Influence of Thin Low-velocity Layer on Elastic Waves

It is important for in-site rock mass estimation and fractured zone exploration with seismic method to clarify the elastic wave propagation in rock mass including thin low-velocity layer such as water-saturated cracks and thin fractured zone. In-siteP wave velocity is commonly used for estimation of rock mass including cracks, because P wave velocity decreases as the thickness of cracks and the number of them increase.But the time average formula is not applicable to in-site rock mass including cracks. The reason is as follows. The wavel ength of P wave used for rock mass estimation is much longer than the thickness of the cracks. Therefore, transmitted wave isf ormed by the superposition of many waves. The one is the wave which transmits without any reflection at the contact plane between rock and the material in the cracks, and the others are the waves which have multiple reflections within the cracks in their wave paths.In this study we make sure experimentally and numerically what is mentioned above by using one dimensional plane wave, and we discuss about amplitude and waveform of reflected and transmitted waves at the thin low-velocity layer.