Geophysics provides a mine of information

Phase successfully used geophysics to locate mineshafts at a site near Bradford at a fraction of the cost of an intrusive survey. Prompting this response from the client, ‘…I’m impressed. The information you’ve provided gives us some degree of confidence in the records and we are probably going to change our site layout as a result’.

Coal Authority records had placed four mineshafts in and adjacent to a site that was due to be developed. Mining records can often be inaccurate and incomplete and so the client needed verification to confirm the number and accurate location of any mineshafts on the site.

Phase recommended a geophysical survey comprising magnetic and electromagnetic (EM) techniques. Combining two techniques, which identify different properties of sub-surface features, is often the best way of maximising the information from a geophysical survey.

The site of 0.5 ha was surveyed in just over a day, despite the presence of steep slopes and heavily waterlogged ground. The same survey grid was used for both techniques and this was tied-in to local topographic features using a total station.

Using the topographic data recorded on site the survey data and information from the Coal Authority plans were positioned onto a map base. This revealed that the assumed positions of two of the mineshafts were under a stable block at the edge of the site and so were not covered by the survey. However two anomalies were identified within the site that indicated the presence of the remaining two mineshafts. The position of the anomalies suggested that the shafts as shown on the Coal Authority plans were reasonably accurate in terms of their relative positions but they were 6 m to 7 m out when it came to their actual position. Providing this information allowed the client to determine how best to proceed with their design and future investigation work.