Incidence-angle-dependent acoustic reflections from liquid-saturated porous solids

Angle of incidence amplitude variations of acoustic waves reflected from an interface is increasingly important in acoustic sea floor imaging and seismological studies. Such observations are almost solely interpreted assuming elastic wave theory. However, wave propagation through, and hence reflectivity from, liquid-saturated porous solids is complicated by the presence of the slow longitudinal (P2) wave. There have only been limited quantitative experimental tests of porous media reflectivity as a function of angle of incidence. Here, the acoustic reflectivity from a water-saturated porous plate is measured as a function of the angle of incidence using a specially developed ultrasonic reflectometer. The observed reflectivity agrees with that predicted using the Biot-type poroelastic theory; this work confirms the use of boundary conditions that allow fluid transfer across the reflecting interface. It is found that simpler elastic expressions based on equivalent-elastic solid cannot be reconciled with the observations.