The ability to detect underground objects is of significant importance in certain aspects of civil<br>(geotechnical) and environmental engineering. A number of geophysical techniques have shown excellent<br>results in location of certain types underground obstacles (anomalies). The Spectral-Analysis-of-Surface-<br>Waves (SASW) method is a seismic nondestructive technique, that has so far been typically used in the<br>evaluation of elastic moduli and layer thicknesses of layered systems, like soils and pavements. One of<br>the assumptions of the test is that the system tested consists of infinite homogeneous layers of a constant<br>thickness. This assumption is violated whenever heterogeneity of the system exists due presence of buried<br>objects and cavities, presence of discontinuities, like pavement edges [Sheu et al. (1988)], variation of<br>layer thicknesses, etc. The problem is of special importance in SASW testing of pavements in an urban<br>environment, where obstacles like utility conduits, subways, and foundation structures cause wave<br>reflections that can affect significantly test results.<br>The effects of anomalies on propagation of elastic waves have been examined by a number of<br>authors. Cooper and Ballard (1988) reported a distinct travel time increase during refraction surveying in<br>soil with voids present close to the surface. Belesky and Hardy (1986) report successful implementation<br>of reflection survey in deep cavity detection, but significant difficulties in identification of shallow<br>cavities. On the other hand, Haupt (1977), Dravinsky (1983) and Curro (1983) demonstrated numerically<br>and experimentally that surface waves are sensitive to anomalies close to the surface. Recently Al-Shayea<br>et al. (1994) demonstrated experimentally that results of the SASW test are affected by underground<br>objects. They showed that an underground cavity artificially created in a homogeneous sand layer can<br>cause significant decrease in the phase velocity of the Rayleigh wave in a broad frequency range. Later,<br>numerical simulations by Gucunski et al. (1996a and 1996b) confirmed findings of Al-Shayea et al.<br>(1994). The following paragraphs discuss the most important findings on the effects of underground<br>obstacles on the Rayleigh wave dispersion obtained from the SASW test, and the application of the test<br>in detection of buried objects.


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