LABORATORY STUDIES OF SEISMIC-WAVE PROPAGATION IN INHOMOGENEOUS-MEDIAUSING A LASER-DOPPLER VIBROMETER

We performed physical model experiments by utilizing a laser doppler v ibrometer (LDV). Because LDV converts velocity of vibration to the Dop pler shift frequency, it enables very precise measurements of ultrason ic waves without any resonating element that conventional transducers usually include. A piezoelectric transducer (PZT) was used as a source of elastic waves, and the waveform was measured in a very small area of about 400 mu m in diameter by focusing the beam. We can easily perf orm very precise measurements of wave field in a physical model, and t hus physical model experiments of wave propagation can simulate realis tic seismic field observations. For models of inhomogeneous material, we used three granitic rocks with different grain sizes: Westerly gran ite (fine grained), Oshima granite (medium grained), sind Inada granit e (coarse grained). Large rock prisms, 300 X 300 X 80 to 90 mm, were u sed to prevent contamination by multiple reflections from the side end s in the earlier portion of waveforms. The direct P and S waves and re flected waves were identified by their travel times. Observations were made by long in-line, circular, and small-aperture arrays. When the r ock grain size becomes comparable to the wavelength, transmitted waves are strongly attenuated by backward scattering, and a large amount of wave energy is transferred to the coda portion. Semblance plots in th e time-slowness plane obtained from the small-aperture array suggest t hat incoherent waves become dominant as the grain size becomes large a nd comparable to the wavelength.