Leg 174B Operations Synopsis


The ship departed at 1600 hr (EST) on 21 July, 1997 from New York City. The 1768 nmi (3274 km) sea voyage to Hole 395A was completed in 154.5 hr at an average speed of 11.4 kt. All subsequent times reported in this operations section are local time (UTC-2hr), unless otherwise noted.


The ship arrived on location at Hole 395A at 0700 hr on 28 July. Following deployment of a beacon, reentry/logging bottom-hole assembly (BHA), and reentry video system, the seafloor was tagged at 4494 mbrf or 4482.5 mbsl. The reentry cone was located and reentered in 3 hr, nearly 100 m northeast of the last reported coordinates. The bit was then positioned at 32 mbsf for logging. The DVTP was run first on the coring line, logging temperatures during 5-min station stops every 20 m down the hole. This was followed by three Schlumberger logging runs, including two advanced sondes never before deployed in ODP holes: the Azimuthal Resistivity Imager (ARI) and Dipole Sonic Imager (DSI). The first tool string included the ARI, spectral gamma ray (HNGS), and Lamont high-resolution TLT sondes; excellent data were acquired from 603 mbsf to the bottom of casing at 113 mbsf. The second tool string included the spectral gamma ray (NGT), DSI, and FMS. Three passes of this string in the open hole section were run using the DSI in conventional monopole, in-line and cross-dipole, and Stoneley wave recording modes, respectively. Good to excellent FMS and compressional and shear waveform data were collected during the first two passes, with the exception of two enlarged intervals near 120 and 420 mbsf. Data from the third pass are of lesser quality, partly because of an electronic fault that precluded further use of the wireline heave compensator. The third triple-combo tool string included the spectral gamma ray (HNGS), Advanced Porosity Sonde (APS), Lithodensity Sonde (LDS), and Digital Induction Tool (DITE). The entire hole was logged up to the seafloor without using the wireline heave compensator, and a repeat log was run from 136 to 106 mbsf. The data are of excellent quality, with the exception of the two hole enlargements near 120 and 420 mbsf, where the density and neutron porosity tools lost contact with the borehole wall. An SP log was also acquired over the open hole interval. Throughout the logging operations, good hole conditions were encountered down to 603 mbsf, and it was never necessary to run the bit beyond 32 mbsf and into open hole for cleanout operations.

After the logging operations, Hole 395A was successfully sealed with a CORK, instrumented with long-term data logger, pressure gauges above and below the seal, and a cable with 10 thermistors (at 98, 173, 248, 298, 348, 398, 448, 498, 548, and 598 mbsf). The data logger was positively latched into the CORK body, but the CORK body could not be mechanically latched into the casing. However, the CORK was seen on video to be in proper position, and the lack of a mechanical latch should not compromise the experiment; the seals are in proper position and the 21-yr history of downhole flow in Hole 395A suggests that there is virtually no possibility that the formation will develop positive pressures large enough to displace the CORK and breach the seals. The CORK running assembly arrived on deck at 0230 hr on 1 August, ending operations in Hole 395A.

HOLE 1074A

After successful deployment of the CORK, almost two days of operational time were still available. This extra time was applied to the contingency plan for Leg 174B, coring the sediments in North Pond. The ship was moved 2.4 nmi (4.44 km) northwest in dynamic positioning (DP) mode, and a beacon was deployed. An advanced hydraulic piston corer (APC) BHA was assembled, and a brief survey with the VIT frame camera was conducted to confirm the seafloor condition, which was flat and featureless. Based on recovery of the mudline core, the water depth is 4445.5 mbsl. Hole 1074A was spudded at 1315 hr, 1 August and APC Cores 1074A-1H through 7H were taken from 0 to 63.5 mbsf (103.6% recovery). Adara heat-flow measurements were made on Cores 3H through 6H, and the Tensor tool was run on Cores 3H through 7H. The inner core barrel used to cut Cores 2H, 4H, and 6H contained a 3-m-long nonmagnetic section. Core 7H was a partial stroke, encountering a hard layer (basalt clast) with increased torque at 8.5 m (62.5 mbsf); hence, the lower 1.53 m of Core 7H is probably highly disturbed (flow-in). The hole was drilled down to 63.5 mbsf, 1.5 m below the APC shoe penetration. We switched to the extended core barrel (XCB) coring system and Core 8X was cut from 63.5 to 69.5 mbsf. High torque stalled the rotary repeatedly, requiring increased pump rates, a mud sweep, and repeated reaming of the hole. Despite the hole problems, Core 8X was retrieved with 0.58 m of basalt (9.7% recovery).

During the connection after Core 8X, the beacon signal was lost for positioning. Shortly thereafter the internet connection to the global positioning system (GPS) Glonass and GPS systems in the underway lab were lost, and the positioning back-up reference was also lost. The drill pipe was pulled and the ship was held to a 45 m maximum excursion using dead reckoning until the bit cleared the seafloor at 0520 hr on 2 August, ending Hole 1074A.

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The 1726 nmi (3197 km) sea voyage to Las Palmas was completed in 166 hr at an average speed of 10.4 kt, which included 8.75 hr at 6 kt while conducting seismic streamer tests over the Madeira/Cape Verde abyssal plains. The first line was ashore at 1500 hr on 9 August, officially ending Leg 174B.

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