Observation and Analysis of Low-Frequency Sound Horizontal-Diffraction Using Vector Sensor in the Reefs Water

A significant phenomenon of bottom-interaction in reefs water, at low-frequency sound propagation, is the horizontal diffraction of energy caused by the noteworthy changes of the topography. To observe and analyze the horizontal diffraction phenomenon, an experiment was carried out in the North Reef of the South China Sea in 2018. Although simulated results illustrated that no effective acoustic path can reach the station placed behind the reef, the explosive signals were still recorded by Ocean Bottom Seismometer (OBS). Using the vector information received by the velocity sensor of the OBS, the back-azimuths of this signal arrival can be accurately estimated. Meanwhile, the sound field distribution in the experimental area was simulated by a three-dimensional parabolic equation method. The back-azimuth estimation and the simulated sound field distribution show that the received sound signal behind the reefs comes from the horizontal diffraction at the edge of the reef, rather than the source-receiving path given by GPS. The horizontal diffraction phenomenon can make part of low-frequency energy diffract to the back of the reefs and continue to spread, then reduce the acoustic shadow zone behind the reefs.