Ocean crustal drilling at the Hawaii-2 Observatory

摘要

In preparation for a permanent borehole seismometer at the Hawaii-2 Observatory (H2O) site, Ocean Drilling Project Leg 200 drilled Hole 1224D 1.48 km northeast of the H2O junction box. A reentry cone and cemented casing were installed through 28 m of soft, red clay and 30 m into basaltic basement. The cased basement interval consisted of massive basalt flows that had been cemented by calcite and it should provide good coupling for the seismometer to true earth motion. We also drilled a second single-bit hole, which was cored and logged, within 20 m of the first to a depth of 145 m into basement. The second hole was left with a free-fall funnel so that it also could be reentered using the wireline reentry technology to carry out other borehole experiments at the site. A suite of shipboard physical properties, well log, chemical and microbiological analyses that can be used to characterize the crust surrounding the observatory, was carried out. Measurements of ship noise during the drilling operations were also acquired on the shallow buried seismometer at the Hawaii-2 observatory.