The application of time-lapse azimuthal apparent resistivity measurements for the prediction of coastal cliff failure

The erosion of coastal cliffs is inevitable. The resultant cliff collapses are a hazard, a problem for coastal land use planners and limit the use of the coastline as an amenity. There has been very little research into physical property changes within the rock mass behind a cliff face prior to collapse. If any such changes could be monitored with geophysical techniques then it might be possible to identify sections of a cliff that are becoming unstable. In cliffs composed of a highly fractured rock, such as chalk, any sub-vertical fractures will gradually dilate with time until a collapse is initiated. Since fractures often occur in sets with a preferred orientation, they impose anisotropic physical properties on the rock mass. With care, azimuthal apparent resistivity can be used to map fracture orientations and a factor of anisotropy can be calculated that should vary with time if the fractures are dilating. Azimuthal apparent resistivity data have been collected at three cliff top sites over a period of 2 years in order to quantify any such effects. The results indicate that a cliff-parallel fracture set develops in response to the presence of a free surface at the cliff face. A large reduction in anisotropy near the cliff face was measured as a result of a cliff fall. This has been interpreted as a release of fracture dilatancy. However, it is likely that the tectonic fractures within the cliff limit the lateral extent of the cliff fall and the lateral extent of the fracture dilatancy. Hence a consistently high value of anisotropy near the cliff edge may constitute a long-term warning of impending cliff collapse. Many of the sites showed seasonal variations in anisotropy, with peaks in the summer and troughs in the winter. A correlation has been observed between these variations and changes in rock temperature. It appears that changes in rock temperature lead to an expansion and contraction of the rock mass, with a maximum expansion in the winter. The expansion leads to fracture contraction with associated minimum values of anisotropy in the winter