Shear-wave anisotropy from dipole shear logs in oceanic crustal environments

Abstract

The deployment of a down-hole dipole shear sonic tool in Hole 395A and Hole 735B marked the first two opportunities to measure high-resolution shear-wave velocity and VS anisotropy profiles in oceanic crustal rocks. In Hole 395A near the Kane Fracture Zone, dipole sonic logs were recorded from 100–600 mbsf, and allow azimuthal anisotropy to be determined as a function of depth in the crust. The magnitude of VS anisotropy varies with depth, from less than 3.2% in low-porosity flows at the bottom of the hole, to approximately 15.5% in highly fractured pillow basalts and breccias. The orientation of the fast VS direction also varies over depth, with a mean value between 75°N and 80°E, and aligns with the strike of steeply dipping structures observed by down-hole electrical and acoustic images. This fast VS angle orientation is locally oblique to the plate-spreading direction and to the Mid-Atlantic Ridge axis. In Hole 735B, drilled near the Atlantis Fracture Zone, dipole sonic logs from 23 to 596 mbsf indicate that VS anisotropy varies with depth, with averages of 5.3% in the foliated and deformed gabbros recovered at the bottom of the hole; 4.5% in undeformed olivine and oxide-rich gabbros around 300 mbsf; and 6.8% in highly deformed mylonitic zones at shallow depths. The fast VS angle also varies with depth, giving a mean orientation of approximately S45°E for well-resolved estimates in the upper interval of the hole. This direction aligns with the strike of steeply dipping fractures observed by down-hole imaging, and is locally oblique to the Southwest Indian ridge axis. Although the effects of regional stresses and local deformation of these holes may introduce anisotropy in the dipole sonic data, we conclude that crustal morphology in the vicinity of the holes contributes significantly to the magnitude and orientation of VS anisotropy.

Data report: analysis of shear wave anisotropy in upper oceanic crust, ODP/IODP Hole 1256D

“…Iturrino et al (2005) analyzed sonic logs from Deep Sea Drilling Project Hole 395A near the Kane Fracture Zone on the Mid-Atlantic Ridge and ODP Hole 735B near the Atlantis Fracture Zone on the Southwest Indian Ridge (slow to intermediate crustal spreading rates). The results revealed a complex pattern of varying degree of anisotropy with depth from 1% to 15%, with the mean direction of the fast shear wave velocity (V S ) oriented oblique to (Hole 395A) and perpendicular to (Hole 735B) nearby ridge segments, but varying widely about the mean V S at different depths.…”

“…Due to limited spatial resolution, these seismic studies could constrain anisotropy directions only within a 10° azimuthal range, at best. An alternative technique with potentially higher azimuthal resolution is provided by dipole sonic logging tools, which record cross-dipole shear wave fields around boreholes using orthogonal source and receiver pairs (Esmersoy et al, 1994;Iturrino et al, 2005 Hole 1256D is one of the few deep holes that penetrate through the upper oceanic crust and offers a unique opportunity to study anisotropy as a function of crustal depth. The site is located on the Cocos plate on the eastern flank of the East Pacific Rise (Fig.…”