Ultrasonic Velocities in Lightly-Loaded Natural and Synthetic Granular Media*

The seismic properties of near surface soils are of interest for a variety of problems, including pore fluid identification and tracking, wave propagation modeling, geotechnical site characterization, static corrections for reflection seismology, and locating underground objects. Ultrasonic velocities increase rapidly when a granular material is loaded as contact stress and area and coordination number of grain contacts increase. Porosity, grain size and distribution, grain shape, and mineralogy all play a role in determining this nonlinear response. We adapted the ultrasonic pulse transmission method to measure compressional (P) and shear (S) wave velocities at ultrasonic frequencies (100-500 kHz) for lightly loaded artificial soils (to 0.1 MPa maximum). Samples were fabricated from Ottawa sand (some with montmorillonite added), Santa Cruz beach aggregate, artificial glass beads, and alumina spheres. All materials were characterized with the SEM before the experiments. We focused on packing, mineralogy, and hysteretic effects in our study and found that compressional velocities vary from ~200 to ~700 m/s over the narrow loading range investigated as a result of these effects. In light cyclic loading of pure Ottawa sand we observed hysteretic effects in the shear mode velocity, implicating sticking of the grains. Our measurements demonstrate a cubic relationship between stress and compressional wave velocity for pure quartz sand, as predicted by Hertzian contact theory when grain roughness is incorporated. The sand/clay mixtures were found to have very different properties from pure sand. The clay bridged sand grains creating more area at the contacts and higher sound speeds over the narrow loading range, but suppressed the strong nonlinear behavior predicted by Hertzian contact theory. *This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract number W-7405-ENG-48 and was supported specifically by the Environmental Management Science Program of the Office of Environmental Management and the Office of Energy Research.