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 layer. For thin weathering layers, information about previous anisotropic events may be distorted; when the thickness of this layer approaches some physically defined limit, however, a previous layer's anisotropic 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 visual hodogram analysis when the time delay between the fast and slow shear-waves is less than tau/8. When two azimuthally anisotropic layers with different symmetry axes exist, however, interpretations generated through both hodogram analysis and Alford rotations begin to deteriorate when the time-delay generated by the second layer is greater-than-or-equal-to tau/8. Recent field work has shown that the weathering layer may possess differential shear-wave birefringence in excess of 25 percent. If we assume a dominant frequency of 40 Hz and shear-wave velocities of 357 m/s (V(S2)) and 442 m/s (V(S1)), then an azimuthally anisotropic weathering layer may be as little as 5.8 m (19 ft) thick when it begins to overwrite a previous layer's anisotropic response. When the time delay generated by a second anisotropic layer is greater-than-or-equal-to tau (46.4 m, 152 ft thick), information about earlier anisotropic events are completely overwritten.