Lucy S. Oxby

Geoelectrical monitoring of simulated subsurface leakage to support high-hazard nuclear decommissioning at the Sellafield Site, UK

A full-scale field experiment applying 4D (3D time-lapse) cross-borehole Electrical Resistivity Tomography (ERT) to the monitoring of simulated subsurface leakage was undertaken at a legacy nuclear waste silo at the Sellafield Site, UK. The experiment constituted the first application of geoelectrical monitoring in support of decommissioning work at a UK nuclear licensed site. Images of resistivity changes occurring since a baseline date prior to the simulated leaks revealed likely preferential pathways of silo liquor simulant flow in the vadose zone and upper groundwater system. Geophysical evidence was found to be compatible with historic contamination detected in permeable facies in sediment cores retrieved from the ERT boreholes. Results indicate that laterally discontinuous till units forming localized hydraulic barriers substantially affect flow patterns and contaminant transport in the shallow subsurface at Sellafield. We conclude that only geophysical imaging of the kind presented here has the potential to provide the detailed spatial and temporal information at the (sub-)meter scale needed to reduce the uncertainty in models of subsurface processes at nuclear sites.

Optimised sequential experimental design for Geoelectrical Resistivity Monitoring Surveys

Sequential experimental design methods use previous data and results to guide the choice and design of future experiments. This paper describes the application of a sequential design technique to produce optimal resistivity imaging surveys for time-lapse geoelectrical monitoring experiments. These survey designs are time-dependent, and are optimised to focus a greater degree of the image resolution on the regions of the subsurface that are actively changing than static optimised surveys that do not change over time. The sequential design method is applied to a synthetic 2.5D monitoring experiment comprising a well-defined cylindrical target moving along a trajectory that changes its depth and lateral position. The data are simulated to be as realistic as possible, incorporating survey design constraints for a real resistivity monitoring system and realistic levels and distributions of random noise, in order to match a forthcoming experimental test of the method. The results of the simulations indicate that sequentially designed optimal surveys yield an increase in image quality over and above that produced by using a static (time-independent) optimised survey.

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.

Long-term Geoelectrical Monitoring to Support Nuclear Decommissioning at the Sellafield Site, UK

The reduction of risk and hazard associated with the UK’s civil nuclear legacy is a strategic priority for the UK’s Nuclear Decommissioning Authority (NDA). A full-scale field trial using 4D ERT has been completed successfully at the Magnox Swarf Storage Silos at the Sellafield Site, which has monitored a controlled injection experiment. This paper summarises the achievements of the trial and lays out future plans for geoelectrical monitoring at Sellafield. The potential of ERT as a long-term monitoring tool is explored, and electrode longevity estimates are made based on the results of the MSSS trial.

Automated Electrical Resistivity Tomography Monitoring of Quarry Dewatering

A study has been undertaken to demonstrate proof-of-concept for the use of time-lapse ERT for the monitoring of changing groundwater conditions at mineral extraction sites undergoing active dewatering using a well-point system. Monitoring was undertaken using a permanently installed geoelectrical imaging system, which comprised an advanced data logger (referred to here as “ALERT”) capable of the autonomous collection of geoelectrical data, and a network of surface electrodes. ALERT monitoring is demonstrated as an effective means of monitoring groundwater rebound and drawdown, although a time-lag is observed between hydraulic head levels recorded in the piezometers and the bulk saturation information provided by ERT.