ATTENUATION MECHANISMS IN SANDS - LABORATORY VERSUS THEORETICAL (BIOT) DATA

The velocity and attenuation of compressional (Q(p)-1, V(p), respectively) and shear waves (Q(s)-1, V(s), respectively), determined with the Pulse Transmission technique at a frequency of about 100 kHz, are compared with the grain size, shape, porosity, density, and static frame compressibility of dry and water-saturated sands. Except for V(s), all the quantities V(p), Q(p)-1 and Q(s)-1 are dependent on grain size and are higher in coarser grains than in finer grains. Q(s)-1 decreases significantly with increasing differential pressure in coarse-grained sediments, but the same sediments show an anomalous increase with differential pressure in Q(p)-1 at low pressures. We have also modeled the V(p), V(s), Q(p)-1, and Q(s)-1 of these samples to understand the mechanisms governing the observed changes. The Contact Radius model with surface force effects predicts both V(p) and V(s) to be dependent on grain size. Frictional losses in unconsolidated coarse-grained sands must also be considered at small strains (10(-7)). Velocity and losses measured in saturated sands are higher than those predicted by the Biot model, which does not account for any grain size dependence of the seismic qualities.