Analysis of Seismic Anisotropy Parameters for Sedimentary Rocks and Strata

The most prominent feature of sedimentary basins is often its layered structure. The effective elastic properties of the layered media are anisotropic. Mudstones account for most of the volume of the sedimentary rocks, and the elastic properties are usually approximated by transverse isotropy. Therefore, seismic anisotropy is very important for exploration geophysics. Study of the relationships between the anisotropy parameters may supply very useful constraints for the estimation of the anisotropy parameters and be critically important for simplification of the problems in anisotropic-seismic-data processing and interpretation. Based on the theory of anisotropic elasticity and observation from the mechanical measurements on mudstones, tight physical constraints on c13 by c11, c33, and c66 are derived. The physical constraints on c13 are used to evaluate the quality control of the laboratory anisotropy-measurement data from the literature. It is found that there are significant uncertainties in the determination of c13 and delta. When the measurement data of bad quality control are identified and discarded, the intrinsic relationship between c13 and other TI elastic constants, and the relationship between delta and the other Thomsen parameters are revealed. Considering the difficulty and significant uncertainty in laboratory anisotropy-measurements, a practical and robust method for laboratory determination of c13 and delta is proposed. Based on laboratory ultrasonic-measurement data of sedimentary rocks and using Monte Carlo simulation and Backus averaging, the layering effects on seismic anisotropy are analyzed. If the sedimentary strata consist of fully brine-saturated isotropic rocks, delta is usually negative; and if the sedimentary strata are gas bearing, delta increases. For an effective TI medium consisting of isotropic layers, c13 can be determined by other TI elastic constants; and therefore, delta can be predicted from the other Thomsen parameters with confidence. The layering effects on seismic anisotropy at the high-frequency limit are compared with those at the low-frequency limit. Relative to the Backus averaging, ray theory usually underestimates the Thomsen parameters epsilon and gamma and overestimates delta. For the sedimentary strata consisting of isotropic layers, the differences are very significant.Earth and Atmospheric Sciences, Department o