There are estimated to be around 405,000 abandoned mine entries in the UK, of which only 150,000 are recorded. Approximately 15 mineshafts collapse each year and with so many unrecorded shafts, mines can have a huge and unexpected impact on future development projects.
While some of these mineshafts have been simply abandoned, others have been capped and backfilled. Furthermore, mining records sometimes show the location of a suspected shaft, however the position is only approximate and there is often very little or no detail on whether the mine has been capped, let alone with what material.
A range of geophysics techniques can be used to find abandoned mineshafts, which rely on the different physical properties of the cap (if present) and the shaft. For example steel used in the cap, or from the cut-off anchors of the mine head gear, can be detected by magnetometry. Similarly, the presence of the mineshaft itself, whether backfilled or filled with air or water, would cause an anomaly in the electrical conductivity of the site, compared to the surrounding geology which a ground conductivity survey could identify. At the same time, the buried structural remains are likely to change the dielectric properties of the mineshaft itself, which ground probing radar could pick up.
Ground Conductivity Survey
An abandoned shaft infilled with different material to the surrounding in-situ ground can be detected by a Ground Conductivity Survey. The data is displayed as a contour plan, in which a mineshaft may appear as a small, discrete, high or low conductivity anomaly that stands out from the surrounding background.
Detailed Magnetic Survey
A magnetic survey can detect ferrous metal as well as soils, rocks and artefacts containing weakly magnetic minerals. A shaft that has been capped with iron or steel elements can be easily identified with this method, as can scrap metal which may have been dumped into the upper levels of a mineshaft.
Above : Ground conductivity data (left) and Magnetic data (right) showing a combined ground conductivity C1 and magnetic anomaly M1 close to a nominal mineshaft position (MS-5).
Ground Penetrating Radar Survey (GPR)
A GPR survey can identify several things which may indicate the presence of a mineshaft. For example, a mineshaft capped by reinforced concrete can be detected as a discrete, buried reinforced concrete slab, whilst an uncapped shaft, or one capped with timber or other non-metallic materials may be identified by the presence of a large internal void. Similarly, if the shaft has been backfilled, then detection relies on the contrast between the backfilled material in the mineshaft and the surrounding in-situ ground.
GPR data can be displayed in plan and in section view as in the example below.
Above : GPR Radargram (section) showing possible mineshaft at c. 1m depth.
The selection of the appropriate Geophysical technique is often dependent upon the size of the survey area and the ground conditions present, as well as any information that is available about the shafts themselves. Certainly, the more techniques that can be deployed the better, as several targets found at the same location by different techniques will reinforce the confidence of the interpretation made.
Geophysical Engineering Surveys
Air Raid Shelters
Cellars & Basements
Post Tension Cables
Underground Storage Tanks
Air Voids
Chimney Flues
Rebar
Water Engineering
Badger Setts
Earthing Installations
Sinkholes
Burials
Geological Application
Soil Thermal Resistivity Testing
Buried Foundations
Landfills
Structural Investigations
Buried Manholes
Mineshafts
