Distinctive acoustic multipath propagation over a low velocity layer of sediments in deep ocean

Very-low-frequency (VLF) (in the range 1-100 Hz) sound propagation is significantly influenced by the properties of the seafloor, especially the presence of low velocity bottom (LVB) layers. During a VLF sound propagation experiment in the northwestern sub-basin of the South China Sea (about 3800 m), a distinctive acoustic multipath excited by a source near the sea surface was observed for the first time. This phenomenon is characterized by the arrival of low-frequency components preceding high-frequency components with bandpass effects around 30-60 Hz, as recorded by an ocean-bottom-seismometer that received signals from broadband explosive sources deployed at approximately 200 m. Numerical simulations reveal that this multipath is formed by the sediment borne mode in the LVB layer, which corresponds to the distinctive acoustic path that refracts in the water column and interacts with the LVB layer at a small grazing angle. Unlike previous studies in an isovelocity profile, the sediment borne mode oscillates with a certain amplitude distribution rather than exponentially decaying in the water column under the incomplete sound channel, leading to the observation of distinctive multipath even when the source is near the surface. Furthermore, through geoacoustic inversion, the sound velocity of the LVB layer aligns well with previous in situ measurements.