About 2 days were devoted to logging in Hole 395A during Leg 174B, with the overall purpose of documenting the in situ physical properties of the upper oceanic crust at the site, particularly as they relate to the hydrologic properties. The hole had been logged reasonably well during Legs 78B and 109, so Leg 174B focused on deploying improved tools made available to ODP since then. These included temperature, Formation MicroScanner (FMS), and density-porosity-resistivity logs, as well as two new tools deployed in oceanic crust for the first time during Leg 174B, an azimuthal resistivity imager (ARI) and digital shear imager (DSI).
Figure F3 shows the downhole temperature log taken when Hole 395A was first reentered during Leg 174B, a log that is very similar to past temperature logs in the hole and illustrates the virtually isothermal profile in the upper 300-400 m, characteristic of downhole flow of ocean bottom water. This figure also shows the Leg 174B log of spontaneous potential (SP), which is quite sensitive to flowing fluids. The SP log suggests that the primary zone accepting downhole flow was at ~420 m below seafloor (mbsf), with several other shallower zones and possibly a deeper zone accepting lesser amounts of the flux. The resistivity log, shown in Figure F4, indicates that the inflow zones are associated with low resistivities between cyclic zones of high resistivity. Hyndman and Salisbury (1984) and Moos (1990) reported this cyclicity in earlier logging data from the hole and suggested that it represents eruptive cycles. If so, the temperature and SP logs suggest that most of the considerable permeability in basement at Site 395 is concentrated in the zones between eruptive cycles.
The advanced logs collected during Leg 174B provided an overwhelming amount of high-quality data, various aspects of which are analyzed in papers submitted to or in preparation for outside journals. Bartetzko et al. (in press) analyzed the log data to interpret a volcanic stratigraphy from a synthetic "electro-facies" log (EFA) based on characteristic log responses. From this EFA log (Fig. F4), they can identify most of the lithostratigraphic types (but not the chemical stratigraphic types) originally identified by Melson, Rabinowitz, et al. (1979) from the relatively poor core recovery (average = 18%) and can present a continuous lithostratigraphic interpretation based on the continuous log data. Bartetzko et al. (in press) also clearly recognize the cyclicity in log data and further develop the interpretation of eruptive cycles in terms of the model of Smith and Cann (1992, 1999) for construction of crust formed at slow spreading rates.