Pre-collapse identification of sinkholes in unconsolidated media at Dead Sea area by 'nanoseismic monitoring' (graphical jackknife location of weak sources by few, low-SNR records)

The sudden failure of near-surface cavities and the resulting sinkholes have constituted a recent hazard affecting the populations, lifelines and the economy of the Dead Sea region. This paper describes how seismic monitoring techniques could detect the extremely low-energy signals produced by cavitation in unconsolidated, layered media. Dozens of such events were recorded within a radius of 200 m during several night-time experiments carried out along the western Dead Sea shores. The absence of prior knowledge about cavitation-induced events in unconsolidated media required an initial signal characterization, for which a series of source processes were simulated in the field under controlled conditions. The waveform analysis by sonograms recognizes two main groups of seismic events: impacts on dry material and impacts in liquid. Our analysis demonstrates that the discrimination between both types of source functions is robust despite the extreme nature of the scatter media. In addition to their association with specific source processes, these events can be precisely located by a graphical, error-resistant jackknifing approach. Using an extended M L scale, their source energy can be quantified, and related to standard seismic activity. In summary, it is now possible to monitor subsurface material failures before sinkhole collapse since the discrimination of impact signals on the basis of their frequency content is indicative of the maturity of the cavitation process.