Final Technical Report DE-FG02-99ER14933 Inversion of multicomponent seismic data and rock physics interpretation

Abstract

Summary of Accomplishments and ResultsWe completed our planned work on anisotropy in loose unconsolidated sediments.An important accomplishment was to understand the seismic velocity anisotropy resulting from the combined roles of depositional stratification and stress in unconsolidated sands. This work has lead to one Ph.D. dissertation, and two conference presentations (SEG, 2003). The expanded abstracts from the conference proceedings were attached in the annual report for [2003][2004]. The completed Ph.D. dissertation is forms part of this final report as attachment A.The dissertation presents an experimental study of velocity anisotropy in unconsolidated sands at measured compressive stresses up to 40 bars, which correspond to the first hundred meters of the subsurface. Two types of velocity anisotropy are considered, that due to intrinsic textural anisotropy, and that due to stress anisotropy.Three types of tests were performed: (1) hydrostatic pressure, (2) quasihydrostatic stress, in which, cubic samples are placed in the polyaxial cell and approximately equal forces are applied in each of the three principal directions; and (3) uniaxial strain, in which, cubic samples are placed in the polyaxial cell and an axial stress is applied in the Zdirection, while the horizontal platens are held fixed, at approximately zero displacement. The hydrostatic pressure test is used to compare the standard velocity measurements with the velocity anisotropy measured in the polyaxial cell. The quasi-hydrostatic stress test is used to study the velocity anisotropy resulting from intrinsic anisotropy of the granular materials. The uniaxial strain test is used to study the velocity anisotropy resulting from stress anisotropy in sands. We found that intrinsic and stress-induced anisotropy can be detected in sands using Vp. The intrinsic velocity anisotropy was studied using P-wave velocities and the textural anisotropy was studied using the spatial autocorrelation function of sediment images for sands and glass beads. The results suggest that P-wave velocity anisotropy and textural anisotropy are related for grain segregation or stratification. We found that sand samples display a bi-linear dependence of velocity anisotropy with stress anisotropy. There exists a transition stress beyond which the stressinduced anisotropy outweighs the intrinsic anisotropy for three different sands. In addition, the dissertation discusses the problem of extrapolating acoustic velocities measured under hydrostatic pressure to quasi-hydrostatic stress. Measured Vp 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 in the apparatus, and due to complexity of the stress field inside of the granular material samples. Finally, we found that the model of Norris I find that intrinsic and stress-induced anisotropy can be detected in sands using Vp. I study the intrinsic velocity anisotropy usi...