Scattering in glass beads: Effects of frame and pore fluid compressibilities

Abstract

Ultrasonic compressional wave phase velocities and attenuation have been measured as functions of frequency in packings of unconsolidated glass beads. External confining pressure was varied so as to modify the physical interactions between grains. Samples were studied when vacuum dry and when saturated with different pore fluids. Pressure/pore fluid combinations were chosen so as to minimize nonscattering loss mechanisms. In each experiment, phase velocity exhibited negative dispersion and attenuation was proportional to the fourth power of frequency. Increasing confining pressure (to 15 MPa) significantly reduced the scattering losses (by an order of magnitude at a given frequency in dry glass beads). Decreasing pore fluid compressibility produced a corresponding decrease in the scattering losses. This work shows that theoretical models based on suspensions are not adequate when the grains are interacting to form a solid frame. The present data, along with previous data for fused glass beads, suggest that the scattering strength is related to the contrast between the acoustic properties of the composite and of the grains, rather than to pore/grain or pore/composite contrasts.