Alan L. Weller

Hydrogeophysical Monitoring of Landslide Processes Using Automated Time-Lapse Electrical Resistivity Tomography (ALERT)

Geoelectrical techniques, such as resistivity and self-potential are being increasingly applied to study the hydraulics of landslide processes. The great strengths of these techniques are that they provide spatial or volumetric information at the site scale, and are sensitive to hydraulic changes in the subsurface. In this study we described the development and installation of an automated time-lapse electrical resistivity tomography (ALERT) system on an active landslide at a site near Malton, North Yorkshire, UK. The overarching objective of the research is to develop a 4D landslide monitoring system that can characterise the subsurface structure of the landslide, and reveal the hydraulic precursors to movement. In this paper we describe the installation of the ALERT system on an active landslide, and present initial results showing the 3D structure of the landslide and subsurface resistivity variations that occurred between static conditions and an active phase of slope failure.

Geophysical anatomy of the Hollin Hill landslide, North Yorkshire, UK.

Geophysical methods are playing an increasingly important role in the investigation and monitoring of landslides; such methods are proving to be particularly effective for revealing the 3D structure, failures surfaces, and the hydrogeological regimes associated with rock and earth slides. In this paper we present the results of a geoelectrical reconnaissance survey of the Hollin Hill landslide, UK. This work was undertaken in advance of the installation of a permanent geophysical and geotechnical monitoring system, and was designed to assess the suitability of resistivity (resistivity mapping and 2D/3D ERT) and self-potential methods (profiling and mapping) for investigating and monitoring this site. In particular, we were concerned to assess the electrical property contrasts and the magnitude of SP response across the study area. The surveys revealed that there was a good resistivity contrast between the slipped material and sandstone bedrock, which allowed us to use resistivity mapping data and ERT models to define the geometry of the landslide. An SP signature consistent with the movement of groundwater through the landslide was observed at the site, and was used to identify seepage patterns associated with the slipped material.

Sand and Gravel Deposit Evaluation Using Electrical Resistivity Tomography

In this study, we assess the suitability of ERT for UK sand and gravel deposit assessment. To this end, we have reviewed the characteristics of deposits in terms of geological setting, thickness and heterogeneity to inform our survey design strategy. We have collated existing data on the electrical properties of UK sand and gravel, reviewed previous examples of ERT sand and gravel surveys, and undertaken detailed geophysical studies at seven potential or active sand and gravel extraction locations in East Anglia and the East Midlands.

Bedrock Interface Detection for Sand and Gravel Mineral Reserve Assessment Using 3D ERT

A study describing the use of 3D ERT to determine depth to bedrock, and hence mineral thickness, at a sand and gravel extraction site within river terrace deposits is presented. Two approaches are considered which automatically extract interface depths from 3D ERT models. The first assumes that the interface is located at the maximum slope of the resistivity-depth curve, and is therefore referred to as the ‘steepest gradient method’ (SGM). The second uses an intrusive sample point to calibrate the model by identifying the resistivity iso-surface associated with the interface, and is referred to here as the ‘known interface method’ (KIM). The results of these two approaches have been tested against direct GPS observations of the interface position that were made after the bedrock had been exposed by quarrying of the river terrace deposits. In this case, the use of intrusive data for model calibration (i.e. the KIM) was essential for recovering accurate depth information from the 3D ERT model. Reliance upon using the steepest resistivity gradient as an indicator of a geological boundary (i.e. the SGM) produced a severe overestimate of interface elevation.

High Resolution 3D Geoelectrical Imaging of Quaternary River Valley Deposits at a Brownfield Site, Midlands, UK

A case study is described in which 3D electrical resistivity tomography (ERT) is used to investigate a chlorinated hydrocarbon contaminated site. The geophysical survey formed a component of a wider site investigation programme that included intrusive geological, hydrogeological and geochemical sampling. The primary goal of the 3D ERT survey was to characterise the thickness and lithological variations within the superficial deposits covering the site, and to assist in identifying suitable targets for the intrusive sampling programme. Interpretation of the 3D resistivity model has been aided by data collected from borehole sampling and cone penetration tests (CPT).