Geological void and mine entry collapse can be a very high risk within certain areas of the country and of particular concern during brownfield development.
Mine shafts are a particular issue as many are not recorded on mine risk maps, and those that are recoded may not be accurately placed on modern mapping. About 1000 incidents regarding void collapse are reported every year, roughly a half related to mine collapse.
1.5 million properties lie above workings where coal has been mined at depths less than 30m.
These voids may not be detectable using GPR, conductivity or magnetic methods due to them being too deep or having no detectable electrical contrast to surrounding material. A microgravity survey solves this problem as the method detects small changes in the Earths gravitational field due to the mass of material missing within the void.
The method requires meticulous data collection in the field and a complex data correction process, but can even detect infilled voids. We work on the process of detecting one mine entrance per day, or roughly 40 to 50 readings depending on field conditions.
Data is presented in 2D plan view maps, showing relative gravity highs and lows. Often a microgravity survey is completed simultaneously with a magnetic survey and electromagnetic conduction survey to add confidence to interpreted voids.