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An experiment was performed just off the research pier at the Scripps Institute of Oceanography to determine the acoustic effects of small bubbles in very shallow water (∼6 m depth). The distance offshore was ∼300 m. The propagation lengths were 2-10 m, and the frequency range was from 39 to 244 kHz. During the experiment, rip currents passed through the field of measurement instruments. These rip currents were laden with bubbles created in the surf between the instruments and the shore. The effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed. From the attenuation data, the bubble distributions are calculated using a new iterative approach [Caruthers et al., in press, J. Acoust. Soc. Am.] that is based on the well-known resonant bubble approximation. Calculated bubble distributions varied from an essentially uniform lack of bubbles during quiescent periods to highly inhomogeneous and dense bubbly regions within rip events. Such observed distributions were consistent with measurements made by other investigators during the experiment. © 1999 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

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 model developed by Jackson et al. [J. Acoust. Soc. Am. 79, 1410-1422 (1986)] were compared to scattering strength measurements taken at 20, 40, and 60 kHz. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

High-frequency shallow-water reverberation statistics were measured from a seafloor that was covered by a thick layer of coarse shells. The reverberation statistics are presented as a function of frequency (20-180 kHz), grazing angle (8.8°-28°), and source beamwidths (1.2°-16°). In most cases the reverberation statistics did not follow a Rayleigh fading model. The model dependence of the reverberation statistics ranged from near Gaussian (high grazing angle) to beyond log normal (low grazing angle). The results suggest that small scale variations in bottom scattering properties and system beamwidth will significantly change the model dependence of the reverberation statistics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Continuous wave (cw) signals at frequencies between 20 and 180 kHz were backscattered from a shallow-water ocean bottom. Even though the source and receiver were mounted on a stable platform, the short-range (50-300 ft) bottom backscattering envelopes exhibited significant amplitude variations that increased as the grazing angles became smaller. Evidence suggests that the oceanic fine structure caused frequency-dependent shifts in the location of the insonified bottom area. These small shifts resulted in changes in the bottom scattered ping-to-ping envelope structure. [ABSTRACT FROM AUTHOR]

Two environmental acoustic scattering experiments have been conducted in shallow water off the coast of Florida. The ocean bottom ranged from a smooth homogeneous medium sand to one where the surface roughness was entirely due to a coarse surficial shell layer. The acoustic transmitting system used a pair of narrow-beam parametric sources operating at secondary frequencies from 20-180 kHz. The receiving system consisted of a 16-hydrophone two-dimensional spatial array with broadband capabilities to 250 kHz. Ping-to-ping correlation estimates are presented as a function of frequency, grazing angle, azimuthal angle, and environmental conditions. Cross-correlation estimates as a function of hydrophone separation, frequency, and environmental conditions are also presented. [ABSTRACT FROM AUTHOR]

Seafloor relief has been measured at very high (<1-cm) resolution with underwater stereo photogrammetry in the course of environmental characterization for high-frequency acoustic experiments. Information concerning bottom microrelief for the purpose of validating backscatter models has been heretofore expressed as rms height roughness. The validity of using Gaussian statistical methods to describe bottom relief is examined with the Hinich bispectrum test [Brockett et al., J. Acoust Soc. Am. 82, 1386-1394 (1987)] for normality in continuous data. Because bottom backscatter models such as the composite roughness model require the slope of the two-dimensional roughness power spectrum as an input parameter, generation of the two-dimensional relief spectrum is preferable to utilization of the rms relief or estimation from the one-dimensional spectra. Two-dimensional roughness spectra generated from photographs of rippled and smooth bottoms are compared to one-dimensional roughness spectra produced from the same photographs. Model and data comparisons are made for bottom backscattering using both roughness spectra. [ABSTRACT FROM AUTHOR]

High-frequency measurements of the complex Young's modulus was achieved by use of Schlieren photography. A sample plate is immersed in a water tank and insonified by an acoustic beam. A laser beam is expanded and used to illuminate the acoustic field at right angles. The resulting diffraction of light by the sound field is imaged by a video camera. Calibration of the light intensity with the sound intensity gives a quantitative measurement of the sound intensity in the acoustic medium. Thus, the incident, reflected, and transmitted wave intensities are measured. These data are used to compute the complex Young's modulus of the rubber sample. Measurements were made for frequencies between 100 kHz and 2 MHz for a sample of rubber. [ABSTRACT FROM AUTHOR]

A series of high-frequency bottom backscattering experiments were conducted off the coast of Panama City, Florida. Reverberation results will be presented as a function of frequency (20 to 180 kHz) and grazing angle (5°-30°). Geoacoustic parameters taken during the experiments will be used as inputs to an existing seafloor backscattering model initially developed by D. R. Jackson et al. [J. Acoust. Soc. Am. 79, 1410-1422 (1986)], and modified by A. Lyons et al. [J. Acoust. Soc. Am. 95, 2441-2451 (1994)]. The experimental data will then be compared to these model simulations. [ABSTRACT FROM AUTHOR]

Using a closed circuit television camera (CCTV), a method was developed to directly obtain quantitative sound pressure measurements from the sound fields imaged in a Schlieren system. By using the exact timed format of the composite video signal and a series of time delay circuits, it was possible to take vertical intensity scans of a Schlieren image at any position on the television raster. The method used the camera's horizontal blanking pulse to trigger a broadband, high-speed sample-hold amplifier. This amplifier was triggered to sample each of the horizontal scan lines at a selected time in the horizontal scan sweep and to feed this value into a transient recorder. The signal was digitized and a continuous analog voltage signal corresponding to the black-white information from a Schlieren image was displayed on a scope. The same analog signal was also plotted on an X-Y recorder. The ease with which the Schlieren system can be operated is evident from the fact that only a single calibration curve at each frequency was needed. The curves plotted applied transducer voltage against the video output level. Thus, by comparing the video output levels for several vertical image intensity scans on a Schlieren image, relative sound pressure measurements were routinely obtained. [ABSTRACT FROM AUTHOR]

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.