Ocean Drilling Program, United States

Brittle structures (open fractures and veins) from basaltic oceanic crust drilled at Ocean Drilling Program (ODP) Site 1256 (Guatemala Basin, Pacific Ocean) during Leg 206 were reoriented to the geographic coordinates by (1) correlating structures observed on the core with unoriented images of the exterior of the core and (2) correlating core structures and unoriented images with oriented borehole images. The images of the exterior of the core were obtained by scanning whole-core pieces with the Deutsche Montan Technologie Digital Color Core-Scan system. In the unrolled core images, nonhorizontal planar structures (e.g., veins, faults, or fractures) produce sinusoidal-shaped curves. These can be matched to similar-shaped features imaged along the borehole wall. The borehole images were obtained by the Formation MicroScanner (FMS)-sonic (Dipole Sonic Imager) tool string and the Ultrasonic Borehole Imager (UBI). The FMS provides high-resolution electrical resistivity-based images of borehole walls. FMS images are oriented to magnetic north using the General Purpose Inclinometer Tool. This allows the dip and azimuth of geological features intersecting the hole to be measured from the processed FMS image. The UBI features a high-resolution transducer that provides acoustic images of the borehole wall. The UBI was used in hard rocks for the first time in the history of the ODP during Leg 206.