The application of real-time, non-destructive electrical tomographic imaging to heritage conservation

Significant advances have been made in recent times with the non-invasive electrical tomographic imaging of the shallow subsurface. These emerging technologies are analogous to magnetic resonance imaging (MRI) or CT scans used in medical physics. Electrical Resistivity Tomography (ERT) is increasingly used to underpin studies in waste management, contaminated land characterisation and remediation, monitoring groundwater resources and the monitoring of geohazards or safety-critical plant. There is significant potential for using some of these techniques for the non-destructive, real-time monitoring of important heritage sites or buildings to assess the impact of environmental change on their physical integrity and resilience. This paper will describe some of the technologies developed by BGS, including our sensors, image reconstruction algorithms and survey methodologies, and how these might be adapted for heritage conservation purposes, viz: • Electrical Resistivity Tomography (ERT). New survey design and array optimisation schemes have been developed to give improved volumetric images at a range of scales. • Capacitive Resistivity Imaging (CRI). Capacitive electrodes now permit continuous high-density, non-intrusive scans below highly resistive artificial or engineered surfaces (such as masonry, walls, foundations and pavement). For large areal coverage, our CRI surveys are integrated with real-time kinematic global positioning systems to provide accurate navigation and location recovery. CRI (like GPR) is entirely non-contacting so surfaces are not disturbed by this technique. • Self-Potential Tomography (SPT). SPT is one of the few geophysical techniques that can detect electro-filtration effects associated with fluid flow and hence SPT can detect and image water infiltration pathways within structures. • Automated time-Lapse Electrical Resistivity Tomography (ALERT). ALERT can monitor temporal changes in subsurface electrical properties using permanently installed instrumentation and electrode arrays or sensor networks. Sites can be interrogated from the office "on demand" using wireless telemetry (GSM, GPRS, internet or satellite), thereby eliminating the need for expensive repeat surveys, visual inspections or manual intervention. The impact of transient events (e.g: from storms, flooding or extreme desiccation) can now be captured in real-time allowing early remedial or preventative maintenance. The volumetric nature of 3D or time-lapse 4D ERT also allows the detection of degradation that could be easily missed by isolated or infrequent “spot” sampling. Automated ALERT monitoring technology is likely to be increasingly important in the future as climate change, and high levels of anthropogenic activity, impact on our cultural heritage