Analysis of Thomsen parameters for finely layered VTI media
Since the work of Postma and Backus, much has been learned about elastic constants in vertical transversely isotropic (VTI) media when the anisotropy is due to fine layering of isotropic elastic materials. Nevertheless, there has continued to be some uncertainty about the possible range of Thomsen's anisotropy parameters {epsilon} and {delta} for such media. The authors use both Monte Carlo studies and detailed analysis of Backus' equations for both two- and three-component layered media to establish the results presented. They show that {epsilon} lies in the range {minus}3/8 {le}{epsilon}{le}1/2[(v{sub p}{sup 2})(vp2{sup {minus}2})-1], for finely layered media; smaller positive and all negative values of {epsilon} occur for media with large fluctuations in the Lame parameter {lambda} in the component layers. They show that {delta} can also be either positive or negative, and that for constant density media, sign ({delta}) = sign ((v{sub p}{sup {minus}2}) - (v{sub s}{sup {minus}2})(v{sub s}{sup 2}/v{sub p}{sup 2})). Monte Carlo simulations show that among all theoretically possible random media, positive and negative {delta} are equally likely in finely layered media. (Of course, the {delta}s associated with real earth materials may span some smaller subset of those that are theoretically possible, but answering this important question is beyond the authors present scope.) Layered media having large fluctuations in {lambda} are those more likely to have positive {delta}. This is somewhat surprising since {epsilon} is often negative or a small positive number for such media, and the authors have the general constraining that {epsilon} - {delta}{gt}0 for layered VTI media. Since Gassmann's results for fluid-saturated porous media show that the mechanical effects of fluids influence only the Lame parameter {lambda}, not the shear modulus {mu}, these results suggest that small positive {delta} occurring together with small positive {epsilon} (but somewhat larger than {delta}) may be indicative of changing fluid content in a layered earth.
- Research Organization:
- Lawrence Livermore National Laboratory, Livermore, CA (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 20003859
- Journal Information:
- Geophysical Prospecting, Vol. 47, Issue 6; Other Information: Paper presented at the 67th SEG Annual Meeting, Dallas, TX (US); 1997; PBD: Nov 1999; ISSN 0016-8025
- Country of Publication:
- United States
- Language:
- English
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