Hydrogeological and geophysical study for deeper groundwater resource in quartzitic hard rock ridge region from 2D resistivity data

Electrical resistivity method is a versatile and economical technique for groundwater prospecting in different geological settings due to wide spectrum of resistivity compared to other geophysical parameters. Exploration and exploitation of groundwater, a vital and precious resource, is a challenging task in hard rock, which exhibits inherent heterogeneity. In the present study, two-dimensional Electrical Resistivity Tomography (2D-ERT) technique using two different arrays, viz., pole–dipole and pole–pole, were deployed to look into high signal strength data in a tectonically disturbed hard rock ridge region for groundwater. Four selected sites were investigated. 2D subsurface resistivity tomography data were collected using Syscal Pro Switch-10 channel system and covered a 2 km long profile in a tough terrain. The hydrogeological interpretation based on resistivity models reveal the water horizons trap within the clayey sand and weathered/fractured quartzite formations. Aquifer resistivity lies between ～3–35 and 100–200 Ωm. The results of the resistivity models decipher potential aquifer lying between 40 and 88 m depth, nevertheless, it corroborates with the static water level measurements in the area of study. The advantage of using pole–pole in conjunction with the pole–dipole array is well appreciated and proved worth which gives clear insight of the aquifer extent, variability and their dimension from shallow to deeper strata from the hydrogeological perspective in the present geological context.     

Abandoned mine galleries detection using electrical resistivity tomography method over Jharia coal field,India

Land subsidence is a serious problem in Indian coalfields due to old underground mine workings. Unfortunately, most of these are uncharted as no mine plans are available. The hidden galleries, goafs, shafts etc. may pose great threat for future mine development as well as to the local environment. The mine workings should be charted to undertake an effective preventive action. In the present study, 2D electrical resistivity tomography (ERT) technique has been used to detect underground mine workings, mainly air or water filled galleries. Initially, the whole exercise has been executed through a synthetic model study. Gaussian random noise of 5mV/A has been added with synthetic data to demonstrate field condition which provides realistic results. ERT survey was conducted over a part of Jogidih coal mine of Jharia coal field in India for a first time. Four electrode configurations, Wenner, Schlumberger, dipole-dipole and gradient were considered for this study. The results indicate the presence of sub-surface water and air filled cavity due to high resistivity contrast with surroundings.     

Karst Sinkholes Stability Assessment in Cheria Area,NE Algeria

This research work deals with the problem of karst sinkhole collapse occurring in the last few years in Cheria area (NE Algeria). This newly revealed phenomenon is of a major constrain in land use planning and urbanization, it has become necessary to locate and assess the stability of these underground features before any planning operation. Several exploration methods for the localization of underground cavities have been considered. Geological survey, discontinuity analysis, resistivity survey [ground penetrating radar has not been used as most of the Mio-Plio-Quaternary filling deposit covering Eocene limestone contains clay layers which limits the applicability of the method (Roth et al. in Eng Geol 65:225–232, 2002)] and borehole drilling were undertaken in order to locate underground cavities and assess their depth, geometry, dimensions, etc. Laboratory testing and field work were also undertaken in order to determine both intact rock and rock mass properties. All the rock mechanics testing and measurement were undertaken according to the ISRM recommendations. It has been found that under imposed loading, the stability of the karst cavities depends on the geo-mechanical parameters (RMR, Rock Mass Rating; GSI, Geological Strength Index; E, Young modulus) of the host rock as well as the depth and dimensions of the gallery. It increases with RMR, GSI, E and depth and decreases as the cavity becomes wider. Furthermore, the calculation results show that a ratio (roof thickness to gallery width) of 0.3 and more indicate, a stable conditions. The results obtained in this work allow identifying and assessing the stability of underground karst cavities. The methodology followed in this paper can be taken as a road map in the establishment of a hazard map related to the studied phenomenon. This map will be a useful tool for the future urban extension planning in Cheria area.     

Hydrogeophysical investigation and estimation of groundwater potentials of the Lower Palaeozoic to Precambrian crystalline basement rocks in Keffi area, north-central Nigeria, using resistivity methods

The use of resistivity sounding and two-dimensional (2-D) resistivity imaging was investigated with the aim of delineating and estimating the groundwater potential in Keffi area. Rock types identified are mainly gneisses and granites. Twenty-five resistivity soundings employing the Schlumberger electrode array were conducted across the area. Resistivity sounding data obtained were interpreted using partial curve matching approach and 1-D inversion algorithm, RESIST version 1.0. The 2-D resistivity imaging was also carried out along two traverses using dipole–dipole array, and the data obtained were subjected to finite element method modeling using DIPRO inversion algorithm to produce a two-dimensional subsurface geological model. Interpretation of results showed three to four geoelectrical layers. Layer thickness values were generally less than 2 m for collapsed zone, and ranged from 5 to 30 m for weathered bedrock (saprolite). Two major aquifer units, namely weathered bedrock (saprolite) aquifer and fractured bedrock (saprock) aquifer, have been delineated with the latter usually occurring beneath the former in most areas. Aquifer potentials in the area were estimated using simple schemes that involved the use of three geoelectrical parameters, namely: depth to fresh bedrock, weathered bedrock (saprolite) resistivity and fractured bedrock (saprock) resistivity. The assessment delineated the area into prospective high, medium and low groundwater potential zones.     

综合物探在北京地区岩溶塌陷勘查中的应用

北京市海淀区四季青镇香山村普安店在近几年中发生了多起地面塌陷灾害,给当地村民的生命和财产安全造成严重威胁。要查明地面塌陷灾害的成因,必须先要寻找地下隐伏的空洞。在勘查工作区内,采用高密度电阻率法、孔间电磁波CT法相结合的综合物探手段,成功探测出两处地下空洞,经钻探验证为岩溶发育所致。综合研究分析认为:工作区内下伏基岩为灰岩,属于可溶性碳酸盐类岩石;地下水在基岩面附近,且基岩面的高低变化大,在漫长的潜蚀过程中将土中物质带入溶洞中,在上覆粘性土层中形成"土洞",一旦塌陷至地表,就有可能成为灾害。     

Imaging of wastewater percolation in heterogeneous soil using electrical resistivity tomography (ERT): a case study at east of Tenth of Ramadan City,Egypt

The objective of this study is to evaluate the effectiveness of DC resistivity surveys for imaging the wastewater percolation around the stabilization ponds in the Tenth of Ramadan City, the desert fringes of East Nile Delta, Egypt. Detailed resistivity surveys, including DC soundings and electrical resistivity tomography (ERT), were carried out along several profiles. Furthermore, synthetic modeling of ERT was designed to optimize the survey configurations and interpretation of the results. A 2D modeling of smoothness-constrained least-squares inversion scheme was applied to delineate the possible wastewater infiltration zones from oxidation ponds. Because the geoelectrical interpretation has a degree of non-uniqueness, the resistivity inversion was constrained using borehole lithological information and soil sample laboratory measurements. The DC inversion results indicate decreasing resistivity down to a depth of 15 m around waste disposal sites. The inferred soil zone close to the oxidation ponds was a mixture of sand, silt and clay. Moreover, the clay minerals were characterized by moderate swelling that could have reduced the vertical infiltration speed, causing wastewater seepage, especially around unlined disposal sites and open surface drains. Accordingly, the medium-to-low resistivity values can be attributed to wastewater leakage in clayey sand soil. Because the area slopes generally toward the northeast, the surface seepage was dominant in the shallow impermeable sandy clay subsoil. Therefore, measuring soil parameters is a complementary method to optimize resistivity interpretation, with potential for mitigating environmental hazards from wastewater leakage around disposal ponds.     

Integrated geoelectrical survey for groundwater and shallow subsurface evaluation: case study at Siliyin spring, El-Fayoum, Egypt

Siliyin spring is one of the many natural fresh water springs in the Western Desert of Egypt. It is located at the central part of El-Fayoum Delta, which is a potential place for urban developments and touristic activities. Integrated geoelectrical survey was conducted to facilitate mapping the groundwater resources and the shallow subsurface structures in the area. Twenty-eight transient electromagnetic (TEM) soundings, three vertical electrical soundings (VES) and three electrical resistivity tomography (ERT) profiles were carried out around the Siliyin spring location. The dense cultivation, the rugged topography and the existence of infra structure in the area hindered acquiring more data. The TEM data were inverted jointly with the VES and ERT, and constrained by available geological information. Based on the inversion results, a set of geoelectrical cross-sections have been constructed. The shallow sand to sandy clay layer that forms the shallow aquifer has been completely mapped underneath and around the spring area. Flowing of water from the Siliyin spring is interconnected with the lateral lithological changes from clay to sand soil. Exploration of the extension of Siliyin spring zone is recommended. The interpretation emphasizes the importance of integrating the geoelectrical survey with the available geological information to obtain useful, cheap and fast lithological and structural subsurface information.     

高寒山地岩溶发育特征——以云南石卡山为例

为了解海拔4 000 m以上高原夷平面灰岩裸露的山体岩溶作用发育情况及表现形式,文章以云南香格里拉石卡山为例,结合高海拔宇宙射线选址,对此问题进行了现场调查和初步研究。结果发现,在海拔4 000 m以上面积约2.4 km 2 高原面场地范围内,石炭系灰岩中发育了118个以上落水洞、陡崖可见6个近水平溶洞。高密度电法探测结果显示,岩溶以低电阻率为特点:地表落水洞在50~60 m深度上连成一体,100~180 m深度低阻区范围更大。研究区地下水系统处于大气降水垂直入渗补给区,上游（西部）灰岩和下游（东部）泥岩阻隔影响到水的径流路径和排泄形式,地表明显积水洼地12处,泉水点10个。地下水埋藏较浅,排泄快。这些岩溶发育特征为该场地规划利用提供了地质依据。      

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.     

Geostatistical integration using 2-D electrical resistivity and 3-D gravity methods for detecting cavities in a karst area

Two-dimensional (2-D) electrical resistivity and three-dimensional (3-D) gravity explorations were undertaken to estimate the 3-D distribution of karst cavities at the area of Yongweol-ri in Korea where ground subsidence has occurred. Although the gravity method is a low-cost way of analyzing a 3-D continuous structure, its vertical resolving power is poor. In contrast, the electrical resistivity method can provide a 2-D and/or 3-D subsurface structure with a much higher lateral and vertical resolution than the gravity method. Accordingly, geostatistical methods and density information were used to enhance the 2-D resistivity structure revealed by the electrical resistivity method into a 3-D structure. The assumptions are, first, that each data set senses the same underlying geological structure in terms of different material properties and, secondly, that two different material properties are correlated locally or globally throughout the entire target area. As a result, the distribution of limestone cavities can be estimated under the assumption that they are mostly filled with groundwater and clayey soils and have abnormally low levels of resistivity and density. The estimated distribution corresponds with the grouting, borehole imaging and monitoring data. In this example, it can be seen that the integration analysis of 2-D electrical resistivity and 3-D gravity methods is a very powerful tool for 3-D subsurface imaging and that the method can provide enhanced imaging capabilities for 3-D cavities.     

Hydrogeophysical approach for contamination assessment in NamSon landfill,Hanoi, Vietnam

Self potential (SP) and electrical resistivity tomography (ERT) methods are used together with the results of groundwater samples hydrogeochemical analysis to assess the impact of the water leak from the landfill garbage site at NamSon located in Northern Hanoi on causing pollution to the surrounding environment and affecting geological structure. Selected survey area covers an area of 180 × 300 m lying in the low land of the NamSon site with a slope ranging about 8 m in direction NW–SE. There are three geophysical measurements lines denoted as T1, T2 and T3. Processing 180 SP data points has allowed to draw maps of equipotential epoch in the two periods in 2015 and 2016. The maps show four zones of SP positive anomalies with maximum amplitudes of about +20 mV where the groundwater flow direction is downward and five zones of SP negative anomalies with minimum values in a range from ?180 to ?260 mV where the groundwater flow direction is upward. Resistivity values of the subsurface layers of soils and rocks have been aquired from 2D inverse model for measuring ERT in March 2015 and March 2016. The results of the ERT allowed to define the low resistivity in the range 15–20 Ωm related to leachate plume from NamSon landfill site. Results of the physico-chemical analysis of groundwater samples from the existing six boreholes show increases in concentration of the measured pollutant parameters indicating contamination of the groundwater as a result of solid waste leachate accumulation. This result is affirmative evidence for the survey results by geophysical technique. The rapid decrease in quality of groundwater over the last year is probably due to the influence of the leachate from the NamSon landfill site.     

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.     

Formation mechanism of large sinkhole collapses in Laibin,Guangxi, China

On June 3, 2010, a series of karst sinkholes occurred at Jili village surrounded by Gui-Bei highway, Wu-Ping highway and Nan-Liu High-Speed Railway in Laibin, Guangxi, China. The straight-line distances from an large sinkhole pit, 85 m in diameter and 38 m in depth, to the above mainlines are 200, 600 and 500 m, respectively. Several investigation methods including geophysical technology, borehole and well drilling, groundwater elevation survey and hydrochemistry analysis of groundwater were used to study the formation mechanisms of these sinkholes. Based on the results, the spatial distribution of the Jili underground river was confirmed with a strike of SN along the middle Carboniferous limestone bedrock and the Quaternary deposits controlled the sinkhole formation. In addition, both historical sinkhole events and analysis of the groundwater–air pressure monitoring data installed in the underlying karst conduit system indicate that sinkholes in this area are more likely induced by extreme weather conditions within typical karst geological settings. Extreme weather conditions in the study area before the sinkhole collapses consisted of a year-long drought followed by continuous precipitation with a daily maximum precipitation of 442 mm between May 31 and June 1, 2010. Typical geological conditions include the Jili underground river overlain by the Quaternary overburden with thick clayey rubble. Especially in the recharge zone of the underground river, a stabilized shallow water table was formed in response to the extreme rainstorm because of the presence of the thick clayey rubble. When the underground conduit was flooded through the cave entrance on the surface, air blasting may have caused the cave roof collapse followed by formation of soil cavities and surface collapses. Borehole monitoring results of the groundwater–air pressure monitoring show that the potential karst sinkhole can pose threats to Shanbei village, the High-Speed Railway and the Wu-Ping highway. Local government needs to be aware of any early indicators of this geohazard so that devastating sinkholes can be prevented in the future. The results also suggest that groundwater–air pressure monitoring data collected both the Quaternary deposits and the bedrock karst system provide useful indicators for potential sinkhole collapses in similar karst areas where sinkholes usually occur during rainy season or karst groundwater level is always under the rockhead.     

Hydrogeophysical investigations for assessing the groundwater potential in part of the Peshawar basin,Pakistan

A large number of valleys and basin systems are present in the northwestern part of the Himalayas in Pakistan which form significant aquifers in the region. Hydrogeophysical investigations in the western part of Nowshera District, a part of the intermontane Peshawar basin, were undertaken to help to determine the availability of groundwater resources in the region. Thirty vertical electrical resistivity soundings (VES) were acquired using a Schlumberger expanding array configuration with a maximum current electrode spacing (AB/2) of 150 m in delineating the groundwater potential in the study area. The results of the interpreted VES data using a combination of curve matching technique and computer iterative modeling methods suggest that the area is underlain by 3 to 5 geo-electric layers. The interpretation results showed that the geo-electrical succession consists of alluvium comprising of alternating layers of clay, silty clay, fine to coarse sands, sand with gravels and gravels of variable thickness. High subsurface resistivity values are correlated with gravel–sand units and low resistivity values with the presence of clays and silts. The modeled VES results were correlated with the pumping tests results and lithological logs of the existing wells. The pumping test suggests the transmissivity of the aquifer sediments is variable corresponding to different sediments within the area. The gravel–sand intervals having high resistivity value show high transmissivity values, whereas clay–silt sediments show low transmissivities. It is concluded that majority of the high resistive gravel–sand sediments belong to an alluvial fan environment. These gravel–sand zones are promising zones for groundwater abstraction which are concentrated in the central part of the study area.     

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.     

Monitoring of wastewater percolation in unsaturated sandy soil using geoelectrical measurements at Gabal el Asfar farm,northeast Cairo,Egypt

The use of wastewater for irrigation in sandy soil increases the pollution risk of the soil and may infiltrate to the shallow groundwater aquifer. In such environment, some important parameters need to be obtained for monitoring the wastewater in the unsaturated zone over the aquifer. These parameters include clay content, heterogeneities of the upper soils, depth to the aquifer and the variations of groundwater quality. In the present work, the efficiency of DC resistivity method in forms of 1-D and 2-D measurements was studied for wastewater monitoring in the Gabal el Asfar farm, northeast of Cairo, Egypt. Forty-one Schlumberger soundings (VES) were performed then followed by three pole-dipole 2-D profiles along some considered regions within the area. The resistivity measurements were integrated with the boreholes, hydrogeological and hydrochemical (surface and groundwater samples) information to draw a clear picture for the subsurface conditions. The obtained results were presented as cross sections and 3-D visualization to trace the clay intercalations within the unsaturated zone. In addition, a vulnerability map was created using the obtained results from 1-D Schlumberger survey and confirmed with the 2-D resistivity profiling. The obtained results have shown that the 2-D resistivity imaging technique is a powerful tool for mapping the small-scale variability within the unsaturated zone and the wastewater infiltration. However, limitations of resistivity techniques were observed in the area with limited resistivity contrast such as thin clay layers with brackish water background. Under that condition, the measured pattern of resistivity distributions depends on the applied electrode array, electrode spacing and using the available geological information during the inversion process.     

Electrical resistivity investigation of potential cavities underlying a proposed ash disposal area

Measurements of electrical resistivity anomalies using a Wenner electrode array in a profiling mode were conducted over a 105-acre area near Crystal River, Florida Analyzed results of collected data were used to determine the likelihood of subsurface solution cavities beneath a proposed ash disposal area “Sounding” measurements with electrode spacing values extending up to 100 feet and Lee electrode measurements were used to identify and locate specific anomaly maxima identified anomalies on the order of 200 chms-feet contrasted with expected values of 300 to 400 ohms-feet and were attributed to lower resistivities inherent in water-filled solution cavities below a shallow water table Prominent cavity complexes at 5 to 15 feet depth were predicted in areas of low resistivity anomalies in the otherwise geoelectrically uniform, Eocene Ocala limestone Shallow test borings confirmed the presence of a significant concentration of shallow cavities in the predicted areas This study demonstrated the feasibility of electrical resistivity measurements as a reconnaissance tool to economically determine subsurface cavities and to aid in developmental and environmental planning     

Assessment of integrated electrical resistivity data on complex aquifer structures in NE Nuba Mountains – Sudan

Two geophysical techniques were integrated to map the groundwater aquifers on complex geological settings, in the crystalline basement terrain in northeast Nuba Mountains. The water flow is structurally controlled by the northwest–southeast extensional faults as one of several in-situ deformational patterns that are attributed to the collision of the Pan-African oceanic assemblage of the Nubian shield against the pre-Pan African continental crust to the west. The structural lineaments and drainage systems have been enhanced by the remote sensing technique. The geophysical techniques used are: vertical electrical soundings (VES) and electrical resistivity tomography (ERT), in addition to hydraulic conductivity measurements. These measurements were designed to overlap in order to improve the producibility of the geophysical data and to provide a better interpretation of the hydrogeological setting of the aquifer complex structure. Smooth and Block inversion schemes were attempted for the observed ERT data to study their reliability in mapping the different geometries in the complex subsurface. The VES data was conducted where ERT survey was not accessible, and inverted smoothly and merged with the ERT in the 3D resistivity grid. The hydraulic conductivity was measured for 42 water samples collected from the distributed dug wells in the study area; where extremely high saline zones were recorded and have been compared to the resistivity values in the 3D model.     

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。     

Geotechnical investigation of the El-Elb dam site,northwest Riyadh,Saudi Arabia,using 2D resistivity and ground-penetrating radar techniques

In the present study, the existence of cavities, voids, and fractures was verified at the site of the El-Elb Dam, which is located to the northwest of Riyadh City across Wadi Hanifa, using 2D electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) techniques. For this purpose, four ERT profiles were measured on the downstream side of the El-Elb Dam using the Syscal Pro Switch-72 resistivity meter. In addition, a GPR survey using a 400-MHz antenna and a SIR-3000 instrument was conducted along five profiles above the stilling basins on the downstream side of the dam and one radar profile was measured outside the stilling basins area across the course of the wadi. The resultant geophysical data were interpreted with the aid of information from a field-based structural and stratigraphic evaluation of the outcropped bedrock on the banks of the wadi course. The analysis of the inverted ERT and filtered radar sections revealed several resistivity and electromagnetic reflection anomalies that are identified laterally and vertically across the measured sections. These anomalies indicate the presence of fractures and karst features affected the limestone bedrock in the dam site. These near-surface karstified and fractured strata represent a critical hazard to the structural safety of the dam.