885095988

9:25

4EA3. Underground sound: Applied seismic waves. J. E. White (Geophys. Dept., Colorado School of Mines, Golden, CO 80401)

The term underground sound is meant to encompass the instrumentation, observational methods, and interpretive techniques that have been developed for the utilization of seismic waves in the earth, quite analogous to the field of underwater sound. Wave propagation in the shallow crust of the earth is morę complex than waves in water, simply because of the presence of shear waves in addition to compressional waves. Either type of wave is severely attenuated in travcling through earth materials, compared with similar distances of travel through the ocean. Finally, many rocks arc anisotropic in average properties, resulting in further complexities. In exp!oration for petroleum, remarkably detailed images of geologie boundaries have been achieved by processing observcd data so as to enhance reflected compressional waves and suppress shear waves and rcverberations. Altematively, shear-wave reflections can be observed by using a vibratory source which radiates strong shear waves and processing the data so as to suppress comprcs-sional “noise” waves. Used together, the two reflection methods yield lithologic information, such as distinguishing between gas saturation and water saturation in a porous rock. For soils and near-surface rocks, shear-wavc speeds are directly useful for evaluation of building foundations and dam sites. Attenu-ation of seismic waves is both a fortunate characteristic and a strict handicap. If attenuation were quite smali, multiple reflections among layers would result in uninterpretable rcverberations. Since the actual attenuation inereases with frequency, the resulting loss of high frequencies places a severe limit on the resolution obtainable with seismic waves. The presence of attenuation requires that the wave speed depend on frequency. When averaged over distances of a few meters, many geologie formations behave as “homo-geneous” layers. However, the average properties may depend on dircction and the layers must be rccog-nized as anisotropic solids. Such anisotropy may be due to fractures caused by previous episodes of defor-mation, due to unequat stresses existing at present, or due to fine layering. Anisotropy clearly complicates seismic data processing. Because so many measurements are madę in oil wells and test boreholes, suitable sourccs and detectors of seismic waves have been developed and the coupling of seismic wave$ to a fluid-filled borehole has been treated mathematically.

9:50

4EA4. Wave propagation and scattering in chiral elastic media. Vijay K. Varadan and Vasundara V. Varadan (Res. Ctr. for the Eng. of Electron, and Acoust. Mater., Penn State Univ._t 149 Hammond Bldg., University Park, PA 16802)

Optical activity is exhibited by media whose molecular configurations are handcd or chiral. Since the geometry is the basis for chirality, it can be probed by transverse waves, but not by longitudinal waves. Composite elastic media can also be madę chiral because the elastic field in solids consist, in generał, of both longitudinal and transverse componcnts. The displacement field u in such isotropic, noncentrosymmetric (chiral) elastic solid has to be supplemented by the independent microrotation field <0. Six different wave numbers are possible in chiral solids. Of these, two represent longitudinal fields and the remaining four are circularly polarized. Composites can thus be tailor-made by suitably designing the microstructure and volume fraction of chiral elements. In this talk, the theoretical foundation for wave propagation and scattering in chiral elastic solids will be outlined. Pertinent experimental results on scattering of acoustic waves by chiral elastic composites containing piezo-chiral elements will also be presented.

10:15

4EA5. Transient and steady-sUte scattering of acoustic waves from elastic objects in fluids, and of elastic waves from inclusions in solid media—Direct and inverse scattering aspeets. G. C. Gaunaurd (Naval Surface Warfare Ctr., Res. Dept. (R-42), White Oak, Silver Spring, MD 20903-5000)

This paper considers scattering problems associated with: (a) the simpler case of acoustic wave scattering by an elastic body in a fluid, and (b) the morę complex situation of elastic waves scattered by a spherical (fluid or elastic) inhomogeneity in a solid host medium. The analysis is first presented in the frequency domain, in generał, for all frequencies. Later, particular emphasis is given to the midfrequency rangę: 5    25, which coincides with the “resonance region” of impenctrable scatterers. It is in this

region that a number of simplifying approximations valid at Iow or high frequcncies do not hołd and the analysis is hardest. The above two cases were the ones originally treated by the RST. Since there are also resonances in the high-frequency region and in the Raylcigh region (kaź 5), the concept of ‘‘resonance region” has to be broadened for penetrabie scatterers. However, resonances in the Rayleigh region have relativcly simple (viz., either multipole or curvature) origins that will be reviewed here. The analysis is then extended (still direct scattering) to the time domain. The targefs transient responses to selected types of incident pulses are computed, measured, and interpreted. Finally, selected aspeets of the irwerse scattering problem are considered. Also examined (in various cases) is how certain features in the targefs time or frequency “signature” are related to specific physical target characteristics (viz., shape, composition,...) that permit its remote and unambiguous classification. [Work supported by ONR and NSWC.]

1900

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

121st Meeting: Acoustical Sodety of America

1900