885095934

Acoustic volume reverberation measurements were madę with a 12-kHz sea beam multibeam echo sounder by recording quadrature sarn-ples of the echocs reccived on cach of ihe systerrfs sixteen 2.66" beams. For initial inspection, the signal amplitudcs from three channels (a center channel and outcr channels on either side) were displayed as a function of depth and along-track distance in a grey lcvel image quan-tized to 4 bils. Deep scattering layers identifiable in thesc images werc analyzed by integrating the echocs rcceived on each beam ovcr a 50-m depth slice containing a layer or sel of layers and by correcting for beam pattern and time spread effects as a function of angular direction. Re-sulls from data rccordcd during night timc periods in the Northern Pacific show a fairly consistent volume scattering picture with variations averaging about 10 dB over 4-km scgments along the ship’$ track, and 2 to 3 dB over a few hundrcd metcrs across track. The ccntroid of the returns in the 50-m window was also calcu la ted for each beam and showed an along-irack wavy pattern with an amplitudę of about 10 m and a "period” of roughly 120 to 140 m. These rcsults are discussed along with applications for 3-D mapping of volumc scattering and patchiness distribution. [Research sponsored by ONR.J

10:30-10:45

Break

10:45

6UW7. Laboratory measurcments of forward and backscattering from striated surfaces. H. E. F. Williams, R. Kille (Dcpt. of Physics, American Univ., Washington, DC 20016), and T. C. Yang (Naval Res. Lab., Washington, DC 20375)

A tank experiment was devised to measure forward and backscat-tering from single and multiple rods distributed on the water-air inter-face orientcd normal to the piane of scattering. The cylindrical rods madę of acrylic simulate the soft half-cylindrical proluberances used by Twersky in calculating scattering [J. Acoust. Soc. Am. 22, 539-546 (!950)J. (The Burke-Twersky model has been used frequcntly to model under-ice scattering in the Arctic Ocean.) To measure the scattering function (including thespecular reflection and backscattering) from the cylindrical rods, a new experimental setup using an array of smali di-ameter (0.05 in.) transducers was used to receive the scattering returns at different angles simultaneously. The sourcc is a large diameter (3 in.) transducer operating al 100 kHz, with a narrow beam ( < 10 deg) to minimize "leakage” into backscattering angles. With the cylindrical rod radius of 1.6 nim, the scattering corresponds to ka = 0.67, which is the equivalent of low-frequency ( — 20 Hz) scattering from the under-ice ridges. Prcliminary resuits of scattering from cylindrical rods are in reasonably good agreement with the Twersky calculations. (Work sup-ported by ONR.)

1 IKK)

6UW8. Seismo-acoustic efTect of trapped air pockets undemeath a floating ice piąte. Jacques R. Chamuel (Sonoquest Advanced Ultrasonic Res., P.O. Box 153, Wellesley Mills, MA 02181-5339)

Little is known about the characteristics, distribution, and role of trapped air pockets present underneath the Arctic ice covcr. Laboratory ultrasonic modeling resuits are presented demonstrating striking effects of smali trapped air pockets undemeath a floating platc on broadband pulsed flcxural wavcs. The thickness of the trapped air pockel is smali compared to a wavelcngth. The expcrimental studies were conducted on ice, Plexiglas, and glass plates floating on water. The air pockets in-crease the fiexural wave velocity causing horizontal refraction and shadow zones in the piane of the floating piąte. A variety of air pocket sizrs and eluster configurations havc bccn investigatcd. The prc««ne« of a shallow-water pool on the top surface of the floating piąte has a negligible effect on the flcxural wave compared to an air pocket of the same dimensions located underneath the piąte. A number of different phenomena occur causing the amplitudę of the detected flexura! wave to be attenuated or simplified. The ncw findings indicate that trapped air below the Arctic ice covcr may play a significant role in Arctic acous-tics. [Work sponsored by ONR.)

11:15

6UW9. A comparative experiment on under-ice acoustic scattering in the marginal ice /.one. Michacl J. Buckingham (Marinę Physical Lab., A-013, Scripps Inst. of Oceanography, La Jolla, CA 92093)

For scveral ycars a serics of ambient noise esperiments has bcen conducted in the marginal ice zonę (MIZ) off the east coast of Green-land from a fixed-wing airborne platform. In the spring of 1991, in addition to ambient noise measurements, a propagation experiment will also be attempted, aimed at establishing the effcct of scattering due to the under surface of the ice cover. Parallel to the ice edge, two lines of omnidirectional sonobuoys will be deployed over a distance of about 100 km, one linę about 1 km out in the open ocean and the other I km within the ice field. Explosive shots will be detonated at bołh ends of the dual linę. Being so close, the primary difference bctwccn the propagation conditions down the two lines is in tlić scattering from the ice cover relative to that from the open-sea surface—bathymetry and sound-speed profiles being essentially the same. To support this experimeni AXBTs will be deployed along both lines of buoys, although, in view of salinity variations in the vicinity of the ice, airborne cxpendable sound vcloci-meters would be preferable. These arc planned for futurę Arctic exper-iments. An airborne tcchnique for measuring the under-ice profile would also be highly desirabie.

11:30

6UW10. Acoustic backscattcr measurements of suspended sediments as part of the 1988 STRESS experiment James F. Lynch (Woods Hole Occanographic Insi., Woods Holc, MA 02543), Thomas F. Gross (Skidaway Inst. of Oceanography, Savannah, GA 31416), Blair Brumley (R. D. Instruments, San Diego. CA 92131), and Christophcr Sherwood (Univ. of Washington, Scattle, W A 98105)

As part of the 1988 STRESS (Sediment Transport Events on Shelves and Slopes) experiment, an upward looking l-MHz backscatter sonar was deployed in 90 m of water off the California coast for a period of 2 months. Dubbed “AHSS" (Acoustic Back Scatter System), this instrument madę vcrtical profiles of the suspended sediment from 1.5 to 27.0 m above bonom. Temporal sampling allowcd both slow (evcry half-hour) sampling to look at the long time cvolution of the bottom boundary layer and fast (0.5 Hz over 2 min) sampling to look at indi-vidual wave-induced suspension. During the course of the dcvelopment, Iwo major transport events (storms) were scen. In this talk, the ABSS measurement resuits, the correlation of the suspended sediment profiles

1963

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

121st Meeting: Acoustical Society of America

1963