-in drill collars, a tapered drill collar, six joints of 5
-in drill pipe, and one crossover sub. A lockable float valve was used instead of the standard float assembly if the possibility of logging existed.
At all Leg 202 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 leg 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 station keeping 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. At the end of operations at a site, the beacon was retrieved and the thrusters were retracted before the vessel proceeded to the next site at full speed.
Two coring systems were used during Leg 202: the advanced piston corer (APC) and the extended core barrel (XCB). The APC is a "cookie-cutter" type 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 attached 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. Whenever possible, the APC was used because it yields the least disturbed cores.
When "APC refusal" occurs in a hole before the target depth is reached, the XCB is used to advance the hole. The XCB is a rotary system with a small cutting shoe that extends below the large rotary bit. The smaller bit can cut a semi-indurated core with less torque and fluid circulation than the main bit and thus optimizes recovery. If the formation becomes even more indurated, the rotary core barrel (RCB), which is the most conventional drilling system, is used but was not needed on Leg 202.
Both the APC and XCB systems use the same bottom-hole assembly (BHA), and switching from one to the other does not require a pipe trip. The standard BHA used at all Leg 202 sites comprised an 117/16-in APC/XCB 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 8-in drill collars, a tapered drill collar, six joints of 5-in 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 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 in order to free the APC barrel). This strategy allows a hole to be advanced much farther with the APC, the favorite drilling tool. A total of 101 core barrels were drilled over during Leg 202, which is a record number for one leg.
Each cored interval was about 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 "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 the case where sediments of substantial thickness cover the seafloor (as at all sites during Leg 202), 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 a 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.
As soon as cores arrived on deck, gas void samples and headspace samples were taken by means of a syringe, if applicable, 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 splitting, whole-round core sections were run through the multisensor track (MST) and thermal conductivity measurements were taken. 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 Leg 202 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 both black-and-white and color film a whole core at a time. 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. The last few hundred meters of core were stored in the unrefrigerated hold because we ran out of refrigerated space. At the end of the leg, the cores were transferred from the ship into refrigerated trucks and to cold storage at the Gulf Coast Core Repository of the Ocean Drilling Program (ODP) in College Station, Texas.
Numbering of sites, holes, cores, and samples followed the standard ODP procedure. A full curatorial identifier for a sample consists of the following information: leg, 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 "202-1239A-1H-1, 10-12 cm" would represent 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 1239A during Leg 202.
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 from a sample from the top of the subsequent core, where the latter does correspond to the drilled core top datum.
During Leg 202, multiple APC holes (typically three) were drilled at a site to construct a continuous composite section. This resulted in a "meters composite depth" 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").
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 202IR-102
