Introductory review of potential applications of nanoseismic monitoring in seismic energy characterization

Major terrestrial hazards are associated with the formation of fractures that evolve with time and lead to structural collapse. The fracture signals emitted in response to stress accumulation and its release, if properly understood, provide sound background for the development of Early Warning Systems (EWSs). Different attempts have been made in the past for their proper understanding, but in this study Nanoseismic Monitoring (NM) is being discussed, in terms of its sensor employment and signal processing modules. NM is a method dedicated to the detection, localization, and characterization of very low-energy seismic signals (ML<1) at short distances (<10 km). Data are acquired by small aperture (max 200m) seismic arrays that are easy to install and consist of one central three component (3C) sensor surrounded by three vertical one-component (1C) sensors in a tripartite layout that are suited for the beam forming processing. Detection and location of weak events are done by dedicated software: the NanoseismicSuite, which was developed at Stuttgart University, Germany. The signals are processed by sonograms (i.e., spectrograms with a frequency-dependent noise adaptation). The sonograms enhance the display of weak signal energy down to the noise threshold and allow supervised pattern recognition of weak target events in the frequency domain. Locations of weak events are supported by a graphical jackknifing approach. The case studies have shown that NM can successfully detect various weak fracture signals induced by the (stress relief mechanisms of near-surface geoprocesses) landslide dynamics, structural health, hydraulic fracturing, erosional features, pre-mature sinkholes, pending rock fall, and micro-seismicity associated with active faults.