Observations of Water Column and Bathymetric Effects on the Incident Acoustic Field Associated With Shallow-Water Reverberation Experiments

The Target and Reverberation EXperiment 2013 (TREX13) included a comprehensive reverberation field project in the frequency band of 2-10 kHz, and was carried out off the coast of Panama City, FL, USA, from April 21 to May 17, 2013. A spatially fixed transmit and receive acoustic system was used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties.Extensive in situ measurements, including a multibeam bathymetric survey, chirp sonar subbottom profiling, gravity/diver cores, sediment sound speed and attenuation, interface roughness, windgenerated sea surface waves, and water column properties, were made to support studies of environmental effects on RL. Beamformed RL data are categorized to facilitate studies emphasizing physical mechanisms of 1) bottom reverberation; 2) sea surface impact; and 3) biological impact. This paper is an overview of RL over the entire sea trial, intending to summarize major observations and provide both a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis is placed on the dependence of RL on local geoacoustic properties and sea surface conditions.

Section: Oceanographic Influence On Target Echomentioning

confidence: 99%

Overview of Midfrequency Reverberation Data Acquired During the Target and Reverberation Experiment 2013

Yang

Tang

Hefner

et al. 2018

“…Using (20), I r may now be extrapolated over the entire water column and sediment depth where we require a depth-dependent density ρ(z). An analogous operation is applied to energy density, with both I r and E p + E k as functions of depth shown for mode 2 at frequencies 30 and 45 Hz (see Fig.…”

Section: Resultsmentioning

confidence: 99%

“…Circularity may also be interpreted as the normalized curl of active intensity [20]. For many geometries describing an underwater waveguide, it is both practical and realistic to discuss circularity as a scalar metric with horizontal and vertical particle velocities establishing particle trajectory in the range-depth (r − z) plane.…”

Section: Circularity θ N (Z F )mentioning

confidence: 99%

Vector Acoustic Analysis of Time-Separated Modal Arrivals From Explosive Sound Sources During the 2017 Seabed Characterization Experiment

Dahl

Dall’Osto

2020

Self Cite

The Intensity Vector Autonomous Recorder (IVAR) is a system that records four coherent channels of acoustic data continuously: one channel for acoustic pressure and three channels associated with a triaxial accelerometer from which acoustic particle velocity is obtained. IVAR recorded the vector acoustic field in broadband signals originating from Signal, Underwater Sound (SUS) (Mk-64) charges deployed at 5-13-km range from the fixed IVAR site (mean depth 74.4 m) as part of the 2017 Seabed Characterization Experiment (SBCEX) designed to study the acoustics of fine-grained muddy sediments. Sufficient geometric dispersion at these ranges permitted unambiguous identification of up to four modes as a function of frequency for frequencies less than 80 Hz. From time-frequency analysis of the dispersed arrivals, a single mode (n) and single-frequency (f i) properties are identified at peaks in the narrowband scalar field, with time dependence corresponding to mode group speed. At these time-frequency addresses, four quantities derived from the vector acoustic measurements are formed by coherent combination of pressure and velocity channels: first, modal phase speed; second, circularity, a measure of the normalized curl of active intensity; third, depth-dependent mode speed of energy; and fourth, vertical component of reactive intensity normalized by scalar intensity. A means to compute these quantities theoretically is provided, and a comparison of model results based on a notional geoacoustic representation for the SBCEX experimental area consisting of a single low-speed mud layer over a half-space area versus a Pekeris representation based on the same half-space shows a striking difference, with the field observations also clearly at variance with the Pekeris representation. A fundamental property of mode 2, observed at the IVAR location, is a change in sign for circularity and vertical reactive intensity near 37 Hz that is posited as a constraint observation for mode 2 that must be exhibited by any geoacoustic model that includes a low-speed mudlike layer applied to this location.

“…This study uses a nondimensional form of the data known as circularity Θ(t, f ), which can be interpreted as the normalized curl of active intensity [4]. As in the case for the phase of the cross-spectrum used in [3], the estimates of circularity as a function of time and frequency are conveniently independent of ship source level.…”

mentioning

confidence: 99%

“…Equation ( 5) is a bounded indicator of the vector acoustic field (between ±1), taking on the maximum absolute value in the regions of destructive interference and shown to be effective in tracking the frequency dependence of waveguide interference features, including study of the waveguide invariant β [4].…”

mentioning

confidence: 99%

Range-Dependent Inversion for Seabed Parameters Using Vector Acoustic Measurements of Underwater Ship Noise

Dahl

Dall’Osto

2022

Self Cite

The Intensity Vector Autonomous Recorder (IVAR) measures acoustic particle velocity and pressure simultaneously. IVAR was deployed on the seabed during the 2017 Seabed Characterization Experiment (SBCEX) with the primary objective to study sound propagation within underwater waveguides for which the seabed consists of fine-grained, muddy sediments. In this study, a Bayesian framework is applied to underwater noise recorded by IVAR from a cargo ship traversing the central region of the SBCEX2017 area for the purpose of inversion to characterize sediment properties. The vector acoustic data are in the form of a bounded, nondimensional form known as circularity, a quantity that is independent of the ship noise-source spectrum and that can be interpreted as the normalized curl of active intensity. The inversion model space for the seabed consists of a low-compressional speed layer and underlying basement half-space, with each having compressional and shear components. The interpretative model for producing a replica of the data is based on the plane wave reflection coefficient for a layered, elastic seabed in conjunction with the depth-dependent Green's function that is integrated in the complex wave number plane to obtain pressure and particle velocity fields. The small change in water depth between the location of the ship source and IVAR is addressed using adiabatic mode theory. The inversion results exhibit slow variation over the 20-min observation period, representing approximately 5 km of travel by the ship source.