Leg Objectives

Leg 191 had two primary objectives. The first was to drill a cased reentry hole into basement at proposed site WP-2 (Site 1179) and install a broadband seismic observatory with a battery assembly and data-recording unit. The second was to perform engineering tests of the HRRS on a submarine igneous outcrop at a depth between 1000 and 2000 m. Leg 191 HRRS testing had three primary objectives: (1) characterization of the Model 260 fluid hammer operating parameters (i.e., flow rates, pump pressures, and weight on bits); (2) characterization of the hammer-drill and bit-spudding capabilities without casing; and (3) testing of the entire HRRS system by drilling in 20 m or more of 13.375-in casing in several slightly different fractured hard rock environments. These hard rock environments were to include (1) little or no overlying sediment or talus and little or no slope, (2) little or no overlying sediment but with a slope, and (3) a sloping surface covered with sediment or talus.

There were also numerous ancillary scientific goals that were envisioned related to cores or wireline logs obtained at the proposed drill sites. From Site 1179 drilling, we hoped to address the following areas: structure, geochemistry, and isotopic characteristics of the upper ocean crust, Pacific plate paleolatitude and tectonic drift, the age of magnetic Anomaly M8, and the microbiology of the abyssal sediment column. It was also hoped that cores would be recovered from Shatsky Rise during the course of the engineering test and that those cores would yield important data about the age and geochemistry of that large igneous province.

The sediment column and igneous basement at Site 1179 were cored successfully, and the borehole seismometer was emplaced as planned. Consequently, it should be possible to accomplish the scientific objectives related to coring and placement of the seismic observatory. The logging program, however, was seriously curtailed owing to collapse of the borehole that did not allow either the lower part of the borehole to be logged or most logs to be run. In addition, because of a streak of bad luck that included a medical emergency and a broken part on the drawworks, it was necessary to move the HRRS test to a site close to Guam. As a result, the Shatsky Rise objectives were not addressed and the HRRS test was shortened. However, the abbreviated test went well and the HRRS test goals related to hammer operation and spudding characteristics were successfully completed (Table 1, Table 2).