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doi:10.2204/iodp.proc.303306.102.2006 Site U1302–U1308 methods1Expedition 303 Scientists2IntroductionSite locationsAt all Integrated Ocean Drilling Program (IODP) Expedition 303 sites, Global Positioning System (GPS) coordinates from precruise site surveys were used to position the vessel on site. The only seismic system used during the cruise was the 3.5 kHz profiler, which was monitored on the approach to each site to compare the seismic characteristics of the sediments with those from the precruise survey. Once the vessel was positioned at a site, the thrusters were lowered and a reference beacon was deployed. Although the automated stationkeeping system of the vessel usually uses GPS data, the beacon provides a backup reference in case of problems with the transmission of satellite data. The final site position was the mean position calculated from the GPS data collected over the time the site was occupied. Drilling operationsOne coring system was used during Expedition 303: the advanced piston corer (APC). The APC is a “cookie-cutter” system that cuts cores with minimal coring disturbance. The drill pipe is pressured up until the failure of one or two shear pins that hold the inner barrel to the outer barrel. The inner barrel strikes out and cuts the core. The driller can detect a successful cut, or “full stroke,” on the pressure gauge on the rig floor.
The standard bottom-hole assembly used at all Expedition 303 sites was composed of an 11 Each cored interval was ~9.5 m long, which is the length of a core barrel. In some cases, the drill string was drilled or “washed” ahead without recovering sediments to advance the drill bit to a target depth where we needed to resume core recovery. Such advances were necessary in multiple holes per site to ensure that coring gaps in one hole were covered by cored intervals in adjacent holes. The amount of advance was typically 1–4 m and accounted for drilling depth shift caused by tides, heave, and other factors (see “Composite section”). Drilled and cored intervals are referred to in meters below rig floor (mbrf), which are measured from the dual elevator stool (DES) on the rig floor to the bottom of the drill pipe. In cases where sediments of substantial thickness cover the seafloor (as at all sites during Expedition 303), the mbrf depth of the seafloor is determined with a mudline core, assuming 100% recovery for the cored interval in the first core. If the first core recovered a full barrel of sediment (it “missed the mudline”), the seafloor reference depth of a previous or later hole was used. Water depth was calculated by subtracting the distance between the DES and sea level (typically 10–11 m, depending on the ship’s load at a given time) from the mbrf depth. The water depth determined in this way (drill string measurement) usually differs from precision depth recorder measurements by a few to several meters. The meters below seafloor (mbsf) depths of core tops are calculated by subtracting the seafloor depth in mbrf from the core-top depth in mbrf. The core-top datums from the driller are the ultimate depth reference for any further depth calculation procedures. Core handling and analysisAs soon as cores arrived on deck, gas void samples were taken for immediate analysis as part of the shipboard safety and pollution prevention program. Core catcher samples were taken for biostratigraphic analysis. When the core was cut in sections, whole-round samples were taken for shipboard interstitial water examinations. In addition, headspace gas samples were immediately taken from the ends of cut sections and sealed in glass vials for light hydrocarbon analysis. Before the cores were split, whole-round core sections were run through the multisensor track (MST) and thermal conductivity measurements were taken. In addition, whole cores were run through a magnetic susceptibility (MS) core logger (MSCL; also known as the “Fast Track”), originally equipped with two magnetic susceptibility loops but later reduced to one (see “Physical properties”) to facilitate real-time drilling decisions to maximize stratigraphic overlap between holes (see “Composite section”). The cores were then split into working and archive halves, from bottom to top, so investigators should be aware that older material could have been transported upward on the split face of each section. The working half of each core was sampled for both shipboard analysis (i.e., physical properties, carbonate, and bulk X-ray diffraction mineralogy) and shore-based studies. Shipboard sampling was kept at a minimum during Expedition 303 to allow construction of a detailed sampling plan after the composite section was built (see “Composite section”). The archive-half sections were scanned on the digital imaging system (DIS), measured for color reflectance on the archive multisensor track (AMST), described visually and by means of smear slides, run through the cryogenic magnetometer, and finally photographed with color film a whole core at a time. Digital close-up photographs were taken of particular features for illustrations in site reports, as requested by scientists. Both halves of the core were then put into labeled plastic tubes, sealed, and transferred to cold storage space aboard the ship. At the end of the expedition, the cores were transferred from the ship into refrigerated trucks and to cold storage at the IODP Bremen Core Repository in Bremen, Germany. Curatorial procedures and sample depth calculationsNumbering of sites, holes, cores, and samples followed the standard IODP procedure. A full curatorial identifier for a sample consists of the following information: expedition, site, hole, core number, core type, section number, and interval in centimeters measured from the top of the core section. For example, a sample identification of “303-U1302A-1H-1, 10–12 cm” represents a sample removed from the interval between 10 and 12 cm below the top of Section 1 of Core 1 (H designates that this core was taken with the APC system) of Hole U1302A during Expedition 303. The “U” preceding site or hole designates site/holes drilled by the IODP riserless vessel. Cored intervals are also referred to in “curatorial” mbsf. The mbsf of a sample is calculated by adding the depth of the sample below the section top and the lengths of all higher sections in the core to the core-top datum measured with the drill string. A soft to semisoft sediment core from less than a few hundred meters below seafloor expands upon recovery (typically 10%–15%), so the recovered interval does not match the cored interval. In addition, a coring gap typically occurs between cores, as shown by composite depth construction (see “Composite section”) (i.e., some cored interval was lost during recovery or was never cut). Thus, a discrepancy exists between the drilling mbsf and the curatorial mbsf. For instance, the curatorial mbsf of a sample taken from the bottom of a core is larger than that of a sample from the top of the subsequent core, where the latter does correspond to the drilled core-top datum. During Expedition 303, multiple APC holes were drilled at a site to construct a continuous composite section. This resulted in a “meters composite depth (mcd)” scale for each site that accommodates core expansion and drilling gaps through interhole correlation of closely spaced measurements of core physical properties (see “Composite section”). |
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inch rotary bit, a bit sub, a seal bore drill collar, a landing saver sub, a modified top sub, a modified head sub, a nonmagnetic drill collar, five 8¼ inch drill collars, a tapered drill collar, six joints of 5½ inch drill pipe, and one crossover sub. A lockable float valve was used instead of the standard float assembly if the possibility of logging existed. APC refusal is conventionally defined in two ways: (1) the piston fails to achieve a complete stroke (as determined from the pump pressure reading) because the formation is too hard, and (2) excessive force (>60 klb) is required to pull the core barrel out of the formation. In cases where full stroke can be achieved but excessive force cannot retrieve the barrel, the core barrel can be “drilled over” (i.e., after the inner core barrel is successfully shot into the formation, the rotary bit is advanced to total depth to free the APC barrel). This strategy allows a hole to be advanced much farther with the APC, the preferred coring tool. A total of 46 core barrels were drilled over during Expedition 303. Nonmagnetic core barrels were used during all conventional APC coring. When using the drillover technique, standard steel core barrels were used because they are more robust and less costly to replace if damaged.