A PHYSICAL MODEL STUDY OF MICROCRACK-INDUCED ANISOTROPY

A laboratory study of the effects of oriented penny-shaped inclusions embedded in a solid matrix on the propagation of seismic shear waves shows good agreement with theoretical predictions for some polarizations and poor agreement for polarizations at large crack densities. The models are constructed of a solid matrix of epoxy resin with inclusions of thin rubber discs of approximately equal cross-sectional areas. The Theoretical basis for these experiments is the theory of Hudson, in which the wavelength is greater than the dimensions of the individual cracks and their separation distance, and the cracks are in dilute concentration. By a pulse transmission method, seismograms were gathered in models free of inclusions and models with inclusions. Seismic measurements of velocity anisotropy, for variations in both a polarization and propagation direction, were performed on physical models with inclusions (cracks) representing five different crack densities (1, 3, 5, 7, and 10 percent). Variations in velocity anisotropy at different crack densities have been evaluated by using Thomsen's parameter (gamma) which relates velocities to their elastic constants, [GRAPHICS] Comparisons between experimental and theoretical results indicate that with wave propagation parallel to the aligned inclusions, shear waves polarized parallel to the plane containing the inclusions (SI) agree well with the theoretical model. However, shear waves for the same propagation direction but polarized perpendicular to the plane containing the inclusions (S2) produced results that agree well with the theory for crack densities up to 7 percent, but disagree for higher crack densities. The deviation of gamma at 10 percent crack density suggests that crack-crack interaction and their coalescence may be observable and could lead to seismic techniques to differentiate between microcracks and larger macrocracks.