Transverse isotropic velocity estimates from slowness and displacement measurements.

摘要

Near offset Vertical Seismic Profiling (VSP) is an established technique to obtain interval velocities for lithology characterization, time-to-depth calibrations, and seismic data processing. The objective of this study is to investigate a technique to measure interval anisotropic elastic properties from multiple-offset VSP travel-times and displacements for lithology discrimination and correlation with petrophysical data.; Independent estimates of vertical and horizontal slowness are made for each receiver depth and source offset pair. These provide the magnitude and direction of phase velocity for calculating in-situ elastic properties from the dispersion relation for transversely isotropic media. This technique requires the assumption of lateral homogeneity because horizontal slowness components are related to surface properties between sources and not to rock properties between receivers at depth. P-wave polarization of the direct arrival is estimated from the initial discontinuity in particle acceleration. Differences between these directions and the phase velocity direction (wave surface normal) may help confirm the presence of transverse isotropy.; Factors that influence these measurements, such as source elevation differences, dipping layers and vertical heterogeneities are evaluated with a dynamic ray-tracing method. Rays are traced through a 2-D medium that consists of homogeneous layers described by five elastic constants of a transversely isotropic medium and a rotation angle of the symmetry axis. Synthetic results indicate that shallow lateral variations can introduce significant errors in estimates of slowness and percent anisotropy. Vertical heterogeneities, where lithologic properties change abruptly, can produce local uncertainties in estimates of both slowness and displacement polarization.; Six impulsive source P-wave offset VSPs in a 663 m well in east Texas are analyzed; a near offset SH-wave vibrator VSP provides vertical shear wave phase velocities. Strong transverse isotropic velocity variations observed in the Tertiary and Cretaceous sediments penetrated by this well could be due in part to dipping layers, vertical heterogeneities and unknown local surface variations. Although these results may not indicate exact intrinsic elastic properties, a good correlation with lithology types suggests that the technique is sensitive to local receiver properties. Percent 'phase velocity' anisotropy appears to increase from 6% to 20% for P-waves and 10% to 50% for SV-waves with increasing amounts of calcareous material and shale in the rock. (Percentages are defined as the ratio of the horizontal to vertical velocity for P-waves and the ratio of 45{dollar}{bsol}sp{bsol}circ{dollar} to vertical velocity for SV-waves). Sandstones and chalk exhibit the least amount of anisotropy, while the shales and calcareous shales the most. Analysis of the P-wave polarization direction does not agree with the traveltime results in this study and show significant scatter. Poor sonde coupling with the borehole wall is most likely responsible for the limited accuracy in obtaining 'true' particle motion.