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tion of thc shell. Thesc questions are revisited here and the elastic re-sponsc of the shell in terms of the modes of a fluid-loaded, infinite cylindrical shell is analyzed. In this work, it is particularly useful to study thc evolution of the scattering as the thick shell limit is ap-proached with reference to the scattering solution (and modes) of a solid elastic target. By examining the evolution of the prominent fea-tures of the scattering solution, the evolution of the underlying modes of thc system can be inferred. This evolution shows a strong analogy to the evolution of the modes of a spherical shell presented previously.

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6PAa5. Abstract withdrawn.

9:45

6PAa6. Differential scattering cross sections at arbitrary outgoing angles and their relation to target boundary conditions. Jacob George and M. F. Werby (NOARL, Codę 221, Stennis Space Ctr., MS 39529)

It is usual to examine differential scattering cross sections in the baekward direction as a function of frcqucncy variation. When this is done in the asymptotic limit the resulting function is referred to as a form function. The form function proves to be scnsitive to the boundary conditions of the target and may manifest resonances and circuinferen-tial diffraction effects typical of certain targets. However, such quanti-ties at exit angles such as those in the forward direction prove to be somewhat less scnsitive to the objeefs properties with inereasing fre-quency. This characleristic is undcsirahlc if one wishes to examine the materiał naturę of the target. On the other hand, it can be valuable from the rnodeling point of view if one wishes to obtain forward scattering target strengths. In that case one may use the simplest target to obtain such a property. Forward and other exit angle target scattering strengths were cxamined over a frequency region to determine when the strengths begin to convergc and thus when thc simple approximations can be madę.

lfcOO

6PAa7. Determination of materiał composition from time-domain and frequency-domain resonance echoes of submerged clongated elastic targets. C. E. Dean and M. F. Werby (NOARL, Codę 221, Stennis Space Ctr., MS 39529)

When scattering from elastic targets backscattered echoes yield in-teresting information in the resonance region. In particular, resonance scattering theory in thc frequency domain along with the circumfercn-tial naturę of resonances imply that materiał constituency is a charac-

1950 J. Acoust. Soc. Am.. Vol. 09, No. 4, R. 2. April 1991 teristic of resonance location. Moreover, recent discussions of resonance signatures in the time domain [sce UberalPs book to be published on resonance scattering] can also yield information conccrning resonance widths and average phase velocities which can also be related to materiał characteristics. By adjusting thc orientation of thc target over a suitable angular region it is possible to ascertain certain symmetries of the target if they exist, particularly if one varies the frequencies over a suitable rangę of resonances. If one observes axial symmetry through such a process, then it is possible to obtain both the dimensions of the object and the aspect ratio of the object (ratio of łength to width). This is assuming that the target is in a “free” environment; that is, thc boundaries of thc target are not a factor in calculation. Time-domain responses for specific pulse types also yield information and it is easy to sce how a serics of questions can form the basis of a scenario that can rule out certain targets or lead to a probability (confidence level) that specific targets are present. To determine the extcnt that this can be done, targets are examincd which are composed of five materials for elastic solid spheroids for aspect ratios of 3 to 1 and 6 to 1 for end-on incidence and for the case of 4 to 1 for all incident angles.

10:15

6PAa8. Constrained total Ieast-squares-based extraction of acoustic scattering resonances of elastic targets. Theagenis J. Abatzoglou and Roger H. Hackman (Pało Alto Res. Labs., Orgn. 91-50, Bldg. 251, 3251 Hanovcr St, Pało Alto, CA 94304-1191)

The analysis of the resonance response of elastic targets to an inson-ifying beam has been a central theme in acoustic scattering investiga-tions in recent years. The motivation for this work is that thc resonance structure of such targets is dircctly related to both the geometry and materiał properties of the target and this has a bearing on target clas-sification. The development of robust techniques to estimate the resonance frequencies from the transient scattered field in the presence of noise is an important and open problem that is the subject of the current study. In the following, the efficacy of several competing tcchniques are examined (i.e., the constrained total least squares [T. J. Abatzoglou et ai, IEEE Trans. ASSP (May 1991)], the Prony, and a recently devel-oped singular value decomposition based technique), with theoretically calculated scattering data. To simulate scattering data, the spheroidal coordinate based T-matrix formallsm is utilized (R. H. Hackman and D. G. Todoroff, J. Acoust. Soc. Am. 78, 1058-1071 (1985)] to compute the impulse response of large aspect ratio, solid elastic cylinders for target aspects ranging from end-on to broadside; additivc Gaussian noise is included in the time domain representation. Such scatterers pose realistic challenges in that their target signature varies strongly with frequency and target aspect. Both monostatic and bistatic scattering are considcred.

10:30

6PAa9. Determination of the vibration modę by a short pulse method. P. Rembert, P. Pareige, G. Maże, and J. Ripoche (Lab. d’Acoust. Ultrasonore et d’Electron., URA CNRS 1373, Univcrsitć du Havre, Place R. Schuman, 76610 La Havre, France)

It is well known that several experimental methods using quasihar-monic insonification allow a direct verification of the resonance scattering theory. They all providc, for elastic cylindrical iurgcts immersed in water, the resonance frequencies and the modę number of cach one. So far, pulscd techniques only allowed resonance isolation. Presented here is a new method: the short pulse method of isolation and identification that completes these works and alłows a total comparison with theoret-ical and quasiharmonie experimental results. This method consists in the digitizing of thc time signals that characterize the scattering of an elastic target insonified by a short pulse for different angular positions of the receiving transducer. For each so obtained signal, a resonance spectrum is obtained after a spectral amplitudę analysis. From these data, a Computer processing allows the plotting of identification pattems, and, by the same way, the knowiedge of the modę of vibration at a given resonance frequency. This method has been used for the identification of

121st Meeting: Acoustical Society of America 1950