Ultrasound propagation in wet and airless non-consolidated granular materials

This paper deals with an experimental description of the acoustic behaviour of non-consolidated granular materials submitted to static force. The aim of this work is to investigate the effect of a small amount of an interstitial fluid on the acoustic propagation. Measurements of the velocity and of the transmission of the coherent wave are performed for different values of the applied force. It is shown that the behaviour of the speed of the ultrasonic coherent wave according to pressure have a slope close to the one of the Hertz-Mindlin's model in the case of a dry medium. When a small amount of a low viscosity fluid is added in a mono-disperse granular medium, the speed of the ultrasonic wave increases according to the power 1/6 to the force applied following the Hertz-Mindlin law (v∼P1/6). Moreover, measurements of the velocity and of the transmission of the ultrasonic wave are strongly dependent on the nature of the interstitial fluid. In order to quantify its effect on the propagation, measurements are performed using various fluids having different characteristics. In a first step, silicon oils of different viscosities (from 50×10-3 to 10Pas) are used, showing that with increasing viscosity, the wave velocity no longer varies according to the power law 1/6. The transmission coefficient also increases with the viscosity, showing a better propagation of the wave through the medium. Then, measurements are done in the vacuum allowing a comparison with ultrasonic propagation in presence of an interstitial fluid. This experiment shows a strong increase of the transmission coefficient while velocity remains the same as in the dry case. The study of scattered waves in vacuum shows also a significant increase in amplitude and duration of these typical waves. Then, different saturating inert gases are added to the medium showing that the propagation of the scattered wave is not influenced by their different characteristics.