Detection of fracture dilatancy on the cliff top using the azimuthal apparent resistivity technique

摘要

Hard rock cliffs erode through an initial catastrophic collapse along pre-existing discontinuitiesudin the rock mass. These may be ancient faults or fractures, orientated at a variety of angles to theudcliff face, or relatively new tension fractures formed during cycles of cliff recession, sub-paralleludto the cliff face. It is likely that an approaching cliff fall will be associated with increasingudfracture dilatancy within the fracture network. Hence if the change of dilatancy can be measuredudthen it may be possible to generate alerts of impending cliff collapse.udSince fractures often occur in sets with a preferred orientation they impose anisotropic physicaludproperties on the rock mass. Hence, the apparent resistivity of the rock will vary with azimuthudreflecting the dominant fracture orientation. Measures of anisotropy can be calculated from theudmeasurements and would be expected to vary with time if the fractures are dilating.udWork package one of the 5th Framework co-funded project ‘PROTECT’ (PRediction Of TheudErosion of Cliffed Terrains) was to detect fracture dilatancy. Azimuthal apparent resistivity dataudwere collected at five research sites in the UK, France and Denmark, all situated on outcroppingudchalk. At each research site, data were collected with the Square array at three locations near theudcliff edge and at a Control site set back from the cliff edge by about 50 m. Data were collectedudapproximately every two months for two years to create a temporal data set. After processing theuddata to remove the effect of the infinite resistance afforded by the cliff face, the data were fittedudto an ellipse in order to test for anisotropy. Measures of anisotropy were then calculated fromudthese data.udThe anisotropy has been interpreted as fracturing and indicates a number of tectonic fractureudorientations that agree with geological mapping. At several of the research sites a cliff-paralleludfracture set was identified in a zone 10 to 20 m wide adjacent to the cliff edge. It is assumed thatudthis fracture set develops in response to the stress relief at the cliff face. At the Birling Gapudresearch site a cliff collapse within the zone of resistivity measurements produced a dramaticuddrop in the magnitude of the post-collapse calculated measures of anisotropy. However, otherudcliff falls that occurred outside of the immediate zone of resistivity measurements did notudgenerate appreciable changes in the calculated measures of anisotropy. It appears that theudtectonic fractures that limit the lateral extent of the cliff fall may also limit the fracture dilatancyudwithin the cliff parallel fracture set. At some sites there was a seasonal variation in the measuresudof anisotropy with peaks in the summer and troughs in the winter. It appears that the most likelyuddriver for these variations is rock temperature that is itself controlled by the external airudtemperature.udOverall, the research has been successful in establishing that there are measurable changes in theudrock mass prior to a collapse. However, the methodology is not yet advanced enough to be ableudto develop technology for the reliable early warning of a cliff fall. The next stage of any researchudwould be to install a system for continuous monitoring in order to establish the magnitude of theudchanges in the measures of anisotropy immediately prior to a cliff collapse.