Résumé
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.