The description of the logged intervals in Holes 994D, 995B, and 997B are divided into three "logging units" on the bases of obvious changes in the natural gamma-ray (NGT), bulk-density (HLDT), acoustic velocity (LSS-SDT), and electrical resistivity measurements (DITE) (Table 2; Fig. 4A-C). The elemental yield data from the geochemical combination tool (GLT) has also been used to assess the mineralogy of the sediments in the delineated logging units. A more detailed assessment of the GLT interpreted mineralogy at Sites 995 and 997 have been included in Collett and Wendlandt (Chap. 21, this volume).
Logging Unit 1 is characterized by relatively low gamma-ray, density, velocity, and resistivity log values (Fig. 4A-C). All of the recorded logs are affected by the enlarged borehole, which exceeds the maximum recording size (46.9 cm) of the caliper throughout most of Unit 1. In most cases, the gamma-ray log shows an abrupt upward step in value at the boundary between Unit 1 and Unit 2. The caliper log (Fig. 3) shows that the hole diameters are reduced across the boundary from Unit 1 to Unit 2. The bulk-density and acoustic velocity increase more gradually across the boundary between Units 1 and 2. Within Unit 1, the weight percent of K and Al are relatively low and remain constant. Analyses of K and Th elemental yields reveal that the clays in Unit 1 are predominately montmorillonites and some illites (Collett and Wendlandt, Chap. 21, this volume).
Logging Unit 2 is characterized by increasing velocities (1.65 km/s at the top to over 2.0 km/s at the bottom) with depth. The natural gamma-ray and bulk-density logs are nearly constant throughout Unit 2. Both the acoustic velocity and resistivity logs are characterized by a distinct baseline shift to relatively higher values throughout Unit 2. The resistivity logs reveal several conspicuous high electrical resistivity intervals near the top of Unit 2 in all three boreholes. At the base of Unit 2, across the boundary into Unit 3, the acoustic velocity and resistivity logs step down to lower values. In Hole 997B (Fig. 4C), the acoustic log (DTLF) has been used to precisely select a depth for the boundary between Units 2 and 3. This acoustic velocity (DTLF) boundary does not exactly match the drop in resistivity observed near the base of Unit 2. This discrepancy of about 8 m is likely a result of the presence of a significant amount of free gas below the deepest gas hydrate occurrence. Analyses of K and Th elemental yields suggest that the clays in logging Unit 2 are mostly montmorillonites (Collett and Wendlandt, Chap. 21, this volume).
Logging Unit 3 is characterized by consistently lower velocities and resistivities with respect to Unit 2. Anomalous low-velocity intervals are seen in Unit 3; velocities within these anomalous intervals decrease to below 1.5 km/s, which suggests the presence of free gas. The higher resistivities, near the boundary between Units 2 and 3 in Hole 997B, may be a result of the presence of free gas, which supports the acoustic log observations. In logging Unit 3, the natural gamma-ray, density, and electrical resistivity logs show slight increases with depth, which is characteristic of a normally compacting sedimentary section.