Subsurface acoustic ducts in the Northern California current system.

This study investigates the subsurface sound channel or acoustic duct that appears seasonally along the U.S. Pacific Northwest coast below the surface mixed layer.

Acoustic resonances within the surficial layer of a muddy seabed.

This is an investigation of sound propagation over a muddy seabed at low grazing angles. Data were collected during the 2017 Seabed and Bottom Characterization Experiment, conducted on the New England Mud Patch, a 500 km area of the U.S.

The mutual scattering cross section.

A generalization of the conventional interface scattering cross section is introduced. This new object will be called the mutual scattering cross section, and, like the conventional cross section, can be used in narrow-band sonar applications. It can treat both sea-surface and seafloor scattering and is useful in cases where large arrays are employed as well as in multipath environments.

Macroscopic observations of diel fish movements around a shallow water artificial reef using a mid-frequency horizontal-looking sonar.

The twilight feeding migration of fish around a shallow water artificial reef (a shipwreck) was observed by a horizontal-looking, mid-frequency sonar. The sonar operated at frequencies between 1.8 and 3.

A time-domain model for seafloor scattering.

Bottom scattering is important for a number of underwater applications: it is a source of noise in target detection and a source of information for sediment classification and geoacoustic inversion. While current models can predict the effective interface scattering strength for layered sediments, these models cannot directly compute the ensemble averaged mean-square pressure. A model for bottom scattering due to a point source is introduced which provides a full-wave solution for mean-square scattered pressure as a function of time under first-order perturbation theory.

Reverberation clutter induced by nonlinear internal waves in shallow water.

Clutter is related to false alarms for active sonar. It is demonstrated that, in shallow water, target-like clutter in reverberation signals can be caused by nonlinear internal waves. A nonlinear internal wave is modeled using measured stratification on the New Jersey shelf.

Application of small-roughness perturbation theory to reverberation in range-dependent waveguides.

A rough-interface reverberation model is developed for range-dependent environments. First-order perturbation theory is employed, and the unperturbed background medium can be layered and heterogeneous with arbitrary range dependence. To calculate the reverberation field, two-way forward scatter due to the slowly changing unperturbed environment is handled by fast numerical methods.

Modeling interface roughness scattering in a layered seabed for normal-incident chirp sonar signals.

Downward looking sonar, such as the chirp sonar, is widely used as a sediment survey tool in shallow water environments. Inversion of geo-acoustic parameters from such sonar data precedes the availability of forward models. An exact numerical model is developed to initiate the simulation of the acoustic field produced by such a sonar in the presence of multiple rough interfaces.

Mid-frequency geoacoustic inversion using bottom loss data from the Shallow Water 2006 Experiment.

Geoacoustic inversion work has typically been carried out at frequencies below 1 kHz, assuming flat, horizontally stratified bottom models. Despite the relevance to Navy sonar systems many of which operate at mid-frequencies (1-10 kHz), limited inversion work has been carried out in this frequency band. This paper is an effort to demonstrate the viability of geoacoustic inversion using bottom loss data between 2 and 5 kHz.

Internal waves as a proposed mechanism for increasing ambient noise in an increasingly acidic ocean.

The effect on the ambient noise level in shallow water of the ocean growing more acidic is modeled. Because most noise sources are near the surface, high-order acoustic modes are preferentially excited. Linear internal waves, however, can scatter the noise into the low-order, low-loss modes most affected by the changes in acidity.

Mid-frequency acoustic propagation in shallow water on the New Jersey shelf. II. Intensity fluctuation.

The scintillation index and the intensity cumulative distribution function of mid-frequency (2-10 kHz) sound propagation are presented at ranges of 1-9 km in a shallow water channel. The fluctuations are due to water column sound speed variability. It is found that intensity is only correlated over a narrow frequency band (50-200 Hz) and the bandwidth is independent of center frequency and range.

Mid-frequency acoustic propagation in shallow water on the New Jersey shelf: mean intensity.

Mid-frequency (1-10 kHz) sound propagation was measured at ranges 1-9 km in shallow water in order to investigate intensity statistics. Warm water near the bottom results in a sound speed minimum. Environmental measurements include sediment sound speed and water sound speed and density from a towed conductivity-temperature-depth chain.

Mid-frequency sound propagation through internal waves at short range with synoptic oceanographic observations.

Preliminary results are presented from an analysis of mid-frequency acoustic transmission data collected at range 550 m during the Shallow Water 2006 Experiment. The acoustic data were collected on a vertical array immediately before, during, and after the passage of a nonlinear internal wave on 18 August, 2006. Using oceanographic data collected at a nearby location, a plane-wave model for the nonlinear internal wave's position as a function of time is developed.

Direct measurement of sediment sound speed in Shallow Water '06.

Knowledge of sediment sound speed is crucial for predicting sound propagation. During the Shallow Water '06 experiment, in situ sediment sound speed was measured using the Sediment Acoustic-speed Measurement System (SAMS). SAMS consists of ten fixed sources and one receiver that can reach a maximal sediment depth of 3 m.

Internal wave effects on the ambient noise notch in the East China Sea: model/data comparison.

The vertical directivity pattern of the ambient noise field observed in shallow water is typically anisotropic with a trough in the horizontal. This trough, often called the ambient noise notch, develops because downward refraction steepens all rays emanating from near the sea surface. Variability in the environment has the potential to redistribute the noise into shallower angles and thereby fill the notch.