The Use Of Time-Lapse Microgravity To Investigate And Monitor An Area Undergoing Surface Subsidence; A Case Study

Northwich is a town located on the Cheshire Basin. Salt was discovered there in 1670 and was<br>mined extensively during the 19th Century. A legacy of this is the continuing subsidence of the<br>surrounding area. This subsidence has restricted development of the town and resulted in the demolition<br>of many buildings. Peter Street is an area of terraced houses suffering from subsidence, thought to be<br>related to the mine workings of the 19th Century. Microgravity and Resistivity profiling have been used<br>in Peter Street as non-invasive techniques to investigate the cause of this subsidence. Repeat (or Timelapse)<br>Microgravity has been used to assess the stability and evolution of the low-density areas. Timelapse<br>Microgravity uses the characteristics of anomaly size and gradient to track the development of<br>cavities as they propagate to the surface. It is possible to create gravity time slices and to model the<br>increase in cavity thickness. High resolution can be obtained using this differential method as the large<br>terrain effect corrections and latitude corrections are automatically removed. Resistivity imaging has<br>been used to investigate the conductivity of the of near surface. Results from both techniques suggest<br>that areas of low density are present at a depth of ~6m in the area experiencing subsidence. Modelling of<br>the gravity data suggests that there has been an upward migration of one of the voids over the<br>monitoring period.