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Seventeen papers based on talks and posters presented at 7IWSA are included in this book: In “General constitutive relations for layered media,” Schoenberg provides a combination of matrix algebra and group theory to offer a formalism for evaluating of the properties of the unique homogeneous anisotropic medium that is statically equivalent to a stationary distribution of homogeneous, but generally anisotropic, fine layers. The method is valid for systems representing fluid permeability, heat conductivity, and electrical properties such as electrical conductivity, dielectric permittivity, and magnetic permeability. Additionally, the formalism extends to higher dimensional elastic layered systems so that bi-anisotropic electromagnetic and piezoelectric layered systems can be studied. “Upscaling: Elastic anisotropy from ultrasonic laboratory measurements to borehole seismic surveys,” by Hornby, applies the equivalent media theory of Backus (1962) to resolve discrepancies in anisotropic parameters for transversely isotropic media derived from ultrasonic laboratory measurements and seismic-scale VSP surveys. Hornby argues that the inversion of laboratory experiments generally leads to larger values of the δ parameter of Thomsen (1986), as well as smaller values for the normalized anisotropy parameter η proposed by Alkalifah and Tsvankin (1994). The difference is attributed to the effect of fine-scale layering on the larger wavelength VSP measurements. Hornby tests this hypothesis by the upscaling of elastic constants determined from in-situ depth-continuous borehole measurements. Although borehole measurements cannot resolve the complete set of elastic constants, remaining parameters are estimated from representative laboratory measurements of borehole samples. The resulting values for the anisotropic parameters δ and η are comparable to those determined from the VSP survey with remaining differences attributed to the effect of unresolved finer-scale layering. In “Can we separate the effects of anisotropy and structure from surface seismic data?” Kuehnel and Li offer a practical method of correcting traveltimes for reflections in transversely isotropic media. They present a method of decomposing the traveltime equation for a reflected wave in a transversely isotropic layer in terms of dip and anisotropy, based on the traveltime of individual wavetype using an iterative procedure applied to a ray-tracing code. In “ P -wave traveltime anomalies below a dipping anisotropic thrust sheet,” Leslie and Lawton raise the awareness of the complications that can arise from this common subsurface configuration and begin to quantify the anisotropic effect in depth migration. Results from physical modeling and numerical ray tracing show that isotropic depth migration and velocity analysis produce significant artifacts in the synthetic data from such a model. Therefore, it is suggested that care be taken when using the isotropic assumption in depth migration even with the presence of mild anisotropy. |
