Keeping in PRIME condition: monitoring infrastructure using geophysics
Jim Whiteley, Arnaud Watlet and Jon Chambers, Shallow Hazards and Earth Observation, British geological Survey.
Identifying, managing and mitigating the processes of internal deformation in infrastructure assets is key to maximising their serviceability. But how can we see the unseeable? Like medical imaging, geophysics allows us accurately diagnose hidden issues before they become bigger problems.
In 1895, Wilhelm Röntgen discovered that the outlines of solid objects could be imaged by exposing them to a “new type of ray” that he accidentally discovered while researching light emission phenomena. The discovery of the ‘Röntgen ray’, better known as the X-ray, ushered in a paradigm shift in diagnosing bone fractures, organ damage, tumours and other hitherto unobservable ailments. The discovery of X-rays spawned the field of medical imaging and revolutionised diagnostic medicine. Not only was it now possible to assess acute trauma reliably and accurately, but people’s health could be more easily monitored, allowing early detection of diseases to prevent unnecessary harm and suffering. It is almost unthinkable now, but prior to the discovery of X-rays, assessing and addressing these problems largely relied on two sources of information: expert medical opinion and cutting open patients.
Today, when it comes to diagnosing problems inside infrastructure assets, ‘expert opinion’ and ‘cutting open’ are still the main ways we assess acute trauma, although in this case, acute trauma may be a deterioration in condition leading to imminent slope failure. To determine the cause of the problem, intrusive investigation of failing embankments, dams and cuttings is required at great cost (and sometimes) risk. For many years, this approach has been the main means of informing the expert opinions of engineers responsible for repair works. Like life-saving surgery, when infrastructure is at imminent risk of failing, rapid intervention is necessary; but what if we could prevent these critical states being reached? Can we proactively monitor the internal health of assets rather than reactively fix their problems? What options are available to us to help make our healthcare plans, and prevent unnecessary harm and economic suffering?
Geophysical monitoring systems are an emerging option for infrastructure management. Unlike remote sensing methods limited to surface-only observations, and sensor measurements that provide extremely localised information, time-lapse geophysics, like modern medical imaging, provides high spatial and temporal volumetric images of the subsurface. A particularly powerful method for doing this is Electrical Resistivity Tomography (ERT), where an electric current is injected into the ground through electrodes at the ground surface. By deploying arrays of electrodes across the surface of an asset, we can build up a 3D picture of the internal structure, and by taking repeated measurements, we can create 4D (or time-lapse) images to monitor changes within the internal structure. ERT is particularly well suited to monitor hydrological processes, as water in pore spaces decreases resistivity measurements. This water can play a significant role in the deterioration and destabilisation of cuttings, dams and embankments.
The PRoactive Infrastructure Monitoring and Evaluation system, or PRIME system, developed by the British Geological Survey combines low-cost and low-powered ERT imaging technology with intelligent monitoring, web portal access, data telemetry and end-user data products (Fig. 1). The PRIME system controls the acquisition of ERT measurements across an array of electrodes deployed to monitor the subsurface of an asset. These electrode arrays can vary in size, from targeted deployments focusing on zones with persistent problems, or to whole-asset scale providing observation of the entirety of a structure (Fig. 2). Knowing where water is entering, accumulating, moving through and exiting an asset can help identify potential deterioration, optimise inspection schedules and target remediation works (Fig. 3). Like medical imaging, time-lapse geophysical imaging of assets will not stop deterioration or remove the need for treatment, but it gives us the opportunity to target resources and efforts to keep our assets in a healthy condition and extend their serviceability.
Key: Left to right drawings and image
Figure 1: The PRIME system workflow, showing the deployment of field equipment, transmission of command files and data to and from the field, data products and end-user early-warning dashboard. Reproduced from Holmes et al. (2020) under a Creative Commons CC BY license.
Figure 2: A BGS PRIME system (foreground) powered by an adjacent solar panel, and attached to an array of buried electrodes covering the face of a leaking dam (background). The minimally invasive nature of the electrode array and small size of the solar-powered PRIME system allow for deployments at a wide range of asset types and scales.
Figure 3: Time-lapse ERT images from the PRIME system, with the expanding area of low resistivity values over time showing the progressive infiltration and saturation of a slope.
Holmes, J., Chambers, J., Meldrum, P., Wilkinson, P., Boyd, J., Williamson, P., Huntley, D., Sattler, K., Elwood, D., Sivakumar, V., Reeves, H. & Donohue, S. 2020. Four‐dimensional electrical resistivity tomography for continuous, near‐real‐time monitoring of a landslide affecting transport infrastructure in British Columbia, Canada. Near Surface Geophysics, 18, 337-351.