Hodgkiss
William S.
Hodgkiss
William S.
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ArticleMaximum entropy inference of seabed properties using waveguide invariant features from surface ships(Acoustical Society of America, 2022-04-28) Knobles, David P. ; Neilsen, Tracianne B. ; Wilson, Preston S. ; Hodgkiss, William S. ; Bonnel, Julien ; Lin, Ying-Tsongcoustic data were recorded on two vertical line arrays (VLAs) deployed in the New England Mud Patch during the Seabed Characterization Experiment 2017 in about 75 m of water. The sound recorded during the passage of merchant ships permits identification of singular points for the waveguide invariant β for mode pairs (1,𝑛):𝛽1,𝑛,for 𝑛=2,3,4,5, in the 15–80 Hz band. Using prior geophysical information and an acoustic data sample from the merchant ship KALAMATA, a geoacoustic model 𝔐 of the seabed was developed. Then, using data samples from other merchant ships, a feature-ensemble maximum entropy method is employed to infer the statistical properties of geoacoustic parameter values for the sound speeds in a surface mud layer and a deep sand layer. Technical challenges include a sparsity of observed singular points, the unique identification of mode pairs for an observed singular point, and the deviation of the waveguide from horizontal stratification. A geoacoustic model 𝔐 is developed that reproduced the observed 𝛽≈−1 for f < 20 Hz and mode cutoff features at about 15 Hz. The statistical low-frequency inference of the singular point structure from multiple ships provides evidence of an angle of intromission at the water sediment interface with an average sound speed ratio of about 0.986 and an average sound speed for the deeper sand layer of about 1775 m/s.
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ArticleAcoustic ducting by shelf water streamers at the New England shelfbreak(Acoustical Society of America, 2023-08-01) Johnson, Jennifer J. ; Lin, Ying-Tsong ; Newhall, Arthur E. ; Gawarkiewicz, Glen G. ; Knobles, David P. ; Chaytor, Jason D. ; Hodgkiss, William S.Greater sound speed variability has been observed at the New England shelfbreak due to a greater influence from the Gulf Stream with increased meander amplitudes and frequency of Warm Core Ring (WCR) generation. Consequently, underwater sound propagation in the area also becomes more variable. This paper presents field observations of an acoustic near-surface ducting condition induced by shelf water streamers that are related to WCRs. The field observations also reveal the subsequent disappearance of the streamer duct due to the passage of a WCR filament. These two water column conditions are investigated with sound propagation measurements and numerical simulations.
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ArticleTrans-dimensional inversion for seafloor properties for three mud depocenters on the New England shelf under dynamical oceanographic conditions(Acoustical Society of America, 2024-03-06) Bonnel, Julien ; Dosso, Stan E. ; Hodgkiss, William S. ; Ballard, Megan S. ; Garcia, Dante D. ; Lee, Kevin M. ; McNeese, Andrew R. ; Wilson, Preston S.This paper presents inversion results for three datasets collected on three spatially separated mud depocenters (hereafter called mud ponds) during the 2022 Seabed Characterization Experiment (SBCEX). The data considered here represent modal time-frequency (TF) dispersion as estimated from a single hydrophone. Inversion is performed using a trans-dimensional (trans-D) Bayesian inference method that jointly estimates water-column and seabed properties along with associated uncertainties. This enables successful estimation of the seafloor properties, consistent with in situ acoustic core measurements, even when the water column is dynamical and mostly unknown. A quantitative analysis is performed to (1) compare results with previous modal TF trans-D studies for one mud pond but under different oceanographic condition, and (2) inter-compare the new SBCEX22 results for the three mud ponds. Overall, the estimated mud geoacoustic properties show no significant temporal variability. Further, no significant spatial variability is found between two of the mud ponds while the estimated geoacoustic properties of the third are different. Two hypotheses, considered to be equally likely, are explored to explain this apparent spatial variability: it may be the result of actual differences in the mud properties, or the mud properties may be similar but the inversion results are driven by difference in data information content.
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ArticleFeature-based maximum entropy for geophysical properties of the seabeda)(Acoustical Society of America, 2024-06-03) Knobles, David P. ; Hodgkiss, William S. ; Chaytor, Jason D. ; Neilsen, Tracianne B. ; Lin, Ying-TsongThe coherent recombination of a direct and seabed reflected path is sensitive to the geophysical properties of the seabed. The concept of feature-based inversion is used in the analysis of acoustic data collected on a vertical line array (VLA) on the New England continental shelf break in about 200 m of water. The analysis approach for the measurements is based on a ray approach in which a direct and bottom reflected path is recombined, resulting in constructive and destructive interference of the acoustic amplitudes with frequency. The acoustic features have the form of prominent nulls of the measured received levels as a function of frequency as a broadband (500–4500 Hz) source passes the closest point of approach to the VLA. The viscous grain shearing (VGS) model is employed to parameterize a two-layer seabed model. The most likely seabed is a sand sediment with a porosity of about 0.42. There is a possibility of a thin (less than 0.5 m) surface layer having a slightly higher porosity between 0.45 and 0.50. Using the estimates for the VGS parameters inferred from the short-range frequency features, a normal mode model is used to predict the received acoustic levels over larger range scales.