Pre-Collapse Detection of Failure in Cavities by Nanoseismic Monitoring

Nanoseismic Monitoring (NM) is a new development in applied seismology, which aimed at monitoring low to extremely-low energy source processes (ML> −4.0). NM was proved successful in detecting incipient failures generated with instable cliffs and active sinkholes. The feasibility of NM to detect pre-collapse failure of underground structures was tested within the ancient (ca. 1100 years old) bell-shaped caverns at the Bet Guvrin National Park, Israel. This site was selected because of the homogenous and massive nature of the host rock, the presence of spaced discontinuities and abundance of shallow underground openings. A series of field investigations with instrument deployment were carried out between March and November 2008. Data analysis shows that it is indeed possible to detect extremely weak spiky signals, whose characterization and location indicate that they are generated by brittle failure within the chalk material of the bell-shaped caves.