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

Abstract

Integrated Ocean Drilling Program Hole 1256D is one of the few deep holes that penetrate through the upper oceanic crust and offers a unique chance to study its anisotropic properties as a function of depth. In this report, we present the results of anisotropy analysis using downhole sonic logging data acquired in Hole 1256D. The analysis is based on the detection of shear wave splitting in azimuthally anisotropic formations; however, poor borehole conditions, high levels of noise, and insufficient tool rotation presented significant challenges in this analysis. Anisotropy could be reliably evaluated only over a few select depth intervals, and they suggest very low shear wave anisotropy, within a range of 0%-1%. In particular, the deepest logged section within the sheeted dike interval is characterized by good borehole conditions and high coherence of shear wave fields, but shear wave velocity appears to be isotropic and/or characterized by weak anisotropy below the detection limit of the Dipole Sonic Imager used to acquire the data. IntroductionUnderstanding in situ properties of oceanic crust has been an important goal of marine geology and geophysics. Seismic studies, in particular, have provided crucial insights into structure and intrinsic properties of oceanic crust (e.g., Carbotte et al., 2008;Harding et al., 1989;Tolstoy et al., 2008;Vera and Diebold, 1994). Although seismic velocity profiles in oceanic crust are well understood (e.g., Spudich and Orcutt, 1980;White et al., 1992), the anisotropy of seismic waves has received less attention. Observations of seismic anisotropy may provide useful insights into intrinsic properties of crust and mantle, such as preferred orientation of mineral fabric and structural features, and the distribution of in situ stress and strain (e.g., Russo and Silver, 1994;Savage, 1999;Schoenberg and Sayers, 1995;Silver and Chan, 1988). In the upper crust, seismic anisotropy is usually attributed to microfracturing and large-scale fractures (Rasolofosaon et al., 2000;Stephen, 1985), whereas in the low crust and mantle, it is related to the crystal preferred orientation or fabric of anisotropic minerals (Ko and Jung, 2015;Rasolofosaon et al., 2000). Several active-source seismic studies of P and S body waves have reported 1% to 5% azimuthal anisotropy in the upper crust, at- tributed to near-vertical water-filled cracks in Layer 2 (Dunn and Toomey, 2001;Stephen, 1985;White and Whitmarsh, 1984). No apparent spreading-rate dependence of the anisotropy magnitude was observed, but it was reported to decrease with depth due to gradual crack closing in the upper 2 km of the crust (Dunn and Toomey, 2001). Observed anisotropy direction varied from roughly ridge-parallel (Dunn and Toomey, 2001) to ridge-oblique by 20°-60° (Stephen, 1985;White and Whitmarsh, 1984). 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...