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Contributed Papers

2:15

7UW3. Measurements of tbe acoustfc vector wavc field in the shallow ocean madę by a single ocean sub-bottom seismomcter (OSS). Tokuo Yamamoto (Geo-Acoust. Lab., Univ. of Miami, RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149), Thomas Nyc (Univ. of Miami, Miami. FL), and Dean Goodman (Univ. of Miami, FL and Nakajima, Japan)

In the shallow oceans, acoustic waves are strongly coupled with the seabed. If the vector wave field within the seabed is measurcd at a point, the direction of acoustic wave propagation can be accurately deter-mined. To test this idea, we have measured the vector wave fields that are induced by ULF/VLF (0.01-1 Hz) ambient acoustic noise and acoustic pulses (0.05-1000 Hz) generated by an airgun using a single ocean sub-bottom seismometer in shallow water 13 m deep. The maxi-mum entropy principle Ls applied to the three orthogorial components of the vector field induced by the ambient noise to calculatc the directional spectra. We were capable of detecting the noise field propagating from many directions that were substantiated from other independent mea-surements. The measured acoustic vector wave field induced by a mov-ing source were compared with the model predictions by a WKBJ codę. In the model calculations, the geoacoustic data obtained from the bot-tom shear modulus profiler method (Yamamoto and Trevorrow, in this conference) were used. Excellent agreemenls are obtained between the measured seismograms and the synthetic seismograms indicating that moving acoustic sources can be accurately determined using only two OSSs in the shallow oceans. [Work supported by ONR.J

2:30

7UVV4. Laboratory scalę measurements of low-frequency underwater sound propagation ovcr a sedimcnt layer with a hard basement. Allen J. Hundley (Science Applications Int. Corp., Campus Point Facility, San Diego, CA) and Stewart A. L. Gegg (Honda Atlantic Univ., Boca Raton, FL 33431)

Recent measurements of low-frequcncy sound propagation in a region where a thin sediment layer overlays a hard rock basement [Hughes et al., J. Acoust. Soc. Am. 88, 283-297 (1990)] showed high transmission losscs at ccrtain freąucncies, which were not well predicted using a safari prediction codę. It was suggested that the high transmission loss levels were caused by resonances in the sediment layer. This paper will describe laboratory measurements in a similar environment using a model whose scalę was ~ l/1000th of the ocean experiment. The sediment was modeled using a layer of epoxy of uniform thickness on a concrcte base, and transmission loss was measured as a function of depth rangę and freąuency. High transmission losses were found at Iow frequencies which have similar characteristics to those observed by Hughes et al. at fuli scalę. This paper will describe the observations of the sound field at Iow frequencies and compare the measurements with predictions madę using safari.

ing over 450 m horizontally. Continuous measurements of the sound-speed profile showed fluctuations of the thermocline boundary caused by long period (3.5 h) intemal waves with 10-m amplitudę. Simulta-neously, acoustic measurements were madę of the deviations from lin-earity of the phase front along the array. Using spatial Fourier analysis, it was seen that the variability of the phase front showed several scales. Calculations showed that long period phase fluctuations were produced by long period interval waves and short period fluctuations were produced by the interferencc of direct waves and waves reflected from the shore. It was also possible to account for some of the paramelers of the intemal waves and the reflected waves.

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7UW6. Sound propagation conditions in the equatorial South Pacific Ocean. David G. Browning (New London Lab., Naval Underwater Syst. Ctr., New London, CT 06320) and Ronald N. Denham (Defence Scientific Establishment, Auckland, New Zealand)

A major part of the South Pacific Ocean is impacted by a cold water circulation induced by the Antarctic Circumpolar Current. This results in either a double or a very broad deep sound channel axis [R. N. Denham et ai, J. Acoust. Soc. Am. 81, 787-789 (1987)]. However, nearer the equator this impact is reduced and a series of equatorial currents and counter currents come into play. An analysis is madę of existing oceanographic data to determine the resulting sound-speed profile sliape and sound channel axis depth. A comparison is madę to profiles from the temperate regions of the South Pacific Ocean.

3:15

7UW7. Experimental detection of a slow acoustic wave in sediment at shallow grazing angles. Frank A. Boyle and Nicholas P. Chotiros (Appl. Res. Labs., Univ. of Texas at Austin, Austin, TX 78713-8029)

Following recent experimental results at sea (N. P. Chotiros, Proc. Oceans '89) that suggest the cxistence of a previously undetccted typc of acoustic wave in sandy sediments, an experiment was designed to detect and measure the speed of acoustic waves in an isolatcd environment. The expcriment was conducted in a laboratory tank containing 1 m of unwashed river sediment under a 3-m water column. Observations were madę of the travel time and attenuation of a pulse from an acoustic source located above the water-sediment interfacc to a set of probes below the interface. It was observed that, at normal incidence the pulse traveled at about 1750 m/s, while at shallow grazing angles, the pulse traveled through the sediment at close to 1200 m/s. An interesting possible cxplanation exists in the Biot model [M. A. Biot, J. Acoust. Soc. Am. 28, 168-191 (1956)], which predicts a slow acoustic wave in porous materials. [Work funded by ONR under NOARL management.]

2:45

7UW5. An experimental investigation of the horizontal refraction of low-frequency acoustic waves in shallow water. A. Yu. Shmelerv, V. G. Petnikov, A. A. Migulin (Institute of General Physics, Moscow, USSR), and James F. Lynch (Woods Hole Oceanographic Inst., Woods Holc, MA 02543)

In shallow water, the measurement of horizontal refraction is a convenient method for the sludy of ocean mesoscale processes. In this paper, obscrvations of horizontal refraction over a long, fixed acoustic track in shallow water are presented. A tonal signal at 100 Hz was transmitted from a near bottom source to a long, fixed receiving array 70-km distant. The receiving array consisted of 48 hydrophones extend-

3:30

7UW8. VLF bottom loss measurements in the Blake-Bahama basin.

Jeffrey L. Becklehimer, Stephen 0'Hara, and L. Dale Bibee (Naval Oceanographic and Atmospheric Res. Lab., Codę 362, Stennis Space Ctr., MS 39529)

Broadband (8-25 Hz) measurements of ocean-bottom reflection loss were madę using airgun sound sources and a vertical hydrophone array for grazing angles between 5 and 55 deg. Frequency-wave-number analysis was used to partition the energy received by the hydrophone array into the energy spectrum due to the direct water path and that from the path with one bottom reflection. Bottom loss esti-mates were then calcu la ted as the ratio of the energy spectrum from these two propagation paths. Comparisons of theoretical and measured

1982

J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2, April 1991

121st Meeting: Acoustical Society of America

1982