Tools and Methods
Time-lapse electrical resistivity tomography (ALERT)
An innovative 4D electrical imaging system known as ALERT (Automated time-Lapse Electrical Resistivity Tomography) has been developed by BGS which uses permanent in-situ electrode arrays and “smart” microprocessor-controlled instrumentation to remotely monitor temporal changes in subsurface properties. Such changes may be related to changes in fluid flow, hydrogeochemistry, or saturation levels. Complex dynamic processes can now be studied at unprecedented sampling rates without manual intervention. Volumetric images of the shallow subsurface are captured “on demand” by wireless telemetry (GSM/3G, GPRS, satellite and/or internet) from an office PC or laptop anywhere in the world – thereby eliminating the need for expensive repeat surveys. A centrally managed network server and web-portal are used for data capture, processing, modelling, visualisation and databasing. The entire process from data capture to visualization is automated and seamless. It is proposed to install ALERT technology at a site in the Witbank Coalfield, South Africa to:
- Study the environmental impacts of mine waste, and acid mine drainage on surface waters, groundwaters and vulnerable ecosystems, to depths of tens of metres or more, thereby complementing satellite imagery (hyperspectral imaging, SAR) which has a penetration depth of only a few centimetres.
- Relate the temporal tomographic images (in both space and time) to all components in the hydrologic continuum (climate, soil, surface water, groundwater, salinization and acid mine drainage). A better characterisation of spatial and temporal hydraulic behaviour at depth is essential to assess the impact of anthropogenic or climatic forcings. Specific focus will be given to minewater rebound (in the case closed mines); surface-groundwater interactions and leachate flowpaths from surficial spoil heaps, low permeability backfill, tailings lagoons or underground workings.
- Provide early warning of minewater break-out, subsurface pollution plumes, and potential threats to containment barriers (e.g: tailings dams). It is proposed that ALERT also be assessed as a potential technology for the automated real-time remote monitoring of mine-related operations to detect these potential problems before visible manifestation. Mine managers or Health & Safety Authorities could be notified of these threats by cell phone text message, visual alarms on the web-portal, or by email.
To-date, none of the above applications have been tested or demonstrated in areas impacted by mining.