Imaging artificial salt water infiltration using electrical resistivity tomography constrained by geostatistical data

in Journal of Hydrology (2012), 438-439

Electrical resistivity tomography is a well-known technique to monitor fresh-salt water transitions. In such environments, boreholes are often used to validate geophysical results but rarely used to ... [more ▼]

Electrical resistivity tomography is a well-known technique to monitor fresh-salt water transitions. In such environments, boreholes are often used to validate geophysical results but rarely used to constrain the geoelectrical inversion. To estimate the extent of salt water infiltration in the dune area of a Natural Reserve (Westhoek, Belgium), electrical resistivity tomography profiles were carried out together with borehole electromagnetic measurements. The latter were used to calculate a vertical variogram, representative of the study site. Then, a geostatistical constraint, in the form of an a priori model covariance matrix based on the variogram, was imposed as regularization to solve the electrical inverse problem. Inversion results enabled to determine the extension of the salt water plume laterally and at depth, but also to estimate the total dissolved solid content within the plume. These results are in agreement with the hydrogeological data of the site. A comparison with borehole data showed that the inversion results with geostatistical constraints are much more representative of the seawater body (in terms of total dissolved solids, extension and height) than results using standard smoothness-constrained inversion. The field results obtained for the Westhoek site emphasize the need to go beyond standard smoothness-constrained images and to use available borehole data as prior information to constrain the inversion. [less ▲]

Image appraisal tools for electrical resistivity tomography

in Proceedings of SAGEEP (2011), 24

Image appraisal is a problem frequently encountered in electrical resistivity tomography (ERT), and more generally in non-linear geophysical inversion. It may include several aspects such as the ... [more ▼]

Image appraisal is a problem frequently encountered in electrical resistivity tomography (ERT), and more generally in non-linear geophysical inversion. It may include several aspects such as the identification of the geometry of buried structures, the detection of numerical artefacts, the estimation of the depth of investigation or the exactitude of inverted parameters. Geophysicists can rely on several tools published in the literature to address these issues. However, few studies offer a quantitative comparison on the performance of these tools concerning the different mentioned aspects. Moreover, to our knowledge, there is no commonly accepted methodology to handle image appraisal. In this contribution, we compared quantitatively the ability of different image appraisal indicators to reach different objectives (geometry, artefacts, depth of investigation, parameter resolution). Among possible image appraisal tools, the model resolution matrix (MRM), the cumulative sensitivity matrix (CSM) and the depth of investigation index (DOI) are the most cited ones and were studied here. We compared them first on numerical models representing different geological situations. This numerical benchmark showed that indicators based on the MRM and CSM were the more appropriate to appraise ERT images in terms of the geometry of structures and the exactitude of inverted parameters, DOI providing mainly qualitative information. On this basis, we propose a methodology to appraise field ERT images focusing on the resolution and geometric aspects (others being implicitly studied). First, True Synthetic Models (TSM), representing simplified cases of field ERT images, are built using available information. Then, through forward modelling, synthetic ERT data are computed and inverted to provide the Inverted Synthetic Models (ISM). Afterwards, a comparison between TSM and ISM (or their gradients for geometry) is made in order to define the errors on inverted parameters. This discrepancy is then plotted with respect to resolution indicator values and points out in every tested cases a resolution range over which the errors abruptly increase allowing the definition of threshold values. The final step consists in applying the threshold values on the field ERT images and to validate the results with a posteriori knowledge. [less ▲]

Joint and sequential inversion of geophysical and hydrogeological data to characterize seawater intrusion models

in Teresa Condesso de Melo, Maria; Lebbe, Luc; Virgílio Cruz, José (Eds.) et al Proceedings SWIM21, 21st Salt Water Intrusion Meeting (2010, June)

The integrated water resource management problems require studying efficiently seawater intrusion at local and regional scale and identifying in real time the seawater/freshwater interface dynamic ... [more ▼]

The integrated water resource management problems require studying efficiently seawater intrusion at local and regional scale and identifying in real time the seawater/freshwater interface dynamic. Hydrogeological modeling is widely used to predict seawater intrusion if additional natural or man‐made factors are modified. These models are currently calibrated using measured heads and salt mass fractions in boreholes, which generally result in sparse data coverage. Within this scope, non to minimally invasive geophysical techniques like electrical resistivity tomography (ERT) or time‐domain electromagnetic method are becoming increasingly popular, given their relatively greater spatial horizontal resolution compared to borehole observations. We present a comparison between both a sequential and joint approach to calibrate seawater intrusion models using ERT. The former consists of constraining hydrogeological parameters using ERT derived parameters and relies on sequential inversions of the geophysical and hydrogeological data using a given petrophysical relationship. The second approach in based on simultaneous inversion of petrophysical and hydrogeological data using electrical resistance data as data. It is performed by coupling an inversion code with two hydrogeological and geophysical modeling codes through a petrophysical conversion. This investigation was performed on a densitydependent flow and transport numerical (three/two‐dimensional) simulation study from complex and realistic heterogeneous models. In the sequential approach, the simulations showed that only the shallow salt concentration of the seawater/freshwater transition zone could be recovered for different time‐lapse, due to poorly resolved regions in depth. The capability of image appraisal indicators (cumulative sensitivity and resolution) has been analyzed to emphasize the discrepancy between the targeted and imaged parameter values. On the other hand, the preliminary coupled inversion avoids the regularization bias introduced by ERT and addresses the non‐stationarity of the petrophysical relationship. [less ▲]

Methodology and setup of the adopted groundwater vulnerability assessment method

Scientific conference (2010, February 04)

Prospection géophysique de la nappe alluviale dans la région de Nasso, Bodo – Dioulasso (Burkina Faso)

Report (2009)

Afin d’appuyer la gestion et la protection des ressources en eaux souterraines au Burkina Faso, dans la région de Bobo-Dioulasso, une mission géophysique relative à la caractérisation du site de la plaine ... [more ▼]

Afin d’appuyer la gestion et la protection des ressources en eaux souterraines au Burkina Faso, dans la région de Bobo-Dioulasso, une mission géophysique relative à la caractérisation du site de la plaine alluviale dans la localité de Nasso a été effectuée. Plusieurs méthodes géophysiques ont été employées afin de déterminer l’hétérogénéité des terrains superficiels, l’épaisseur de la nappe alluviale, ainsi que la profondeur du bedrock. Les trois méthodes déployées pour atteindre ces objectifs sont la tomographie électrique, le bruit de fond H/V, et le radar géologique. Ce dernier n’a pas donné de résultats satisfaisant tandis que les deux autres méthodes se sont montrées concluantes pour déterminer la profondeur du bedrock et détecter des hétérogénéités dans les terrains superficiels et profonds. [less ▲]

Physically-based groundwater vulnerability assessment using sensitivity approach: Synthetic and real case studies based on the Israeli test-site

Conference (2009, April 28)

DL 45 : Application and validation of the general methodology and concept of groundwater vulnerability assessment at regional scale on the Israeli coastal aquifer

Report (2008)

This deliverable consists in applying the concepts and methodologies developed in Deliverable D43 and D44 to synthetic and real case studies. Our researches have focused on the generalized groundwater ... [more ▼]

This deliverable consists in applying the concepts and methodologies developed in Deliverable D43 and D44 to synthetic and real case studies. Our researches have focused on the generalized groundwater vulnerability assessment methodology, and more precisely on evaluating, under the framework of physically-based indicators, the groundwater sensitivity/vulnerability to stress factor considering artificial recharge as a potential response to the degradation of the groundwater resource. In the context, different approaches have been identified in the literature and implemented in appropriate modelling tools (i.e. HydroGeoSphere) for calculating the various sensitivity/vulnerability coefficients. These approaches are the influence coefficient method, the sensitivity equation method and the adjoint operator method. The two first methods show relevant results on both the considered synthetic case studies that relate groundwater vulnerability to (1) quantity issues and (2) to sea water intrusion. They illustrate the way of applying the methodology to “real case studies”.These first applications should be the object of more complex but strongly related case studies. [less ▲]

Evaluation and ranking of different artificial recharge scenarii for controlling salt water intrusion in the Israeli coastal aquifer

Master's dissertation (2007)

Coastal aquifers are under a continuous threat of pollution due to the danger of sea water intrusion. In many cases, this threat has become a reality, as a result of excessive pumping beyond some safe ... [more ▼]

Coastal aquifers are under a continuous threat of pollution due to the danger of sea water intrusion. In many cases, this threat has become a reality, as a result of excessive pumping beyond some safe yield. Israeli coastal aquifer is a good example. In some places around the world, existing wells, especially close to the coast, have to be abandoned, and the available fresh water of the coastal aquifer decreases. Also, the area above the intruding sea water wedge is lost as a source of water by natural recharge or natural replenishment. Artificial recharge may constitute a response against seawater intrusion. Because of over exploitation of pumping wells, hydraulic depressions appeared along the coast and seawater intrusion advanced inland. The topic of the thesis merges two methodologies linked to these problems. The first one aims the evaluation of different artificial recharge scenarii for controlling sea water intrusion in the Israeli coastal aquifer. Thus, two sites have been selected to apply the first methodology, namely Tel Aviv/Yafo city and Ashqelon town. Secondly, it concerns a new method to assess the vulnerability of pumping/injection wells facing with sea water intrusion and finally for ranking the different mentioned artificial recharge scenarii. The first of both of them is applied with the use of VASP and FEAS software including the existing model of the Israeli coastal aquifer and the second includes the DPSIR concept. VASP is a piece of software for the generation and post-processing of models in addition to the analysis and visualization of environmental data. FEAS is a piece of software that is based on the finite elements techniques, developed for simulating sea water intrusion in two- and three-dimensional coastal aquifers, and upconing problems. The second approach has been developed in prospect to be applied later once the necessary tools would be available. [less ▲]