the initial value about 3 h after salt injection. Time‐lapse electrical resistivity inversions show two broad zones at the approximate conduit depth where resistivity decreased and then recovered in general agreement with in situ measurements. Combined salt injection and electrical resistivity imaging are a promising tool for locating karst conduits. The method is also useful for gaining insight into conduit geometry and could be expanded to include multiple electrical resistivity transects. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrical Imaging of Impact Structures

Electrical imaging provides important subsurface information for the construction of hypervelocity impact models. We here provide an overview and evaluation of the current electrical imaging methods used in impact cratering studies. Although apparent resistivity models are commonly used in the geoelectrical imaging of impact structures, the reliability of these models has not hitherto been determined. In order to assess these imaging approaches in impact cratering, we investigate for the first time the discrepancies between the apparent resistivity and true resistivity models of an impact structure. To this end, we present (1) a new true resistivity model of the Araguainha impact structure in central Brazil by applying L2-norm inversion to previously published data, (2) apparent resistivity model of the impact structure, and (3) models obtained from different stages of the iterative tomographic inversions. Our results show that changes in vertical resistivity gradient are significantly better defined in the true resistivity models than in the apparent resistivity model. On the basis of these results, we outline a new approach that true resistivity models can be effectively assessed by applying both L1- and L2-norm inversion schemes together with the monitoring of intermediate models from iterative inversion. The results of our study highlight the importance of tomographic inversion of resistivity data in impact cratering studies, and they provide a data modeling framework and foundation for cost-effective subsurface imaging of impact structures in the future.     

Focused imaging of electrical resistivity data in archaeological prospecting

Electrical resistivity mapping and electrical resistivity profiling are powerful instruments for investigating archaeological structures. Interpretation of geoelectrical data is complicated by near-surface anomalies and the characteristics of the applied electrode arrays. Averaging Wenner α and Wenner β data as an alternative method of focused imaging is presented to overcome these problems. The mechanism of focused imaging is explained using the sensitivity distribution of the combined arrays. Various methods of imaging geoelectrical data are examined with synthetic and field data. In electrical resistivity mapping, inversion of the data is unnecessary when using focused imaging. In electrical resistivity profiling, focused imaging gives a first idea about the subsurface resistivity distribution without achieving the quality obtainable by inversion.     

3D inversion of DC data using artificial neural networks

In this paper, we investigate the applicability of artificial neural networks in inverting three-dimensional DC resistivity imaging data. The model used to produce synthetic data for training the artificial neural network (ANN) system was a homogeneous medium of resistivity 100 Ωm with an embedded anomalous body of resistivity 1000 Ωm. The different sizes for anomalous body were selected and their location was changed to different positions within the homogeneous model mesh elements. The 3D data set was generated using a finite element forward modeling code through standard 3D modeling software. We investigated different learning paradigms in the training process of the neural network. Resilient propagation was more efficient than any other paradigm. We studied the effect of the data type used on neural network inversion and found that the use of location and the apparent resistivity of data points as the input and corresponding true resistivity as the output of networks produces satisfactory results. We also investigated the effect of the training data pool volume on the inversion properties. We created several synthetic data sets to study the interpolation and extrapolation properties of the ANN. The range of 100–1000 Ωm was divided into six resistivity values as the background resistivity and different resistivity values were also used for the anomalous body. Results from numerous neural network tests indicate that the neural network possesses sufficient interpolation and extrapolation abilities with the selected volume of training data. The trained network was also applied on a real field dataset, collected by a pole-pole array using a square grid (8 ×8) with a 2-m electrode spacing. The inversion results demonstrate that the trained network was able to invert three-dimensional electrical resistivity imaging data. The interpreted results of neural network also agree with the known information about the investigation area.     

地-空瞬变电磁法电阻率成像研究与应用

地-空瞬变电磁法(Semi-airborne transient electromagnetic,SATEM)凭借适应能力强、探测深度大、实时性强等特点,适用于湖泊、沼泽、山区等地形复杂地区观测.SATEM接收数据量大、精度要求高,传统成像方法基于地面视电阻率计算未考虑飞行高度,开展SATEM的高精度快速成像研究对实际...     

Characterisation of glacial sediments using geophysical methods for groundwater source protection

A sequence of glacial and alluvial deposits overlying the Cretaceous Chalk in Eastern England was characterised using two geophysical techniques: electrical resistivity imaging and electromagnetic (EM) induction. Extensive geological data were available from trenching and boreholes. Synthetic modelling of the electrical resistivity imaging technique was undertaken to identify its limitations and to optimise survey design. The EM induction method provided a quick and cost-effective reconnaissance technique for identifying large-scale lateral variation in lithology, and for siting resistivity profiles and further boreholes. The resistivity imaging technique provided detailed information on the vertical continuity of permeable units, and was able to identify permeable pathways through the sequence. Certain limitations in detecting thin sand or gravel layers underlying electrically conductive clay were seen in both the synthetic and field data. Nevertheless, the study shows that knowledge of these limitations allows interpretation for the purpose of groundwater vulnerability assessment, given that an appropriate amount of invasive investigation has been conducted.     

Characterisation of glacial sediments using geophysical methods for groundwater source protection

A sequence of glacial and alluvial deposits overlying the Cretaceous Chalk in Eastern England was characterised using two geophysical techniques: electrical resistivity imaging and electromagnetic (EM) induction. Extensive geological data were available from trenching and boreholes. Synthetic modelling of the electrical resistivity imaging technique was undertaken to identify its limitations and to optimise survey design. The EM induction method provided a quick and cost-effective reconnaissance technique for identifying large-scale lateral variation in lithology, and for siting resistivity profiles and further boreholes. The resistivity imaging technique provided detailed information on the vertical continuity of permeable units, and was able to identify permeable pathways through the sequence. Certain limitations in detecting thin sand or gravel layers underlying electrically conductive clay were seen in both the synthetic and field data. Nevertheless, the study shows that knowledge of these limitations allows interpretation for the purpose of groundwater vulnerability assessment, given that an appropriate amount of invasive investigation has been conducted.     

电性源瞬变电磁地空逆合成孔径成像

电性源地空瞬变电磁法具有工作效率高、勘探深度大、采集信号信噪比高、适用于地形地质条件复杂地区等优点.但是,到目前为止,由于尚未建立起该方法的解释系统,大大制约了该方法的发展.本文旨在建立起完整的地空电磁探测系统,丰富整个探测系统的理论.本文围绕地空瞬变电磁法全域视电阻率定义、瞬变电磁虚拟波场的克希霍夫偏移成像、逆合成孔径成像方法三个科学问题进行了系统研究.提出了用磁场强度定义全域视电阻率的迭代算法,理论模型试验结果表明计算出的视电阻率曲线首支趋于第一层电阻率,尾支趋于最后一层电阻率,实现了全空域、全时域视电阻率的计算;在先前研究的基础上,实现了适合电性源地空装置的瞬变电磁虚拟波场的克希霍夫偏移成像;采用相关迭加技术,实现孔径内多测点数据合成,将传统的单点处理方式发展成为逐点推移多次覆盖的逆合成孔径处理方法.层状模型试验表明:(1)全域视电阻率能够光滑、完整、渐变地反映出模型的电性信息变化;(2)当改变三层模型中间层电阻率时,全域视电阻率曲线随着参数的改变分异明显,对电性层的识别容易且直观;(3)由于在电阻率计算中同时考虑了接收机高度、偏移距、时间等各参数的影响,全域视电阻率可实现全空域、全时域的视电阻率计算.含水采空区的复杂模型算例表明:(1)根据不同测线的全域视电阻率结果可以看出,在靠近采空区的位置,全域视电阻率断面可以清晰地反映出采空区的空间位置,随着测线离采空区越来越远,采空区异常越来越弱直至消失;(2)波场变换和偏移成像的结果显示存在两个电性差异较大的界面,上界面指示地表,由于空气和大地之间的电性差异较大,故该界面波场信号反映强烈,遍布整个区域,下界面异常信号则主要集中在中部,向外逐渐减弱,指示采空区;(3)逆合成孔径成像结果表明地表界面在合成前后没有变化,而采空区异常合成后范围明显变小,且异常边界清晰,指示的采空区位置与模型吻合很好.本文借助于逆合成孔径雷达成像的基本思想,建立了一套电性源瞬变电磁地空逆合成孔径成像方法.基于反函数思想结合迭代算法提出的电性源地空瞬变电磁法的全域视电阻率定义方法,实现了全空域、全时域的视电阻率计算;借鉴瞬变电磁拟地震偏移成像算法,实现了瞬变场的三维成像;借鉴逆合成孔径雷达的思想,提出电性源瞬变电磁地空逆合成孔径算法,进一步提高了成像的分辨率.采空区模型算例表明相关叠加合成确实具有增强有用信号、提高信噪比、提高分辨率的诸多优点,证实了瞬变电磁地空逆合成孔径成像方法的有效性.     

Explicit expressions and numerical calculations for the Fréchet and second derivatives in 2.5D Helmholtz equation inversion

In order to perform resistivity imaging, seismic waveform tomography or sensitivity analysis of geophysical data, the Fréchet derivatives, and even the second derivatives of the data with respect to the model parameters, may be required. We develop a practical method to compute the relevant derivatives for 2.5D resistivity and 2.5D frequency-domain acoustic velocity inversion. Both geophysical inversions entail the solution of a 2.5D Helmholtz equation. First, using differential calculus and the Green's functions of the 2.5D Helmholtz equation, we strictly formulate the explicit expressions for the Fréchet and second derivatives, then apply the finite-element method to approximate the Green's functions of an arbitrary medium. Finally, we calculate the derivatives using the expressions and the numerical solutions of the Green's functions. Two model parametrization approaches, constant-point and constant-block, are suggested and the computational efficiencies are compared. Numerical examples of the derivatives for various electrode arrays in cross-hole resistivity imaging and for cross-hole seismic surveying are demonstrated. Two synthetic experiments of resistivity and acoustic velocity imaging are used to illustrate the method.