Velocity anisotropy in shale determined from crosshole seismic and vertical seismic profile data

Abstract

Crosshole and vertical seismic profile (VST) data made possible accurate characterization of the elastic properties, including noticeable velocity anisotropy, of a near‐surface late Tertiary shale formation. Shear‐wave splitting was obvious in both crosshole and VSP data. In crosshole data, two orthologonally polarrized shear (S) waves arrived 19 ms in the uppermost 246 ft (75 m). Vertically traveling S waves of the VSP separated about 10 ms in the uppermost 300 ft (90 m) but remained at nearly constant separation below that level. A transversely isotropic model, which incorporates a rapid increase in S-wave velocities with depth but slow increase in P-wave velocities, closely fits the data over most of the measured interval. Elastic constants of the transvesely isotropic model show spherical P- and [Formula: see text]wave velocity surfaces but an ellipsoidal [Formula: see text]wave surface with a ratio of major to minor axes of 1.15. The magnitude of this S-wave anisotropy is consistent with and lends credence to S-wave anisotropy magnitudes deduced less directly from data of many sedimentary basins.

The elastic anisotrophy of shales

“…This is apparently the case for the near-surface late Tertiary shale studied by Winterstein and Paulsson [1990]. The use of a limited number of Wtmn therefore corresponds to the specification of the orientation distribution by its first few moments.…”

“…These examples, therefore, all have the same best fitting isotropic reference medium in the sense ofFederov [1968] Paulsson [1990]Federov [1968] Paulsson [1990]…”

Vertical seismic profile at Pike's Peak, Saskatchewan, Canada: turning rays and velocity anisotropy

“…Previous authors have used VSPs to estimate the velocity anisotropy of rocks in situ (e.g., Uhrig and van Melle, 1955;Vander Stoep, 1966;Gaiser, 1990;Winterstein and Paulsson, 1990;de Parscau, 1991;Kirtland Grech et al, 2002). Uhrig and van Melle (1955) used traveltimes measured at a single receiver in a borehole to estimate their ''anisotropy factor A'', which they define as the ratio of bedding-parallel velocity to bedding-perpen- …”

Any P-wave kinematic algorithm for vertical transversely isotropic media can be ADAPTed to moderately anisotropic media of arbitrary symmetry type

“…The presence of seismic anisotropy in sedimentary basins is now commonly accepted (e.g., Winterstein and Paulsson, 1996;Thomsen, 1986). For instance, the fraction of papers involving seismic anisotropy in international conventions such as those of the Society of Exploration Geophysicists (SEG) or of the American Geophysical Union (AGU) is steadily growing, keeping a seismic anisotropy community very active (e.g., Brown and Lawton, 1993;Fjaer et al, 1996;Rasolofosaon, 1998;Ikelle and Gangi, 2000).…”