3-D Electrical Resistivity Tomography for Cliff Stability Assessment at Pointe du Hoc in Normandy, France

Pointe du Hoc overlooking the English Channel in Normandy, France was host to one of the most important military engagements of World War II. While the site is a valuable historic cultural resource, it is vulnerable to cliff collapses that already have endangered the observation post and Rudder?s command post. The observation post has been closed to visitors for some time due to safety concerns. Geophysical techniques have been used increasingly in recent years for slope stability investigation purposes. The objective of this study is to apply advanced 3?D resistivity tomography toward a detailed site stability assessment with special attention to the two at-risk buildings. 3?D resistivity tomography datasets at Pointe du Hoc in the presence of extreme topography and dense cultural clutter have been successfully acquired, inverted, and interpreted. The cliff stability in the areas around the two at?risk buildings has been analyzed. A hazard assessment scheme has been designed in which regions of high resistivity are interpreted as zones of open, dry fractures with a moderate mass movement potential. Regions of low resistivity are zones of wet, clay?filled fractures with a high mass movement potential. The observation post tomography results indicate that the highest mass movement hazard appears to be associated with the marine caverns at the base of the cliff that are positioned at the point of strongest wave attack. These caverns likely occupy the future site of development of a sea arch which will definitely threaten the observation post building. A high probability of a soil wedge failure is on the east?facing cliff edge close to the observation post that could damage or destroy the building. The mass movement potential at the Rudder?s command post area is low to moderate. The greatest risk is associated with soil wedge failures at the top of the cliffs. The resistivity geophysical data add great value to the natural geohazard assessment at Pointe du Hoc and constitute an integral component of an interdisciplinary approach to the problem of cultural resource preservation at the site. Geophysics is a non?invasive and relatively inexpensive technology that provides unique constraints which are unobtainable using traditional engineering geology methods for site characterization. However, the technology is difficult to master and the inherent limitations must be carefully understood to ensure a reliable geotechnical interpretation.