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Abstract
This study showed the importance of laboratory velocity measurements for intrinsic rock velocity and velocity anisotropy and the effect of pressure on velocity and anisotropy. Without the laboratory-determined intrinsic velocity and pressure dependence combined with the in situ velocities from the vertical seismic profile (VSP), it would be impossible to understand the full effect of voids and fluid pressure. While earlier studies found the importance of fluid on seismic wave propagation in the upper crust, this study consolidates and further emphasizes the effect. With increasing depth, the departure of laboratory velocities from those of the VSP qualitatively corresponds to the increase in fluids and disaggregation described by Soviet scientists in earlier studies (Kozlovsky, 1987). Similar results of porosity and fracturing, though related to a major fault zone, were described from the KTB (Rabbet et al., 2004) without deep rock samples but with a very extensive VSP. Anisotropy greatly confounds the application of Vp/Vs to constrain rock compositions. This anisotropy is the effect of lattice preferred orientation (LPO), microcracks and possibly layering and fractures. For travel paths at 45° to the symmetry axis (the regional dip of the layering of the geologic sections), S-wave splitting is near maximum, causing the greatest spread in possible Vp/Vs values, but at this angle, Vp is still near maximum (for this one particular mafic rock). The main contribution of this study is understanding the effect of anisotropy on Vp/Vs and resulting compositional estimates, to integrate previous laboratory velocity measurements, borehole logging, and seismic data, to clarify the importance of voids on upper crustal velocities, to seismically confirm the occurrence of overpressuring in the deepest part of the upper crust. Fluids and overpressuring extend to at least 12 km depth and possibly to 17 km depth where strong sub-horizontal reflections are observed.
