Abstract
An anomalous dispersion, e.g., when low frequencies arrive earlier whereas the high frequencies arrive later, was observed in the signal arrivals recorded by a single deep-sea bottom-mounted vector sensor. Numerical simulations and modal analyses, based on a three-layer range-independent model, are applied to interpret the anomalous dispersion. Results indicate that the arrival with anomalous dispersion corresponds to trapped modes in the low sound speed sediment and can be observed when both the source and receiver are deployed near the seafloor. Furthermore, the cutoff frequencies, dispersion characteristics, and energy distributions of trapped modes are also performed in this paper.
Highlights
Due to the transport and deposition of seawater, a sediment layer containing sand, silt, clay, or biological debris will be formed overlaying the basement
All of the above studies on low sound speed sediment focused on the signals corresponding to waterborne modes travelling in shallow water, while few studies focused on the signals propagating in deep sea or corresponding to sediment borne modes
This paper described an early arrival with anomalous dispersion observed for the first time in a deep-sea experiment conducted on the South China Sea (SCS) northern continental slope
Summary
Due to the transport and deposition of seawater, a sediment layer containing sand, silt, clay, or biological debris will be formed overlaying the basement. Most deep-sea surficial sediment types are clayey silt and silty clay, and they have sound speeds less than that in the water overlying the bottom.. The low sound speed layer overlying a fast sub-bottom forms a secondary waveguide with a cutoff frequency above which sediment borne modes are supported, and a frequency response with bands of high loss was observed and used to invert bottom properties in Yellow Shark inversion experiments.. All of the above studies on low sound speed sediment focused on the signals corresponding to waterborne modes travelling in shallow water, while few studies focused on the signals propagating in deep sea or corresponding to sediment borne modes. We will emphasize the arrival with anomalous dispersion observed in a deep-sea experiment conducted over the northern continental slope of the South China Sea (SCS). Some in situ sediment acoustic measurements reported that the sediment sound speeds in the SCS northern continental slope are less than the seawater sound speed, and sometimes even less than 1450 m/s.12,13 The goals of this study are to understand the physical mechanisms for the arrival with anomalous dispersion and to investigate its propagation characteristics and energy distributions
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