P.I. Meldrum

Monitoring Saline Intrusion of a Coastal Aquifer with Automated Electrical Resistivity Tomography

An Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the remote long-term monitoring of vulnerable earth systems. This ALERT technology 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 river bed. The maximum depth of investigation is about 150 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. Spatial 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 automated and seamless. The ALERT technology provides early warning of potential threats to the aquifer 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.

Geoelectrical Monitoring of Seasonal Moisture Content Changes in an Earth Embankment

Earth structures, such as embankments, require ongoing monitoring and maintenance to identify potential failure zones and to compensate for the effects of settlement. Extreme weather events leading to prolonged periods of desiccation or saturation are becoming more frequent and threaten embankment stability. In this paper we develop electrical resistivity tomography (ERT) as a non-invasive tool for characterising and monitoring earth embankments. A study is described in which ERT was applied alongside conventional intrusive techniques to investigate and monitor a section of Victorian Great Central Railway embankment, near Nottingham, UK. A number of modes of deployment were considered including linear 2D ERT arrays both parallel and perpendicular to the long-axis of the embankment and 3D imaging arrays. The resulting ERT images, when calibrated using intrusive geotechnical testing and core samples, revealed the spatial variability of the embankment soils. Parallel ERT sections were used to identify major discontinuities between material types at locations associated with poor track geometry. Perpendicular ERT sections also revealed significant internal heterogeneity, and were used to monitor seasonal changes in the moisture content within the embankment.

Comparison of fundamental modes of illumination for cross-hole electrical impedance tomography: Part II - Synthetic modelling

Cross-borehole electrical surveying, in which current and potential electrodes are placed downhole in two horizontally separated boreholes and moved over a range of depths, yields information about the variation of electrical properties in the subsurface between the boreholes.

Rapid inversion of data from 2D resistivity surveys with electrode displacements

Resistivity monitoring surveys are used to detect temporal changes in the subsurface using repeated measurements over the same site. The positions of the electrodes are typically measured at the start of the survey program and possibly at occasional later times. In areas with unstable ground, such as landslide-prone slopes, the positions of the electrodes can be displaced by ground movements. If this occurs at times when the positions of the electrodes are not directly measured, they have to be estimated. This can be done by interpolation or, as in recent developments, from the resistivity data using new inverse methods. The smoothness-constrained least squares optimisation method can be modified to include the electrode positions as additional unknown parameters. The Jacobian matrices with the sensitivity of the apparent resistivity measurements to changes in the electrode positions are then required by the optimisation method. In this paper, a fast adjoint-equation method is used to calculate the Jacobian matrices required by the least squares method to reduce the calculation time. In areas with large near-surface resistivity contrasts, the inversion routine sometimes cannot accurately distinguish between electrode displacements and subsurface resistivity variations. To overcome this problem, the model for the initial time-lapse dataset (with accurately known electrode positions) is used as the starting model for the inversion of the later-time dataset. This greatly improves the accuracy of the estimated electrode positions compared to the use of a homogeneous half-space starting model. In areas where the movement of the electrodes is expected to occur in a fixed direction, the method of transformations can be used to include this information as an additional constraint in the optimisation routine.

Array Optimisation for Multi-channel Electrical Resistivity Tomography Instruments

In recent years there has been considerable research into the selection of near-optimal arrays of electrode configurations that enhance the resolution of electrical resistivity tomography (ERT) images. Several algorithms have been developed that select resistivity measurements based on their contribution to the cumulative sensitivity of the array (Furman et al., 2004; Hennig and Weller, 2005) or its model resolution matrix (Stummer et al. , 2004; Wilkinson et al., 2006a; 2006b). Homogeneous subsurface resistivity distributions were assumed for these studies, although better results can be obtained using the same algorithms if the resistivity distribution is known a priori (Anthansiou, 2006). When compared to standard arrays, such as dipole-dipole or Wenner- Schlumberger, optimised arrays can substantially improve the resolution of the ERT image for the same number of measurements (Wilkinson et al., 2006b). The driver for researching array optimisation techniques has been the development of computer controlled ERT systems that can address arbitrary combinations of current and potential electrodes. Unfortunately all the published optimisation algorithms share a problem that is likely to impede their wider use: the arrays that they produce are inherently ‘single channel’ (SC). Since they do not take advantage of the multichannel (MC) capability of modern ERT instruments, the optimised arrays that they produce are rather inefficient to use compared to many standard arrays that are well suited to MC operation. However, we have developed a simple extension that constrains our previous algorithm to choose near-optimal configurations that also fit well into a MC measurement scheme. This extension could easily be adapted to work with the other optimisation schemes cited above.

Monitoring Leachate Levels in Landfill Sites Using Automated Time-Lapse Electrical Resistivity Tomography (ALERT)

Automated time-Lapse Electrical Resistivity Tomography (ALERT) is a concept that aims to provide tomographic images of landfill sites at regular intervals or on demand. The British Geological Survey has developed an ALERT system specifically for the deployment on landfills and has undertaken a research programme focussing on leachate monitoring techniques based on ERT. Simple scale experiments demonstrate the capabilities of the ALERT technique to accurately monitor liquid levels in a porous medium between wells. ALERT data provide additional control and information about inter-well properties and processes, thus contributing to an improved understanding of landfill hydrology.

Remote Condition Assessment of Geotechnical Assets Using a New Low-power ERT Monitoring System

The development of a new low-cost, low-power, electrical resistivity tomography system (PRIME - Proactive Infrastructure Monitoring and Evaluation) is described. PRIME is designed to provide continuous near-real-time information on the internal condition of the earthworks, helping to predict failures and enable timely intervention. Conventional asset monitoring involves examining the surface (either by people on the ground or from aerial photos) and using point sensors, such as moisture content and tilt meters, which only give information in the immediate vicinity of the sensor. But PRIME uses imaging for the volumetric tracking of moisture content changes and the detection of ground movement, thereby identifying problems at a much earlier stage. An example of the deployment of PRIME to monitor a leaking canal embankment is also provided.

Imaging of Moisture Dynamics within an Operational Railway Cutting – The Effects of Vegetation

The British railway network is one of the busiest in the world. A major part was constructed more than a century ago, prior the advent of modern soil mechanics theories. Today about 50% of the earthworks are in poor or marginal condition. There is a need for cost-effective, remote condition monitoring technology to evaluate their state. A novel Proactive Infrastructure Monitoring and Evaluation (PRIME) system has been developed, which employs electrical resistivity tomography monitoring. We present data obtained from an operational railway cutting over the time of more than one year. At site a now grass-covered relict landslide is in contrast to the otherwise densely vegetated cutting, offering the opportunity to study the effect of vegetation on the shallow moisture dynamics in infrastructure earthworks. Our results show that evapotranspiration and canopy lead to strongly increasing resistivities in summer, indicative for a significant loss in moisture, followed by a fast saturation in winter, when biological activities are at their minimum and no canopy exists. Resistivity, and thus moisture dynamics show significantly higher amplitudes in the vegetated than in the grass-covered part. These insights into the moisture dynamics will aid engineers in designing infrastructure slopes and intervention strategies for unstable slopes.

Performance Assessment of Novel Electrode Materials for Long-term ERT Monitoring

We present results of a laboratory study of novel electrical resistivity tomography (ERT) sensor materials, whose performance has been assessed in terms of suitability for long-term geoelectrical monitoring. The study has addressed concerns over the longevity of buried ERT sensors required to support nuclear decommissioning at the Sellafield Site in the UK. Electrodes made from three candidate materials and installed in a bentonite grout were subjected to accelerated measurements and electrochemical analyses were carried out on both pristine and used electrodes after extraction from the laboratory tanks. Electrical contact resistance showed significantly different behaviour for stainless steels compared with platinised titanium. Pt-Ti sensors displayed outstanding properties and their stability under operational conditions was remarkable. Their susceptibility to ERT noise, which was expected to be worse due to their higher nobility, was only marginally greater than that of stainless steels. No tangible advantage in terms of electrical performance was found for using higher-grade varieties of stainless steel over a conventional 316L-based design. Crucially, both steel types were affected by the growth of carbonate scales when buried in bentonite. This fundamental process may well be (at least partially) responsible for the frequently encountered increase in contact resistance of stainless-steel electrodes over time.

4D ERT Monitoring of Simulated Leaks from a Nuclear Storage Silo at Sellafield, UK

Results are presented from a 2-year full-scale field trial of Electrical Resistivity Tomography (ERT) monitoring of simulated leaks from a waste storage silo at Sellafield, UK. The trial included the monitoring of a series of injections of conductive tracer fluids to simulate leaks, as well as an extended monitoring period to assess the fate of the resulting saline plume. The trial constituted the first application of ERT subsurface monitoring at a UK nuclear licensed site. The work was undertaken to evaluate the Technology Readiness of ERT in terms of detecting, spatially characterising and monitoring potential leaks to the subsurface of silo liquor from a storage structure. Despite the weak conductivity contrast of the tracer fluids compared to the groundwater, and the loss of one of the ERT boreholes during the trial, full 4D inversion proved sufficiently sensitive to detect and image the passage of the tracers. Images of resistivity changes relative to a reference baseline have revealed likely pathways of tracer flow in the vadose zone and upper groundwater system. These pathways were found to be in close agreement with the locations of historic contamination detected in sediment cores retrieved from the ERT boreholes.