Cavity Investigation by Surface Wave Tomography and Microgravity
Résumé
Shallow cavities, such as karstic caves in carbonate bedrock and near-surface underground mine workings and tunnels, constitute serious hazards for persons and existing constructions due to the risk of collapse and subsidence involving damages to buildings and increased urban development costs. Detection of karstic features, voids, surficial dissolution, fissuring, alteration and unconsolidated material, is thus a major challenge for geophysical methods. Density and resistivity are known to be good indicators of the alteration and fissuring of draining or saturated epikarst, and electric behaviour may be amplified by the presence of shallow groundwater and strong water circulation. Ground penetrating radar (GPR) can be also suitable where soils or argillaceous cover, that absorbs radar waves, are absent or highly discontinuous. Recent investigations have reported good results for cavity imaging using surface seismic methods (Samyn et al., 2013, Leparoux et al., 2000; Vogelaar, 2001, Gucunski et al., 1996; Park et al. 1996). Surface wave have several properties that make them useful for near surface characterization. They are usually the largest amplitude waves generated by a surface impact, and their dispersive property allows for quantification of shear wave velocity with depth. Based on the relative advantages and disadvantages of the different geophysical techniques the most prudent imaging approach is the use of two or more complementary techniques. For example, the exploration of a karst network in the Swiss Jura was conducted by combining GPR which provides a precise image of the most surficial voids and structures, with gravimetry which is sensitive to the presence of deeper and larger heterogeneities (Beres et al., 2001). Debeglia et al. (2006) proposed microgravity and Multi-channel Analysis of Surface Waves (MASW) coupling to detect and characterise karst structure in an urban environment. This study evaluates the potential of using surface wave tomography (Long, L.T., and Kocaoglu, A.H., 1999, Abbott et al. 2006) with microgravity to image complex near surface cavities. The study area is located in Souzay-Champigny, France, and consists of an abandoned underground quarry network in cretaceous chalky limestone. The cavities are accessible and were partly mapped. The study focuses on the largest mapped cavity.
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