5pUW9. Improved empirical descriptions of the dependence of surface backscatter on air-sea environmental descriptors. Michael. Nicholas, Peter M. Ogden, ...
discontinuities. Thus the pulse length dependence of bottom scattering could be a useful measure of the lateral length scales of sub-bottom volume inhomogeneities. [Work supported by ONR/AEAS.]
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5pUW9. Improved empirical descriptions of the dependence of surface backscatter on air-sea environmental descriptors. Michael Nicholas, Peter M. Ogden, and Fred T. Erskine (Naval Res. Lab., Code 7142, 4555 Overlook Ave. SW, Washington, DC 20375-5350) Between August 1988 and March 1992 six at-sea experiments were performed in which sea surface backscattering strengths were measured from ~70 to ~ 1000 Hz utilizing explosive (SUS) charges. (These were part of the Critical Sea Tests.) The latter of the six experiments (CST-7) was particularly interesting in that wind speeds as high as 17.5 m/s (at 10-m height) were encountered, which is 4 m/s higher than in any of the earlier experiments. Here, results obtained from CST-7 will be compared with those from the five earlier tests, and some important differences will be presented. Some new empirical formulas will be presented that describe the results from all six of the CST tests better than the Ogden-Erskine algorithm. (The Ogden-Erskine algorithm was formulated using data from the first five tests only and is now widely regarded as a much better description than the older Chapman-Harris formula.) Possibilities for improving still further the prediction of surface scattering strengths will be discussed. [Work supported by ONR.]
2:05 5pUW10. Anomalous frequency behavior in low-frequency sea-surface backscatter. Peter M. Ogden, Fred T. Erskine, and Michael Nicholas (Naval Res. Lab., Code 7142, 4555 Overlook Ave. SW, Washington, DC 20375-5350) Surface backscattering strengths have been measured between 70 and 1500 Hz using broadband SUS charges as sources in the Critical Sea Test 7 at-sea test. While the results of the measurements for frequencies of 1 kHz and below have been presented previously, the extension of the results up to 1.5 kHz have shown some unexpected frequency behavior in the scattering strengths as a function of wind speed. For wind speeds of 15 m/s and higher, scattering strengths at a fixed grazing angle are found to peak in the 600- to 1000-Hz region and then decline with increasing frequency, while for lower wind speeds the scattering strengths increase roughly monotonically with increasing frequency. Possible explanations for this behavior will be discussed. [Work supported by ONR.]
2:10 5pUWll. Low-frequency acoustic wave generation in a resonant bubble layer. O. Druzhinin,a) L. Ostrovsky,a) and A. Prosperetti (Johns Hopkins Univ., Baltimore, MD) The enhancement of frequency transformation of acoustic waves (in particular, in parametric arrays) by bubbles added to water has been discussed for about a decade. However, the resonance losses usually degrade the efficiency of the process. Here, the possibility of using small, nonresonant bubbles is considered and the effect by exploiting the resonance of the whole bubble layer is enhanced. It is assumed that a plane wave is incident on such a layer and the transmitted and reflected fields, including the low-frequency waves are investigated, both analytically and numerically. For bubble volume fractions /3~ 10 - 3 , at a frequency ratio of about 10 and for pump amplitudes of the order of 10" 1 atm, the power of the low-frequency signal (including waves radiated in both the directions) may reach 10% of the total power of the incident biharmonic wave. This is a very good efficiency for processes of this type. Further improvements are difficult due to the formation of shocks. [Work supported by the Science Opportunities and Ocean Acoustics Programs of ONR.] a)Permanent address: Inst, of Appl. Phys., Russian Acad, of Sci., Nizhny Novgorod, Russia. 3019
J. Acoust. Soc. Am., Vol. 95, No. 5, Pt. 2, May 1994
2:15 5pUW12. Single surface integral equation for penetrable wedge scattering. Anthony M. J. Davis (Math. Dept., Univ. of Alabama, Tuscaloosa, AL 35487-0350) and Robert W. Scharstein (Univ. of Alabama, Tuscaloosa, AL 35487-0286) The transmission problem of time-harmonic acoustic scattering by the velocity/density contrast wedge is formulated, through symmetry arguments and with the construction of suitable Green's functions, as a pair of uncoupled surface integral equations, each with one unknown function defined on a single wedge face. The convergent solution to the Fredholm integral equation of the first kind is expressed as a Galerkin series of Laguerre polynomials, and the development takes account of the distant behavior anticipated from the asymptotic solution to the related Sommerfeld half-space problem. The required inner products for the Galerkin projection scheme, which are integrals of products of the weighted Laguerre polynomials and the pair of Green's functions for the separate homogeneous regions, are written as Taylor series coefficients of an auxiliary function. Efficient numerical implementation of the physically based analysis produces an accurate and intuitive picture of the wave interactions with this canonical scatterer. [Work supported by the Naval Research Laboratory, Stennis Space Center.]
2:20 5pUW13. Forward and inverse scattering from rough surfaces at low grazing incidence. Mark Spivack (Dept. of Appl. Math. & Theor. Phys., Univ. of Cambridge, Cambridge CB3 9EW, UK) In acoustic wave scattering from rough surfaces at low grazing angles the usual approximations are known to break down. In the forwardscattering regime this case is often treated by the parabolic integral equation method. The scattering of plane waves within this formulation is considered, and effective reflection coefficients are derived for slightly rough surfaces. Of the higher-order statistics, the dependence of the diffuse and specular components upon incident angle is also examined. The problem of surface reconstruction can be expressed in terms of coupled integral equations; it is explained how in the parabolic case these are easily treated. A major problem, however, is backscatter; a nonparabolic approach to this problem for low-grazing angles is described. [Work supported by Smith Institute Research Fellowship.]
2:25 5pUW14. Time domain visualizations of the acoustic scattering from two-dimensional (2-D) rough surfaces. Richard S. Keiffer (NRL Code 7181, Stennis Space Center, MS 39529-6000), Jorge C. Novarini (Planning Systems, Inc., Slidell, LA 70458), and Guy V. Norton (NRL, Stennis Space Center, MS 39529-6000) The impulse response of a 2-D rough surface to a point source is calculated via the wedge assemblage (WA) method. Periodic, doubly periodic, and randomly rough 2-D surfaces having both Dirichlet and Neumann boundary conditions are considered. The WA method is used to generate time-evolving visualizations of the scattering process by mapping onto all parts of the scattering surface a color that represents a surface elements contribution to the instantaneous scattered field as measured at the receiver. Successive snapshots of this color-coded spatial mapping of the individual impulse responses are generated and displayed sequentially to form a movie. In addition to the movie, a new tool for the analysis of the scattering process is introduced. This new tool uses the fact that, for the particular surface discretization employed, only four distinct types (directions) of wedge can occur. Because the contributions of the different wedge types to the overall impulse response are readily separated via the WA model, one can study the role of surface curvature in different directions for various scattering geometries. In particular, this unique capability is exploited to gain further insight into the role of out-of-plane scatterers for various geometries. [Work supported by ONR.] 127th Meeting: Acoustical Society of America
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