Pre-failure event location and waveform characterization by nanoseismic monitoring

The Nanoseismic Monitoring (NM) technique was studied in laboratory by monitoring incipient microcracking generated by unreinforced concrete beams and limestone plates undergoing four-point bending tests. These tests show that progressive loading triggers a wide range of impulsive signals whose frequency and rate patterns evolve until complete material failure occurs. Later, NM technique was applied to monitor microcracking generated by unstable archeological caverns excavated in natural chalk. Although signals can be detected in unfavorable SNR conditions by a single array at slant-distances beyond 102 m, reliable locations are obtained when several mini-arrays are deployed in the vicinity of caverns suspected to be unstable. Epicentral locations of microcracking events tend to cluster near free boundaries and in zones of high tensile stress as predicted by numerical models computed for these caverns.