S. Holyoake

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

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.

Volumetric Monitoring of Dynamic Moisture Distribution in an Aged Railway Embankment

The condition of aged embankments relates to the engineering geological properties of the source materials, the internal heterogeneity produced during construction and how these factors have affected long term processes leading to deterioration in integrity and performance. Remotely operated, automated monitoring systems providing non-invasive geophysical measurements provide insight into the processes driving long term deterioration compromising stability, such as dynamic moisture movement throughout embankments. Automated time Lapse Electrical Resistivity Tomography (ALERT) technology provides high resolution information relating to the internal structure of an embankment. The dynamic moisture distribution throughout the embankment can be interpreted from a series of time lapse, differential resistivity images based upon robust resistivity-moisture content relationships. A 3D ALERT system was installed to monitor moisture movement within a 22 m section of embankment operated by the Great Central Railway (Nottingham) Ltd. This included: 12 cables of 31 m length laid from toe to toe, across the transect of the embankment, spaced at 2 m, each comprising 32 electrodes with a 1 m spacing. This paper presents baseline 2D and 3D images of the resistivity distribution within a section of the embankment that will be used to assess the impact of vegetation and drainage characteristics upon moisture movement.

Volumetric Remote Monitoring of an Earth Embankment: Part II - Visualisation and Condition Assessment

We describe a study in which time-laspe electrical resistivity tomography (ERT) has been used to investigate an aged (>100 years old) section of earth embankment on an operational railway (Great Central Railway (GCR), Nottingham, UK). Monitoring has been undertaken using a permanently installed ERT array extending across the embankment from toe to toe. Part I of the study (Munro et al., 2012) describes our approach to ERT data collection and processing, including corrections for seasonal temperature changes, and the implementation of a procedure to translate time-lapse resistivity models into gravimetric moisture content. Here, in Part II of the study, we use the derived models of gravimetric moisture content to assess the internal condition of the earth embankment. These results show that this can be monitored noninvasively, and at a high spatial resolution, using ERT.

Volumetric Remote Monitoring of an Earth Embankment: Part I - Geophysical – Geotechnical Property Translations

In this work we describe a study where automated time-lapse electrical resistivity tomography (ALERT) monitoring technology has been installed on a section of Victorian embankment on the Great Central Railway (Nottingham, United Kingdom). Raw datasets collected by the ALERT system have been processed/filtered, and inverted to yield a 3D resistivity distribution which is temperature corrected and converted to gravimetric moisture content using a relationship established by laboratory testing. Electrical resistivity tomography monitoring has been used to characterize the internal structure of the embankment, and image moisture content changes and wetting front development at a high spatial resolution. Monitoring has been undertaken at the test site to determine seasonal temperature changes in the subsurface; these data have been used to correct for temperature effects. We fitted the resistivity data as a function of gravimetric moisture content by modifying the Waxman-Smits model. Using results from laboratory testing, a best fit is computed and used to establish a resistivity, gravimetric moisture content relationship, used to facilitate property translation from temperature corrected resistivity to gravimetric moisture content. These results indicate that ERT has potential to identify structures and processes related to instability at an early stage in their development.

4D Geoelectrical Monitoring of Natural Attenuation Processes at a Contaminated Former Gas-works Site

A permanent automated geoelectrical imaging system was installed at a contaminated land site to monitor resistivity changes associated with groundwater quality after the completion of a remediation programme. The former gasworks site had been designated statutory contaminated land due to the risks of pollution of an underlying minor aquifer. The system collected data at regular scheduled intervals from a network of electrodes arranged in vertical borehole arrays on the boundaries of the site. The incoming data from the system were automatically inverted to produce 4D resistivity images. Analysis was carried out over one year, revealing resistivity fluctuations in the infilled ground beneath the tarmac that were strongly temperature dependent. But in the underlying sand and gravel aquifer there was a steady increase in resistivity that was suggestive of a reduction in groundwater contamination after the removal of the contaminant source zones. A tracer test was also undertaken to investigate the groundwater flow velocity and demonstrate rapid 4D geoelectrical monitoring of natural attenuation processes. The motion and evolution of the tracer were visualised directly in high-resolution volumetric images in near realtime. Seepage velocities measured from the images agreed closely with estimates based on the piezometric gradient and assumed material parameters.