Structural data were determined from RAB images using Schlumberger's Geoframe software. Geoframe presents RAB data as a planar, "unwrapped" 360° resistivity image of the borehole with depth. Horizontal features are horizontal within these images, whereas planar dipping features are sinusoidal. Sinusoids are interactively fitted to beds and fractures to determine their dip and azimuth, and exported from Geoframe for further analysis.
Methods of interpreting structure and bedding differ considerably between Leg 131, Site 808 and Leg 190, Site 1173 (core analysis and wireline Formation MicroScanner [FMS] images) and Leg 196, Sites 808 and 1173 (RAB image analysis). Resolution is considerably lower for RAB image interpretation (5-10 cm at best, compared with millimeters within cores and 0.5 cm for FMS images), and therefore identified features are likely to be different in scale. For example, microfaults ("small faults," <1 mm width) and shear bands (1-2 mm, up to 1 cm width) identified at Leg 131, Site 808 are not visible in Leg 196 data. This should be considered when directly comparing reports.
Fractures were identified within RAB images by their anomalous resistivity or conductivity and from contrasting dip relative to surrounding bedding trends. Differentiating between fractures and bedding planes is problematic in places, particularly in Hole 808I, as both may be steeply dipping and with similar orientations, based on comparison with the results of Leg 131, Site 808 (Shipboard Scientific Party, 1991b) and seismic data. The interpretations presented here are based on the above criteria but it is acknowledged that fractures and bedding planes may have been misinterpreted in some cases.