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doi:10.2204/iodp.pr.305.2005
Expedition 305 began when the first line was placed ashore at Pier 12 in Ponta Delgada on the island of São Miguel, Azores Islands, Portugal, at 2120 h on 7 January 2005. The three pacing items for the port call were an American Bureau of Shipping (ABS) Annual and Statutory survey, an overhaul of the drawworks transmission, and the replacement of the active heave compensator (AHC) hydraulic hose bundle.
The ABS survey was completed by the morning of 11 January. The replacement hose for the AHC was determined to be defective, so the AHC was not operational for Expedition 305. The completion of the drawworks transmission repair was dependent upon the arrival of hardware from the Transocean warehouse in Houston, Texas (USA). Customs clearance in Lisbon, Portugal, delayed arrival of these parts until ~1530 h on 12 January. At 1715 h, 12 January, the last line was released from the dock, and, after clearing the harbor entrance, the pilot was released at 1721 h and the vessel began the journey to Site U1309. The duration of the port call was 4.8 days.
The transit to Site U1309 covered 1002 nmi at an average speed of 10.7 kt. During the transit, the ship's clocks were retarded two h to –3 h Universal Time Coordinated. On approach to Site U1309, we conducted a 17.8 nmi towed magnetometer survey near the site. The survey line began at 30°4.7'N, 41°47.0'W, and extended west-northwest to 30°07.9'N and 42°07.1'W.
Initial Deepening of Hole U1309D
The vessel was positioned over Hole U1309D at 1600 h on 16 January using the Global Positioning System and the beacon that was deployed during Expedition 304. At 0027 h on 17 January, we reentered Hole U1309D and deployed the water-sampling temperature probe (WSTP) to the bottom of the hole. To minimize contamination or disturbance of the borehole water column, the drill string was lowered to ~10 m off the bottom of the hole with minimum rotation and no circulation. After the WSTP was retrieved, ~3 m of fill was cleaned from the bottom of the hole.
Between 17 January and 30 January, we cored from 401.3 to 837.4 mbsf (Table T1) using four C-7 rotary core bits. The average rate of penetration steadily decreased downhole, from 2.6 to 1.6 m/h, averaging 2.2 m/h for the 14 day coring operation, with recovery averaging >80%. Based on a conservative estimate of bit life, we elected to change bits after nominally 50 h of rotation. At the end of each bit run, the bits returned worn but essentially undamaged. Mud sweeps (20 bbl) were circulated every 10 m of advance to clean the hole. Fluorescent microspheres and perfluorocarbon tracers (PFTs) were deployed episodically during cored intervals at depths where microbiology samples were requested. However, PFT use was discontinued after Core 305-U1309D-90R at the request of the shipboard microbiologist, owing to insufficient time to rapidly process samples. At the end of the fourth bit run, the hole was conditioned for logging, including displacing the borehole fluid with freshwater to improve the logging signal.
In preparation for logging, the bottom of the pipe was set at 170 mbsf. This allowed overlap with previous logging runs performed during Expedition 304 and positioned the pipe below an interval where minor obstructions were encountered in the borehole during reentry. Six logging runs were completed, including the triple combo, FMS-sonic, VSP, Ultrasonic Borehole Imager, Goettingen Borehole Magnetometer, and a test of the logging wireline heave compensation system. In accordance with IODP policy, prior to the VSP, a 1 h visual survey of the water within a 700 m radius of the vessel was undertaken to ensure that no marine mammals were present prior to the start of the VSP experiment. Also consonant with the policy, the generator injector gun was soft-started (gradually increased intensity for the first 30 min of operation) at the initiation of testing. The marine mammal watch was maintained until the VSP was secured, and no marine mammals were sighted during the experiment. Logging operations were completed at 1040 h on 2 February.
Between 2 February and 23 February, we cored from 837.4 to 1415.5 mbsf, using one C-7 and five C-9 bits. The C-9 bits are designed for harder formation coring. We elected to use C-9 bits in order to preserve the last two C-7 bits in inventory in the event we encountered softer rock or noted deteriorating performance of the C-9 bits at depth. Average rate of penetration continued at 1.6–2.4 m/h, with recovery averaging 78%. We experienced no significant difference in bit performance between the styles of coring bits employed. Our routine procedure included 20-bbl mud sweeps after every other cored interval to clean the hole. Coring was completed at 1015 h on 23 February.
During our midcruise logging run, a temperature of ~60°C was measured by one of the magnetometer tool's sensors. We used two different approaches to verify borehole temperature prior to our final logging experiment. In our first attempt, an assortment of calibrated, heat-sensitive adhesive strips were affixed to a modified advanced piston corer (APC) brass core catcher spacer deployed in a dedicated core barrel run. The core barrel was deployed at 1162 mbsf (a few meters above the bottom of the hole at the time) and allowed to equilibrate for 17 min without circulation. Upon recovery, the temperature strips indicated a minimum temperature of ~70°C. During the ensuing bit trip, we deployed the WSTP to collect a water sample and temperature measurement. Before deploying the sampler, our technicians determined the thermistor on the WSTP was broken and no exact replacement was available. The shipboard electrical technician fabricated an ad hoc arrangement of the APC Temperature (APCT) tool on the WSTP and found an uncalibrated thermistor to install on the WSTP. In addition to these two temperature measuring devices, an assortment of the adhesive temperature recording strips were also affixed to the WSTP. Owing to a failed O-ring, the APCT failed to record a temperature. The thermistor on the WSTP recorded a maximum temperature of ~60°C. A subsequent bench test determined 60°C was the maximum recording temperature for the new WSTP thermistor/datalogger assembly. The adhesive temperature strips indicated a minimum temperature of 110°C.
After hole conditioning on 23 February, the borehole was filled with drill water to improve the logging signal in resistive rock. A logging bottom-hole assembly was deployed, and the bit was set at 194 mbsf. Two passes were completed with the triple combo tool string. The second logging run utilized the FMS-sonic tool string (two passes including overlap with our midcruise logging passes); however, the sonic tool was not deployed because of tool failure. A marine mammal watch was instituted at daybreak 24 February in preparation for a VSP experiment. The three-component VSP tool failed, and, after deployment of the single-component tool, the sea state (>5 m heave) had deteriorated to the point where we were required to terminate logging. A single station was occupied and shots recorded with the single-component well-seismic tool. Operations in Hole U1309D concluded on 26 February.
Prior to departure from Site U1309, a towed magnetometer survey was conducted. The 950 nmi transit required 104 h at an average speed of 9.1 kt. Expedition 305 concluded with the first line ashore at Ponta Delgada at 1805 h on 2 March 2005.