161 Preliminary Report
(Proposed site MedSap-6A)
The 380-nmi sea voyage to Site 975 required 47 hr at an average speed of 8.1 nmi/hr. We conducted a pre-site seismic survey, lasting about 6 hr, over Site 975. We deployed a beacon at 38°15.612'N, 4°44.990'E, on 0100 hr, 14 May. We then deployed a second beacon at 0300 hr,
14 May. The elevation of the DES above sea level was 10.93 m for Holes 975A and 974B, 10.99 m for Hole 975C, and 11.02 m for Hole 975D.
We spudded Hole 975A at 0700 hr, 14 May, at 38°53.795'N, 04°30.587'E. The bit was positioned at 2428.0 mbrf, and Core 161-975A-1H recovered 9.92 m (104.4% recovery).
Since this core overshot the mud line, an exact seafloor depth could not be calculated and the hole was terminated. The bit cleared the seafloor at 0730 hr, 14 May.
We raised the bit 7 m to 2421.0 mbrf and spudded Hole 975B at 0745 hr, 14 May. Core 161-975B-1H recovered 4.1 m of sediment; therefore, the seafloor was defined to be at 2416 mbsl. Cores 161-975B-1H to -16H were taken to 2573 m (146.6 mbsf) and recovered 151.68 m (103% recovery). Cores 161-975B-3H through -16H were oriented using the Tensor tool. While retrieving Core 161-975B-6H, the sand line broke before the core was brought onto the rig floor. The core barrel was dropped twice during attempts to fish the core barrel and sand line from inside the drill pipe. After the core was recovered and split, it was noted that the lower section of this core was disturbed. In addition, the next core (Core 161-975B-7H) also had some disturbance caused by Core 161-975B-6H having penetrated into the top of Core 161-975B-7H, as well as from the additional drill pipe motion during the 6 hr while Core 161-975B-6H was being retrieved.
Cores 161-975B-14H to -16H were partial strokes, even though the cores were full when recovered. After an overpull of 60,000 lb was recorded for Core 161-975B-16H, we switched
to XCB-coring. XCB Cores 161-975B-17X to -34X were taken from 146.6 to 317.1 mbsf.
We XCB-cored 170.5 m and recovered 147.3 m (86%). Several of the first few XCB cores (Cores 161-975B-17X, -19X, and -20X) had lower recoveries. The total APC/XCB recovery
in Hole 975B was 94%.
A hard layer was encountered at about 307.5 mbsf (2734 mbrf). This layer appears to correlate with a strong reflector in the seismic reflection data collected just prior to drilling at this location. Previous velocity analyses suggested that this reflector (M reflector) was at a depth of 350 mbsf. Hole 975B was terminated after advancing to 317.1 mbsf (about 9.6 m into the hard gypsum material) to abide by PPSP recommendations.
We offset the ship 20 m to the north-northeast and spudded Hole 975C at 0230 hr, 16 May.
Core 161-975C-1H recovered 2.43 m of sediment with the bit position at 2419.0 m; therefore,
the seafloor was defined to be at 2415 mbsl. Cores 161-975C-1H to -16H were taken to 2571 m (144.9 mbsf) and recovered 152.54 m (104% recovery). ADARA temperature measurements were made while taking Cores 161-975C-3H, -6H, -9H, -12H, and -15H. Cores 161-975C-14H through -16H were recorded as partial cores with overpull exceeding 45,000 lb.
XCB Cores 161-975C-17X to -34X were cut from 144.9 to 313.7 mbsf and recovered 155.12 m (92% recovery). The hard gypsum "M" reflector was encountered at 2735.1 m (309 mbsf). Total APC/XCB recovery for Hole 975C was 307.66 m (98%).
We then conditioned the hole for logging with a short wiper trip. No drag was observed, and only
7 m of fill was encountered in the bottom of the hole, indicating good hole conditions for logging. The hole was displaced with seawater. The bit was positioned at 2497.53 m (71.43 mbsf) to log and was raised approximately 20 m more just before the logging tools reentered the drill pipe, allowing more of the upper portion of the borehole to be logged. Three tool strings were run in the following order: (1) quad-combo, (2) FMS, and (3) GLT. The logs were run to 2735, 2730, and 2723 mbrf, respectively. No significant operational or hole stability problems were reported by the logger. The log data indicated hole deviations <2°. There was a noticeable difference in hole diameters between the APC and XCB portions of the hole. The XCB portion exhibited a larger and more ragged diameter as compared to the APC section. Logging was finished at 1000 hr, 18 May, and the bit cleared the seafloor at 1015 hr, 18 May.
We offset the ship 20 m north-northeast and spudded Hole 975D with the bit at 2424.0 mbrf at 1045 hr, 18 May. Core 161-975D-1H recovered 7.4 m of sediment; therefore, the seafloor was defined to be at 2415 mbsl. Cores 161-975D-1H to -16H were taken to 149.9 mbsf and recovered 157.13 m (105% recovery). Cores 161-975D-11H, -12H, and -16H were partial strokes with up to 50,000 lb overpull. The bit cleared the seafloor at 2245 hr, 18 May, and the BHA was secured for the transit to proposed site Alb-2 (Site 976) at 0330 hr, 19 May. The 11-7/16-in. Security bit (S/N 478458) had negligible tooth wear, and the bearings were still in good shape despite having been used for 37.7 hr.
To Operations Site 976
161 Table of Contents