Tracing subsurface oil pollution leakage using 2D electrical resistivity tomography

This study provides an evaluation of the subsurface oil contaminant flow in an oil production field at the Wadi Feiran fan, along the Gulf of Suez, Egypt. The 2D electrical resistivity tomography technique has been applied along four profiles surrounding the spill location of oil dump site. Theoretical soil resistivity response versus fluid resistivity for different cation exchange capacity values has been studied. Then, a physical model for the current situation of flowing the oil over the groundwater (sea invaded water) of saturated fluvial sediments has been introduced. It was supposed that the crude oil has higher resistivity values and turned to be conductive far away from the oil spill source under the effect of microbial biodegradation. Such conditions of contaminated distribution and its resistivity values have been considered during the inversion and interpretation of the acquired data sets. The processed 2D resistivity profiles reveal that oil plumes are accumulated in the seawater invasion direction, whereas it has low signatures along the profiles parallel to the shore line.     

Deciphering Geothermal resources in Deccan trap region using electrical resistivity tomography technique

Exploration of Geothermal resources is important from energy point of view. Western margin of volcanic Deccan traps, also known as Western Ghats, is characterized with the presence of numerous hot springs. The resistivity contrast for the geothermal reservoir rock and the surrounding host rock is significantly high in volcanic terrain which is the case of present study. In such cases the resistivity associated with geothermal reservoirs usually varies from <5 to 15 Ohm-m regardless of how high resistivity is outside the reservoir zone. Direct current (DC) resitivity method is proved to be more suitable method for delineation of groundwater reservoirs. The present work describes the results of electrical resistivity tomography survey carried out at four hot spring sites located at Unhavare (Khed), Tural, Rajwadi and Aravali villages of Chiplun taluk in Ratnagiri district of Maharashtra for delineation of the geothermal reservoirs and associated geological features like faults and fractures responsible for vertical movement of geothermal water with the sole purpose of harnessing geothermal energy. In addition to this, the present study is also aimed to delineate the groundwater reservoirs with normal temperature for exploration purpose to meet the local water supply demand. The interpreted results of the field data suggest the presence of two potential geothermal reservoirs at Unhavare (Khed) and one each at Tural and Aravali. Potential groundwater zones with normal temperature are also delineated for groundwater exploration at Rajwadi and Aravali sites.     

基于电阻率孔压静力触探的浅层气可靠度识别

地下浅层气的存在会对地铁项目的勘察、施工以及后期正常运营构成安全隐患。采用基于地质统计学和可靠性分析的概率方法,对杭州某地铁站场地的电阻率静力触探测试数据进行分析。通过回归分析移除电阻率数据的趋势项,以变差函数为工具,描述电阻率残差在竖向和水平向上的各向异性,采用克里格插值对未采样点参数值进行最优线性无偏估计,采用一阶可靠度方法获取基于电阻率的地下浅层气分布三维概率图。分析表明：电阻率残差具有显著的各向异性,在竖直和水平方向的自相关距离分别为5.1 m和55.6 m。在考虑不同土体电阻率背景值不同的前提下,采用一阶可靠度方法建立了浅层气三维概率分布图。该场地竖直方向上浅层气分布于25 m附近及30～35 m深度区间内,水平方向上则集中分布于RCPTU1点位附近。可基于该结果提出针对性的防治对策,为布设排气孔、浅层气监测等治理措施提供依据。     

Delineation of groundwater resources using electrical resistivity tomography

Chandrabhaga basin of the Nagpur district is a part of the drought prone Vidarbha region of Maharashtra, India. This region is facing acute shortage of water for drinking and irrigation purposes. The basin is located at the eastern fringe of the Deccan traps consisting of volcanic rocks. Presently, dug wells penetrating composite layers of the weathered mantle and highly fractured rocks overlying stratified basaltic lava flows are the main source of water supply. However, water available in the dug wells is inadequate to meet the ever increasing demand for water causing considerable economic losses to the farming community. In this work, we present the results of an electrical resistivity tomography (ERT) survey carried out in the basin for the delineation of groundwater potential zones. Validity of the hydrogeological setup obtained from the ERT results has been confirmed by bore well drillings at two investigated sites. The study demonstrates the efficacy of the ERT technique in delineation of groundwater potential zones in the hydrogeologically complex basaltic terrain of the Deccan traps.     

Application of electrical resistivity tomography technique for investigation of landslides: a case from Turkey

Electrical resistivity imaging is a widely used tool in near surface geophysical surveys for investigation of various geological, environmental and engineering problems including landslide. In this study, an electrical resistivity tomography (ERT) survey was conducted in a landslide area, located in the Söke district of Aydın, Turkey. In 2003, the Neogene-aged units on the slope next to a newly built school building became unstable due to an excavation work and moved after a heavy rainfall. The resulting landslide partly covered the school. The authors carried out a 2-D resistivity survey along three profiles over the landslide mass using a Wenner configuration. It yielded useful information about the geometry and characteristics of the landslide. In addition, a 2-D synthetic resistivity modelling study was carried out to understand the response of the resistivity method to a landslide problem before the field surveys. Eight boreholes were also drilled in the landslide area. Both the drilling and resistivity results indicated the presence of a fault in the site. Also, the resistivity data from the line measured along the axis of the landslide revealed the surface of rupture.     

Interpretation of shallow electrical resistivity images of faults: tectonophysical approach

A new approach to interpretation of shallow electrical resistivity tomography (ERT) data discussed for the case of the Olkhon area (western Baikal region) stems from tectonophysical ideas of faulting phases and deformation levels in rocks. The deformation levels, identified statistically from ERT responses, constrain fault boundaries and subboundaries associated with the formation of main and subsidiary fault planes. Information of this kind creates a basis for solving various fundamental and applied problems of tectonics, mineral exploration, and engineering geology.     

基于高密度电法的城市岩溶地下水通道三维电性成像

岩溶地下水通道是隐伏岩溶区常见的地质现象,开展城市隐伏岩溶通道探测对城市地下空间开发和地质灾害防治具有重要意义。岩溶通道常具有高度的空间变异性,常规二维探测难以对其进行较好的表征。基于此,文章采用三维高密度电法对城市隐伏岩溶地下通道进行了精细探测,结合地球物理数值模拟和应用实例,分析三维高密度电法对不同充填类型岩溶地下通道的成像效果。结果表明：三维高密度电法较二维探测在数据量和分辨率上均有较大提升,可更直观地表征目标体三维电性结构特征,该探测方法对岩溶地下水通道成像具有优势;通过对武汉市源泉村岩溶地下水通道三维电性成像,揭示了该低温热泉的地下水运移特征,可为城市地热勘探开发提供参考。     

Characterization of shallow karst using electrical resistivity imaging in a limestone mining area

The Guangxi area is famous for its high degree of karstification due to a long period of groundwater erosion and the development of fracture networks. Karst collapse appeared during the mining process and caused many environmental problems. Applying electrical resistivity imaging (ERI) is the most cost-effective method to study the characteristic of the subsurface karst environment. In this area, more than 24 km of ERI profiles around a mining area is carried out to present the development of karst which will be used to evaluate the effects on the ground water and the surrounding environment. The area represents a classical limestone rock which is of high quality and is used for cement production. In this study, the ERI is used to determine the geometry of the karst range at depth. The results are in good agreement with drilling experiments. The ERI survey provided near-surface resistivity information, which is very useful for establishing the geometry and the position of potential karst.     

从高密度电阻率成像法到三维空间上的包气带水文学

高密度电阻率成像法是新近出现的通过测定介质电传导特性的地球物理勘探方法.它方便、快速和多尺度,可对探测对象进行无损的、三维的和动态的监测.由于包气带中发生的各种物理、化学、生物和水文过程不可避免的都会引起介质电阻率的变化,通过高密度电阻率成像法可以对这些过程进行三维的动态监测,从而为我们在三维空间上认识各种包气带过程创造了条件.本文首先介绍了高密度电阻率成像法在半无限的介质表面上应用时的电极布置、测定和反演计算方法,然后介绍了影响介质电阻率的因素.最后,从电阻率空间分布的差异性和时间上的变化性方面,介绍了目前高密度电阻率成像法的应用领域和研究方向,以期能促进高密度电阻率成像法的广泛应用和推动三维空间上的包气带水文学的研究.     

高密度电阻率成像法在水文地质领域中的应用

高密度电阻率成像法(ERT)作为一种高效、准确的地球物理探测手段,在水文地质领域得到了广泛的应用。本文通过文献追踪和分析,重点讨论了电阻率与含水率、渗透系数、含盐量等水文地质表征参数之间的相关性,并就该方法在地下水资源勘查、溶质运移动态监测以及水文地质参数反演等方面的应用进行了剖析,阐述了ERT探测过程中的主要影响因素及各因素之间的相互关系。最后,针对ERT方法在应用过程中存在的问题提出相应的改进建议。     

Application of electrical resistivity tomography (ERT) technique to detect underground cavities in a karst area of South Korea

Recently, the electrical resistivity tomography (ERT) technique has been increasingly applied to underground cavity detection filled with groundwater and/or clay. In this study, an ERT survey was conducted to examine the spatial distribution and shape of underground cavities in a karst area in Korea. Numerical ERT modeling and inversion were carried out to select appropriate field survey parameters. Two geological structures, similar to those that exist in the field survey area, were modeled: a sinkhole model and a vein-type structure. Electrode configurations using pole–pole, pole–dipole, dipole–pole, and dipole–dipole arrays were tested with the two geologic models and compared. The numerical modeling showed that the resistivity distribution of the ERT tomogram using the pole–dipole array produced the best representation of the two models. The pole–dipole and pole–pole array configurations were applied in an ERT field survey using six boreholes. The field ERT results show that cavity areas filled with clay or groundwater appeared as low-resistivity anomalies in the limestone formation. This was particularly evident in the inclined vein-type structures attributed to fracture zones, where the field data were similar to the modeling results. Therefore, the ERT technique could be effectively used for detecting underground limestone cavities under similar geological conditions as those at our study area.     

基于高密度电法的天然边坡水分运移规律研究

地下水是滑坡变形与失稳的重要影响因子,目前关于天然边坡水分运移过程的研究多借助传感器监测或数值模拟,但尚不能充分满足无扰、快捷的测量要求。针对该类问题,以天然边坡为研究对象,利用高密度电法结合时域反射技术,通过测量边坡土体含水率与电阻率,构建两者的对应关系,据此反演并分析边坡介质地下水的分布特征及迁移规律,验证高密度电法技术在探究天然边坡水分运移规律的有效性。现场试验结果显示:研究边坡土体含水率与电阻率呈明显对数关系;该边坡有效入渗或蒸发深度为2 m,大于该深度范围内,边坡土体含水率随深度增加逐渐减小;中雨强度下埋深为1.3~2.0 m范围内土体含水率激增的主要原因在于后缘裂缝的优势流补给;边坡高含水率(低电阻区)与低含水率(高电阻区)过渡区与后期滑坡滑动面位置高度吻合。     

Monitoring hydraulic processes with automated time-lapse electrical resistivity tomography (ALERT)

Hydraulic processes in porous media can be monitored in a minimally invasive fashion by time-lapse electrical resistivity tomography (ERT). The permanent installation of specifically designed ERT instrumentation, telemetry and information technology (IT) infrastructure enables automation of data collection, transfer, processing, management and interpretation. Such an approach gives rise to a dramatic increase in temporal resolution, thus providing new insight into rapidly occurring subsurface processes. In this paper, we discuss a practical implementation of automated time-lapse ERT. We present the results of a recent study in which we used controlled hydraulic experiments in two test cells at reduced field scale to explore the limiting conditions for process monitoring with cross-borehole ERT measurements. The first experiment used three adjacent boreholes to monitor rapidly rising and falling water levels. For the second experiment, we injected a saline tracer into a homogeneous flow field in freshwater-saturated sand; the dynamics of the plume were then monitored with 2D measurements across a 9-borehole fence and 3D measurements across a 3 × 3 grid of boreholes. We investigated different strategies for practical data acquisition and show that simple re-ordering of ERT measurement schemes can help harmonise data collection with the nature of the monitored process. The methodology of automated time-lapse ERT was found to perform well in different monitoring scenarios (2D/3D plus time) at time scales associated with realistic subsurface processes. The limiting factor is the finite amount of time needed for the acquisition of sufficiently comprehensive datasets. We found that, given the complexity of our monitoring scenarios, typical frame rates of at least 1.5–3 images per hour were possible without compromising image quality.     

Assessment of seawater mixing in a coastal aquifer by high resolution electrical resistivity tomography

Seawater intrusion is a major problem in urbanized coastal regions of India which is due to over exploitation of groundwater for various purposes. This study was carried out with the objective of assessing the zone of mixing between seawater and groundwater in the coastal aquifer in south of Chennai, Tamil Nadu, India using high resolution electrical resistivity tomography. High resolution electrical resistivity tomography was carried out in five profiles perpendicular to the sea using IRIS make SYSCAL Pro-96 system with 2.5 m or 5 m inter-electrode separation. The maximum length of the profile was 170 m which resulted in a depth of investigation of 28.7 m. The apparent resistivity measured in this area varies from 0.3 ohm-m to 30,000 ohm-m. The apparent resistivity of saturated zone decreases towards the sea, indicating the influence of seawater. This was also confirmed by measuring the electrical conductivity of groundwater, which gradually increases from 156 μS/cm to 3430 μS/cm towards the sea. Further, the concentration profiles of electrical conductivity, sodium, chloride and chloride / bicarbonate ratio are compared with the high resolution electrical resistivity tomography profile. The distance of influence of seawater is comparatively high in northern part than in southern part of the area. The high resolution electrical resistivity tomography was effectively used to determine the effect of seawater mixing with groundwater.     

高密度电阻率法探测DNAPLs污染的适宜性探讨

重非水相流体（DNAPLs）的电阻率一般较高,在地下介质中与周围的地下水形成明显的电性差别。利用这一特性,非侵入式的高密度电阻率法（ERT）在探测DNAPLs污染场址中污染范围展现出一定优势。然而在实际场址的调查应用中,ERT有时无法探测到显著的高阻异常,或探测的高阻异常是由于介质的非均质性产生,从而导致对DNAPLs污染分布的误判。针对该问题,文章首先基于有效介质电阻率模型,计算了不同DNAPLs饱和度情形下的地层总电阻率值,从理论上解释了当DNAPLs污染程度较低时不存在显著高阻异常的原因。进一步构建理想ERT正演模型,探讨ERT探测DNAPLs污染的适宜性。研究结果表明:介质的非均质性（例如高阻的砾石等）对ERT探测具有很强的干扰,可造成DNAPLs分布范围的误判,因此传统静态ERT探测DNAPLs污染仅适用于均质介质或者非均质介质中DNAPLs污染程度较高的条件。对于非均质介质DNAPLs污染程度较低时,需采用基于时间域的差分反演方法屏蔽介质非均质性的影响,从而准确判定DNAPLs分布。     

岩柱中水体入渗过程的高密度电阻率成像法研究

本文应用高密度电阻率成像法探讨了室内实验尺度岩柱中的这一人渗过程。首先建立了电法勘探的电位场数学模型，用有限元法进行了电位场正问题的求解。然后应用基于序列二次规划(SQP)方法的电阻率反演算法，由测量数据反演得到了岩石电阻率在二维空间上的分布。由岩柱中水体人渗过程的电法测量数据反演得到的电阻率图像反映了岩柱中水分的空间变化过程。岩柱中电阻率的空间分布及其随时间的变化表明水体的人渗是不均匀的，在岩柱周边渗流较快，而在中部存在滞后现象。得到的相对电阻率图像也反映了岩柱周边存在的微裂隙。     

Reliability of dipole-dipole electrical resistivity tomography for defining depth to bedrock in covered karst terranes

 Sinkhole collapse is one of the main limitations on the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Because of the irregular distribution of pinnacles and cutters on the bedrock surface, uncertainties arise when "hit-or-miss" borehole drilling is used to locate potential collapse sites. A high-resolution geophysical technique capable of depicting the details of the bedrock surface is essential for guiding the drilling program. Dipole-dipole electrical resistivity tomography (ERT) was used to map the bedrock surface at a site in southern Indiana where limestone is covered by about 9 m of clayey soils. Forty-nine transects were conducted over an area of approximately 42,037 m². The electrode spacing was 3 m. The length of the transects varied from 81 to 249 m. The tomographs were interpreted with the aid of soil borings. The repeatability of ERT was evaluated by comparing the rock surface elevations interpreted from pairs of transects where they crossed each other. The average difference was 2.4 m, with a maximum of 10 m. The discrepancy between interpreted bedrock-surface elevations for a transect intersection may be caused by variations in the subsurface geology normal to the transect. Averaging the elevation data interpreted from different transects improved the ERT results. A bedrock surface map was generated using only the averaged elevation data at the transect junctions. The accuracy of the map was further evaluated using data from four exploratory boreholes. The average difference between interpreted and actual bedrock surface-elevations was less than 0.4 m. The map shows two large troughs in the limestone surface: one coinciding with an existing sinkhole basin, while the other is in alignment with a small topographic valley. Because sinkholes were observed at the same elevation interval in similar valleys in the vicinity, the delineated trough may have implications for future land use at the site. Received: 4 January 1999 · Accepted: 8 March 1999     

Application of electrical resistivity tomography methods for delineation of groundwater contamination and potential zones

Identification of damages/changes that are affecting the underground water quality due to the effect of anthropogenic activities is often done after environmental problems have become evident or water portability being strongly affected. Two main applications of electrical resistivity tomography (ERT) methods have been addressed covering characterization of leachate movement from a composting area of a Sugar Mill in Southern India. Good correlation has been obtained between ERT and groundwater quality assessments as well as from groundwater monitoring data sets. The study helped in conceptualization of hydrogeologic framework in basaltic terrain. Impacts on groundwater regime associated with urbanization and industrialization can easily be assessed through the variation of resistivity in the inverse resistivity pseudo-section model of the ERT investigations. Assessment of groundwater potential in an upcoming Urban Node, Greater Hyderabad city has been illustrated in the second example. Identification of good thickness of weathered regolith for location of water harvesting through Green Fingers evolved. The small infrastructure would help carrying of enhanced surface runoff as well as to sustain groundwater yield in the urban node and thereby ensuring sustainable groundwater resource exploitation. The above studies have illustrated immense potential of the ERT tools in the assessment of groundwater contamination as well as groundwater potential.