Ultrasonic velocities of unconsolidated sand: Evaluating the microstructure and contact based models

Abstract

Microstructure and grain shape factors largely affect the seismic velocities through unconsolidated sands. Moreover, parameters such as coordination number (CN) and contact surface area are dependent on the grain sorting and grains shape factors. We use 3D micro-CT images to quantify the microstructure in terms of CN and contact surface area, and grain shape factors in terms of sphericity and roundness to understand the results of the laboratory experiments on the ultrasonic wave propagation through four different sorted unconsolidated sand samples. We have found that sample with a higher CN and large total contact surface area has higher ultrasonic velocities (both compressional VP and shear VS). CNs calculated for all samples provide a good match between contact based model for effective bulk modulus and dynamic effective bulk modulus obtained from measured velocities. Following the contact based effective elasticity models that incorporate fraction of no slip contacts between the grains, we analyze the relationship of it with porosity and compaction stress.