4D electrical resistivity tomography monitoring of soil moisture dynamics in an operational railway embankment

Abstract

The internal moisture dynamics of an aged (> 100 years old) railway earthwork embankment, which is still in use, are investigated using 2D and 3D resistivity monitoring. A methodology was employed that included automated 3D ERT data capture and telemetric transfer with on-site power generation, the correction of resistivity models for seasonal temperature changes and the translation of subsurface resistivity distributions into moisture content based on petrophysical relationships developed for the embankment material. Visualization of the data as 2D sections, 3D tomograms and time series plots for different zones of the embankment enabled the development of seasonal wetting fronts within the embankment to be monitored at a high-spatial resolution and the respective distributions of moisture in the flanks, crest and toes of the embankment to be assessed. Although the embankment considered here is at no immediate risk of failure, the approach developed for this study is equally applicable to other more high-risk earthworks and natural slopes.Geophysical ground imaging techniques offer the potential to complement existing approaches by spatially characterizing and monitoring the internal conditions of earthworks to provide highresolution information of subsurface property changes and hence precursors to slope failure. Resistivity imaging, or electrical resistivity tomography (ERT), holds particular promise due to its sensitivity to both lithological variations (e.g., Shevnin et al. 2007) and changes in soil moisture, which can be imaged by applying appropriate petrophysical relationships linking resistivity and saturation (e.g., Cassiani et al. 2009;Brunet et al. 2010). Twodimensional ERT is now a well-established technique for investigating natural slopes with numerous recent examples of the use of the technique for structural characterization and hydrogeological investigations (e.g

Computation of optimized arrays for 3-D electrical imaging surveys

“…This allows the array used to traverse across the entire grid instead of being limited to three adjacent lines. Such a system has been developed for special purposes, such as long-term monitoring of hydraulic processes and soil movement (Merrit et al 2013;Chambers et al 2014). This allows the use of the 'quadruple line' configurations, as well as the other configurations, with the electrodes spread over several lines (Fig.…”

Interpolation of landslide movements to improve the accuracy of 4D geoelectrical monitoring

“…Below these near surface artefacts (> 2 70 m depth), deeper features of the opposite polarity are found extending to a depth of about 7 m 71 below ground level (bgl). As resistivity ratios are commonly used to show changes in moisture 72 conditions (Jomard et al 2007; Chambers et al 2014) which, in terms of landslide monitoring, can be 73 used as proxy to slope stability (Lebourg et al 2010), methodologies have to be developed to 74 estimate electrode movements to minimize these artefacts and improve ERT monitoring applied to 75 landslides. 76 GPS measurements of the moved electrodes were included.…”

Measurement and modelling of moisture—electrical resistivity relationship of fine-grained unsaturated soils and electrical anisotropy

“…The conversion between the measured sample temperature in the laboratory and MAT was made assuming that 2% =1°C -1 (Hayley et al, 2010;Hayley et al, 2007;Chambers et al, 2014;Brunet et al, 2010).…”

“…It can provide information on lithological variability and is also sensitive to changes in soil moisture content. It is particularly effective when deployed as a time-lapse monitoring tool where, once the influence of temperature has been determined (Hayley et al, 2007), changes in resistivity can be related to subsurface moisture dynamics (Brunet et al, 2010;Chambers et al, 2014). ERT monitoring has therefore proven to be a useful tool for investigating slope hydrology when installed on rainfall-induced landslides (Jongmans et al, 2007;Di Maio et al, 2011;Lebourg et al, 2005;Supper et al, 2014).…”