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doi:10.2204/iodp.proc.303306.110.2006 Site U1312–U1315 methods1Expedition 306 Scientists2IntroductionSite locationsAt all Expedition 306 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 that the site was occupied. Drilling operationsThe advanced piston corer (APC) was the only coring system used during Expedition 306. The APC is a “cookiecutter” type system that cuts soft sediment cores with minimal coring disturbance relative to other Integrated Ocean Drilling Program (IODP) coring systems. The drill pipe is pressured up until the failure of two shear pins that hold the inner barrel attached to the outer barrel. The inner barrel advances into the formation and cuts the core. The driller can detect a successful cut, or “full stroke,” from the pressure gauge on the rig floor.
The standard bottom-hole assembly (BHA) used at all Expedition 306 sites comprised 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 core recovery needed to be resumed. Such advances were necessary in multiple holes at a 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 also “Stratigraphic correlation”). Drilled and cored intervals are referred to in meters below rig floor (mbrf), which is 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 306), 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 probably “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 from this 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, headspace samples were taken using a syringe for immediate hydrocarbon 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 splitting, whole-round core sections were run through the multisensor track (MST) and thermal conductivity measurements were taken. In addition, most whole cores were run through a “Fast Track” magnetic susceptibility core logger (MSCL) equipped with two magnetic susceptibility loops to facilitate real-time drilling decisions to maximize stratigraphic overlap between holes (see “Stratigraphic correlation”). 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 [XRD] mineralogy) and shore-based studies. Shipboard sampling was kept at a minimum during Expedition 306 to allow construction of a detailed sampling plan after the composite section was built. 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 summary 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 then 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 “306-U1312A-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 U1312A during Expedition 306. The “U” preceding hole numbers indicates the hole was drilled by the United States Implementing Organization (USIO) platform. 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 a few percent to as much as 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 (i.e., some cored interval was lost during recovery or was never cut) (see “Stratigraphic correlation”). 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 deeper than that from a sample from the top of the subsequent core, where the latter does correspond to the drilled core-top datum. During Expedition 306, multiple APC holes (typically three) 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 “Stratigraphic correlation”). |
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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, four 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 the case where full or partial 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. Nonmagnetic core barrels were used during all conventional APC coring. When utilizing the drillover technique, standard steel core barrels were used because they are more robust.