2D Electrical Tomography Research Articles

Evaluation of wet front under foundation at “El Chocón dam” using electrical tomography

El Chocón dam is located on Limay River in the province of Neuquén (Argentina) and is one of the most important in the country and in Latin America due its reservoir capacity. Currently, the dam is at full operation stage, while its structural health is monitored through a significant number of instruments installed in the dam body and its foundation. In the sector of the closure of the dam on its left margin, the measurements of some piezometers that monitor the deep rock foundation, showed some changes that gave rise to the hypothesis of the advance of wet front, despite fact that the work has practically 50 years from commissioning. To confirm or rule out this hypothesis, research work was started using the 2D electrical tomography method. The methodology was selected based on the good correlation that exists between the electrical resistivity and the volumetric content of water in a rock formation. Tomographic image obtained made it possible to determine different sectors in which wet front is detected and to define its extension, also evidencing a good correlation with what was recorded in the rock auscultation instruments, confirming the slow advance of wet front to downstream.

Study of multi-electrode excitation mode for 3D electrical resistance tomography

Multi-electrode excitation is an effective way to improve the performance of 3D electrical tomography (ET) system compared with single electrode excitation. This paper systematically discusses various sensing strategies for multi-electrode excitation with different combinations in radial and axial directions. A typical 3 × 8 3D ERT sensor is adopted to analyze the influence of different excitation modes on performance indexes, such as number of independent measurements, dynamic range of measurements, sensitivity distribution and correlation coefficient of image reconstruction. On the basis of radial rotation angle between the excitation electrodes mapped to the same plane and whether the excitation electrodes are located in the same axial layer, five typical electrode combination modes are designed under dual-electrode excitation and single-electrode measurement protocol. The results show that the variation trend of different indexes influenced by excitation mode is inconsistent. The entropy-based method is applied to comprehensively evaluate the different excitation modes with the selected multiple performance indexes. Among all above modes, the mode in which excitation electrodes are vertically aligned is better universal one. The experimental results show that the position characteristics of the preferable mode can obtain better image reconstruction quality in different distribution models. Furthermore, the better excitation mode will provide guidance for design of other multi-electrode excitation in 3D ET systems with different structures.

Electrical Tomography Hardware Systems for Real-Time Applications: a Review

This paper presents a review of two-dimensional (2D) and three-dimensional (3D) electrical tomography (ET) hardware accelerators for real-time applications. While many recent review papers have discussed various algorithms for image reconstruction or acquisition systems, none of them has considered state-of-the-art hardware implementations of the associated image reconstruction algorithms to achieve real-time performance, especially for 3D ET where the computation requirement is excessively high. A 3D ET is useful in various applications such as robotics, autonomous vehicles, and process control, but it is computationally very expensive with respect to its 2D counterpart. Most implementations are based on single or multi-core CPUs and, to a lesser extent, on either graphics processing units (GPUs) or field programmable gate arrays (FPGAs). However, there is a clear gap between the currently available processors, whose computation power exceeds hundreds of teraflops per second (TOPS) at a reasonable low power consumption, and the ones recently used in ET systems. This gives great potential for next-generation ET systems to achieve real-time 2D and 3D ET reconstruction within a small form factor. The paper summarizes the most recent ET hardware systems with respect to their performance in terms of quality and processing frame rate, reconstruction methods, along with optimization and future directions.

GEOPHYSICAL INVESTIGATION FOR POST-FOUNDATION ASSESSMENT WITHIN EKITI STATE UNIVERSITY, ADO EKITI, SOUTHWESTERN NIGERIA

This study has evaluated the post-construction subsoil integrity of a distressed building in Ado-Ekiti, Southwestern Nigeria, to decipher the in-situ lithology and structural disposition. Four traverses of 100 m length were established in approximately E-W direction with an inter-station spacing of 10 m. Two Geophysical methods were deployed including the ground magnetic method using Proton Precession Magnetometer and Electrical Resistivity method utilizing 2D Electrical Imaging and Vertical Electrical Sounding (VES) techniques. Twelve (20) VES station points were acquired within the investigated area using Schlumberger configuration which gave relevant information on layer stratification and geoelectric parameters. The obtained results from the VES showed four geo-electric layers comprises topsoil, clayey layer, weathered layer, fractured bedrock, and fresh basement. Low apparent resistivity was delineated from the 2D imaging exhibiting unfit materials at distance between 50 to 75 m and 80 to 100m. The magnetic results showed a series of bedrock ridges and depressions. The integration of results revealed that the majority of the study areas are unsuitable except for central parts which indicate appreciable competence and stability.

Groundwater prospecting using remote sensing and geoelectrical methods in the North Cameroon (Central Africa) metamorphic formations

Remote sensing imagery and geoelectrical surveys used in this study help to evaluate the groundwater potentiality in the metamorphic formations of the North Cameroon. The methodology of the work is then mainly based on the extraction and identification of lineaments or fracture zones which acts as the passageway for groundwater. Manual and automatic extraction technics applied on the Landsat-8 images made it possible to find sixty one (61) lineaments. The main directions of these lineaments are N-S and N22°E (NNE-SSW) and the secondary directions are N10°E, NE-SW, then W-E to N100°E. According to the lineament density map, high density values are located at the centre and at the western part of the study area. The lineament density contrasts show the level of fracturing in the area. This fracturing is also evidenced by the geoelectrical survey. Indeed, results of the electrical resistivity profiling and 2D electrical imaging around or near lineaments, reveal the existence of fractures and shear zones characterized by the conductive discontinuities in the electrical current responses. Usually, the discontinuities sus-evocated above form potential groundwater targets. So, based on the combined results from remote sensing and geoelectrical method, the present study thus demonstrates the effectiveness of using this approach for assessing the availability of potential groundwater zones in the metamorphic formations. However, methods used in this study should be supported by the drill hole methods for the validity of results.

Landslide vulnerability of the Debre Sina-Armania road section, Central Ethiopia: Insights from geophysical investigations

Geophysical investigations using 2D electrical imaging and magnetic methods have been conducted to study the landslide problem on the road located at Dokakit Kebele near Sar Amba Kidane Mhiret locality between Debre Sina and Armania towns in Central Ethiopia. From field observations, the landslide problem in the Debre Sina-Armania road section at Dokakit is due to a combination of factors, the most important of which are topography, the high degree of weathering, hydrology of the area, and human activities. Among these factors, human activities that include deforestation for housing, agricultural activities, search for building materials, and road construction are generally assumed to be the triggering factors for slope failure in the area. From the interpretation of the geophysical results, the near-surface geology of the study area is found to be composed of massive deposits of fragmented basalts and structurally disturbed layers together with expansive clay soils, which considerably contribute to the sliding problem. Another contributing factor to the subsurface instability is the high gradient slope in the area and the lack of vegetation/wooded tree cover. From the geophysical survey results, it has further been possible to map zones of weakness in the subsurface over which the geologic layers slide as well as their vertical and lateral extents. Areas of active sliding surfaces and extensive subsurface cracks, especially at the two ends of the profiles, were mapped. A combination of numerous weak zones, deep weathering conditions of the lithologic units, the action of rainfall, springs, and the high slope gradient of the study area were the contributing factors for this sliding problem. Remedial measures to stabilize a section of the affected area and construction of a detour road are proposed as mitigation measures for the road segment's recurrent and rather expensive failure over the locality.

The ancient settlement of Piammiano‐Statonia (Bomarzo, Italy): New data from 2D and 3D electrical tomography

AbstractThis study presents new archaeological data concerning the ancient layout of Piammiano, an Etruscan and Roman settlement located in the Tiberine internal Etruria (probably corresponding to the ancient town of Statonia), still little known. These data have been obtained by geophysical surveys performed using the Electrical Resistivity Tomography technique. These investigations were carried out in the central sector of the plateau where the settlement lies, and they have documented the presence of ancient buried structures at a depth of between about 0.5 and 2.5 m. These structures has the same orientation of the main road of the ancient settlement, and they are also consistent with the orientation of the structures documented in previous geophysical surveys and archaeological excavations carried out in the westernmost sector of the investigated area. Specifically, the new geophysical surveys have highlighted the presence of the buried remains of the road and numerous walls, some of which are pertaining to a large building with porticoes on the two sides overlooking the Tiber valley. Furthermore, a channel network perhaps linked to a cistern was documented at more depth.

Application of the Electrical Resistivity Method for Site Investigation in University of Anbar, Ar-Ramadi City, Western Iraq

The 2D resistivity imaging technique was applied in an engineering study for the investigation of subsurface weakness zones within University of Anbar, western Iraq. The survey was carried out using Dipole-dipole array with an n-factor of 6 and a-spacing values of 2 m and 5 m. The inverse models of the 2D electrical imaging clearly show the resistivity contrast between the anomalous parts of the weakness zones and the background resistivity distribution. The thickness and shape of the subsurface weakness zones were well defined from the 2D imaging using Dipole-dipole array of 2 m a-spacing. The thickness of the weakness zone ranges between 9.5 m to 11.5 m. Whereas the Dipole-dipole array with a-spacing of 5 m and n-factor of 6 allocated the geoelectrical stratigraphic layers sequence in low-accuracy of weakness zones, but deeper than the inverse model of 2 m a-spacing. This survey was made to explain the correlation between the weakness zone and the deeper layers in the study area. It points out that the deeper layers were not affected in the weakness zones. The inverse model was produced using the Standard Least-Squares Inversion Method and the Robust Inversion Model Constraints Method. The first method had a gradational boundary of the weakness zones and the second had sharper and straighter boundaries of fractures and voids within the weakness zones.

Comparison between Dipole-dipole and Pole-dipole Arrays in Delineation of Subsurface Weak Zones Using 2D Electrical Imaging Technique in Al- Anbar University, Western Iraq

The presence of natural voids and fractures (weak zones) in subsurface gypsiferous soil and gypsum, within the University of Al-Anbar, western Iraq. It causes a harsher problem for civil engineering projects. Electrical resistivity technique is applied as an economic decipher for investigation underground weak zones. The inverse models of the Dipole-dipole and Pole-dipole arrays with aspacing of 2 m and an n-factor of 6 clearly show that the resistivity contrast between the anomalous part of the weak zone and the background. The maximum thickness and shape are well defined from 2D imaging with Dipole-dipole array, the maximum thickness ranges between 9.5 to 11.5 m. It is concluded that the 2D imaging survey is a useful technique and more effective for determining and mapping subsurface weak zones (voids, fracture and cavities), when taken in consideration using the suitable a-spacing and n-factor for each electrode array, especially with the Dipole-dipole array which provided the best imaging of thesubsurface shape of the weak zones.

Resistivity Imaging: Application of 2D Resistivity Survey for Under Groundwater Prospecting in New Campus of Sulaimani University and Surrounding

Groundwater, the hidden source of life, it is so important to be estimated quantitatively and qualitatively before extracting and using. New campus of university of Sulaimani locates atthe west of the city, has a vast area, so that it needs to more water supplies for daily uses and irrigation. Therefore, conducting of geophysical survey was decided to ensure the existenceof underground water then drilling water wells. Next, a 2D electrical tomography was carried out along seven profiles of 720m using Syscal instrument with its accessories in andout of the university. After that, the obtained data were processed then interpreted using Res2Dinv software to approach a final conclusion. Finally, the results showed in profiles 1,2, and 3, the resistivity value was about 5-157 ohm.m and about 20-30 ohm.m in the water bearing layer with nearly 20m depth, deep well of depths ranging from 100 to 125 arerecommend near the location of electrode are indicated on the resistivity sections, the expected productivity is moderate to low. At profile 4, resistivity of 2-300ohm.m wasobtained and 20-100ohm.m in the water bearing layer of more than 20m depth, deep well is not recommended, the expected productivity is low. In geophysical point of view, thesurveyed locations beneath the profiles 5 and 6 not suitable for drilling an artesian well, a Marley limestone of low resistivity 9-40ohm.m was detected extending 70-150m of depth.Also, profile 7, is not suitable for drilling water well because of clay layers of low resistivity ranging 18-30ohm.m in depth of 30m.

Efficient uncertainty analysis of the 3D electrical tomography inverse problem

We have evaluated the uncertainty analysis of the 3D electrical tomography inverse problem using model reduction via singular-value decomposition and performed sampling of the nonlinear equivalence region via an explorative member of the particle swarm optimization (PSO) family. The procedure begins with the local inversion of the observed data to find a good resistivity model located in the nonlinear equivalence region. Then, the dimensionality is reduced via the spectral decomposition of the 3D geophysical model. Finally, the exploration of the uncertainty space is performed via an exploratory version of PSO (RR-PSO). This sampling methodology does not prejudge where the initial model comes from as long as this model has a geologic meaning. The 3D subsurface conductivity distribution is arranged as a 2D matrix by ordering the conductivity values contained in a given earth section as a column array and stacking parallel sections as columns of the matrix. There are three basic modes of ordering: mode 1 and mode 2, by using vertical sections in two perpendicular directions, and mode 3, by using horizontal sections. The spectral decomposition is then performed using these three 2D modes. Using this approach, it is possible to sample the uncertainty space of the 3D electrical resistivity inverse problem very efficiently. This methodology is intrinsically parallelizable and could be run for different initial models simultaneously. We found the application to a synthetic data set that is well-known in the literature related to this subject, obtaining a set of surviving geophysical models located in the nonlinear equivalence region that can be used to approximate numerically the posterior distribution of the geophysical model parameters (frequentist approach). Based on these models, it is possible to perform the probabilistic segmentation of the inverse solution found, meanwhile answering geophysical questions with its corresponding uncertainty assessment. This methodology has a general character could be applied to any other 3D nonlinear inverse problems by implementing their corresponding forward model.

Time-Lapse 3D Electric Tomography for Short-time Monitoring of an Experimental Heat Storage System

A borehole thermal energy storage living lab was built nearby Torino (Northern Italy). The aim of this living lab is to test the ability of the alluvial deposits of the north-western Po Plain to store the thermal energy collected by solar panels. Monitoring the temperature distribution induced in the underground and the effectiveness of the heat storage in this climatic context is not an easy task. For this purpose, different temperature evolution strategies are compared in this paper: Local temperature measurements, numerical simulations and geophysical surveys. These different approaches were compared during a single day of operation of the living lab. The results of this comparison allowed to underline the effectiveness of time-lapse 3D electric resistivity tomography as a non-invasive and cost-effective qualitative heat monitoring tool. This was obtained even in a test site with unfavorable thermo-hydrogeological conditions and high-level anthropic noise. Moreover, the present study demonstrated that, if properly calibrated with local temperature values, time-lapse 3D electric resistivity tomography also provides a quantitative estimation of the underground temperature.

3D electrical imaging of the inner structure of a complex lava dome, Puy de Dôme volcano (French Massif Central, France)

Lava domes result from extrusion of massive lava, frequent explosions and collapses. This contribution focuses on a complex trachytic lava dome, the Puy de Dôme volcano, located in the Chaîne des Puys volcanic field (French Massif Central, France). We performed Electrical Resistivity Tomography (ERT) acquisitions on the entire edifice in order to investigate its overall inner structure as well as to detail its summit area. The resulting large ERT dataset integrated a recently developed 3D inversion code based on an unstructured discretization of the geometrical model. The 3D inversion models obtained refine the existing geological model of the Puy de Dôme's inner structure obtained by previous geophysical studies. These results also highlight the strong fracturing and fumarolic alteration that affect the summit part of the volcano.

Non-invasive Geophysical Surveys in Search of the Roman Temple of Augustus Under the Cathedral of Tarragona (Catalonia, Spain): A Case Study

An integrated geophysical survey has been conducted at the Tarragona’s Cathedral (Catalonia, NE Spain) with the aim to confirm the potential occurrence of archaeological remains of the Roman Temple dedicated to the Emperor Augustus. Many hypotheses have been proposed about its possible location, the last ones regarding the inner part of the Cathedral, which is one of the most renowned temples of Spain (twelfth century) evolving from Romanesque to Gothic styles. A geophysical project including electrical resistivity tomography (ERT) and ground probing radar (GPR) was planned over 1 year considering the administrative and logistic difficulties of such a project inside a cathedral of religious veneration. Finally, both ERT and GPR have been conducted during a week of intensive overnight surveys that provided detailed information on subsurface existing structures. The ERT method has been applied using different techniques and arrays, ranging from standard Wenner–Schlumberger 2D sections to full 3D electrical imaging with the advanced Maximum Yield Grid array. Electrical resistivity data were recorded extensively, making available many thousands of apparent resistivity data to obtain a complete 3D image after a full inversion. In conclusion, some significant buried structures have been revealed providing conclusive information for archaeologists. GPR results provided additional information about shallowest structures. The geophysical results were clear enough to persuade religious authorities and archaeologists to conduct selected excavations in the most promising areas that confirmed the interpretation of geophysical data. In conclusion, the significant buried structures revealed by geophysical methods under the cathedral were confirmed by archaeological digging as the basement of the impressive Roman Temple that headed the Provincial Forum of Tarraco, seat of the Concilium of Hispania Citerior Province.

Assessment of landfill leachate in Nam Son, Hanoi using 3D electrical imaging and self-potential data

The main objective of this study was to assess the environmental impact of the Nam Son Landfill on the subsurface geological structure studied using 3D electrical imaging and self-potential investigations. The map of the equipotential was based on self-potential data and was indicative of concentrations of redox-active components, and the map of the electrical conductivity provided information about the mineralization of the groundwater. The electrical conductivity of the subsurface was obtained from 3D-electrical imaging analysis conducted using a Wenner-based array configuration. An inversion of the data was carried out using 3D regularized least-squares optimization methods with robust model constraints. Three profiles of 3D electrical imaging and 180 self-potential data points, scattered within the study area near the disposal site, were implemented. The self-potential method supports the results of the geoelectrical method relating to a contaminated zone in the survey area with a large negative self-potential anomaly of ∼−260 mV with respect to the reference of a contaminant plume. Interpretations of geophysical data showed a low resistivity zone (<10 Ω m), which appeared to be a fully saturated zone with leachate from the Nam Son open dumpsite. The correlation between geophysical investigations and hydrochemical analysis proved to show high conductivity zones related with landfill leachate.

Uncertainty analysis and probabilistic segmentation of electrical resistivity images: the 2D inverse problem

ABSTRACTIn this paper, we present the uncertainty analysis of the 2D electrical tomography inverse problem using model reduction and performing the sampling via an explorative member of the Particle Swarm Optimization family, called the Regressive‐Regressive Particle Swarm Optimization. The procedure begins with a local inversion to find a good resistivity model located in the nonlinear equivalence region of the set of plausible solutions. The dimension of this geophysical model is then reduced using spectral decomposition, and the uncertainty space is explored via Particle Swarm Optimization. Using this approach, we show that it is possible to sample the uncertainty space of the electrical tomography inverse problem. We illustrate this methodology with the application to a synthetic and a real dataset coming from a karstic geological set‐up. By computing the uncertainty of the inverse solution, it is possible to perform the segmentation of the resistivity images issued from inversion. This segmentation is based on the set of equivalent models that have been sampled, and makes it possible to answer geophysical questions in a probabilistic way, performing risk analysis.

TWO-PHASE FLOW REGIME THREE-DIMENSONAL VISUALIZATION USING ELECTRICAL CAPACITANCE TOMOGRAPHY – ALGORITHMS AND SOFTWARE

This paper presents the software for comprehensive processing and visualization of 2D and 3D electrical tomography data. The system name as TomoKIS Studio has been developed in the frame of DENIDIA international research project and has been improved in the frame of Polish Ministry of Science and Higher Education Project no 4664/B/T02/2010/38. This software is worldwide unique because it simultaneously integrates the process of tomographic data acquisition, numerical FEM modeling and tomographic images reconstruction. The software can be adapted to specific industrial applications, particularly to monitoring and diagnosis of two-phase flows. The software architecture is composed of independent modules. Their combination offers calibration, configuration and full-duplex communication with any tomographic acquisition system with known and open communication protocol. The other major features are: online data acquisition and processing, online and offline 2D/3D images linear and nonlinear reconstruction and visualization as well as raw data and tomograms processing. Another important ability is 2D/3D ECT sensor construction using FEM modeling. The presented software is supported with the multi-core GPU technology and parallel computing using Nvidia CUDA technology.

Geophysical Investigations at Cairo’s Oldest, the Church of Abu Serga (St. Sergius), Cairo, Egypt

ABSTRACTThe results of an integrated geophysical survey at the archaeological site of Abu Serga church, Cairo, Egypt are presented and discussed. The aim was to investigate the ground conditions of the Church of Abu Serga (St. Sergius), the Cairo’s oldest, dated from 4th Century church, which is located at Qasr el-Shama in old Cairo in Egypt. In particular the objective is to study the subsurface geological structures at the location of the church, and to detect and possibly map any ancient remains concealed under the monument. The survey was conducted using two geophysical methods: the ground penetrating radar (GPR), which is a fully non-destructive method, and the electrical resistivity tomographies (ERTs). The usefulness of combining conventional geophysical mapping techniques and high resolution imaging methods in delineating shallow targets of archaeological interest at such complex archaeological sites, is studied. Ground penetrating radar time slices and 3D electrical tomography depth slices were used for the verification of specific anthropogenic anomalies, which were detected on the geophysical maps Processing of geophysical maps included filtering with the gradient and first derivative operators in the space domain and the upward continuation and Butterworth filters in the wave number domain. The integration of the geophysical measurements revealed that the present Crypt is not the original holy Crypt. The anomalous reflector is detected at depth of about 5 m below the sanctuary floor, in the form of buried ceiling of the original Crypt. The present Crypt is just a small low subterranean church belong to the 2nd century. High resistivity anomalies and distinct GPR signals were also observed deeper in the inner parts of the church. They are attributed to possible remains of ancient walls and surrounding tunnels, or other man-made structures concealed under the floor of the monument. The geophysical survey at Abu Serga church also demonstrates that the general features of the foundation soil are heterogeneous with abundance of fractures; the water table is very high at 1.8m below the sanctuary floors. The benefits of combined geophysical surveys in case of archaeological investigations at complex sites are highlighted.

Soil–plant interaction monitoring: Small scale example of an apple orchard in Trentino, North-Eastern Italy

Accurate monitoring and modeling of soil–plant systems are a key unresolved issue that currently limits the development of a comprehensive view of the interactions between soil and atmosphere, with a number of practical consequences including the difficulties in predicting climatic change patterns. This paper presents a case study where time-lapse minimal-invasive 3D micro-electrical tomography (ERT) is used to monitor rhizosphere eco-hydrological processes in an apple orchard in the Trentino region, Northern Italy. In particular we aimed at gaining a better understanding of the soil–vegetation water exchanges in the shallow critical zone, as part of a coordinated effort towards predicting climate-induced changes on the hydrology of Mediterranean basins (EU FP7 CLIMB project). The adopted strategy relied upon the installation of a 3D electrical tomography apparatus consisting of four mini-boreholes carrying 12 electrodes each plus 24 mini-electrodes on the ground surface, arranged in order to image roughly a cubic meter of soil surrounding a single apple tree. The monitoring program was initially tested with repeated measurements over about one year. Subsequently, we performed three controlled irrigation tests under different conditions, in order to evaluate the water redistribution under variable root activities and climatic conditions. Laboratory calibration on soil samples allowed us to translate electrical resistivity variations into moisture content changes, supported also by in-situ TDR measurements. Richards equation modeling was used also to explain the monitoring evidence. The results clearly identified the effect of root water uptake and the corresponding subsoil region where active roots are present, but also marked the need to consider the effects of different water salinity in the water infiltration process. We also gained significant insight about the need to measure quantitatively the plant evapotranspiration in order to close the water balance and separate soil structure effects (primarily, hydraulic conductivity) from water dynamics induced by living plants.

2D Electrical Imaging Surveys for Leachate Plume Migration at an Old Dump Site in Ibadan South Western Nigeria: A Case Study

The site surveyed is along a popular ring road in Ibadan. It is an old dump site which had been closed over a long period of time. Development into industrial and residential house schemes had crowded around the site. This development resulted in the exploitation of groundwater within the thick regolith that characterize the area. The investigation was carried out to ascertain leachate plume generation and migration and its impact on the surrounding soil and the groundwater. 2D electrical resistivity imaging using Wenner array was used to delineate the plume and probable trend of migration. The method involves the injection of low frequency DC current into the subsurface and measuring the potential difference set up by another pair of electrodes called potential electrodes. All the traverses show areas of very low resistivity values ranging between 4 ohm-m and 13.8 ohm-m; in addition to this, the water samples collected from hand dug wells that surround the dump site show high total dissolved solids (TDS) with an average value of 2033 mg/L, high conductivity with an average value of 2868 mg/L, and high nitrate value (63 mg/L); these values are above the World Health Organization (WHO) permissible values. This poses a health challenge to the residents living around this site.