Utilizing High-Frequency Seismic Signals (HFSS) and other geophysical methods to locate potential sinkhole collapses: a case study from West of Alexandria, Egypt

Analysis of microtremors, especially the high-frequency seismic signals (HFSS), emitted from shallow sources in the frequency domain 20–20,000 Hz, can be used in investigating various hazard causes, such as landslides, sinkholes, avalanches, shallow cavities and many other applications. These seismic signals can provide knowledge about shallow Earth structure in soil investigations aimed at reducing geological hazards. This technique integrated other geophysical techniques to determine the possible locations of future sinkhole collapses in the Gharabanyat area, located west of Alexandria, Egypt, which is extremely vulnerable to this kind of hazard. Approximately 1 km profile of electrical resistivity tomography, multichannel analysis of surface waves, seismic refraction tomography, and deep ground penetrating radar, combined with borehole drilling data, were used to reconstruct how these sinkholes occurred within the study area. No single geophysical tool was able to predict impending collapse events; however, their integration highlighted the possible locations of collapses expected in the near future, and the study of high-frequency seismic signals (HFSS) in the audible frequency range of 20–20,000 Hz proved particularly helpful for this purpose. This was achieved by recording "high-frequency microtremor signals" at 42 stations, whose analysis showed a possible correlation between areas where only low-frequency signals are detected and the areas of previous collapses. It was concluded that low-frequency signals can be used to identify the possible locations of future collapses and as part of a warning system. The phenomenon of sinkhole collapses is expected to keep taking place unless an adequate solution is found to control the flow of water into the subsurface of the Gharabanyat region.