Edgar Kennedy

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. >

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.

Measurements of high-frequency shallow-water acoustic phase fluctuations

A shallow-water high-frequency (HF) acoustic propagation experiment was conducted just off shore in Panama City, FL. Several broad-band high-resolution sources and receivers were mounted on stable platforms and deployed in water depths of 8-10 m. Signals covering the frequency range from 20 to 200 kHz were transmitted from the sources to two spatially separated receivers. The data were analyzed to provide estimates of the signal phase variances as a function of frequency and source-to-receiver range. These phase variabilities are correlated with small-scale water column thermal variabilities and ocean swell conditions.

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.

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.

Panama City 2003 Acoustic Coherence Experiments: Environmental Characterization

During June 2003, the Naval Research Laboratory conducted a series of acoustic propagation experiments to measure both high (20 to 150 kHz) and low (1 to 10 kHz) frequency spatial and temporal coherence in very shallow water. Environmental data collected to support the acoustic measurements included water column current, bottom current, sea‐surface wave height, tide height, CTD water column profiles and mid‐water time series, two‐dimensional micro‐scale seawater temperature, and weather parameters. Wave periods varied from 3 to 7 seconds and wind speeds ranged from 4 to 35 knots throughout the experiment. Temperature and salinity profiles characterized periods when the water column was isovelocity and periods when the water column was stratified with a strong depth dependence of temperature and salinity. Current magnitudes were always less than 25 cm/s. Experimental geometry and methods of environmental data collection are briefly described and environmental conditions and their impact on the propagation ...

VARIABILITY OF SHALLOW-WATER BISTATIC BOTTOM BACKSCATTERING

High‐frequency bistatic bottom backscattering measurements were made in an area 26 miles east of Jacksonville, FL. Bistatic scattering variability is presented as a function of frequency (20–180 kHz), grazing angles (8.8°–28°), and small horizontal and vertical bistatic scattering angles. Results indicate that bistatic variabilities tend to decrease with decreasing grazing angle and show only a weak frequency dependence. Possible explanations for these functional dependencies are also presented.

A High‐Speed, Multi‐Channel Data Acquisition System

The Naval Research Lab is currently conducting research programs in MCM detections and classifications using both low and high frequency acoustics. These include target detections, target imaging, proud and buried target detections and classifications using structural clues. To determine the limitations that a fluctuating environment places on these target detection methods, a data acquisition system was developed. The data acquisition system consists of multi‐channel, high‐speed A/D’s with remote, variable gain control, and FPGA technology. Each A/D is synchronously sampled at a rate of 1 MHz and using time ‐division multiplexing techniques, is sent down an optical fiber at 1.3 Gbps. The sampled data is then separated back to its original channel and recovered back to an analog signal along with the original clock. Precision filters and high speed transient recorders utilizing fast CAMAC crate controllers are then employed to sample, simultaneously, all data channels with sample rates up to 3Msps. Acoust...

High-frequency acoustic phase stability measurement system

During March and April 1995 the Naval Research Laboratory (NRL) conducted a series of high-frequency acoustic phase stability measurements to determine the limits that the shallow-water (20-40 ft) environment places on synthetic aperture (SAS) sonar system. The paper describes the acoustic systems and experiments that took place in the shallow-waters off Panama City, Florida. The instrumentation consisted of several linear arrays mounted on three stable towers that were deployed on the shallow-water ocean bottom. These arrays were designed at the Applied Research Laboratory/Penn. State University and had vertical beamwidths that ranged from 1 to 3 degrees with 28 dB sidelobe suppression. Direct path phase stability measurements were taken as a function of frequency (20-200 kHz), path length, and environmental conditions. Several examples of the instantaneous signal phase versus small scale thermal variabilities are shown.

Attenuation measurements across surface‐ship wakes and computed bubble distributions and void fractions: A tribute to the contributions by Ralph Goodman on oceanic bubbles.

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.