Logging tool strings deployed on Leg 164 (Table 1) included the Schlumberger quad- and split-combination (NGT, LSS-SDT, DITE, CNT-G, HLDT), Formation MicroScanner (FMS), and the Geochemical Combination Tool (GLT). The split-combination tool string consisted of separate runs of the seismic stratigraphic and lithoporosity combinations (Paull, Matsumoto, Wallace, et al., 1996).
The quality of the log measurements on Leg 164 were moderately to severely degraded by the size and irregular nature (rugosity) of the borehole. The caliper logs in Holes 994C, 994D, 995B, and 997B (Fig. 3) show borehole diameters greater than the 46.9-cm maximum range of the caliper for a significant portion of the hole. The comparison of log data from Holes 994C and 994D (Shipboard Scientific Party, 1996a), reveals that the log data from Hole 994D are of superior quality; therefore, we have focused our interpretive efforts at Site 994 on the log data from Hole 994D. The natural gamma-ray spectrometry (NGT), lithodensity (HLDT), and compensated neutron porosity (CNT-G) tools are particularly susceptible to adverse affects from large and irregular hole diameters. The NGT logs from Hole 995B were not significantly affected by the size of the borehole. However, the NGT logs from Holes 994D and 997B are highly degraded because of enlarged borehole sizes and the rugosity of the borehole. The HLDT, which is an excentered device, has a caliper arm that forces the tool against the wall of the borehole. If the hole is larger than the maximum reach (46.9 cm) of the caliper arm the density tool may lose contact with the formation. In general, the density logs in all of the holes drilled on the Blake Ridge were degraded because of poor tool contact with the borehole wall. Data from the density logs have been used to calculate sediment porosities; however, the results of these calculations are unsatisfactory (additional information on density log porosity calculations is provided later in this report). The compensated neutron porosity logs (CNT-G) are severely affected by enlarged boreholes in all of the holes drilled on the Blake Ridge, and all of the neutron porosity data from Leg 164 have been disregarded in this study (additional information on neutron porosity data is provided later in this report). The acoustic velocity (LSS-SDT) and electrical resistivity (DITE) logs provided invaluable information on sediment porosities and gas-hydrate saturations in all of the Leg 164 Blake Ridge drill sites. The GLT also provided useful information; however, the GLT measurements have also been degraded by the enlarged borehole conditions, which required extensive shore-based processing to rectify (Paull, Matsumoto, Wallace, et al., 1996).
The absolute depths, relative to seafloor, for all of the logs were fixed by identifying the gamma-ray signal associated with the seafloor and depth shifting the log data appropriately. The natural gamma-ray log pick for the seafloor in Holes 994D, 995B, and 997B were 2,809.0, 2,786.0 and 2,775.0 m below the dual elevator stool (DES) on the drilling mask, which is located on the ship 11.4 m above sea level.