Steve Stanic

High‐frequency acoustic backscattering from a coarse shell ocean bottom

Acoustic bottom backscattering measurements were taken in a coarse shelly area 27 miles east of Jacksonville, Florida Data from sidescan sonar, underwater television, stereo photography, high‐resolution bathymetry, and sediment core analysis were used to locate and classify the experimental area. Bottom backscattering measurements were made as a function of frequency (20–180 kHz), grazing angle (5°–30°), and azimuthal angle. Backscattering strengths were found to follow Lambert’s law, had a slight negative frequency dependence, and were consistent with measurements taken in other shelly areas. There was no azimuthal dependence of the scattered signals over the range of grazing angles and frequencies used. Bottom roughness had a Gaussian distribution and the ping‐to‐ping scattered signal envelope distributions were non‐Rayleigh. Comparison of scattering strengths from several shelly areas showed little correlation with measured rms roughness. Scattering strength predictions made using a composite roughness...

Shallow‐water high‐frequency bottom scattering off Panama City, Florida

A series of bottom backscattering measurements was made in a flat, uniform, and isotropic area 19 miles south of Panama City, FL. Sidescan sonar, underwater television, stereo photography, high‐resolution bathymetry, and sediment core analysis were used to locate and classify the experimental site. A sidescan sonar areal mosaic was contructed detailing the relationship between the experimental area and the surrounding topography. Bottom backscattering measurements were made as a function of frequency (20–180 kHz), grazing angle (5°–30°), azimuthal angle, and environmental conditions. Backscattering strengths were found to follow Lambert’s law with little frequency dependence or measurable anisotropy. For this particular site, scattering strengths at 90 kHz were found to agree with predictions made using the Applied Physics Laboratory—University of Washington (APL—UW) model.

Shallow-water bottom reverberation measurements

High-frequency bottom reverberation measurements were made at an experimental site in the Gulf of Mexico. The acoustic data were taken as a function of frequency (40-180 kHz) and grazing angle (40-33/spl deg/). The measured acoustic reverberation results are compared to predictions made by models developed by Jackson et al. (1986, 1996) and Boyle and Chotiros (1995). The models used inputs from the analysis of sediment cores and stereophotography. The model predictions show differences from each other and from the data. The results show reverberation-level variabilities as a function of frequency that cannot be accurately predicted by these models.

Reverberation fluctuations from a smooth seafloor

High-frequency shallow-water reverberation statistics were measured from a smooth, sandy, featureless seafloor. The reverberation statistics are presented as a function of source frequency (20-180 kHz), grazing angle (30 degrees , 20 degrees , 9.5 degrees ), and source beamwidths (1.2 degrees -2.75 degrees ). Generally, the reverberation statistics did not follow a Rayleigh fading model. The model dependence of the reverberation statistics exhibited a complex behavior that ranged from near Gaussian to beyond log-normal. The results show that small changes in the source frequency, grazing angles, and beamwidths caused large variations in the model dependence of the reverberation statistics. >

At‐sea measurements of sound penetration into sediments using a buried vertical synthetic array

Acoustic bottom penetration experiments were carried out in a medium-grain sandy bottom at a site in St. Andrews Bay, Florida. These investigations used a new buried, vertical, one-dimensional synthetic array system where a small hydrophone was water-jetted into the sediment to a depth of approximately 2 m. Once buried, this hydrophone was mounted to a vertical robotics stage that translated the hydrophone upward in 1-cm increments. A broadband (3 to 80 kHz) spherical source, positioned 50 cm above the sediment-water interface, was used to insonify the sediment. Measurements were made with insonification angles above and below the critical angle by changing the horizontal distance of the source relative to the insertion point. This new measurement system is detailed, and results are presented that include temporal, frequency, and wavenumber analysis for natural and roughened interfaces. The measured compressional sound speed and attenuation are shown to be self-consistent using the Kramers-Kronig relation. Furthermore, only a single fast compressional wave was observed. There was no observation of a second slower compressional wave as predicted by some applications of the Biot model to unconsolidated water-saturated porous media.

Attenuation Measurements Across Surface-Ship Wakes and Computed Bubble Distributions and Void Fractions

A surface ships wake is composed of several hydrodynamic phenomena. A large part of that wake contains a mixture of air bubbles of various sizes in turbulent water. Eventually, as the wake ages, the turbulence subsides and bubbles begin to rise at rates that are determined by their sizes. These bubbles of various sizes and concentrations control the propagation of acoustic signals inside and across a wake. To further our understanding of these phenomena, a series of three continuous-wave (CW)-pulsed signals were transmitted across a wake as the wake aged. Each transmission contained a set of four 0.5-ms-long pulses. The 12 pulses ranged over frequencies from 30 to 140 kHz in 10-kHz steps. The acoustic attenuations across wakes that were due to varying bubble-size densities within the wakes were determined experimentally. From those data, estimates of the bubble densities as functions of the speed of the wake-generating ship, the wakes age, and acoustic frequency were calculated. From the bubble-density results, power-law fits and void fractions are calculated. The attenuation measurements were taken at 7.5-m intervals behind the wake-generating ship and continued for about 2 km. The experiment was run for wakes generated at ship speeds of 12- and 15-kn wakes, and the 15-kn run was repeated for consistence determination. The bubble densities were observed to have power-law forms with varying parameters with the strongest, for early ages, having an exponent of -3.6 and a void fraction of 4 x 10-7 , and with both diminishing for older wakes, as might be expected.

Panama City 2003 Broadband Shallow‐water Acoustic Coherence Experiments

In June 2003 a series of acoustic propagation experiments were conducted off the coast of Panama City, Florida. The experiments were designed to measure and provide an understand of signal phase and amplitude fluctuations, and signal spatial and temporal coherence over several large horizontal and vertical arrays. The propagation measurements were conducted in a water depth of 8.8m and at ranges of 70 m and 150 m. The acoustic measurements cover frequencies from 1 to 140 kHz. The propagation measurements were supported by data obtained by wave rider buoys, CTD’s, thermister chains and current meters. Bottom penetration data was also obtained using a buried hydrophone array. The experiments will be outlined and the data sets described.

Statistics of shallow water, high-frequency acoustic scattering and propagation

During August 1991, the Naval Research Laboratory conducted high-frequency shallow water acoustic scattering experiments in the Gulf of Mexico near Panama City, Florida. The acoustic measurements included surface and bottom reverberation, surface and bottom forward scattering, and direct path propagation. The results reported are confined to the direct and bottom forward reflected paths and include the statistical characteristics of three signals; namely, the direct, the bottom reflected, and the direct plus the bottom reflected. Representative envelopes will be presented that illustrate the complexity of the shallow water environment statistics, including the means, variances, and probability distributions for each signal, are presented to discern any differences that can be exploited in the detection process. The frequency range covered during the experiment was from 20 to 180 kHz. The supporting environmental measurements included sound speed profiles, currents, wave heights, and bottom samples.<<ETX>>

A high-frequency, shallow-water acoustic measurement system

The US Naval Ocean Research and Development Activity has developed a high-frequency acoustic measurement system for use in shallow water. The heart of this system is a pair of submersible towers supporting acoustic transmitting and receiving instrumentation. These towers are transported to an experimental staging area, assembled, and acoustic instrumentation installed. They are towed to a preselected measurement site, then the chambers on each tower are flooded, thereby settling slowly to the ocean bottom. Stability and dynamic response analyses were used to determine the towing and deployment stability envelopes for the towers. The acoustic transmitting system uses a pair of narrow-beam parametric acoustic sources operating at secondary frequencies ranging from 20 to 180 kHz. The acoustic receiving systems consist of a pair of 16-hydrophone, two-dimensional arrays with broadband capabilities up to 250 kHz. These systems have been used to make high-resolution bottom scattering measurements in shallow water off the coast, south of Panama City, Florida. >

High‐frequency bistatic reverberation from a smooth ocean bottom

High‐frequency bistatic reverberation was measured from a smooth, sandy, featureless bottom located 19 miles south of Panama City, FL. Bistatic scattering variability is presented as a function of frequency (20–180 kHz), grazing angles (9.5°–30°), and small horizontal and vertical bistatic scattering angles. Results show that bistatic variabilities tend to decrease with decreasing grazing angles and decreasing source beamwidths. Possible explanations for these decreasing variations are also presented.