HR: 1340h
AN: T13B-1340    [Abstracts]
TI: Effect of Fluid on Seismic Anisotropy
AU: * Bandyopadhyay, K
EM: kaushikb@stanford.edu
AF: Stanford University, Dept. of Geophysics, 397 Panama Mall, Stanford, CA 94305, United States
AU: Mukerji, T
EM: mukerji@stanford.edu
AU: Mavko, G
EM: mavko@stanford.edu
AB: We present the effect of fluid on the anisotropic Thomsen's parameters for rocks with transversely isotropic and orthorhombic symmetry. We use the anisotropic form of Gassmann's equation for fluid substitution and consider the following origins of anisotropy in the rock: (a) anisotropy due to aligned fractures, (b) thin layers of isotropic sands with different elastic properties, (c) thin layer anisotropy with alternate layering of anisotropic sand, and (d) stress induced anisotropy. Our modeling results show that fractured rocks exhibit significant changes in anisotropy due to fluid substitution. The predicted change in such situations can reach even five times the initial anisotropy. The magnitude of change in anisotropy arising from nonhydrostatic stress or layering of different facies is less significant. We find that, it is the initial anisotropy of the medium that controls the fluid effect on anisotropy. Higher initial anisotropy causes larger change in anisotropic parameters due to fluid substitution. We present an approximate fluid substitution equation for vertical velocity in anisotropic medium. We express the changes in vertical P-wave velocity due to fluid substitution by isotropic fluid substitution plus a correction term proportional to the Thomsen's parameter 'delta'. This simplification reduces the number of parameters required for fluid substitution in anisotropic rocks. It also helps us to explain the over- or under-prediction of isotropic Gassmann's prediction in anisotropic rock in terms of the magnitude and sign of 'delta'.
DE: 3909 Elasticity and anelasticity
DE: 4260 Ocean data assimilation and reanalysis (3225)
DE: 5102 Acoustic properties
DE: 5104 Fracture and flow
SC: Tectonophysics [T]
MN: 2007 Fall Meeting