Geoelectrical method in the investigation of groundwater resource and related issues in Ophiolite and Flysch formations of Port Blair, Andaman Island, India

Geoelectrical resistivity investigation using Vertical Electrical Sounding (VES) technique was conducted at Port Blair, South Andaman Island, to locate the fractures in different formations and to decipher its groundwater potential. A total of 40 VES were carried out covering the entire study area using Schlumberger electrode configuration out of which 34 VES fall in Andaman Flysch formations and the remaining VES in Ophiolite formations. The interpreted resistivity results were integrated with nine borehole lithologs for the subsurface analysis. The combination of VES with borehole litholog data has provided a close correspondence on subsurface hydrogeological conditions. The interpreted VES data of various formations showed drastic variations in the resistivity ranging from higher in Ophiolite, moderate in Andaman Flysch and very low in valleys of Andaman Flysch formations. The study further revealed that the weathered and fractured volcanics of Ophiolite groups of rocks and sandstone that occur in the Andaman Flysch formations constitute the productive water bearing zones categorized as good groundwater potential zone. Based on the geoelectrical parameters, viz., thickness of layers and the layer resistivity values, a groundwater potential map was prepared, in which good, moderate, and poor groundwater zones were demarcated. Further, numerical, spatial and litho-geoelectric models of resistivity were analyzed in terms of groundwater potential and these models have thus enabled to prepare a comprehensive groundwater development and management plans proving its efficacy in this art of exploratory investigations.     

Electrical and electromagnetic surveys to locate possible causes of water seepage to ground surface at a quarry open pit near Helwan city,Egypt

Water seepage to ground surface at a limestone quarry located at Wadi Garawy about 20 km south-east of Helwan city in Egypt posed a real threat to the mining activity at the quarry. The quarry area is known to be very dry for decades and away from water utilities and infrastructures that may cause water leaks to the quarry. Geophysical investigation including 1D Vertical Electrical Sounding (VES), 2D Electrical Resistivity Tomography (ERT) and 1D Transient Electromagnetic (TEM) surveys were conducted to characterize the rock sequence and locate what could be a possible source of water seepage to the quarry. The resistivity profiles generated from the VES and TEM surveys mapped the rock units in the area down to depths exceeded 100 m. The ERT profiles acquired from the quarrying zone close to the water seepage spot have imaged the top of groundwater level at few meters below the ground surface at the quarry open pit. The spot of groundwater seepage seemed to occur at an area of limestone dissolution that were filled by finer sediments. The finer sediments acted as a hydrological conduit that allowed an upward seepage of groundwater to ground surface under the capillary action effect.     

Electrical resistivity tomography (ERT) of a coastal carbonate aquifer (Port-Miou, SE France)

As part of a larger regional research program “KarstEAU”, the authors have applied electrical resistivity tomography (ERT) techniques to characterize heterogeneities in the Port-Miou coastal karst aquifer (Cassis, SE France). Field surveys were carried out on intensely fractured and karstified Urgonian carbonates. Extensive research has characterized macro- and micro-scale geology of the Port-Miou area and particularly underground water-filled conduits and fault/fracture and karst systems within a former quarry. The authors applied 2D ERT along two surface profiles of length 420 and 595 m to test capability for delineating subsurface conduits and possibly relationship between conduit and fault/fracture/karst orientation; and 3D ERT with a dense 120 electrode array at 1 m spacing (11 × 10 m) was applied over an area of the quarry that had been profiled using 3D georadar and which has had intensive nearby structural geological interpretation. The 2D profiling imaged several underground conduits at depths to >50 m below ground surface and below sea level, including possibly the main Port Miou submarine spring and smaller springs. The 2D profiling within the quarry provided a better understanding of the connectivity between major fractures and faults on the quarry walls and secondary springs along the coast supporting flow of the secondary springs along interpreted fracture orientations. In addition, 2D inversion-derived conductivity models indicate that high resistivity zones above sea-level are associated with non-saturated zones and low resistivity anomalies in the non-saturated zone are associated with residual clays in paleokarsts. A partitioned lower resistivity zone below sea-level can be associated with a higher porosity/permeability zone with fractures and karstic features. Inversion models of the dense 3D ERT data indicate a higher resistivity volume within the larger surveyed block. The survey characterized the non-saturated zone and the ERT resistivities are correlated with karst features interpreted by 3D georadar and visible in the inferior wall of the quarry.     

基于高密度电阻率成像法的基岩裂隙网络原位识别试验研究

对于以低渗透性基岩为处置围岩的核废物地质处置库来说,基岩中的裂隙网络是地下水进入处置库和放射性核素从处置库向外界环境迁移的主要通道。因此,研究基岩内裂隙网络的空间分布特征,对核废物的地质处置是否安全具有十分重要的意义。本文以花岗岩为例通过野外试验,分析研究了利用高密度电阻率成像法（ERT）,对处于不同状态条件下的基岩中裂隙网络进行三维原位识别的效果及能力。结果表明在以高阻值为主要电特性的基岩中,利用ERT对基岩中三维裂隙网络进行原位识别是可行的,并且裂隙的电阻率特征能较好的反映出裂隙的空间分布位置和形态,但对裂隙网络识别的效果与测线方向有关。     

Intervention of Electrical Resistance Tomography (ERT) in resolving hydrological problems of a semi arid granite terrain of Southern India

Electrical Resistivity Tomography (ERT) has been used in association with several geohydrological, geotechnical and geophysical methods in resolving several site specific problems in hydrological science. Advancement in the field of computers and automation in the field of electronics has jointly resulted in the development of this geophysical innovation which has wide application in groundwater, environmental and engineering problems including monitoring of vadose zone water movement, steam injection etc. In this paper application of ERT in association with geohydrological and other exploration methods in resolving groundwater sustainability problem of a micro-watershed area in semi arid granite terrain is presented as three independent cases. In a hard rock terrain the probability of complexities in understanding the sub-surface lithology and its corresponding hydrological parameters are more. Hence appropriate resistivity survey configuration and suitable inversion of acquired data in congregation with other geo-scientific investigations were carried out to understand the site specific problem. The study demonstrated as 3 independent cases shows the usefulness of the ERT method in hydrological investigations, which is economic, efficient and less time consuming in comparison to the other exploration methods.     

Groundwater exploration using integrated geophysics method in hard rock terrains in Mount Betung Western Bandar Lampung,Indonesia

The presence of hard rock in Mount Betung has caused the misalignment of the groundwater aquifers,and resulted in many drilling failures for groundwater.An integrated geophysics method using gravity survey and Geoelectric Vertical Electrical Soundings(VES)were conducted to study the effect of basement and hard rock on groundwater prospects.From the gravity method,38 mapping points were carried out randomly,with a distance of 1-2 km in-between.Meanwhile,from the geoelectric method,51 VES points were acquired at the foot of Mount Betung.The acquisition was conducted with a Schlumberger configuration with AB/2=1 m to 250 m.The results show the Bouguer Anomaly in the west is 50-68 mgal due to the presence of hard rock in Mount Betung.This anomaly responds to relatively shallow hard rocks near surface.Hard rocks composed of andesite and breccia normally present at the depth of 5-180 m during well construction.Resistivity isopach mapping from VES data(at AB/2=50 m,100 m,and 150 m)shows the dominant constituents of hard rock.Fractures in hard rock contribute to secondary porosity,which could be a prospect zone that transmit groundwater.This finding shows that the fractures are randomly scattered,causing several well failures that have been worked.Furthermore,the fractures in the hard rock at the foot of Mount Betung acts as conduits between recharge at Mount Betung and the aquifer in the Bandar Lampung Basin.     

利用ERT数据推求非均质多孔介质渗透系数初探

高密度电阻率成像法（ERT）能通过改变间距来对不同尺度上的地质体特征进行描述，近年来广泛地应用于地质、工程等领域。本文从电流场和渗流场的相似性出发来研究电阻率和渗透系数之间的关系，简要介绍了利用ERT技术所得的电阻率数据推求渗透系数的基本原理，并利用张家港大新和塘桥两处的ERT测量数据推求出渗透系数。与两地抽水试验结果和岩性取样分析的结果分别进行对比，结果较为吻合。     

Geochemical appraisal of mine discharge and tailing at Malanjkhand copper mine,India

Geophysical investigation using Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT) and Seismic Refraction at a proposed conference center site along Ajibode-Labani road, Ibadan, southwestern Nigeria has been carried out. The investigation aims at characterizing and delineating the subsurface strata to understand the weathered profile at the site. Understanding the weathered profile is essential in determining the suitability of the site for engineering construction of the future conference center. A total of 25 VES and 10 ERT profiles were acquired in a systematic grid pattern using both Schlumberger andWenner configurations with Allied omega terrameter. TheVES data were processed and analyzed using WinResist and the ERT data were inverted using RES2DINV. The data were combined to form a 3-D data set of the site and RES3DINV was used to produce the depth slices. Seismic refraction data were also acquired with an ABEM seismograph and processed using SeisImager and Fajseis software. Seismic data were used in understanding the velocity distribution and thickness. The results of VES, ERT and seismic refraction show good correlation. Four sub-surface layers were delineated: top layer of reworked sand, clayey sand/ lateritic hard pan, clay/ sandy clay and fracture/ fresh basement. The 3-D model permits a pictorial view of the sub-surface in relation to materials that overlie the basement. The thickness of unconsolidated materials to bedrock varies from 2.7 m to 12.2 m which revealed inhomogeneity in weathering under the shallow sub-surface. It is found that the integrated geophysical tool is well suited to characterize and delineate sub-surface structure (weathered profile) for engineering site characterization.     

Geophysical characteristics of Wadi Hanifah water system, Riyadh, Saudi Arabia

Integrated geophysical techniques including resistivity image, vertical electrical sounding (VES), and seismic refraction have been conducted to investigate the Wadi Hanifah water system. The groundwater in Wadi Hanifah has problems caused by the high volumes of sewage water percolating into the ground. The combination of VES, resistivity image, and seismic refraction has made a valuable contribution to the identification of the interface between the contaminated and fresh water in Wadi Hanifah area. The contaminated groundwater has lower resistivity values than fresh groundwater due to the higher concentration of ions which reduces the resistivity. Resistivity image and sounding in this area clearly identified the nature of the lithological depth and proved useful at identifying water-bearing zones. Fresh groundwater was found in the study area at a depth of 100 m within the fractured limestone. Water-bearing zones occur in two aquifers, shallow contaminated water at 10 m depth in alluvial deposits and the deeper fresh water aquifer at a depth of about 100 m in fractured limestone. The interface between the contaminated water (sanitary water) and fresh water marked out horizontally at 100 m distance from the main channel and vertically at 20 m depth.     

Vertical electrical sounding as an exploration technique to improve on the certainty of groundwater yield in the fractured crystalline basement aquifers of eastern Uganda

Groundwater in eastern Uganda mostly occurs in fractures in the crystalline basement rocks and at the interface between the overburden and bedrock. The study was aimed at improving the success rate of boreholes through the use of complimentary geophysical siting procedures in 16 locations in Kamuli District, eastern Uganda. Boreholes that were sited after applying appropriate geophysical techniques yielded adequate quantities of water, whereas those sited where such procedures were not applied were out of service sooner than expected. Techniques to determine the precise location of resistivity anomaly and vertical electrical sounding (VES) models were used to locate water-bearing zones. VESs were undertaken to provide an overview of the geology. The apparent resistivities of the water-bearing zones both from VES and resistivity profiling data, had a relationship with the success rates of the boreholes. Electrical resistivities were correlated with hydrogeological parameters. The majority of successful boreholes had, within water-bearing zones, minimum apparent resistivity values less than 200 and 100 Ohmm, from the resistivity profiling anomalies and VES, respectively. The depth to bedrock was generally greater than 20 m below ground level, which indicates potential for medium yielding boreholes.     

离散裂隙渗流方法与裂隙化渗透介质建模

流体渗流模拟的连续介质方法通常适用于多孔地质体，并不一定适用于裂隙岩体，由于裂隙分布及其特征与孔隙差异较大。若流体渗流主要受裂隙的控制，对于一定尺寸的裂隙岩体，多孔介质假设则较难刻划裂隙岩体的渗流特征。离散裂隙渗流方法不但可直接用于模拟裂隙岩体非均质性和各向异性等渗流特征，而且可用其确定所研究的裂隙岩体典型单元体及其水力传导（渗透）张量大小。主要讨论了以下问题：（1）饱和裂隙介质中一般的离散流体渗流模拟；（2）裂隙岩体中的REV（典型单元体）及其水力传导（渗透）张量的确定；（3）利用离散裂隙网络流体渗流模型研究裂隙方向几何参数对水力传导系数和REV的影响；（4）在二维和三维离散裂隙流体渗流模型中对区域大裂隙和局部小裂隙的处理方法。调查结果显示离散裂隙流体渗流数学模型可用来评价不同尺度上的裂隙岩体的水力特征，以及裂隙方向对裂隙化岩体的水力特征有着不可忽视的影响。同时，局部小裂隙、区域大裂隙应当区别对待，以便据其所起的作用及水力特征，建立裂隙化岩体相应的流体渗流模型。     

Joint analysis of SAR interferometry and electrical resistivity tomography surveys for investigating ground deformation: the case-study of Satriano di Lucania (Potenza, Italy)

We have jointly applied microwave remote sensing imaging and ground-based geophysical methodologies for investigating ground deformation. In particular, the Differential Interferometric Synthetic Aperture Radar (DInSAR) technique referred to as Small BAseline Subset (SBAS) algorithm and the Electrical Resistivity Tomography (ERT) method have been applied. The former is a tool for investigating large areas, allowing us to detect deformation phenomena and to analyse their spatial patterns and temporal evolution; the latter permits us to reconstruct, with high spatial resolution, the geometry of near-surface geological structures and to estimate the electrical resistivity of outcropping terrains. The starting point of our analysis has been a DInSAR survey carried out on an area of about 400 km² centred around the town of Potenza (Basilicata region, Italy), for which SAR data acquired by the ERS-1/2 sensors in the time interval 1992–2000 were available. The DInSAR analysis allowed us to detect an unknown ground deformation effect which involves the urban area of Satriano di Lucania, located close to Potenza. An on-site ERT survey was then carried out in order to characterize in detail the physical properties of the terrains involved in the main deformation pattern detected via the DInSAR processing. The integration of the DInSAR and ERT measurements permitted us to investigate areas where the deformation have space-variant characteristics and allowed us to formulate a hypothesis on the origin of the detected displacement processes that, although not fully conclusive, is consistent with the overall DInSAR and ERT analysis.     

Characterization by electrical and electromagnetic geophysical methods of the shallow hydrogeological system at Hebron (West Bank,Palestine) in a semi-arid zone

Multi-electrode geo-electrical and transient electromagnetic surveys were carried out to characterize the nature of the subsurface infiltration zones (5 to 20 m) related to a series of groundwater outlets, and to reveal the geometry of the different aquifers at Bani-Naim, in the south-eastern foothills of the Hebron area, West Bank, Palestine. The purpose of the surveys was to understand the link between water storage/transfer and the characteristics of the geological formations. The strata in this semi-arid region are composed of alternate layers of chalky limestone, hard limestone, marl and chalk. A total of 30 ERT and 15 TDEM were conducted at Bani Naim-Jahir and Bani Naim-Birein. A correlation between the results indicates various infiltration pathways: fractures, feature heterogeneities, and porous chalk. The local heterogeneity on the eastern side were the major pathways for the water infiltration, whereas the thick marl layer underneath acts as a natural impermeable barrier preventing water from infiltrating deeper. A combination of the different geophysical results identified conductive features that correspond to the infiltration zones supplying the dug wells with water. Furthermore, it was established that the fractured chalk and porous chalky limestone act as an aquifer. A three-dimensional visualization of the resistivity allowed a useful reconstruction of the shallow hydrogeological system. Consequently, these studies contribute to regional sustainable development projects in this semi-arid region.     

Integrated geophysical and geomorphological approach to investigate the snowmelt-triggered landslide of Bosco Piccolo village (Basilicata, southern Italy)

Most of Basilicata region in the southern Italian Apennines is characterized by landslides often developing in clayey–marly formations. Many events have been triggered by extreme rainfall or snowmelt. The most important happened (on February–March 2005) at Bosco Piccolo 5 km far from Potenza. This landslide developed subsequently to rapid snowmelt occurred during alternating short periods of high temperatures and intense and continuous snowfalls. This complex landslide affected 4 ha of surface and reached a maximum depth of 20 m inducing damage and collapse of about 80% of the buildings in the village. An integrated multidisciplinary approach has been adopted to study the landslide. A multitemporal aerial photo interpretation and detailed geomorphological surveys have been carried out. Combined Electrical Resistivity Tomography (ERT) and Self-Potential (SP) measurements have been performed and calibrated with boreholes stratigraphy. Such an integrated approach allowed us to partially reconstruct the geometry of the investigated body and to evaluate the effectiveness of drainage system planned for the area.     

Mapping lithology and assessing recharge characteristics in a granitic hard rock aquifer: Inference from 2D resistivity,induced polarization,tracer and moisture measurements

Two dimensional Electrical Resistivity Tomography (ERT) investigation along with Time Domain Induced Polarization (TDIP) investigation covering 1.6 km line were carried out at 3 natural recharge sites in a overexploited groundwater granite watershed, situated in a semi arid region in the state of Telangana, India. At these sites, shallow and/ or deep moisture influx measurements were also carried out using injected tritium tracer and neutron moisture probe. The watershed is covered by sandy loam to silt loam soil, receives an average annual rainfall of 620 mm with the pre monsoon groundwater level ranging from 8m to 29m bgl. The spot investigations were done to assess and understand the recharge process and groundwater potential in terms of resistivity/conductivity and moisture characteristics of the subsurface rock formation.     

Electrical resistivity imaging of conductive plume dilution in fractured rock

Electrical resistance tomography (ERT) was used to monitor a conductive plume dilution experiment that was conducted in fractured basalt in order to assess its applications in this type of fractured-rock environment. Tap water was injected into an injection well for 34 days to dilute a pre-existing potassium chloride (KCl) plume at a site in Idaho, USA. No further fluids were introduced artificially during a 62-day monitoring period. Both surface ERT and cross-borehole ERT were used to monitor dilution and displacement of the plume. A square grid of land-surface electrodes was used with the surface ERT. Three-dimensional images of surface ERT delineated areas of increased and decreased resistivities. Increasing resistivities are attributed to dilution/displacement of the KCl solution by tap-water invasion or the influx of seasonal recharge. Decreasing resistivities resulted from redistribution of residual KCl solution. Cross-borehole ERT was conducted between the injection well and each of seven surrounding monitoring wells. Polar plots of the injection-well resistivity data in the direction of each monitoring well delineate specific locations where tap water seeped from the injection well via preferential flow paths determined by time-dependent resistivity increases. Monitoring-well data indicate locations of clustered and isolated regions of resistivity changes.     

Identification of groundwater prospective zones by using remote sensing and geoelectrical methods in Jharia and Raniganj coalfields, Dhanbad district, Jharkhand state

The Dhanbad district in Jharkhand faces acute water scarcity and is chronically drought-prone. The groundwater resources in the area have not been fully exploited. The present study was undertaken to evaluate the groundwater prospective zones. Landsat-5 Multi Spectral Scanner (MSS) data of band 2 and band 4 and false colour composite (FCC) of band 2, 3, 4 were interpreted visually to differentiate different hydromorphogeological units and to delineate the major trends of lineaments. The different geomorphic features identified are linear ridges, residual hills, and pediplain, buried pediment and dissected pediplain, besides lineaments. The study shows that the pediplain and buried pediments are promising zones for groundwater prospecting. The occurrence and movement of groundwater is restricted to the unconsolidated material, weathered and fractured rocks. For the selection of tube well sites, geoelectrical resistivity investigations have been carried out at the sites, which were found suitable based on hydrogeomorphological and hydrogeological studies. Twenty-six Vertical Electrical Soundings (VES) have been carried out by using Schlumberger electrode configuration, which have brought out 3 to 7 layered sub-surface layers. The resistivity of water-bearing weathered/fractured rocks varies from 120–150 ohm m. The integrated studies have revealed that the blue colour zones are most promising for groundwater exploration and dug wells may be dug up to depths of 30±5m.     

Hydrogeophysical investigations in a typical khondalitic terrain to delineate the kaolinised layer using resistivity imaging

Investigation for high yielding water wells in the khondalitic terrain (graneti ferrous silliminite gnesiss) is mostly faced with the problem of identification of the extent of the depth of kaolinisation of the aquifer. The traditional Vertical Electrical Sounding survey, Seismic Refraction survey and Very Low Frequency Electromagnetic survey could not identify the kaolinisation of the aquifer in the present investigations. The Two Dimensional (2D) Resistivity and Induced Polarization (IP) Imaging surveys are attempted for the identification of kaolinised layer and depth of kaolinisation. Number of 2D Resistivity and IP Imaging profiles were conducted near Chipurupally in Vizianagaram district of Andhra Pradesh, India along successful and failed wells located within short distances. Resistivity and IP measurements were carried out using an ABEM SAS 4000 Terrameter. The resistivity and I.P. images have provided a clear view of the thickness of the highly weathered zone (kaolinised zone) at both successful and failed wells. The highly weathered zone is identified with the resistivity values below 25 ohm.m. The depth of highly weathered material at failed well is extended about 8–10 m more deeper than the successful wells at some places to as much as 20 m more deep at some other places. This extended deeper kaolinisation of the aquifer is responsible for failure of wells. Layers having resistivities between 25–65 Ohm.m are identified as aquifer layers which are composed of moderately weathered and fractured khondalitic suit of rocks (Garnti ferrous sillimanite/biotite gneiss). Layers with resistivities greater than 65 Ohm.m are interpreted to have basement characteristics belonging to the granite gneiss. Interestingly IP imaging has not provided any greater insights in delineating the kaolinistion of the aquifer when compared to resistivity Imaging, in fact resitivity imaging has shown greater depths of kaolinisation than IP Imaging.     

Efficacy of electrical resistivity tomography technique in mapping shallow subsurface anomaly

Electrical Resistivity Tomography is a versatile, fast and cost effective technique for mapping the shallow subsurface anomaly. It covers a wide spectrum of resistivity ranging from <1 Ohm.m to several thousands of Ohm.m. In this paper applications and utility of two-dimensional Electrical Resistivity Tomography (ERT) technique are discussed to look into huge data density coverage, different signal strengths of data from subsurface and their implications in resolving the aquifer zones, related geological structures etc. of the substratum ranging from alluvium to tectonically disturbed hard rock ridge region of the country. The major advantages and flexibility of ERT over conventional resistivity methods are also discussed.     

An integrated approach to investigate saline water intrusion and to identify the salinity sources in the Central Godavari delta, Andhra Pradesh, India

The Central Godavari delta is located along the Bay of Bengal Coast, Andhra Pradesh, India, and is drained by Pikaleru, Kunavaram and Vasalatippa drains. There is no groundwater pumping for agriculture as wells as for domestic purpose due to the brackish nature of the groundwater at shallow depths. The groundwater table depths vary from 0.8 to 3.4 m and in the Ravva Onshore wells, 4.5 to 13.3 m. Electrical Resistivity Tomography (ERT) surveys were carried out at several locations in the delta to delineate the aquifer geometry and to identify saline water aquifer zones. Groundwater samples collected and analyzed for major ions for assessing the saline water intrusion and to identify the salinity origin in the delta region. The results derived from ERT indicated low resistivity values in the area, which can be attributed to the existence of thick marine clays from ground surface to 12–15 m below ground level near the coast and high resistivity values are due to the presence of coarse sand with freshwater away from the coast. The resistivity values similar to saline water <0.01 Ω m is attributed to the mixing of the saline water along surface water drains. In the Ravva Onshore Terminal low resistivity values indicated up coning of saline water and mixing of saline water from Pikaleru drain. The SO ₄ ^?2 /Cl^?and Na⁺²/Cl^?ratios did not indicate saline water intrusion and the salinity is due to marine palaeosalinity, dilution of marine clays and dissolution of evaporites.