Inversion of multi-temporal geoelectrical data sets: insights from several case studies

in Berichte der Geologischen Bundesanstalt (2012, September), 93

Inversion of multi-temporal geoelectrical field data sets: insights on noise characterization and regularization

Poster (2012, July 11)

Inversion of geoelectrical time-lapse data sets is increasingly growing as monitoring systems are being used in more applications such as seawater intrusion, landslides, remediation of contaminated sites ... [more ▼]

Inversion of geoelectrical time-lapse data sets is increasingly growing as monitoring systems are being used in more applications such as seawater intrusion, landslides, remediation of contaminated sites, landfill operation, shallow geothermal systems, or management of water resources. To date, several inversion strategies exist for taking into account the temporal dimension of the data. The most used nowadays are the independent inversion of multi-temporal data sets, the difference inversion, the temporally-constrained inversion, and the more recent process-based inversion. However, difference inversion schemes generally assume that part of the noise contained in the data cancels out when working with temporal data differences. Temporally-constrained inversion on the other hand assumes that the changes are localized and minor. Process-based inversion requires a more advanced knowledge of the system prior the inversion. In this study we demonstrate that the resolution of the time-lapse inversion scheme is mostly dependent on the quantification of the temporal behavior of the data error, on the resolution of the model-dependent pattern of the survey, and not on the regularization strategy. Our study is based on the imaging results of different data sets with different time and spatial scales, and with different degrees of geological complexity and resistivity contrast, The considered sites are a shallow sandy aquifer and a fractured hard rock aquifer where tracer experiments were performed and monitored using surface arrays. The two studied transport processes are advection, with velocities on the order of 10 m/hour and slower advection/diffusion processes. The strongest improvements were brought by using the data difference and a quantitative estimation of the data error. We found in particular a dependence of the time-lapse data error to the measured resistance (i.e., signal-to-noise-ratio), permitting to formulate an error model to describe the data error present in time-lapse data sets. We used minimum gradient support regularization to invert for model changes with enhanced contrast and found this technique more suited to time-lapse studies than for static images. Noise characterization and error models appear therefore as essential and the most impacting for a successful inversion both for static and time-lapse data whereas different spatio-temporal regularization techniques allowed to decrease artefacts but needs to be coherent with the process. [less ▲]

Geoelectrical investigations (DC) on a contaminated site during biostimulation: monitoring results and resolution analysis

Conference (2012, January 12)

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites ... [more ▼]

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites poses a risk to human or ecosystem, measures need to be taken to clean it up. Among these measures, methods using in situ bioremediation are beginning to become more important because of their ease of implementation and their relatively low cost. However, it is often difficult to ensure their effectiveness except by carrying out extensive drilling and sampling, which can be long and expensive while offering only punctual information. Thus, it becomes necessary to use other techniques to overcome these shortcomings. Recently, an increasing interest is being born to use geophysical methods as tools for remediation monitoring. As part of our work, we conducted several electrical resistivity tomography campaigns on a bus station located in Bassenge (Belgium) which has undergone a contamination of hydrocarbons (gasoline) for several years and on which a biostimulation remediation device was set up in order to clean it up. The aim of our investigations was to study the electrical response of the contaminated area during the remediation phase and whether electrical resistivity tomography allowed to monitor its effectiveness. After a year of monitoring, the time lapse images obtained show a significant decrease of electrical resistivities (up to -40%) during biostimulation at the location of the main contaminant plume and an increase again of resistivities from the time the biostimulation was stopped. The electrical response during the biostimulation is in agreement with the models presented by several authors in the literature. The increase again of resistivities after the stimulation is however more surprising and can be explained by several physico-chemical (sorption-desorption processes) or biological (decrease of conductive biofilms) assumptions. The results obtained tend to suggest that it is possible to use electrical resistivity tomography as a tool for qualitative control during the remediation of a contaminated site. However, for a more quantitative use of resistivity models, it is important to assess their reliability through the use of resolution indicators. We therefore developed a methodology to address this issue based on the creation of synthetic models representing simplified cases of field resistivities and we applied it on our case study. The results obtained provided us important information about the reliable parts of the resistivity models. These findings may lead in the future to the development of mathematical models that can link quantitatively geophysical properties to the level of (de)contamination of a site. [less ▲]

Geoelectrical monitoring on a contaminated site during biostimulation

Poster (2011, December 01)

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites ... [more ▼]

In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites poses a risk to human or ecosystem, measures need to be taken to clean it up. Among these measures, methods using in situ bioremediation are beginning to become more important because of their ease of implementation and their relatively low cost. However, it is often difficult to ensure their effectiveness except by carrying out extensive drilling and sampling, which can be long and expensive while offering only punctual information. Thus it becomes necessary to use other techniques to overcome these shortcomings. Recently, an increasing interest is being born to use geophysical methods as tools for remediation monitoring. As part of our work, we conducted several electrical resistivity tomography campaigns on a site contaminated by LNAPLs (gasoline) on which a biostimulation remediation device was set up. The aim of our investigations was to study the electrical response of the contaminated area during the remediation phase and whether electrical resistivity tomography allowed to monitor its effectiveness. After a year of monitoring, the time lapse images obtained show a significant decrease of the electrical resistivity (up to -50%) at the location of the main contaminant plume. This particular response during the biostimulation, in agreement with the models presented by several authors in the literature, tends to suggest that it is possible to use electrical resistivity tomography as a tool for qualitative control during the remediation. These findings may also lead in the future to the development of models to estimate more quantitatively the level of (de)contamination of a site. [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 ▲]

Use of image appraisal tools and covariance matrix

Conference (2010, July)

Report of geophysical investigations on the contaminated site of Maldegem

Report (2010)

Deliverable D2.5: Decision grid for best approach in terms of modelling concepts/contaminants

Report (2008)