Previous   |   Next

doi:10.2204/iodp.proc.317.102.2011

Operations

Site locations

Global Positioning System (GPS) coordinates from precruise site surveys were used to position the vessel at all Expedition 317 sites. A SyQuest Bathy 2010 CHIRP subbottom profiler was used to monitor the seafloor depth on the approach to each site to reconfirm the depth profiles from precruise surveys. Once the vessel was positioned at a site, the thrusters were lowered and a positioning beacon was dropped to the seafloor. The dynamic positioning (DP) control of the vessel used navigational input from the GPS system and triangulation to the seafloor beacon, weighted by the estimated positional accuracy. The final hole position was the mean position calculated from the GPS data collected over a significant portion of the time the hole was occupied.

Drilling operations

All three standard coring systems—the advanced piston corer (APC), the extended core barrel (XCB), and the rotary core barrel (RCB)—were used during Expedition 317.

The APC system was used in the upper portion of each hole when coring in the top of the hole was the objective. The APC system cuts soft-sediment cores with minimal coring disturbance relative to other IODP coring systems. After the APC core barrel is lowered through the drill pipe and lands near the bit, the drill pipe is pressured up until the two shear pins that hold the inner barrel attached to the outer barrel fail. The inner barrel then 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.

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 or (2) excessive force (>60,000 lb; ~267 kN) is required to pull the core barrel out of the formation. When a full stroke could not be achieved, one or two additional attempts were typically made, and each time the bit was advanced by the length of recovered core. Note that this resulted in a nominal recovery of ~100% based on the assumption that the barrel penetrated the formation by the equivalent of the length of core recovered. When a full or partial stroke was achieved but excessive force could not retrieve the barrel, the core barrel was sometimes "drilled over": after the inner core barrel was successfully shot into the formation, the drill bit was advanced to total depth to free the APC barrel.

Nonmagnetic core barrels were used during all conventional APC coring to a pull force of ~40,000 lb. When the need for drillover was anticipated, standard steel core barrels were used because they are stronger than the nonmagnetic barrels. Most APC cores recovered during Expedition 317 were oriented using the Flexit tool (see "Paleomagnetism"). Formation temperature measurements were made to obtain temperature gradients and heat flow estimates (see "Heat flow").

The XCB system was used to advance the hole when APC refusal occurred before the target depth was reached or when the formation became either too stiff for the APC system or hard substrate such as cemented layers and nodules or chert was encountered. The XCB is a rotary system with a small cutting shoe that extends below the large rotary APC/XCB bit. The smaller bit can cut a semi-indurated core with less torque and fluid circulation than the main bit, optimizing recovery. The XCB cutting shoe (bit) extends ~30.5 cm ahead of the main bit in soft sediments but retracts into the main bit when hard formations are encountered. The XCB system was used extensively during Expedition 317 and to greater depths than anticipated.

The bottom-hole assembly (BHA) used for APC and XCB coring during Expedition 317 was composed of an 11 inch (~29.05 cm) drill 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 inch (~20.32 cm) control length drill collars, and a tapered drill collar, followed by a 5½ inch drill pipe to the surface. A typical tapered drill string was not used during Expedition 317 because of the shallow water depth. A lockable flapper valve was used so that downhole logs could be collected.

The RCB system was deployed when XCB coring rates diminished below an acceptable level or the bit was destroyed by the increasingly hard formation. The RCB is the most conventional rotary drilling system and was used during Expedition 317 to drill and core the deepest sediment hole in the history of the R/V JOIDES Resolution. The RCB system requires a dedicated RCB BHA and a dedicated RCB drilling bit.

The BHA used for RCB coring included a 9⅞ inch RCB drill bit, a mechanical bit release, a modified head sub, an outer core barrel, a modified top sub, and seven control length drill collars, followed by a tapered drill collar to the 5½ inch drill pipe.

Most cored intervals were ~9.5 m long, which is the length of a standard core barrel and the length of a joint of drill pipe. In some cases, the drill string was drilled or "washed" ahead without recovering sediments to advance the drill bit to a target depth to resume core recovery. Such intervals were typically drilled using a center bit installed within the RCB bit.

IODP depth conventions

In the last few decades, Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP), and IODP Phase 1 reports, diagrams, and publications used three primary designations to reference depth: meters below rig floor (mbrf), meters below seafloor (mbsf), and meters composite depth (mcd). These designations were combinations of origin of depth scales (rig floor or seafloor), measurement units (m), and method of construction (composite). The designations evolved over many years based on the needs of individual science parties.

Over the course of ODP and IODP scientific drilling, issues with the existing depth scale designations and the lack of a consistent framework became apparent. For example, application of the same designation to scales created with distinctly different tools and methods was common (e.g., mbsf used for scales measured both by drill string tally and the wireline). Consequently, new scale type designations were created ad hoc to differentiate the wireline logging scale from the core depth scale so that depth-mapping procedures and products could be adequately described. However, the management and use of multiple maps, composite scales, or splices for a hole or a site was problematic, and the requirement to integrate scientific procedures among three IODP implementing organizations amplified the need to establish a standardized and versatile depth framework.

Using the opportunity offered by the hiatus in IODP drilling operations, a new classification and nomenclature for depth scale types was defined in 2006–2007 to provide a starting point from which the implementing organizations could address more specific issues about the management of depth scales, depth maps, and splices (see "IODP depth scales terminology," version 1.0, available at www.iodp.org/program-policies/, for explanation of IODP depth scale definitions) (Table T1).

The primary depth scale types are based on the measurement of drill string length (e.g., drilling depth below rig floor [DRF] and drilling depth below seafloor [DSF]), length of core recovered (e.g., core depth below seafloor [CSF] and core composite depth below seafloor [CCSF]), and logging wireline (e.g., wireline log depth below rig floor [WRF] and wireline log depth below seafloor [WSF]). All units are in meters (m). The relationship between scales is defined either by protocol, such as the rules for computation of CSF from DSF, or user-defined correlations, such as stratigraphic correlation of cores between holes to create a common CCSF scale from the CSF scales used in each hole. The distinction in nomenclature should keep the user aware that a nominal depth value at two different depth scales usually does not refer to exactly the same stratigraphic interval (also see "Curatorial procedures and sample depth calculations").

For editorial convenience, the depth scale type acronym was not repeated with each depth reference in the site chapters. The scale type was declared at the beginning of each section and only mentioned again if a different scale type was used.

Core handling and analysis

The first hole drilled at each Expedition 317 site was a short (<50 m deep) hole dedicated to whole-round sampling for microbiology, geochemistry, and geotechnical studies of the uppermost interval in the hole, where physical and chemical properties change rapidly downhole. Whole-round samples were also taken from cores from the main holes but at much lower frequency. As soon as cores arrived on deck, microbiological whole-round samples as well as headspace samples for contamination testing were taken with sterilized tools (see "Geochemistry and microbiology"). Additional headspace samples as well as gas void (vacuum tube) samples were also taken for immediate hydrocarbon analysis as part of the shipboard safety and pollution prevention program. Additional whole-round samples were taken for interstitial water analysis. Core catcher samples were taken for biostratigraphic analysis.

Core sections were then placed in core racks in the laboratory. When the cores reached equilibrium with laboratory temperature (typically after ~4 h), whole-round core sections were run through the Whole-Round Multisensor Logger (WRMSL; measuring P-wave velocity, density, magnetic susceptibility, and resistivity) and the Natural Gamma Radiation Logger (NGRL). Thermal conductivity measurements were typically taken at a rate of one per section (see "Physical properties").

The cores were then split lengthwise from bottom to top into working and archive halves. Investigators should note that older material may have been transported upward on the split face of each section during splitting. The working half of each core was sampled for shipboard analysis (biostratigraphy, physical properties, carbonate, paleomagnetics, and bulk X-ray diffraction [XRD] mineralogy) and for shore-based studies. The archive half of each core was scanned on the Section Half Imaging Logger (SHIL) and measured for color reflectance and magnetic susceptibility on the Section Half Multisensor Logger (SHMSL). At the same time, the archive halves were described visually and by means of smear slides. Finally, the archive halves were run through the cryogenic magnetometer.

Both halves of the core were then put into labeled plastic tubes that were sealed and transferred to cold storage space aboard the ship. At the end of the expedition, the cores were transferred from the ship to refrigerated trucks and then transported to cold storage at the IODP Gulf Coast Repository in College Station, Texas (USA).

Drilling disturbance

Cores may be significantly disturbed as a result of the drilling process and contain extraneous material as a result of the coring and core handling process. In formations with loose sand layers, sand from intervals higher in the hole may be washed down by drilling circulation, accumulate at the bottom of the hole, and be sampled with the next core. The uppermost 10–50 cm of each core must therefore be examined critically during description for potential "cave-in." Common coring-induced deformation includes the concave-downward appearance of originally horizontal bedding. Piston action may result in fluidization (flow-in) at the bottom of APC cores. Retrieval from depth to the surface may result in elastic rebound. Gas that is in solution at depth may become free and drive core segments within the liner apart. When gas content is high, pressure must be relieved for safety reasons before the cores are cut into segments. This is accomplished by drilling holes into the liner, which forces some sediment as well as gas out of the liner. These disturbances are described in "Lithostratigraphy" in each site chapter and graphically indicated on the core summary graphic reports (standard graphic reports or "barrel sheets").

Curatorial procedures and sample depth calculations

The numbering of sites, holes, cores, and samples followed standard IODP procedure. A full curatorial sample identifier 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 "317-U1351A-1H-2, 10–12 cm" represents a sample taken from the interval between 10 and 12 cm below the top of Section 2 of Core 1 ("H" designates that this core was taken with the APC system) of Hole A of Site U1351 during Expedition 317. The "U" preceding the hole number indicates that the hole was drilled by the United States Implementing Organization (USIO) platform, the JOIDES Resolution.

Cored intervals are defined by the core top depth in DSF and the amount the driller advanced the bit and core barrel in meters. The length of the core is defined by the sum of lengths of the core sections. The CSF of a sample is calculated by adding the offset of the sample below the section top and the lengths of all higher sections in the core to the core top depth measured with the drill string (DSF). 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. Thus, a discrepancy between DSF and CSF depths can exist with regard to a stratigraphic interval. Furthermore, when a core recovers more than 100% of the cored interval, the CSF depth of a sample taken from the bottom of a core will be greater than that of a sample from the top of the subsequent core (i.e., the data associated with the two core intervals overlap at the CSF scale).

If a core has incomplete recovery, all cored material is assumed to originate from the top of the drilled interval as a continuous section for curation purposes. The true depth interval within the cored interval is unknown and should be considered a sample depth uncertainty in age-depth analyses and in the correlation of core data with downhole logging data.

Top of page   |   Previous   |   Next