PROGRAM OF
The 127th Meeting of the Acoustical Society of America
Massachusetts Institute of Technology © Cambridge, Massachusetts ß 6-10 June 1994
NOTE: All Journal articles and Letters to the Editor are peer reviewed before publication. Program abstracts, however, are not reviewed before publication, since we are prohibited by time and schedule.
MONDAY MORNING, 6 JUNE 1994 KRESGE LITTLE THEATRE, 8:30 A.M. TO 12:00 NOON Session laAO
Acoustical Oceanography: Acoustical Monitoring of Environmental Pollution
$ohn R. Proni, Chair
NOAd/AOML/OdD, 4301 Rickenbacker Causeway, MiarnL Florida 33149
Chair's Introductions8:30
Invited Papers
8:35
laAO1. Acoustic monitoring of environmental pollution. Marshall H. Orr (Naval Res. Lab., Code 7120, 4555 Overlook Ave., S.W., Washington, DC 20375-5350)
High-frequency acoustic backscattering systems have been used to detect, track, and estimate dispersion coefficients for particles formed or released during ocean dumping of industrial wastes. The results of work performed in the late 70's will be reviewed to illustrate: the effects of environmental variability on vertical dispersion of released particles; the types of vertical mixing processes that have been observed in the presence of strong season thermoelines (large gradients in the isopycnals); the breakdown of those thermoclines during periods of high wind stress; and the difficulties that can be encountered during attempts to navigate ships and perform acoustically guided chemical water sampling in the presence of strong ocean currents and vertical shear. Calibration of acoustic systems and estimates of system sensitivity to particles of known size distribution and concentration will be discussed.
9:00
laAO2. Current monitoring for environmental pollution studies by acoustic Doppler current profiling. Robert G. Williams (NOAA/NOS/OES33, SSMCA, 6th fl., 1305 East-West Highway, Silver Spring, MD 20910)
NOAA's National Ocean Service (NOS) has been measuring and predicting coastal currents for over i00 years. In earlier times, measurements were made with current poles, or propeller-type current meters. In the late 1980's, acoustic Doppler current profilers (ADCPs) were widely adopted. In Tampa Bay, 10 ADCPs (RD Instruments) were deployed in bottom-mounted platforms; a towed ADCP provided spatial contours of current speed. A real-time system provided 6-rain averages of current to mariners and environmental managers. This system, still in place, provides vital information for tracking land cleanup of pollutant spills. Recently, an ADCP was added to a conventional current meter array deployed by NOAA/AOML at a wastewater outfall off Miami, Florida. The ADCP measurements compared well with earlier observations and facilitated assessing the effects of mooring motion and bio-fouling. Analysis of ADCP profile data revealed a near-linear velocity gradient from surface to bottom, enabling interpolation of velocity profiles from earlier measurements. ADCP backscatter intensity provided a rough estimate of suspended material. Future applications include mapping the three-dimensional circulation around point sources of pollution.
Such maps provide environmental managers with data for effective effluent disposal management at minimum cost.
2801 J. Acoust. Soc. Am., Vol. 95, No. 5, Pt. 2, May 1994 127th Meeting: Acoustical Society of America
9:25
laAO3. Aeonstieal •cattering from plumes of sediment suspended in open waters. Paul R. Ogushwitz (PRO Scientific Consulting, 303 Center Street, Hackettstown, NJ 07840)
Acoustical data from a field survey at a disposal site for dredged materials near Mobile Bay, Alabama, are analyzed. During the survey, two independent acoustical instruments provided measurements of echo strength from vertical plumes of suspended sediments. Particulate concentrations and grain size distributions were determined from in situ water samples. Quantitative analysis of the measurements indicates that the acoustical echoes from the suspended sediments are dominated by Rayleigh backscattering. The best fit to data is achieved when the measured decrease of grain size with decreasing concentration is explicitly accounted for. Attenuation appears to play a significant role at high concentrations (i.e., within the dense part of the plume). For the grain sizes studies here ( 1-200 m), viscous effects account for at least 90% of the attenuation. When the theoretical estimates are subtracted from the data, the residual echo strength is seen to increase with decreasing concentration; some possible causes of this are beam-to-beam variability, effects of grain shape, and factors related to experimental design. One of the instruments measured echo strength on four beams simultaneously; the average beam-to-beam variability was about 3 dB, corresponding to a precision of about 50% in the measured concentration. [Work supported by U.S. Army, Corps of Engineers, Dredging Research Program.]
9:50
ICAO4. Aeotmtieal observations of anthropogenic dlaeharge plumes. John R. Proni (Atlantic Oceanogr. and Meteorol. Labs., Ocean Acoust. Div., 4301 Rickenbacker Causeway, Miami, FL 33149)
Acoustical methods such as backscatter and Doppler current measurements are gaining substantial acceptance in both point source and nonpoint source oceanic pollution studies. Using acoustical backscattered energy, chemical and biological samplers are guided to oceanic water column regions of higher contaminant concentration. In typical discharge operations, e.g., dredged material, sewage effluents, oil-well blowouts, after-discharge backscatter levels are initially at least three to four orders of magnitude above normal background backscatter levels and are available for tracking of plumes and guidance of sampling operations. Additionally, acoustical backscatter may also be used to map naturally occurring surfaces within the oceanic water column along which both discharged material and naturally recurring particulates may accumulate. Thus nonacoustical sampling may continue to be guided indirectly far beyond the time or distance at which plume concentration levels may be considered to have diminished to average water column background levels. Furthermore, such surfaces may be loci of increased probability of exposure to contaminants for plankton, larvae, and juvenile fish and as such should be closely studied. Acoustical detection of incursions of discharged. material along sub-oceanic surfaces for both negatively and positively buoyant plumes will be illustrated using sewage sludge discharges and secondarily treated wastewater effluent discharges. Preferential accumulation surfaces present:
( 1 ) at the boundary region of Hudson/Raritan River water and continental shelf water and (2) within Chesapeake Bay, detected through backscatter and transverse Doppler, measurements will be shown.
10:1S-10:30 Break
10:30
laAOS. Acoustical discrimination of estuarine turbidity structure. Henry Bokuniewicz, Robert Wilson, and Robert Chant (Marine Sci. Res. Ctr., State Univ. of New York, Stony Brook, NY 11794-5000)
The distribution of suspended sediment in estuaries is often concentrated in turbidity maxima which, because of the associ- ation of many contaminants with fine-grained sediment particles, can be important in determining the overall water quality.
Acoustic investigations of the turbidity in the Hudson River estuary were done with a 200-kHz transducer and a 600-kHz broadband acoustic Doppler current profiler. (Water temperature and salinity were also measured and samples of suspended sediment were collected. ) High turbidity here was found to be associated with tidally modulated and geomorphically controlled salinity intrusions. A salinity front is arrested at the bottom by the ebbing tide. As the tide turns, the front migrates upestuary producing an intense, local turbidity maximum. The re-establishment of a front in the original position on the next ebb can leave high turbidity pockets stranded upestuary. This is a novel mechanism for the generation of estuarine turbidity maxima but probably not unique to the Hudson River estuary; it may prove to be of widespread importance. [Work supported by the Hudson River Foundation, New York, NY.]
Contributed Papers
10:45
laAO6. Coastal sediment transport monitoring using high-frequency aeonsties in the 1990-91 STRESS II experiment. James Lynch, James Irish, and Arthur Newhall (Woods Hole Oceanogr. Inst., Woods Hole, MA 02543)
The bottom sediments of many coastal areas are contaminated by a truly awesome witches brew of ingredients (PCB's, heavy metals, ra- dioactive wastes, to name a few). The eventual fate of these contami- nants is intimately tied in with the processes of coastal sediment trans- port. In recent years, acoustical remote sensing has become a powerful tool in measuring the process involved in coastal and deep sea sediment transport. In this talk, acoustical measurements made as part of a major
experiment to understand the processes governing transport on the con- tinental shelves, the "Sediment Transport on Shelves and Slopes"
(STRESS) experiments will be discussed. Of particular interest here are the acoustical images of the near-bottom boundary layer, and their implications for sediment transport modeling. [Work supported by ONR.]
11:00
laAO7. Remote acoustic sensing of manganese nodules on the ocean bottom. A.V. Bunchuk and A. N. Ivakin (N. N. Andreev Acoustics Institute, 4 Shveruik St., 117036 Moscow, Russia)
2802 J. Acoust. Soc. Am., Vol. 95, No. 5, Pt. 2, May 1994 127th Meeting: Acoustical Society of America 2802
Experimental and theoretical investigations of acoustic properties of the deep-sca bottom covered with manganese nodules have been con- ducted. The objectives were ( 1 ) to study remote acoustic detection and outlining nodule deposits and (2) to investigate remote acoustic moni- toring and estimation of the nodules abundance, mcan size, and other parameters. Field experiments were conducted in four regions of the Pacific and Indian Oceans at the frequency range of 2-20 kHz. The field experiments have shown that availability of manganese nodules is dis- played by an increase of both the sound reflectivity and scattering strength and by the appearance of their unusual, comparing nodules- free bottom, frequency-angular dependences. A theoretical model of sound scattering and reflection by a set of nodules laid on the bottom has been developed [A. V. Bunchuk and A. N. Ivakin, Soy. Phys.
Acoust. 35( 1 ), 5-11 ( 1989)] in which the explicit form of connection
between characteristics of bottom reradiated acoustic signal and nodules parameters have been established. As a result, explanation became pos- sible for all the obtained experimental data. The spatial distribution of amplitudes of signals reflected and scattered from the bottom can be mapped. Finally, real-time algorithms for the inverse problem solution were developed and their validity for adequacy was checked.
11:20=12:00
PANEL DISCUSSION:
Panel Moderator: John R. Proni
Panel Members: Henry Bokuniewicz, James F. Lynch, Paul R.
Ogushwitz, Marshall H. Orr, Robert G. Williams
MONDAY MORNING, 6 JUNE 1994
STUDENT CENTER, TWENTY CHIMNEYS 8:00 A.M. TO 12:00 NOON Session laSA
Structural Acoustics and Vibration: Scattering Courtney B. Burroughs, Chair
Applied Research Laboratory, Pennsyloania State Uniuersity, Uniuersity Park, Pennsylvania 16801 Contributed Papers
8..oo
laSAI. Scattering and attenuation of elastic wave by a viseoelastie cylinder. I. P-wave incident. Yin-Bin Liu and Ru-Shan Wu (Inst. of Tectonics, Univ. of California, Santa Cruz, CA 95064)
The analytical expressions for scattered and transmitted fields from a circular elastic cylinder with and without intrinsic attenuation caused by a normally incident plane P wave or a normally incident cylindrical P wave are derived. The corresponding acoustic cases are also given by letting the shear modulus either inside or outside the cylinder be equal to zero. In the Rayleigh wave scattering regime, the solution agrees with standard approximate solutions. Resonance scattering, radiation pat- tern, scattering cross section, and synthetic seismogram are calculated.
For a given mode number m in some discrete frequencies, either scat- tered compressional or scattered shear waves vanish for a nonabsorbing cylinder, and the amplitudes of scattered compressional and shear waves are always nonzero for an absorbing cylinder. The sawtooth shapes of a scattering cross section that appear in a low-velocity non- absorbing cylinder will disappear in the absorbing cylinder. The scat- tered fields are the superposition of the geometrically reflected waves and a series of surface waves that circumnavigate the cylinder azimuth- ally, the amplitudes of scattered compressional waves are much greater than those of scattered shear waves (kay. 1 ). [Work supported by NSF.]
8:15
lnSA2. Comparison of membrane wave coupling coefficients.
Douglas A. Rebinsky (Dept. of Mech. and Aerosp. Eng., Rutgers Univ., P.O. Box 909, Piscataway, NJ 08855-0909)
Comparisons are made between various estimates of the coupling coefficients that characterize the longitudinal wave contribution to acoustic scattering from thin elastic circular cylindrical and spherical shells. A common method of obtaining the coupling coefficient is through the application of the Sommerfeld-Watson transformation to the exact modal series solution. From this result, a high-frequency ap- proximation can be derived that accurately describes the magnitude, but
it does not adequately represent the phase. These are compared to cou- pling coefficients determined using a ray-theoretic approach. This rep- resentation describes both the magnitude and the phase well when com- pared to that obtained from the exact modal series result using the Sommerfeld-Watson transformation. [Work supported by ONR.]
8:30
IaSA3. Acoustic scattering from rigid shells of revolution using the generalized internal source density and SVD method. P. R.
Stepanishen (Dept. of Ocean Eng., Univ. of Rhode Island, Kingston, RI 02881)
Acoustic harmonic nonaxisymmetric scattering from rigid shells of revolution is addressed using the generalized internal source density and SVD method. The development of the generalized ISD/SVD method, which entails the use of various internal multipole source distributions along the axis of the shell, will be presented. The approach is based on first expanding the incident pressure and associated velocity field into circumferential harmonics about the axis of the shell. Integral equations of the first kind are developed for the unknown source distributions. A least-mcan-squared error formulation of the solution naturally leads to using the SVD method to determine the source distributions. The blocked surface pressure for each circumferential harmonic is then readily evaluated by simple quadrature methods and the associated far- field scattered pressures are shown to be simply related to the Fourier transforms of the source distributions. Numerical results for some se-
lected problems are presented and some limitations of the method as a function of azimuthal wave number and shape of the shells are noted.
These limitations are discussed and are readily interpreted in light of the above-noted Fourier transform relationship.
8:45
IaSA4. Modeling of echoes from extended targets in shallow water.
T. Baus, W. Chang, and R. Deavenport (Naval Undersea Warfare Ctr., Code 3111, New London, CT 06320)
2803 J. Acoust. Soc. Am., Vol. 95, No. 5, Pt. 2, May 1994 127th Meeting: Acoustical Society of America 2803
Echoes are generated, and compared, for extended targets in a shal- low water environment using the total target-medium broadband fre- quency response function and the frequency representation of the trans- mitted signal. The two-way medium response is calculated using the generic sonar model (GSM) environmental model. The target fre- quency response is found using a model of distributed targets and elastic spheres. The combination of the target and medium frequency responses gives the full frequency response, or form function, for the target- medium system. The product of the total form function with the fre- quency spectrum of the incident signal is computed and an inverse FFT is performed to predict the received signal, or echo, from the object.
[This work was sponsored by the Office of Naval Research-- Technology Directorate.]
9.'0O
laSAS. Merging of launching or detachment points of weakly damped leaky waves on S-shaped surfaces and complex launching or detachment points. Philip L. Marston (Dept. of Phys., Washington State Univ., Pullman, WA 99164-2914)
In previous work, Fermat's principle and properties of leaky waves on fiat surfaces were combined to obtain a high-frequency approxima- tion to scattering by cylinders of slowly varying curvature [P. L. Mar- stoa, J. Acoust. Soc. Am. 94, 1861 (A) (1993)]. The usual assumption was made that only the first derivative of optical-path length (with respect to path variation) vanishes. The present work considers situa- tions where the second (or higher) derivatives vanish at launching or detachment points. The simplest example is for coupling to (or detach- ment from) a tilted S-shaped surface. Such surfaces may have pairs of launching or detachment points; the pairs can merge at an appropriate surface tilt that results in a k t/6 amplitude enhancement coefficient. The
launching or detachment factor becomes proportional to an Airy func- tion of complex argument and launching or detachment in a shadow
•gion of the Airy factor is described approximately by a complex ray.
Higher-order launching or detachment points may also result from a sufficiently rapid dependence of the leaky-wave phase velocity on posi- tion. [V4ork supported by ONtL]
9:15
lnSA6. Enhancement of the total scattering cross section near the eoineidenee frequeney of thin shells. Philip L. Marston (Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814 and Appl.
Res. Lab., Univ. of Texas, Austin, TX 78713-8029)
The total scattering cross section o is affected by the elastic response of a target and is related to the forward scattering amplitude by the optical theorem. Computations based on the exact partial-wave series for scattering of plane waves by circular-cylindrical and spherical thin shells reveal an enhancement followed by broad oscillations as •a is increased through the coincidence region. This extends previous work on thick shells [S. G. Kargi and P. L. Marston, $. Aenust. SOc. Am. 88, 1103 ( 1990)]. A ray model shows that the coincidence region enhance- ment for thin shells is associated with both the subsonic (a0_) wave and the supersonic (a o) Lamb wave, unlike the corresponding backscatter- ing enhancement that primarily results from the contributions of the subsonic wave [P. L. Marston and N.H. Sun, J. Acoust. Soc. Am. 92, 3315 (1992)]. The phase evoldtion of the coupling coefficient for the subsonic wave limits the aggaeiated nontribution to o for inereaging When cr is normalized by the area of the scatterer's profile, the elastic contribution for spheres is approximately twice that of cylinders in the coincidence region. [Work supported by ONR and by the ARL:UT IR&D Program.]
9:30
lagA7. Group delay in resonant scattering. Hua He (Dept. of Ocean Eng., MIT, Cambridge, MA 02139) and Richard H. Lyon (MIT, Cambridge, MA 02139)
For a slightly damped structural system with low modal overlap,
roughly half the zeros of the transfer function (TF) remain on the pole line and the group delay of the TF is •r/2 times the structural modal density. But in the sound scattering case, the zeros are translated and rotated randomly and far fewer zeros will remain on the pole line. It is then reasonable to conjecture that the phase of resonance scattering function (RSF) he determined by the number of poles alone and thus the group delay of RSF is roughly a factor of 2 times that of the TF. To verify this conjecture, an in-air plate scattering experiment was carried out and the results show the factor of 1.67. Furthermore, the zero density distribution patterns versus modal overlap for both TF and RSF were obtained through numerical simulation of a 20-DOF system. The simulation shows that in the smaller modal overlap case, the RSF group delay is nearly twice that of TF. When modal overlap increases, how- ever, this factor decreases and approaches 1. Physically this means in a high modal overlap system, like room acoustics, the group delay of RSF approaches the energy decay rate.
9:45
IaSA8. Edge effects in finite gratings of cylindrical scatterers.
Douglas R. Denison and Robert W. Scharstein (Elect. Eng. Dept., Univ. of Alabama, Tuscaloosa, AL 35487-0286)
Edge effects in finite arrays are studied via the analysis and inter- pretation of one canonical geometry--a linear grating of thin circular cylinders. The partial field scattered by each element in a finite array is decomposed into three components: ( 1 ) the contribution of the infinite or periodic array, (2} a component due to the presence of the left edge of the array, and (3) a component due to the right edge of the array.
The scattered field due to either array edge is derived from a separate analysis of the appropriate semi-inllnite array and is interpreted in terms of a decaying wave that is launched from the array end. This solution to the weB-posed boundary value problem is numerically verified via boundary condition satisfaction, where the accuracy of the edge-wave decomposition persists for all tested values of element spacing and plane wave incidence angles. [Work supported by NSF.]
10:00-10:.15 Break
10:15
IaSA9. Transmission of euergy across a discontinuity by fluid-loading. Douglas Photiadis (NRL, Code 7131, 4555 Overlook Ave. SW, Washington, DC 20375-5000)
The presence of a heavy adjoining fluid is known to play an impor- tant role in transporting surface vibrational energy across discontinui- ties in a thin elastic structure. These effects are particularly important if the discontinuities in the elastic properties of the structure are large and would hence tend to reflect a significant amount of vibration. The main results that have been previously obtained refer to the reflection of a normally incident flexural wave from a line discontinuity: The depen- dence on the incident angle of the flexural wave relative to the line discontinuity has been only briefly considered. Some simple models to examine this phenomena, fluid-loaded infinite membranes and plates with line discontinuities, have been investigated. It has been found that short-circuiting effects due to the fluid become much weaker as the angle of incidence increases. These results and their phenomenological implications will be discussed.
lO:3O
IaSAI0. Diffraction of short wavelengths by a circular elastic plate in a contrasting baffle. Gerry R. Wickham (Dept. of Math., Univ. of Manchester, Manchester MI3 9PL, England, UK)
The scattering of sound of wavelength 2•'/k by a circular elastic
plate of radius a in an infinite planar contrasting baffle is exactly for-
mulated in terms of a singular integral equation of the second kind using
a novel extension of the Wiener-Hopf technique. The exact solution of
this equation is easily derived analytically as a rigorous convergent
expansion that is also asymptotic in the limit ks--, oo. Thus it is possible
2804 J. Acoust. Soc. Am., VoL 95, No. 5, Pt. 2, May 1994 127th Meeting: Acoustical Society of America 2804
to derive explicit asymptotic formulas for the scattered field in terms of the factorization of the standard Wiener-Hopf kernel corresponding to the canonical diffraction problem for two adjoining semi-infinite com- pliant surfaces. The special cases of an elastic plate in a rigid baffle and a rigid plate in an elastic baffle are considered in detail. It turns out that the asymptotic forms for the diffracted far fields may be simplified dra- matically using a factorization procedure devised by the author and Andrew Norris and that these results are uniform in the fluid loading parameter.
10:45
laSAll. Reduction of backscattering from cylindrical shells by damping treatment. Yueping Guo (Dept. of Ocean Eng., MIT, Cambridge, MA 02139)
For oblique incidence within about 30' off normal incidence, back- scattering from cylindrical shells is known to be dominated by reradi- ation from shear and/or compressional waves in the shell. Thus back- scattering can be reduced by dissipating these shell-borne waves through damping treatment to the shell. This paper discusses some possible forms of damping. Three examples are given, namely, a three-layered shell with a homogeneous viscoelastic core, a three-layered shell with a fiber-reinforced viscoelastic layer, and a shell with cavities filled with highly dissipative viscous fluid. It is shown that these kinds of damping treatment reduce the phase speeds of the radiating waves, as well as increase the effective loss of the composite shell, which lead to signifi- cant reduction in the backscattered field. [Work supported by ONR.]
11:00
