Initial Reports of the Deep Sea Drilling Project - Volume 78B

This volume covers Leg 78B of the cruises of the Drilling Vessel Glomar Challenger. Leg 78B represents the first cruise specifically designed for downhole measurements. The original impetus for the cruise arose from the scarcity of good downhole logs in the ocean crust. This was particularly critical in the few deep crustal holes drilled, because the recovery in such holes was poor and because the physical properties of the core material were clearly not representative of the large-scale properties of the crustal sections penetrated. One of the deepest of these holes and most attractive candidates for re-entry (since it had been cased to basement) was Hole 395A on the Mid-Atlantic Ridge. The hole had been drilled on Leg 45 on the west side of the ridge as part of a long-term program designed to determine the petrological, geochemical, and geophysical properties of the ocean crust and to determine the mechanisms by which it forms and evolves. Although the hole had been continuously cored and the recovered material extensively studied, no downhole measurements had been made, and the core recovery was poor. The primary objective of the Leg 78B scientific program was therefore to study the geophysical and hydrogeological properties of the upper oceanic crust at Site 395 through downhole logging and downhole experiments, for comparison with previous studies of the core and with regional geophysics. On Leg 78B, it was planned to use logging tools designed to measure the diameter of the hole, the temperature of the water in the hole, and the sonic velocity, electrical resistivity, bulk density, porosity, and natural gamma radiation of the formation. The downhole experiment program was planned to include (1) a packer experiment for large-scale permeability tests, in situ pressure tests, large-volume water sampling, and hydrofracture tests; (2) a program to measure the temperature and collect water samples down the hole using the self-contained heat-flow/pore-water sampler; (3) a large-scale resistivity experiment; and (4) the experimental emplacement of a borehole seismometer. Leg 78B began in San Juan, Puerto Rico in March 1981 and ended in Las Palmas, Grand Canary Island in April 1981.