Mysterious seismoacoustic signals of eastern Helwan quarry blasts 2022

Mysterious seismoacoustic events were reported at the beginning of 2022 near Helwan Cairo, Egypt. The majority of these events were recorded by the Egyptian National Seismic Network. The source characteristics of the events were unknown. In May 2022, a temporary infrasound array station was established with a small aperture of 450 m in Helwan. Throughout the 6-month monitoring period, we employed a recursive short-term average/long-term average trigger method across all sensors, leading to the detection of the impulsive seismoacoustic events. Infrasound propagation models, coupled with F-K analysis, further confirmed the locations and directions of the recorded events, providing robust data that could be correlated with planet satellite images of the azimuth directions detected via the infrasound array analyses. The mysterious signals were identified as originating from a major construction project in Egypt: the high-speed railway train corridor. Our study demonstrates the effectiveness of integrating seismic sensors with infrasound arrays for enhanced source characterisation. The combination of these tools enabled precise discrimination of quarry blasts in eastern Helwan. Additionally, our findings suggest that inexpensive sensors can be a cost-effective solution for monitoring higher-frequency events.