Geophysics has ‘adit’ with mine entrance

Our client had a site in County Durham which was in the vicinity of known coal mining activity from Victorian times through to the twentieth century. During the course of their groundworks for redeveloping the site they came across a brick feature. Concerned that there may be unrecorded mining features our client asked Phase SI to carry out a geophysical survey within the rest of the area where they proposed to carry out excavation work.

Only a relatively small area could be surveyed due to the presence of spoil heaps and several areas of very muddy, waterlogged ground. Magnetic and electromagnetic (EM) techniques were chosen as the site was not suitable for a microgravity survey. The geophysical survey identified a linear anomaly in the vicinity of the brick structure but this response only carried on for a short distance. At the same time as carrying out the geophysical survey the client excavated back from the feature and found that it was the crown of a Victorian adit. Further excavation and pumping of groundwater showed that the adit was in excellent condition and could be seen to slope downwards at the point where the geophysical survey lost the response.

Had the geophysical survey been carried out before any excavation work then it would have identified the presence of this feature, which could then have been investigated before the main construction work was carried out. The full extent of the adit would still not have determined using magentics and EM, because of its increasing depth . However, if a microgravity survey had been undertaken before any excavation work then this would probably have determined the full extent and location of the adit.