REFLECTION CHARACTERISTICS OF P1 WAVES AT THE FREE BOUNDARY OF UNSATURATED SOIL

The unsaturated soil is the most common nature soil form on the earth’s surface,unlike the fully saturated soil,four bulk waves exist in this soil: one shear wave (i.e.,S wave) and three compressional waves (i.e.,P1 wave,P2 wave,P3 wave). Among these four type waves,P1 wave is the wave with fastest wave velocity and lowest attenuation coefficient,which is similar to the fast compressional wave in fully saturated soil. Hence the unsaturated poroelasticity theory is used to describe the propagation characteristics of elastic waves in unsaturated fluid-saturated soils,and the reflection problem of plane-P1-waves at the free boundary of unsaturated soils is analyzed. Based on the boundary conditions,the theoretical expressions of the amplitude reflection coefficients and energy reflection coefficients for the four reflected waves at the boundary,i.e.,reflected P1 wave,reflected P2 wave,reflected P3 wave and reflected S wave,are respectively derived and the numerical calculations are then carried out. The effects of the incident angle and saturation degree of the soils on the amplitude reflection coefficients and energy reflection coefficients of the four reflected waves are discussed in the numerical example. It is shown that the amplitude reflection coefficients and energy reflection coefficients of the waves is effected by not only the incident but also the saturation degree. These conclusions are useful for the theoretical research and seismic exploration projects.