Heat as a proxy to image dynamic processes with 4D electrical resistivity tomography

Since salt cannot always be used as a geophysical tracer (because it may pollute the aquifer with the mass that is necessary to induce a geophysical contrast), and since in many contaminated aquifer salts (e.g., chloride) already constitute the main contaminants, another geophysical tracer is needed to force a contrast in the subsurface that can be detected from surface geophysical measurements. In this context, we used heat as a proxy to image and monitor groundwater flow and solute transport in a shallow alluvial aquifer (< 10 m deep) with the help of electrical resistivity tomography (ERT). The goal of our study is to demonstrate the feasibility of such methodology in the context of the validation of the efficiency of a hydraulic barrier that confines a chloride contamination to its source. To do so, we combined a heat tracer push/pull test with time-lapse 3D ERT and classical hydrogeological measurements in wells and piezometers. Our results show that heat can be an excellent salt substitution tracer for geophysical monitoring studies, both qualitatively and semi-quantitatively. Our methodology, based on 3D surface ERT, allows to visually prove that a hydraulic barrier works efficiently and could be used as an assessment of such installations

ERT and the Location of Mining Cavities in Anisotropic Media: A Field Example

Often cave location requires the use of surface indirect techniques, such as geophysical methods. In particular, electrical methods have been applied to cavity exploration with evident success. However, as any other indirect methods, the use of these techniques has advantages and disadvantages. Cavities may be too small, too deep or masked by local geology to be detected. Nevertheless, indirect methods provide non-invasive, low cost and fast techniques to carry out the reconnaissance of an area where the presence of cavities is suspected. Complex geological conditions and formations anisotropy can induce strong orientational variation on ground resistivity measurements and, therefore, mask the presence of caves. Herein a field study in an old mining area demonstrates that 2D resistivity data—electrical resistivity tomography (ERT)—can be strongly affected by local anisotropy that masks the presence of cavities in ERT data modelling. In these cases, specific field strategies must be considered to overcome misleading interpretations and modelling, so that, meaningful results are obtained, uncertainty and interpretation ambiguity are reduced and the correct diagnosis of caves is accomplished

Capability of cross-hole electrical configurations for monitoring rapid plume migration experiments

Cross-hole electrical resistivity tomography is a useful tool in geotechnical, hydrogeological or fluid/gas plume migration studies. It allows better characterization of deep subsurface structures and monitoring of the involved processes. However, due to the large amount of possible four-electrode combinations between boreholes, the choice of the most efficient ones for rapid plume migration experiments (real-time monitoring), becomes a challenge. In this work, a numerical simulation to assess the capabilities and constraints of the most common cross-hole configurations for real-time monitoring is presented. Four-electrode configurations, sensitivity, dependence on the body location and amount of data were taken into account. The analysis of anomaly detection and the symmetry of the sensitivity pattern of cross-hole configurations allowed significant reduction of the amount of data and maintaining the maximum potential resolution of each configuration for real-time monitoring. The obtained results also highlighted the benefit of using the cross-hole AB-MN configuration (with both current - or potential - electrodes located in the same borehole) combined with other configurations with complementary sensitivity pattern

Optimized survey design for electrical resistivity tomography: combined optimization of measurement configuration and electrode placement

Within geoelectrical imaging, the choice of measurement configurations and electrode locations is known to control the image resolution. Previous work has shown that optimized survey designs can provide a model resolution that is superior to standard survey designs. This paper demonstrates a methodology to optimize resolution within a target area, while limiting the number of required electrodes, thereby selecting optimal electrode locations. This is achieved by extending previous work on the ‘Compare-R’ algorithm, which by calculating updates to the resolution matrix optimizes the model resolution in a target area. Here, an additional weighting factor is introduced that allows to preferentially adding measurement configurations that can be acquired on a given set of electrodes. The performance of the optimization is tested on two synthetic examples and verified with a laboratory study. The effect of the weighting factor is investigated using an acquisition layout comprising a single line of electrodes. The results show that an increasing weight decreases the area of improved resolution, but leads to a smaller number of electrode positions. Imaging results superior to a standard survey design were achieved using 56 per cent fewer electrodes. The performance was also tested on a 3-D acquisition grid, where superior resolution within a target at the base of an embankment was achieved using 22 per cent fewer electrodes than a comparable standard survey. The effect of the underlying resistivity distribution on the performance of the optimization was investigated and it was shown that even strong resistivity contrasts only have minor impact. The synthetic results were verified in a laboratory tank experiment, where notable image improvements were achieved. This work shows that optimized surveys can be designed that have a resolution superior to standard survey designs, while requiring significantly fewer electrodes. This methodology thereby provides a means for improving the efficiency of geoelectrical imaging