R.D. Ogilvy

Electrical resistivity tomography applied to geologic, hydrogeologic, and engineering investigations at a former waste-disposal site

A former dolerite quarry and landfill site was investigated using 2D and 3D electrical resistivity tomography (ERT), with the aims of determining buried quarry geometry, mapping bedrock contamination arising from the landfill, and characterizing site geology. Resistivity data were collected from a network of intersecting survey lines using a Wenner-based array configuration. Inversion of the data was carried out using 2D and 3D regularized least-squares optimization methods with robust (L1-norm) model constraints. For this site, where high resistivity contrasts were present, robust model constraints produced a more accurate recovery of subsurface structures when compared to the use of smooth (L2-norm) constraints. Integrated 3D spatial analysis of the ERT and conventional site investigation data proved in this case a highly effective means of characterizing the landfill and its environs. The 3D resistivity model was successfully used to confirm the position of the landfill boundaries, which appeared as electrically well-defined features that corresponded extremely closely to both historic maps and intrusive site investigation data. A potential zone of leachate migration from the landfill was identified from the electrical models; the location of this zone was consistent with the predicted direction of groundwater flow across the site. Unquarried areas of a dolerite sill were imaged as a resistive sheet-like feature, while the fault zone appeared in the 2D resistivity model as a dipping structure defined by contrasting bedrock resistivities.

Noninvasive monitoring of DNAPL migration through a saturated porous medium using electrical impedance tomography.

Electrical impedance tomography (EIT) was used to monitor the movement of a fluorinated hydrocarbon dense nonaqueous phase liquid (DNAPL) through a saturated porous medium within a laboratory column. Impedance measurements were made using a horizontal plane of 12 electrodes positioned at regular intervals around the centre of the column. A 2D inversion algorithm, which incorporated the cylindrical geometry of the column, was used to reconstruct resistivity and phase images from the measured data. Differential time-lapse images of DNAPL movement past the plane of electrodes were generated by the cell-by-cell subtraction of resistivity and phase baseline models from those associated with the DNAPL release stage of the experiment. The DNAPL pulse was clearly delineated as resistive anomalies in the differential time-lapse resistivity images. The spatial extent of the resistive anomalies indicated that in addition to vertical migration, some lateral spreading of the DNAPL had occurred. Residual contamination could be detected after quasi-static conditions were reestablished. Residual DNAPL saturation was estimated from the resistivity model data by applying Archies second equation.Despite significant measured phase changes due to DNAPL contamination, the differential phase images revealed only weak anomalies associated with DNAPL flow; these anomalies could be seen only in the initial stages of the experiment during peak flow through the plane of electrodes.

Automated monitoring of coastal aquifers with electrical resistivity tomography

An Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the long-term monitoring of coastal aquifers. This ALERT system has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and land-use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry riverbed with electrode take-outs at regular intervals of 10 m. The maximum depth of investigation is about 160 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Volumetric geoelectric images of the subsurface can be obtained ‘on demand’ or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is fully automated and seamless. The ALERT technology can provide early warning of potential threats to vulnerable water systems such as over-exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater-freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.

Characterization of seawater intrusion using 2D electrical imaging

We 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.

Fundamentals of the capacitive resistivity technique

Capacitive resistivity (CR) is an emerging geophysical technique designed to extend the scope of the conventional methodology of dc resistivity to environments where galvanic coupling is notoriously difficult to achieve — for example, across engineered structures (roads, pavements), hard rock, dry soil, or frozen ground. Conceptually, CR is based on a four-point array capacitively coupled to the ground. Under certain conditions, capacitive measurements of resistivity are equivalent to those obtained with the dc technique, thus making dc interpretation schemes applicable to CR data. The coupling properties of practical sensor realizations are shown to be a function of their geometrical arrangement. Separate bodies of theory are associated with two complementary but distinct sensor types: the capacitive-line antenna and the plate-wire combination. The use of plate-wire combinations results in localized coupling, which, in conjunction with a quasi-static (low-frequency) formulation of the transfer impedance,...

The Use of 3D Electrical Resistivity Tomography to Characterise Waste and Leachate Distribution within a Closed Landfill, Thriplow, UK

Electrical Resistivity Tomography (ERT) has been used to map the 3D spatial distribution of waste and leachate concentrations within a closed and unconfined landfill. Borehole sampling and a 2D ERT survey down-gradient of the landfill boundary had failed to detect a pollution plume. Accordingly, a 3D survey was undertaken to determine the pattern of leachate drainage within the waste so that a more refined contaminant transport model could be developed. A full 3D survey was undertaken by sub-dividing the landfill into a number of discrete rectangular blocks and acquiring data on multiple parallel lines. The line data were merged into a single x-y matrix file and then inverted using a 3D finite element algorithm. The results are presented as 3D volumetric tomograms to show the inferred waste distribution and leachate flow-paths. The resistivity models indicate that leachate has accumulated at several discrete localities within the landfill. The controlling mechanism appears to be the depth and geometry of ...

Hydrogeophysical imaging of deposit heterogeneity and groundwater chemistry changes during DNAPL source zone bioremediation

Robust characterization and monitoring of dense nonaqueous phase liquid (DNAPL) source zones is essential for designing effective remediation strategies, and for assessing the efficacy of treatment. In this study high-resolution cross-hole electrical resistivity tomography (ERT) was evaluated as a means of monitoring a field-scale in-situ bioremediation experiment, in which emulsified vegetable oil (EVO) electron donor was injected into a trichloroethene source zone. Baseline ERT scans delineated the geometry of the interface between the contaminated alluvial aquifer and the underlying mudstone bedrock, and also the extent of drilling-induced physical heterogeneity. Time-lapse ERT images revealed major preferential flow pathways in the source and plume zones, which were corroborated by multiple lines of evidence, including geochemical monitoring and hydraulic testing using high density multilevel sampler arrays within the geophysical imaging planes. These pathways were shown to control the spatial distribution of the injected EVO, and a bicarbonate buffer introduced into the cell for pH control. Resistivity signatures were observed within the preferential flow pathways that were consistent with elevated chloride levels, providing tentative evidence from ERT of the biodegradation of chlorinated solvents.

Optimization of Array Configurations and Panel Combinations for the Detection and Imaging of Abandoned Mineshafts using 3D Cross-Hole Electrical Resistivity Tomography

Cross-borehole electrical resistivity tomography was used to detect and image a concealed air-filled mineshaft at a greenfield test site. The measurement configurations and panel combinations were selected using a two-stage optimization process. An optimal set of array configurations was selected for each cross-borehole panel on the basis of the model resolution matrix. Subsequently, various combinations of panels were tested with synthetic and field data to determine the effects of coverage and data density on the resulting tomographic image. In the field trials, complicating factors were introduced by the use of resistive cement linings in the boreholes. A resistive feature was detected between the boreholes using a single panel and a 2.5D inversion, but the image quality was too poor to identify this as a mineshaft. A much-improved image was obtained using eight boreholes and eight panels with a full 3D inversion. Only four of these panels intersected the shaft. Crucially, the other panels provided coverage of outlying regions of the model, enabling the inversion algorithm to distinguish between the resistive effects of the borehole linings and the mineshaft.

Time-lapse Monitoring in Single Boreholes Using Electrical Resistivity Tomography

Electrical Resistivity Tomography (ERT) has been applied in single boreholes to study leachate generation within a disused and unconfined landfill. Several boreholes were drilled in the landfill to characterize the waste and to establish the concentration and spatial variability of the leachate. To assist this study, six of these boreholes were instrumented by installing an electrode array on the outside of the PVC casing prior to completion. Resistivity measurements were taken every month from December 1999 to November 2000 to monitor the seasonal hydrochemical changes in the Chalk immediately beneath the landfill base. The results have been modelled using a novel reconstruction scheme tailored to the requirements of single-hole data. Apart from the expected matrix infiltration, the results suggest that contaminant transport in the Chalk may also occur as transient pulses in narrow fractures, when the waste is re-activated by changing groundwater levels. The study highlights the advantages of a volumetri...

Measurement and inversion schemes for single borehole-to-surface electrical resistivity tomography surveys

Several measurement arrangements involving different electrode arrays are considered for the case of single borehole-to-surface ERT surveys. Synthetic data tests indicate that standard measuring and inversion techniques can result in reconstructed images that suffer from artefacts. Based on the measurement sensitivity analysis, we propose a modified inversion scheme that compensates for the symmetric sensitivity of the in-hole measurements and improves the quality of the image. The new scheme has been tested using both synthetic and field data. The best results were obtained using pole–dipole or dipole–dipole arrays. Finally, some recommendations are made to improve the efficiency of field measurement configurations.