Investigation of a fractured limestone cliff (Chartreuse Massif, France) using seismic tomography and ground-penetrating radar
C. Dussauge-Peisser, M. Wathelet, D. Jongmans, D. Hantz, B. Couturier and M. Sintes
Journal name: Near Surface Geophysics
Issue: Vol 1, No 4, November 2003 pp. 161 - 170
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Evaluating the stability state of a rock slope is a complex problem, mainly due to the lack of knowledge of the real state of the rock mass. Geophysical methods appear to be useful for investigating the deep discontinuity pattern, which may be poorly interpreted from surface observations. However, they have seldom been applied on steep rock slopes. The aim of this study is to test seismic tomography and ground-penetrating radar on near-vertical cliffs, and assess the quality of information that they can provide when investigating the characteristics of the fracture pattern inside the massif. The test site is located in the Chartreuse massif, 20 km north-west of Grenoble, France. It is a 15 m high limestone cliff, characterized by one main near-vertical discontinuity set, including some wide open fractures. Seismic tomography has been conducted between the vertical free surface and the plateau above, along three parallel profiles. Results show strong velocity gradients, from 800 to 3500 m/s. Some triangular low-velocity zones can be correlated with field observation of open fractures, but different tests on synthetic models and on real data show that the method is too sensitive to such heterogeneous conditions to provide accurate information on the fracture pattern. Ground-penetrating radar surveys have also been acquired along vertical profiles on the cliff. We used three different antennae with centre frequencies of 35,120 and 500 MHz. The penetration depth varied from 10 m (500 MHz) to about 20 m (35 MHz). The main reflectors are near-vertical and most of them can be correlated with fractures observed on the site. The reflectivity varies strongly along one single reflector, indicating changes in the aperture and/or filling characteristics.