| There are many sedimentary structures of soft sediments that have intrinsic anisotropy and induced anisotropy. In situ measurements of these structures are often made with seismic wave methods (from sources such as explosives, vibrators, or hammer hits). However, how these acoustic waves detect anisotropy in soft sediments is not well understood. The present dissertation presents an experimental study of acoustic wave velocity anisotropy in unconsolidated sands at compressive measured stresses up to 40 bars (approximately 150 m depth).; In this study, I performed three tests: (1) quasi-hydrostatic stress test that is used to study the velocity anisotropy due to intrinsic anisotropy in a sand and three glass bead samples. In this test, cubic samples are placed in a polyaxial cell and approximately equal forces are applied via platens in three perpendicular directions; (2) uniaxial-strain test that is used to study the velocity anisotropy due to stress anisotropy in three sand samples. In this test, the cubic samples placed in the polyaxial cell are loaded with an axial vertical stress, and the horizontal platens are held fixed; and (3) hydrostatic pressure test that is used to compare the standard velocity measured under hydrostatic pressure with the velocities measured in the polyaxial cell in a sand.; The main results and conclusions in this dissertation are as follows: (1) Intrinsic and induced anisotropy can be detected in sands using P-wave velocity. (2) P-wave velocity anisotropy and texture anisotropy are related for grain segregation and stratification. (3) Sand samples display linear dependence of velocity anisotropy with stress anisotropy. (4) There exists a transition stress at which the stress-induced anisotropy outweighs intrinsic anisotropy in unconsolidated sands. (5) Velocity anisotropy is more significantly affected by stress anisotropy than by textural anisotropy, except for very low compressive measured stresses (2 to 5 bars). (6) Vp measured under hydrostatic pressure is higher than Vp measured under quasi-hydrostatic stress in the sand, for the same depositional anisotropy and similar isotropic stress. This difference might be due to boundary effects. Likewise, stress anisotropy is reduced if these boundary effects are taken in account. |
