Case Study - Buried Mineshaft
Multiple-Technique Approach to Buried Mineshaft Location
The approximate location of many mine shafts are recorded on historic mapping and other records held by organisations such as The Coal Authority.
These shafts are unlikely to have been adequately filled or capped and so need exposing to allow checking before development work goes ahead. However, as the recorded positions are normally only approximate it is cost-effective to carry out a non-invasive geophysical survey first to target where the excavation should be made.
Several techniques can be used to survey the shaft area which rely on different physical properties of the shaft and the cap. Steel used in the cap or from cut-off anchors of the mine head gear would be detected by magnetometry.
The presence of the shaft itself, whether backfilled or filled with air or water, would cause an anomaly in the electrical conductivity compared to the parent geology so a ground conductivity survey is often successful. Similarly, the buried structural remains are likely to change the dielectric properties of the ground so ground probing radar is another worthwhile option. Other techniques that can be considered are microgravity, resistivity imaging and seismic survey particularly if the depth of overburden is in excess of 5m. The more techniques that can be deployed the better as several targets found at the same location will reinforce the confidence of the interpretation made.
Example from the location
In the example given here both magnetometry and ground conductivity surveys were carried out. The conductivity survey involved collecting both quadrature and in-phase data.
The former gives a measure of the conductivity and the latter an assessment of the bulk magnetic susceptibility of the ground at each survey point.
The magnetic survey showed a strong, well defined anomaly in the central southern part of the survey area. This was reinforced by a weaker anomaly seen in the quadrature data in the same location. However, the in-phase is less helpful with only a weak anomaly seen nearby to the west.
Upon excavation the shaft was found at the location of the strong magnetic anomaly