David R. Dall'Osto

Vertical coherence and forward scattering from the sea surface and the relation to the directional wave spectrum

Results of an experiment to measure vertical spatial coherence from acoustic paths interacting once with the sea surface but at perpendicular azimuth angles are presented. The measurements were part of the Shallow Water 2006 program that took place off the coast of New Jersey in August 2006. An acoustic source, frequency range 6-20 kHz, was deployed at depth 40 m, and signals were recorded on a 1.4 m long vertical line array centered at depth 25 m and positioned at range 200 m. The vertical array consisted of four omni-directional hydrophones and vertical coherences were computed between pairs of these hydrophones. Measurements were made over four source-receiver bearing angles separated by 90°, during which sea surface conditions remained stable and characterized by a root-mean-square wave height of 0.17 m and a mixture of swell and wind waves. Vertical coherences show a statistically significant difference depending on source-receiver bearing when the acoustic frequency is less than about 12 kHz, with results tending to fade at higher frequencies. This paper presents field observations and comparisons of these observations with two modeling approaches, one based on bistatic forward scattering and the other on a rough surface parabolic wave equation utilizing synthetic sea surfaces.

The underwater sound field from vibratory pile driving.

Underwater noise from vibratory pile driving was observed using a vertical line array placed at range 16 m from the pile source (water depth 7.5 m), and using single hydrophones at range 417 m on one transect, and range 207 and 436 m on another transect running approximately parallel to a sloping shoreline. The dominant spectral features of the underwater noise are related to the frequency of the vibratory pile driving hammer (typically 15-35 Hz), producing spectral lines at intervals of this frequency. The mean-square pressure versus depth is subsequently studied in third-octave bands. Depth and frequency variations of this quantity observed at the vertical line array are well modeled by a field consisting of an incoherent sum of sources distributed over the water column. Adiabatic mode theory is used to propagate this field to greater ranges and model the observations made along the two depth-varying transects. The effect of shear in the seabed, although small, is also included. Bathymetric refraction on the transect parallel to the shoreline reduced mean-square pressure levels at the 436-m measurement site.

Elliptical acoustic particle motion in underwater waveguides

Elliptical particle motion, often encountered in acoustic fields containing interference between a source signal and its reflections, can be quantified by the degree of circularity, a vector quantity formulated from acoustic particle velocity, or vector intensity measurements. Acoustic analysis based on the degree of circularity is expected to find application in ocean waveguides as its spatial dependence relates to the acquisition geometry, water column sound speed, surface conditions, and bottom properties. Vector sensor measurements from a laboratory experiment are presented to demonstrate the depth dependence of both the degree of circularity and an approximate formulation based on vertical intensity measurements. The approximation is applied to vertical intensity field measurements made in a 2006 experiment off the New Jersey coast (in waters 80 m deep) to demonstrate the effect of sediment structure on the range dependence of the degree of circularity. The mathematical formulation presented here establishes the framework to readily compute the degree of circularity from experimental measurements; the experimental examples are provided as evidence of the spatial and frequency dependence of this fundamental vector property.

Measurement of acoustic particle motion in shallow water and its application to geoacoustic inversion.

Within an underwater acoustic waveguide, the interference among multipath arrivals causes a phase difference in orthogonal components of the particle velocity. When two components of the particle velocity are not in phase, the fluid particles follow an elliptical trajectory. This property of the acoustic field can be readily detected by a vector sensor. A non-dimensional vector quantity, the degree of circularity, is used to quantify how much the trajectory resembles a circle. In this paper, vector sensor measurements collected during the 2013 Target and Reverberation Experiment are used to demonstrate the effect of multipath interference on the degree of circularity. Finally, geoacoustic properties representing the sandy sediment at the experimental site are inverted by minimization of a cost function, which quantifies the deviation between the measured and modeled degree of circularity.

On the underwater sound field from impact pile driving: Arrival structure, precursor arrivals, and energy streamlines

Underwater noise from impact pile driving is studied through measurements using a vertical line array (VLA) placed at range 120 m from the pile source (water depth 7.5 m) over which bathymetry varied gradually increasing to depth 12.5 m at the VLA. The data were modeled assuming the pile impact produces a radial expansion that acts as sound source and propagates along the pile at supersonic speed. This leads to the conceptualization of the pile as a discrete, vertical line source for which frequency- and source-depth-dependent complex phasing is applied. Dominant features of the pressure time series versus measurement depth are reproduced in modeled counterparts that are linearly related. These observations include precursor arrivals for which arrival timing depends on hydrophone depth and influence of a sediment sound speed gradient on precursor amplitude. Spatial gradients of model results are taken to obtain estimates of acoustic particle velocity and vector intensity for which active intensity is studied in the time domain. Evaluation of energy streamlines based on time-integrated active intensity, and energy path lines based on instantaneous (or very-short-time integrated) active intensity reveal interesting structure in the acoustic field, including an inference as to the source depth of the precursor.

The sea surface directional wave spectrum and forward scattering from the sea surface

An influence of the directional wave spectrum on acoustic forward scattering from the sea surface is difficult to measure. Here we present results of an experiment to measure vertical spatial coherence from an acoustic path interacting once with the sea surface at two different angles with respect to the wave direction. The measurements were part of the Shallow-Water 2006 program that took place off the coast of New Jersey in August 2006. An acoustic source was deployed at depth 40 m, and signals were recorded on a moored receiving system consisting of two, 1.4 m long vertical line arrays centered at depths 25 and 50 m. Measurements were made over four source-receiver bearing angles separated by 90°, during which sea surface conditions remained stable and characterized by an rms waveheight of 0.17 m and a mixed swell, and wind-wave field originating from different directions. The measurements show a statistically significant difference depending on source-receiver bearing when the acoustic frequency is le...

The effect of bottom layering on the acoustic vector field

A signal reflected from a layered sea-bed contains information pertaining to the sediment properties. Typically, a signal intended to probe the sea-bed is designed to have a large bandwidth to allow for time separation of arrivals from the multiple layers. Depending on the geometry, it may impossible to avoid interference of these arrivals. The interference of these multiple arrivals does establish a pattern observable in the vector intensity. Measurements of the vertical complex acoustic intensity of a near-bottom source (~λ from the seafloor) collected off the coast of New Jersey in 2006 demonstrate the effect of a sub-bottom layer and the observable interference pattern between the first bottom reflection and the sub-bottom reflection. The spatial structure of the complex intensity can be used to infer bottom properties, which are in close agreement with a number of experimental studies at this location. The observable in the complex intensity can also be directly measured with a particle motion sensor...

Implications of signal intensity fluctuations on vector sensor array processing

Vector sensor processing relies on the covariance matrix for both a single vector sensor and a larger matrix from a vector sensor array. The elements of these covariance matrices have physical interpretation in terms of complex intensity. The presence of reactive intensity on the array shows up in the off diagonal elements of the covariance matrix and has significant implications on direction of arrival (DOA) algorithms. Sources of reactive intensity in an underwater waveguide are dependent on the geometry of the system and fluctuations in these quantities affect the ability to increase the array aperture to better resolve arrival angles.

Trans-dimensional geoacoustic inversion on the New England Mud Patch using modal dispersion data

This paper presents trans-dimensional geoacoustic inversion of two low-frequency broadband signals recorded during the Seabed Characterization Experiment (SBCEX). The considered sources are a chirp emitted by a towed J15 source and an underwater impulse created by a combustive sound source (CSS). Corresponding received signals are recorded on single hydrophones, and the two source/receiver configurations are reciprocal, on the same track. Input data for inversion are modal time-frequency dispersion curves, estimated using warping. Inversion is then performed within a Bayesian framework. A trans-dimensional inverse algorithm is used to estimate the model parametrization (i.e., number of seabed layers), and the mode covariance matrices are estimated as a first-order autoregressive error process. Inversion results on the two reciprocal tracks are consistent with the modal information available in each dataset. Results are also consistent with what is known about the area.This paper presents trans-dimensional geoacoustic inversion of two low-frequency broadband signals recorded during the Seabed Characterization Experiment (SBCEX). The considered sources are a chirp emitted by a towed J15 source and an underwater impulse created by a combustive sound source (CSS). Corresponding received signals are recorded on single hydrophones, and the two source/receiver configurations are reciprocal, on the same track. Input data for inversion are modal time-frequency dispersion curves, estimated using warping. Inversion is then performed within a Bayesian framework. A trans-dimensional inverse algorithm is used to estimate the model parametrization (i.e., number of seabed layers), and the mode covariance matrices are estimated as a first-order autoregressive error process. Inversion results on the two reciprocal tracks are consistent with the modal information available in each dataset. Results are also consistent with what is known about the area.

Geoacoustic inversion of the New England Mud Patch using modal time-frequency dispersion

This paper presents geoacoustic inversion of several low-frequency broadband signals recorded during the Seabed Characterization Experiment (SBCEX) that took place on the New England Mud Patch in March 2017. The considered sources are chirps emitted by a towed J15 source and underwater impulses created by MK64 explosions. These sources are low-frequency (f<250 Hz) so that the shallow water environment acts as a dispersive waveguide, and propagation is conveniently described by modal theory. The received signal is recorded on a single hydrophone placed 0.8 m off the bottom. In this context, inversion is carried out by matching modal dispersion curves in the time-frequency domain. Experimental dispersion curves are estimated using a non-linear sampling scheme called warping. Up to six modes can be estimated, and non-linear inversion results are consistent with what is known about the area.