Cynthia J. Sellers

Measurement and prediction of ultralow frequency ocean ambient noise off the eastern U.S. coast.

Ultralow frequency (0.02-2 Hz) acoustic ambient noise was monitored from January to April 1991 at six ocean bottom stations off the eastern U.S. coast. The depths of the stations ranged from about 100 m to 2500 m. The measured spectra are in good agreement with predictions made using Catos theory [J. Acoust. Soc. Am. 89, 1076-1095 (1991)] for noise generation by surface-wave orbital motion after extending the calculations to incorporate horizontally stratified environments. Contributions from both the linear, single-frequency (virtual monopole) and the nonlinear, double-frequency (dipole) mechanisms are clearly recognizable in the data. The predictions make use of directional wave data obtained from surface buoys deployed during the SWADE experiment and an ocean bottom model derived from compressional wave speed data measured during the EDGE deep seismic reflection survey. The results demonstrate conclusively that nonlinear surface-wave interactions are the dominant mechanism for generating deep-ocean ULF noise in the band 0.2-0.7 Hz.

Determination of compressional wave and shear wave speed profiles in sea ice by crosshole tomography—Theory and experiment

Sea ice is a heterogeneous material whose acoustic properties are functions of time and space. Results of a crosshole tomography experiment conducted in multi‐year ice with the objective of determining the spatial structure of the compressional and shear wave speeds from travel time measurements made with high‐frequency pulses are presented here. The results of the experiment indicate that the wave speeds can be determined from such a crosshole experiment with good resolution. The compressional and shear wave speed contour maps indicate that the spatial variations of the wave speeds are complex with regions of low speed. Low‐speed regions observed are likely caused by high brine volume content. Resolution and variance studies performed on the estimates are also presented. Material properties such as Poisson’s ratio, salinity, and elastic and shear moduli of sea ice are obtained from the estimates of compressional and shear wave speeds. By measuring the amplitude of the transmitted and received signals alo...

Modal Mapping Experiment and Geoacoustic Inversion Using Sonobuoys

This paper summarizes the results of an experiment whose primary goal was to demonstrate that reliable geoacoustic inversion results can be obtained in shallow water by postprocessing acoustic data acquired by Global Positioning System (GPS)-capable sonobuoys. The experiment was conducted aboard the R/V Sharp on March 5-18, 2011 off the coast of New Jersey using AN/SSQ-53F sonobuoys with a GPS capability as well as GPS-equipped research buoys originally developed under the Modal Mapping Experiment (MOMAX) project, which provided reliable geoacoustic information to which the sonobuoy results could be compared. It is shown that when low-frequency ( 500 Hz) continuous-wave (CW) signals are acquired on the two types of buoys in a colocated configuration, the geoacoustic models inferred from the sonobuoy data are very similar to those obtained from the MOMAX buoy data. The inversion results also compare favorably with bottom models for the region obtained from other experiments. This work is an important milestone toward achieving the ultimate goal of transitioning a basic research method to an operational scenario in which sonobuoy data are routinely used to infer geoacoustic parameters of the seabed.

Low‐frequency acoustic backscattering by volumetric inhomogeneities in deep‐ocean sediments

An experiment to study acoustic backscattering from deep‐ocean sediments was conducted in July 1993 as part of the Acoustic Reverberation Special Research Program sponsored by the Office of Naval Research. An acoustic source transmitting chirp signals in the frequency range 250–650 Hz and a 24‐element vertical receiving array attached to the source were suspended near the seafloor over a sediment pond in the vicinity of the Mid‐Atlantic Ridge. The array steered in the normal incidence (endfire) direction is used to determine the sediment structure, while the array steered at oblique angles is used to determine backscattering strength. It is found that the sediment is stratified with gentle horizontal changes, except for two irregular regions, with thicknesses of about 20 m, located approximately 18 and 60 m beneath the seafloor. These inhomogeneous regions are shown to be the dominant influence on the backscattered field, and the backscattering strength of each region as a function of grazing angle is est...

The relative effects of sound‐speed variability in the water column versus the seabed on normal‐mode propagation in shallow water

Normal‐mode propagation in shallow water is influenced by variations in the acoustic properties of both the water column and the seabed. Specifically, the sound speed in the water may fluctuate due to the passage of an internal wave, while the sound speed in the bottom may change due to variable geological features. Assessing the relative effects of the water column versus the seabed on the characteristics of modal propagation is critical to the understanding of both the forward and inverse problems in shallow‐water acoustics. In this paper, perturbation theory is combined with a Pekeris waveguide model to provide an analytic formalism for evaluating the delicate interplay between the water column and the seabed in shallow water propagation. In particular, the relative contributions of sound‐speed fluctuations in the water and the bottom to variations in the modal eigenvalues are determined. The results are affected by both the strengths of the fluctuations and the magnitudes of the background modal eigen...

Observations of region-specific fish behavior using long- and short-range broadband (1.5—6 + kHz) active acoustic systems

Two broadband active acoustic systems, in concert with traditional narrowband systems and nets, were used to study distributions of fish in three regions within the Gulf of Maine. The long-range multi-beam broadband system detected fish out to 15 km range and the downward-looking short-range broadband system detected fish throughout the water column close behind the ship. The multi-year (2007–2011) study revealed distinct spatial patterns of fish and corresponding echo statistics in each region—diffusely distributed, sparsely distributed compact patches, and long (continuous) shoals. The broadband capabilities of the sonar systems (each spanning 1.5—6 + kHz) uniquely allow observations of resonance phenomena of the local swimbladder-bearing fish. The observed resonances were consistent with the fish species, sizes, and depths that were concurrently sampled in each area from a second research vessel. Spectral peak analysis also interestingly revealed the presence of distinct modes, which may be useful indi...

Measurements and modeling of midfrequency clutter from fish aggregations over Georges Bank in the Gulf of Maine.

Broadband midfrequency acoustic measurements of fish backscattering were made with two complementary sonar systems in September 2008 at the northern slopes of Georges Bank in the Gulf of Maine. One system, towed near the surface, was downward‐looking (short‐range: < 0.2 km) and was used to image fish at high vertical resolution throughout the water column. The other system was horizontal‐looking (relatively long‐range: 1–10 km), consisting of a vertical source array and a horizontal receive array, and was used to image fish at high horizontal resolution. In this talk, the focus is on characterizing the longer‐range data. Spectrogram analysis showed that the echoes had resonance frequencies in the 2–4 kHz band, consistent with scattering by Atlantic herring, while examination of normalized match‐filter output revealed strong, spatiotemporally variable clutter. The associated probability‐density functions (PDFs) exhibited variable non‐Rayleigh behavior. Several PDF models were fit to the data, with the K an...

High‐resolution, three‐dimensional measurements of low‐frequency sound propagation in shallow water

An overview is presented of the modal mapping experiment (MOMAX), which was conducted in March 1997 in the vicinity of the East Coast STRATAFORM site. Both fixed and moving source configurations were used to transmit several pure tones in the frequency range 50–300 Hz. The magnitudes and phases of these signals were recorded on several freely drifting buoys, each containing a hydrophone, GPS and acoustic navigation, and radio telemetry. High‐resolution, three‐dimensional measurements of the sound field were made out to ranges of 10 km and illustrate the influence of the laterally varying seabed, measured with a chirp sonar system. The precision navigation also enabled the creation of a two‐dimensional, synthetic aperture planar array, parallel to the ocean surface. The pressure field data measured on the array were transformed into the wave number domain, where the lateral variability of the waveguide manifests itself in the spatially evolving spectral content of the modal field. Finally, the phases of th...

Simultaneous reconstruction of velocity structure and boundary geometry in crosshole tomography

In recent years, crosshole tomography has been used to infer the velocity structure in the earth’s interior, ocean sediments, and in sea ice. In all cases, velocity estimates were obtained from travel time data for the earliest arrival. In many instances, the field measurements contain not only the information about this direct path but also about paths that have interacted with the boundaries. If one were to use the information contained in such boundary reflected paths, the resolution of the estimates could be considerably improved. This, however, implies that one knows the boundary geometry, which is generally not the case. An approach that one can take is to jointly estimate the boundary geometry and the velocity structure in the material. Different approaches that can be used to implement this idea and evaluate the method using synthetic data and field data are presented. [Work supported by ONR.]

Microscale, finescale, and mesoscale measurements made during the 2004 Structured Mixing Project (Micro-Tow 04) Cruise

Funding was provided by the Office of Naval Research through Contract nos. N00014-03-1-0335 and N00014-14-1-0223.