THIN-LAYERS AND SHEAR-WAVE SPLITTING

The near-surface weathering layer is considered by many to be strongly anisotropic. Any shear-wave signal passing through this low-velocity layer will inherit, to some degree, the anisotropic response of this l ayer. For thin weathering layers, information about previous anisotrop ic events may be distorted; when the thickness of this layer approache s some physically defined limit, however, a previous layer's anisotrop ic signature is completely overwritten. Hodograms and Alford rotations are typically used to analyze shear-wave splitting in the presence of azimuthal anisotropy. When the time-delay generated by an azimuthally anisotropic layer is greater-than-or-equal-to tau/8, where tau = one period of the wavelet's dominant frequency, distortion of a shear-wave signal is great enough to degrade the accuracy of the interpretation in hodogram analysis. We found that Alford rotations are superior to v isual hodogram analysis when the time delay between the fast and slow shear-waves is less than tau/8. When two azimuthally anisotropic layer s with different symmetry axes exist, however, interpretations generat ed through both hodogram analysis and Alford rotations begin to deteri orate when the time-delay generated by the second layer is greater-tha n-or-equal-to tau/8. Recent field work has shown that the weathering l ayer may possess differential shear-wave birefringence in excess of 25 percent. If we assume a dominant frequency of 40 Hz and shear-wave ve locities of 357 m/s (V(S2)) and 442 m/s (V(S1)), then an azimuthally a nisotropic weathering layer may be as little as 5.8 m (19 ft) thick wh en it begins to overwrite a previous layer's anisotropic response. Whe n the time delay generated by a second anisotropic layer is greater-th an-or-equal-to tau (46.4 m, 152 ft thick), information about earlier a nisotropic events are completely overwritten.