Jason D. Sagers

Evidence of three-dimensional acoustic propagation in the Catoche Tongue

This paper presents observations of two classes of acoustic arrivals recorded on a sparsely populated vertical line array (VLA) moored in the center of the Catoche Tongue, a major reentrant in the Campeche Bank in the southeastern Gulf of Mexico. The acoustic signals were generated by signals underwater sound (SUS) located 50-80 km from the VLA. The first class of arrivals was identified as resulting from a direct (non-horizontally refracted) path. Then following a quiescent period, a second, more diffuse class of arrivals is observed and is believed to be the result of horizontal refraction from the margin of the Tongue. A spectral analysis of the measured data revealed that both classes of arrivals were characterized by the source spectrum associated with SUS. Additionally, the difference in time between the onset of the first and second class of arrivals observed as a function of range from the VLA is consistent with the relative difference in the length of the direct and refracted paths. The observations are further supported by a three-dimensional (3D) acoustic propagation computation that reproduces many of the features of the measured data and provides additional insight into the details of the 3D propagation.

An impulsive source with variable output and stable bandwidth for underwater acoustic experiments

The Combustive Sound Source (CSS) is being developed as an environmentally friendly source to be used in ocean acoustics research and surveys. It has the ability to maintain the same wide bandwidth signal over a 20 dB drop in source level. The CSS consists of a submersible combustion chamber filled with a fuel/oxidizer mixture. The mixture is ignited and the ensuing combustion and bubble activity radiates an impulsive, thus broadband, acoustic pulse. The ability to control pulse amplitude while maintaining bandwidth is demonstrated.

Development of a standing wave apparatus for calibrating acoustic vector sensors and hydrophones

An apparatus was developed to calibrate acoustic hydrophones and vector sensors between 25 and 2000 Hz. A standing wave field is established inside a vertically oriented, water-filled, elastic-walled waveguide by a piston velocity source at the bottom and a pressure-release boundary condition at the air/water interface. A computer-controlled linear positioning system allows a device under test to be precisely located in the water column while the acoustic response is measured. Some of the challenges of calibrating hydrophones and vector sensors in such an apparatus are discussed, including designing the waveguide to mitigate dispersion, understanding the impact of waveguide structural resonances on the acoustic field, and developing algorithms to post-process calibration measurement data performed in a standing wave field. Data from waveguide characterization experiments and calibration measurements are presented and calibration uncertainty is reported.

Numerical analysis of three-dimensional acoustic propagation in the Catoche Tongue

Analysis of modeled time series data is presented to provide insight into propagation physics of horizontally refracted sound in the Catoche Tongue region of the Gulf of Mexico. The analysis is motivated by the observation of out-of-plane arrivals in measured time series data. In particular, the extended duration of the refracted arrivals is shown to be caused by interaction with multiple locations along the steep sides of the Tongue. Comparison of the modeled time series is made to previous work by Sturm [J. Acoust. Soc. Am. 117(3), 1058-1079 (2005)], who examined the frequency dependence of out-of-plane modal arrivals for the wedge-shaped ocean.

Compressional and shear in situ measurements in the Lower Laguna Madre

This presentation describes the apparatus, procedure, and results from a field experiment conducted in July 2017 in the seagrass beds of the Lower Laguna Madre. The Laguna Madre is a shallow hypersaline lagoon on the western coast of the Gulf of Mexico. The relatively flat bathymetry is covered by a thick meadow of Thalassia testudinum. Acoustic piezoelectric transducer probes measured compressional wave speed and attenuation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the underlying sediment. Geoacoustic parameters were measured at depths between 5 cm and 20 cm in 5 cm increments. Compressional wave measurements were made over a frequency range of 20 kHz to 100 kHz, and shear wave measurements were made at 500 Hz and 1 kHz. The compressional wave data from this experiment indicates dispersion within the seagrass canopy and sediment. This dispersion is likely due to the high presence of gas bubbles in both the seagrass and supporting sed...

The effects of ice cover and oceanography on medium-frequency acoustic propagation on the Chukchi Shelf

The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.

Properties of the ambient noise field recorded at the 150 m isobath during the 2016–2017 Canadian Basin Acoustic Propagation Experiment

The Applied Research Laboratories at the University of Texas at Austin (ARL:UT) deployed two passive acoustic recording systems along the 150 m isobath of the Chukchi Shelf during the 2016–2017 Canadian Basin Acoustic Propagation Experiment (CANAPE). The first system was a single-hydrophone recorder located on the seafloor, while the Persistent Acoustic Observation System (PECOS) contained a horizontal line array of hydrophones along the seabed and a vertical line array spanning a portion of the water column. The systems were deployed and recovered during open-water conditions, but remained in place during the ice-formation, ice-covered, and ice-melt time periods. This talk presents initial findings of the statistical ambient noise levels during the year-long experiment, presents beam-noise levels recorded by PECOS, and qualitatively discusses the natural, biologic, and anthropogenic sounds present in the acoustic recordings. [Work sponsored by ONR.]

An introduction to research topics in underwater acoustics

The Acoustical Society of America Technical Committee on Underwater Acoustics (UW) investigates sound wave phenomena in marine environments, including oceans, lakes, and rivers. Research interests span a broad spectrum from the measurement and modeling of acoustic propagation and scattering, to the detection and characterization of underwater sound, to signal processing algorithms and statistics. This diverse technical committee also shares interests with Animal Bioacoustics (AB), Acoustical Oceanography (AO), and Signal Processing (SP). This talk will highlight a few past and present research topics in underwater acoustics, emphasizing the important role of sound as a tool in subsea research and exploration.The Acoustical Society of America Technical Committee on Underwater Acoustics (UW) investigates sound wave phenomena in marine environments, including oceans, lakes, and rivers. Research interests span a broad spectrum from the measurement and modeling of acoustic propagation and scattering, to the detection and characterization of underwater sound, to signal processing algorithms and statistics. This diverse technical committee also shares interests with Animal Bioacoustics (AB), Acoustical Oceanography (AO), and Signal Processing (SP). This talk will highlight a few past and present research topics in underwater acoustics, emphasizing the important role of sound as a tool in subsea research and exploration.

Seabed properties at the 150 m isobath as observed during the 2016–2017 Canada Basin Acoustic Propagation Experiment

Seabed layering and sediment properties impact sound propagation in ocean waveguides, particularly in environments where sound propagation paths repeatedly interact with the seafloor. As part of the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), experiments were designed to investigate seabed layering and sediment properties on the Chukchi Shelf. First, the shallow water experimental region was surveyed with a subbottom profiler to provide information about the overall sediment layering. Second, ship-radiated noise from a research vessel sailing specifically designed tracks was received on the Persistent Acoustic Observation System (PECOS). These recordings provide an opportunity for short- to mid-range geoacoustic inversion for sediment properties. Third, in-situ acoustic sound speed measurements were made with the Acoustic Coring System (ACS) while two to five meter long core samples were simultaneously collected. This talk presents initial findings of the seabed layering and sediment properties from these three experiments. [Work sponsored by ONR.] Seabed layering and sediment properties impact sound propagation in ocean waveguides, particularly in environments where sound propagation paths repeatedly interact with the seafloor. As part of the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), experiments were designed to investigate seabed layering and sediment properties on the Chukchi Shelf. First, the shallow water experimental region was surveyed with a subbottom profiler to provide information about the overall sediment layering. Second, ship-radiated noise from a research vessel sailing specifically designed tracks was received on the Persistent Acoustic Observation System (PECOS). These recordings provide an opportunity for short- to mid-range geoacoustic inversion for sediment properties. Third, in-situ acoustic sound speed measurements were made with the Acoustic Coring System (ACS) while two to five meter long core samples were simultaneously collected. This talk presents initial findings of the seabed layering and sediment ...

A comparison between directly measured and inferred wave speeds from an acoustic propagation experiment in Currituck Sound

An acoustic propagation experiment was conducted in Currituck Sound to characterize low-frequency propagation in a very-shallow-water estuarine environment. The water column properties were homogeneous over the study area, and the emphasis of this work is on understanding the propagation effects induced by the estuarine bed. During the experiment, low-frequency sound propagation measurements of waterborne sound and interface waves were acquired, and direct measurements of the compressional and shear wave properties were obtained at high frequencies. The propagation data consist of signals from a Combustive Sound Source recorded on bottom mounted geophones and a vertical line array of hydrophones. A statistical inference method was applied to obtain an estimate of the sediment compressional and shear wave speed profiles as a function of depth within the estuarine bed. The direct measurements were obtained in situ by inserting probes 30 cm into the sediment. Sediment acoustics models were fit to the high-frequency in situ data to enable comparison with the inferred low-frequency wave speeds. Overall, good agreement was found between the directly measured and inferred wave speeds for both the compressional and shear wave data.