Based on the hypothesis that brittle fracture and brecciation decrease with depth in plutonic layer 3 due to the steep geotherm beneath an ocean ridge, and that fine-grained rocks are unlikely to be encountered lower in the section, it is reasonable to suspect that the overall penetration rate would remain close to that for Leg 118. Five hundred meters were drilled at Hole 735B in 19 days, including setting a guide base and starting with the mud motor rather than the top drive, and using minimum bit weight and a high degree of operational conservatism. Improved bit design subsequent to Leg 118 and the observed lack of bit wear during Leg 118 suggest that operations during Leg 176 should not decrease bit life, which should make up for increased trip time as the hole is deepened.
The priorities for Hole 735B drilling defined by the Planning Committee (PCOM) are as follows:
1. Deepen Hole 735B to at least 1.5 km below the seafloor.
2. Log the deepened hole.
3. Conduct vertical seismic profiling (VSP) experiments in the deepened hole.
In the event difficulties are encountered while drilling, the following priorities will be pursued:
1.A brief 12-hr video survey will be made on the wave-cut terrace to the east and south of Hole 735B, which, in conjunction with the drilling results to date, will provide a geologic basis for an offset drill hole to be sited either on an isochron to the east or close to a north south lithospheric flow line centered on Hole 735B.
2. A hard rock guide base with a casing stub to 10-20 m will be set, and drilled to the maximum depth possible, allowing for an appropriate amount of time for logging and VSP.
3. In the event of further difficulties, the offset drilling strategy will proceed, siting holes as geologically appropriate on an isochron or lithospheric flow line based on the results of the video survey and drilling.
4. In the event that starting a new hole is required close to the end of the leg (6-7 days), and
would not result in sufficient recovery to justify the effort, the priority will be to drill a
single-bit hole at SWIR 6, bit-to-destruction, to recover the basaltic carapace corresponding
to Hole 735B.
To 176 Logging Plan
176 Table of Contents