2D Electrical Tomography Research Articles

Reconstruction of Space Distribution of Characteristics in Compound Structures by General Ray Method

ABSTRACTRecognition of material structures, particularly, identification of electrical properties of materials by Electrical Tomography is very important in different applied problems. In a plane case Electrical Tomography can be mathematically described as a coefficient inverse problem for the Laplace type equation, written in the divergent form. The General Ray (GR) Principle, proposed by the author, reduces the Laplace type equation to the family of ordinary differential equations with respect the traces of the potential function and the permittivity function on the lines, which intersect the plane domain. General Ray Principle was realized as General Ray method and fast algorithm for the plane domains. In presented investigation we apply the plane scheme of GR-method for some space domains to identify distribution of structure characteristics inside it. For this we consider the space domain as assemblage of plane slices. Reconstructing desired distribution in each plane slice we obtain then the space internal distribution of electrical characteristics by 3D spline approximation. We consider here specific variant of the measurement scheme for the 3D Electrical Tomography (ET), based on the variant, proposed by the author for the plane domain. Proposed approach gives, in principle, the possibility to use a large number of electrodes, obtain more values of the input data and reconstruct the desired space structure more perfectly. Computer simulation of this 3D scheme is realized as MATLAB software and justified by numerical experiments on simulated examples.

Direct recovery of the electrical admittivities in 2D electrical tomography by using Calderon's method and two-terminal/electrode excitation strategies

Calderon's method was used to reconstruct electrical admittivity, i.e. conductivity as well as permittivity, for electrical tomography in this paper. It is a direct algorithm of the image reconstruction, as the value of the real and imaginary parts of the admittivity at any position can be obtained independently. A new way to construct the Dirichlet-to-Neumann map from the data collected through two-terminal/electrode excitation strategy was also introduced to calculate the scattering transform. The image reconstruction was implemented through numerical integration. Since the Gauss–Legendre quadrature was applied and can be predetermined, the image reconstruction process was fast and resulted in images of good quality. Both simulated and experimental results validated the feasibility and effectiveness of the proposed method in dual-modality electrical tomography.

Monitoring Soil-plant Interactions in an Apple Orchard Using 3D Electrical Resistivity Tomography

In this paper we present the application of time-lapse non invasive 3D micro-electrical tomography (ERT) to monitor soil-plant interactions in the root zone. The goal of the study is to better understand the soil-vegetation interactions by the use of non invasive technique, and how this dynamical process can be monitored by time lapse geophysical measurements to explain water balance exchanges in the upper subsoil critical zone. We designed, built and installed a 3D electrical tomography apparatus for the monitoring of the root zone of a single apple tree in an orchard located in the Trentino region, Northern Italy. The micro-ERT apparatus consists of 48 buried electrodes on 4 instrumented micro boreholes plus 24 mini-electrodes on the ground surface. We collected repeated ERT and TDR soil moisture measurements for about one year and performed three different controlled irrigation tests: one during a very dry summer and, during a highly dynamic plant growing spring period and one during a very wet autumn period. We also performed laboratory analysis on soil specimens, in order to evaluate the electrical response at different saturation steps. The results demonstrate that 3D micro-ERT is able to characterize subsoil conditions and monitor root zone activities, especially in terms of root zone suction regions. In particular, we note that in very dry conditions, 3D micro ERT is able to image water plumes on the shall subsoil produced using a drip irrigation system. In the very dynamic growing season, under induced widespread irrigation, micro 3D ERT can detect the main suction zones caused by the tree root activity. Complex interactions between soil moisture and pore water salinity complicate the interpretation. The presented approach is a very promising tool for a better understanding of hydrological interactions in the shallow subsoil, and for the quantification of the soil-plant-atmosphere mass exchanges.

Improvement in time‐domain induced polarization data quality with multi‐electrode systems by separating current and potential cables

ABSTRACTMeasuring induced polarization in the time domain with relatively compact multi‐channel multi‐electrode systems is attractive because of the simplicity of the procedure and thus its efficiency in the field. However the use of this technique is sometimes discouraged by the bad quality of the measurements in cases of high electrode contact resistances that can render data interpretation infeasible or at least unreliable. It is proposed that capacitive coupling in the multi‐core electrode cables has a significant role in creating this problem.In such cases separation of current and potential circuits by using separate multi‐conductor cable spreads can yield significant improvement in data quality. The procedure is relatively simple and can be implemented with common resistivity and time‐domain IP equipment.We show here three field examples from Southern Sweden, all measured as 2D electrical imaging sections. The first one is an example where the use of a single cable spread is sufficient thanks to moderate electrode contact resistance and high signal levels. The following two examples are from sites where induced polarization measurements could not yield consistent results using only a single multi‐conductor cable spread. Useful results were subsequently obtained by using separate cable spreads.The first example is a 280 m long line measured over an old covered municipal waste deposit where the waste body stands out as a zone of high chargeability. The second example is a 120 m line measured on a sandy glaciofluvial structure that is host to an aquifer of regional importance. The improvement led to discrimination between materials of different grain sizes, with potential bearing for understanding the aquifer. The third example is a 300–400 m line measured across an esker lying on clay till. The improvement led to a clear visualization of the esker and to the identification of a possible fault in the underlying gneissic bedrock.In all cases pseudosections and examples of chargeability decay curves are shown and discussed as tools for assessing data quality. Inversion results are shown together with background geological information and it is concluded that they are in good agreement.

A methodology for converting traditional vertical electrical soundings into 2D resistivity models: Application to the Saïss basin, Morocco

We demonstrated a simple methodology for the reinterpretation of sets of electrical vertical soundings and the production of 2D resistivity models. The method is applied to the northern central part of the subsiding Saïss basin (Morocco) where a series of vertical electrical soundings (VES) were acquired in 1982 and 1984, using the Schlumberger array with current electrodes semispacings between 3 and 3000 m. We describe a methodology used to estimate and sample apparent resistivities collected from traditional single soundings acquired by using a Schlumberger configuration. We then construct a resistivity pseudosection in a format compatible with a Wenner-Schlumberger array to perform 2D electrical tomography. The methodology takes into account the lateral and vertical spatial correlations shown by the apparent resistivities. The tomographic inversions are performed using a commercial inversion software commonly used in industry. Geostatistical tools are used to quantify the spatial correlation between apparent resistivity data. The result is a series of estimated pseudosections that are inverted to produce 2D resistivity models of the basin along geophysical profiles. We focus our attention on one of these profiles (i.e., one 2D resistivity model). The inverted geoelectrical model allows us to propose a geologic model of the basin. Our methodology allows historical and current data carried out using the Schlumberger configuration, to be rapidly reinterpreted in countries where only low-cost methods, such as VES soundings, can be used due to technology and economic constraints. It can also be applied to cases where continuous or pulled-array continuous vertical soundings method, where a tail of electrodes is towed behind a small vehicle while continuously and simultaneously measuring several electrode configurations, is inoperative due to field conditions (survey length, deep layers, and local environmental conditions).

Imageamento Elétrico 3D em área contaminada por hidrocarbonetono polo industrial de Cubatão - SP

This work presents the results of geophysical surveys performed over an oil contaminated site in the Polo Industrial de Cubatao - Sao Paulo. The aim is to characterize geoelectrical signatures associated to hydrocarbon presence in order to delimit and calculate the volume of the contaminated area. For this study were used Vertical Electrical Sounding (VES) and 3D Electrical Imaging for characterization of geology and geoelectrical response of the contaminant. The results showed that the hydrocarbon presence is associated to conductive anomalies due to products from biodegradation. The conductive anomalies are disseminated over the area, totalizing 1365.3 m3 volume. This volume, however, corresponds only to the residual phase contaminants, since it was not possible to map free-phase hydrocarbons.

An evaluation of optimization strategies to automatically select the optimal set of array configurations for 2D electrical imaging surveys

This paper presents results of tests involving four different strategies used to automatically select the optimal set of array configurations that generate the maximum amount of information regarding a subsurface with 2D electrical imaging surveys invoking the same number of measurements. The first strategy (Compare R) calculates the improvement in the model resolution for each array configuration added to the set of measurements. This strategy generates the best results but requires the greatest amount of time to complete. The second (BGS) and third (ETH) strategies use linear approximations to calculate a goodness function. Both strategies are less time consuming than the Compare R strategy; however, the BGS strategy produces array configurations with model resolution values that are slightly less accurate than the Compare R strategy, while the ETH strategy produces results of the poorest quality. The fourth strategy (Combined BGS–CR) uses a combination of the BGS and Compare R strategies and produces results that are almost as accurate as the Compare R strategy but requires significantly less computing time. The four optimization strategies were tested using model resolution calculations, synthetic models and field surveys. The results obtained using the different optimization strategies were also compared with conventional arrays. The various tests show that arrays selected via the optimization strategies result in subsurface inversion models that have better resolution and identify more accurate depths and shapes compared to conventional arrays.

Fast computation of optimized electrode arrays for 2D resistivity surveys

Four different methods to automatically select an optimal set of array configurations that gives the maximum subsurface resolution with a limited number of measurements for 2D electrical imaging surveys were tested. The first (CR) method directly calculates the change in the model resolution for each new array added to the base data set, and uses this to select array configurations that gave the maximum model resolution. However this method is the slowest. The algorithm used by the CR method for calculating rank-one updates was optimized to reduce computational time by a factor of eighty. The sequence of calculations was modified to reduce the traffic between the computer main memory and the CPU registers. Further code optimizations were made to take advantage of the parallel processing capabilities of modern CPUs. The second (ETH) and third (BGS) methods use approximations based on the sensitivity values to estimate the change in the model resolution matrix. The ETH and BGS methods, respectively, use the first and second power of the sensitivity values to calculate approximations of the model resolution. Both methods are about an order of magnitude faster than the CR method. The results obtained by the BGS method are significantly better than the ETH method, and it approaches that of the CR method. The fourth method (BGS–CR) uses a combination of the BGS and CR methods. It produces results that are almost identical to the CR method but is several times faster. The different methods were tested using data from synthetic models and field surveys. The models obtained from the inversion of the data sets generated by the four different methods confirm that the models generated by the CR method have the best resolution, followed by the BGS–CR, BGS and ETH methods.

Characterization of seawater intrusion using 2D electrical imaging

ABSTRACTWe have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater‐freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in‐situ and laboratory‐derived petrophysical relationships. Surface‐to‐hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results and hence, surface‐to‐hole images are not easily interpretable in terms of larger‐scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non‐stationarity of the petrophysical relationship during the imaging process.

Multiple superimposed probability tomography on a second electrical field

Based on the requirement of probability tomography, we developed two-directional pole–pole measurement on a 2D electrical imaging survey which is able to extract all second electrical field profiles to participate in probability tomography. The concept of charge occurrence probability (COP) is defined as the average of cross-correlation of the space domain scanning (SDS) function with the second electrical field being generated by a whole set of point current sources. To quantitatively analyse the results of the tomography, the COP values are normalized. Finally, in order to test the effectiveness of the tomography, we use a finite element algorithm to synthesize the potential data for 2D models, and the profiles of the second electrical field are extracted by eliminating the primary field from the total field. Using the second electrical field of all profiles, the geometric patterns of normalized COP (NCOP) values are reconstructed by means of multiple superimposed probability tomography. The results are quite satisfactory.

The use of 2D electrical tomography to assess pollution in slurry ponds of the Murcia region, SE Spain

A large quantity of pig slurry is produced in the Murcia Province, raising serious environmental concern. Monitoring the evolution of the slurry ponds with time in the subsoil is a prime requirement for environmental safety. Our aim was to determine the temporary effects of pig slurry ponds on the subsoil in the Murcia Province to establish the degree of slurry pond infiltration in semi‐arid climates. A non‐destructive, geophysical, 2D electrical tomography technique was used to: 1) monitor the vertical movement of pig slurry into the subsoil and 2) determine the possible depth of this movement. The results of our studies showed that the method works well. Areas affected by pig slurries have, indeed, developed environmental problems over time. The method identified the lithological layers with sufficient resolution in order to study the possible pollution of the soils by the slurry, including salts. The method is also capable of monitoring seasonal changes and the time‐dependent behaviour of the polluting plume within different subsoils in the ponds.

Optimisation of electrode arrays used in 2D resistivity imaging surveys

SummaryThis paper describes four different methods to automatically select the optimal set of array configurations that give the maximum resolution of the subsurface with 2D electrical imaging surveys for a limited of measurements. The first method (CR method) directly calculates the improvement in the model resolution for each new array configuration added to the set of measurements. It gives the best results but is the slowest. The second (BGS) and third (ETH) methods use linear approximations (using the second and first power of the sensitivity values respectively) to estimate the change in the model resolution matrix. Both methods are about an order of magnitude faster than the first method. The BGS method produces array configurations with model resolution values that approaches the results obtained by the CR method, while the ETH method gives significantly poorer results. The fourth method uses a combination of the BGS and CR methods. It produces results that are almost identical to the CR method but is several times faster. The different methods were tested using data from synthetic models and field surveys. The results show that the optimised array configuration data sets significantly improve the subsurface resolution achieved by the survey compared to conventional arrays such as the Wenner, Schlumberger and dipole-dipole arrays.

Imagerie combinée géoélectrique–radar géologique des cavités souterraines de la ville de Zaouit Ech Cheikh (Maroc)

The basement of the Zaouit Ech Cheikh city shelters a dense network of underground cavities. These sub-surface voids make many buildings or infrastructures unstable or cause them to collapse. To test the efficiency of geophysical methods in detecting cavities, 2D electrical tomography and ground-penetrating radar (GPR) studies were carried out along the main streets of the city. The obtained results reveal the existence of a large number of underground cavities throughout the investigated area. They also show the interest of such combined geophysical methods as tools for mapping underground holes in an urban area.

3D electrical imaging of an archaeological site using electrical and electromagnetic methods

Floridablanca is an 18th century archaeological site located in southern Argentina. Archaeological investigations at the site began in 1998, and in 2000 we started a project to perform geophysical studies there. In this paper, we report the implementation of electrical and electromagnetic (EM) methods in a sector of the site that corresponds to the settlers' houses. The objective of the project was to characterize the zone and the buried archaeological structures (adobe walls, tiles from a collapsed roof) with 2D and 3D electrical and EM techniques. We first applied an EM induction method covering a 600-[Formula: see text] area with a frequency ranging from 1000 to 19 000 Hz. A 3D visualization of the in-phase and quadrature components gave an initial description of anomalies possibly associated with buried structures. We then performed dipole-dipole profiles and inverted the data to obtain the corresponding 2D and 3D electrical images. Finally, after correlating the information obtained from the analysis of both EM and electrical data, we performed a more localized 3D dipole-dipole mesh (25 [Formula: see text]) to achieve the final electrical image of the most representative buried structure. The combination of both techniques allowed us to map two entire houses and to identify three types of walls: main, separating, and inner. These results have been confirmed by an archaeological excavation.

Integrated geophysical studies at ancient Itanos (Greece)

The results of an integrated geophysical survey at the archaeological site of Itanos (Crete) are presented and discussed. At Hellenistic Itanos, which is located near the seashore, the buried ruins are partially under the saline water table. The purpose of this geophysical survey was to map buried relics of buildings, streets and walls of ancient Itanos. In particular, 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. Magnetic gradient, soil resistance and electromagnetic measurements were taken in grids covering an area of approximately 16,000 m 2. 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 wavenumber domain. Emphasis is given to the mathematical description of the processing steps. The integration of the geophysical measurements revealed the existence of a well and three parallel walls verified by excavation. Ground probing radar and electrical tomography sections crossing the parallel walls showed the vertical extent of these features. 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. A shallow seismic survey was carried out, in an effort to locate and map the ancient port. The combination of a wide spectrum of geophysical prospecting techniques was successful in delineating the buried relics, enhancing the information context regarding the Hellenistic settlement of Itanos, together with its port and necropolis. The geophysical survey at Itanos demonstrates that the integrated approach is appropriate for archaeological investigations at complex sites like Itanos.

Image processing of 2D resistivity data for imaging faults

A methodology to locate automatically limits or boundaries between different geological bodies in 2D electrical tomography is proposed, using a crest line extraction process in gradient images. This method is applied on several synthetic models and on field data set acquired on three experimental sites during the European project PALEOSIS where trenches were dug. The results presented in this work are valid for electrical tomographies data collected with a Wenner-alpha array and computed with an l 1 norm (blocky inversion) as optimization method. For the synthetic cases, three geometric contexts are modelled: a vertical and a dipping fault juxtaposing two different geological formations and a step-like structure. A superficial layer can cover each geological structure. In these three situations, the method locates the synthetic faults and layer boundaries, and determines fault displacement but with several limitations. The estimated fault positions correlate exactly with the synthetic ones if a conductive (or no superficial) layer overlies the studied structure. When a resistive layer with a thickness of 6 m covers the model, faults are positioned with a maximum error of 1 m. Moreover, when a resistive and/or a thick top layer is present, the resolution significantly decreases for the fault displacement estimation (error up to 150%). The tests with the synthetic models for surveys using the Wenner-alpha array indicate that the proposed methodology is best suited to vertical and horizontal contacts. Application of the methodology to real data sets shows that a lateral resistivity contrast of 1:5–1:10 leads to exact faults location. A fault contact with a resistivity contrast of 1:0.75 and overlaid by a resistive layer with a thickness of 1 m gives an error location ranging from 1 to 3 m. Moreover, no result is obtained for a contact with very low contrasts (∼1:0.85) overlaid by a resistive soil. The method shows poor results when vertical gradients are greater than horizontal ones. This kind of image processing technique should be systematically used for improving the objectiveness of tomography interpretation when looking for limits between geological objects.

2D Electrical Imaging of an LNAPL Contamination, Al Amiriyya Fuel Station, Jordan

The detection of organic contaminants such as light non-aqueous phase liquid (LNAPL) in the subsurface using geophysical methods, particularly electrical resistivity methods has been the subject of considerable interest in recent years. Their detection is based principally upon the electrical properties of the hydrocarbons. During 2002, direct current resistivity data were collected with the Iris Syscal R2 resistivity instrument at the location of Al Amiriyya fuel station site, Jordan. The objective of this study was to evaluate the resolution of resistivity techniques in detecting and locating anomalies of organic contamination. Resistivity measurements were carried out utilizing the Wenner and dipole-dipole array configurations in order to achieve good vertical and lateral resistivity distributions for the investigated site. The interpretations obtained from 2D-modelling of the field data show a highly conductive region in areas with LNAPL contamination. This explanation was supported by the presence of hydrocarbon sheens floated on the surface of water filling the subsidence feature and by excavation works. Correlation between field results obtained shows that the dipole-dipole array is the most reliable since it showed details over the fuel station site that were poorly indicated by the Wenner array.

Anisotropic resistivity inversion

We present an inversion method for 3D electrical imaging in media with aninhomogeneous and anisotropic conductivity distribution. The conductivitydistribution is discretized via finite elements and is described by a second-ordertensor at each finite element node. The inversion method is formulated as afunctional optimization with an error functional containing terms measuringdata misfit and model covariance by means of smoothness, anisotropyand deviation from a starting model. Including the model covarianceinformation overcomes the problem of ill-posedness at the expense oflimiting the allowed models to the class of models which are compatible withthe provided model covariance information. The discretized form of theerror functional is minimized by a Levenberg–Marquardt type methodusing an iterative preconditioned conjugate gradient solver. The use ofan iterative solver allows one to bypass the actual computation of theJacobian or an inverse system matrix. The use of a memory efficient iterativesolver together with the implementation on parallel computers allowslarge-scale inverse problems, comprising several hundred thousand nodes withhundreds of sources and receivers, to be solved. The new method is testedusing computer-generated data from two- and three-dimensional syntheticmodels. For each inversion a choice of penalty parameters, gauging thelevel of model covariance information imposed, has to be made and thelevel of regularization required is hard to estimate. We find that runninga suite of inversions with varying penalty parameters and subsequentexamination of the results (including inspection of residual maps) offers a viablemethod for choosing appropriate numerical values for the penalty levels. Inthe applications we found the inversion process to be highly non-linear.Inversion models from intermediate steps of the iterative inversion showstructure in places that do not exhibit structure in the true model andonly at later iterations do anomalies move to the correct location in themodelling domain. This result indicates that linearized inversions that fail tore-linearize during the inversion process will fail to find meaningful inversionimages. The inversion images achieved using the new method recoverthe important features of the true models, including the approximatemagnitudes of the conductivity anomalies and the magnitudes and directions ofanisotropy anomalies. The inversion images are generally ‘blurred’, that issharp edges are smoothed, and the recovered magnitudes of conductivity,anisotropy and anisotropy direction are generally under-estimated.

A comparison of smooth and blocky inversion methods in 2D electrical imaging surveys

Two-dimensional electrical imaging surveys are now widely used in engineering and environmental surveys to map moderately complex structures. In order to adequately resolve such structures with arbitrary resistivity distributions, the regularised least-squares optimisation method with a cell-based model is frequently used in the inversion of the electrical imaging data. The L norm based least-squares optimisation method that attempts to minimise the sum of squares of the spatial changes in the model resistivity is often used. The resulting inversion model has a smooth variation in the resistivity values. In cases where the true subsurface resistivity consists of several regions that are approximately homogenous internally and separated by sharp boundaries, the result obtained by the smooth inversion method is not optimal. It tends to smear out the boundaries and give resistivity values that are too low or too high. The blocky or L norm optimisation method can be used for such situations. This method attempts to minimise the sum of the absolute values of the spatial changes in the model resistivity. It tends to produce models with regions that are piecewise constant and separated by sharp boundaries. Results from tests of the smooth and blocky inversion methods with several synthetic and field data sets highlight the strengths and weaknesses of both methods. The smooth inversion method gives better results for areas where the subsurface resistivity changes in a gradual manner, while the blocky inversion method gives significantly better results where there are sharp boundaries. While fast computers and software have made the task of interpreting data from electrical imaging surveys much easier, it remains the responsibility of the interpreter to choose the appropriate tool for the task based on the available geological information.

Application of integrated magnetic resonance sounding and resistivity methods for borehole implementation. A case study in Cambodia

A geophysical survey was conducted in Cambodia to measure the contribution of geophysics to a running drilling programme. The geology of the area (the province of Siem Reap) mainly consists of recent heterogeneous sediments of sand, silt and clay. The thickness of this formation ranges from 20 to 100 m and lies on Jurassic to Tertiary rocks. Surveys were done with direct current methods [one-dimensional vertical electrical sounding (1D VES), and two-dimensional (2D) electrical imaging], time domain electromagnetic (TDEM) sounding and proton magnetic resonance sounding (MRS). To validate the geophysical results, boreholes were drilled and tested with electrical logs and pumping tests. We found that: (1) The resistivity methods (VES, 2D electrical imaging and TDEM) are very sensitive to the groundwater electrical conductivity which is highly heterogeneous within the province. A preliminary relationship between measured groundwater conductivity and aquifer resistivity is proposed. (2) The MRS gives accurate information on groundwater occurrence for the 5- to 60-m-deep layers. A preliminary relationship between MRS data (aquifer transmissivity estimated from MRS field measurements) and hydrodynamic parameters (aquifer local transmissivity and borehole relative specific capacity estimated from borehole pumping tests) is proposed. (3) The resistivity methods and MRS are complementary, and a joint use is recommended. (4) At the survey scale, the borehole success rate was improved from 56% to 90% by the use of geophysics. Crossing the technical and cost analyses, we propose a geophysical methodology to implement boreholes in the province of Siem Reap. This methodology could both increase the borehole success rate and save money at the programme scale. It consists of the use of: 1. MRS, TDEM and electrical methods jointly where the borehole success rate is less than 30%. 2. MRS and TDEM jointly where the borehole success rate ranges from 30% to 50%. 3. Electrical methods (VES and 2D electrical imaging) as standard methods.