2. Explanatory Notes1

Shipboard Scientific Party2


Information assembled in this chapter will help the reader understand the basis for the preliminary conclusions and also enable the interested investigator to select samples for further analysis. This information concerns only shipboard operations and analyses described in the site reports in the Leg 194 Initial Reports of the Proceedings of the Ocean Drilling Program. Methods used by various investigators for shore-based analyses of Leg 194 data will be described in the individual contributions published in the Scientific Results volume and in publications within various professional journals.

Drilling Operations

Three standard coring systems were used during Leg 194: the advanced piston corer (APC), the extended core barrel (XCB), and the rotary core barrel (RCB). These standard coring systems and their characteristics are summarized in the "Explanatory Notes" chapters of previous Initial Reports volumes as well a number of technical notes. The Leg 139 Initial Reports volume includes a particularly detailed description. In addition, the developmental advanced diamond coring barrel (ADCB) system was deployed for the second time (first time during Leg 193) and is described in some detail below. All systems were applied to maximize core recovery in the sediments being cored. Most cored intervals were ~9.6 m long, which is the length of a standard core barrel. In some cases, the RCB was advanced only part of the interval to maximize recovery. ADCB core barrels were 4.8 m long. In some holes the drill string was drilled, or "washed ahead," without recovering sediments in order to advance the drill bit to a target depth where core recovery needed to be resumed.

Drilled intervals are measured from the kelly bushing on the rig floor to the bottom of the drill pipe and referred to in meters below rig floor (mbrf). Meters below seafloor (mbsf) references for the core tops are calculated by subtracting the seafloor depth. When sediments of substantial thickness cover the seafloor (as at all but one site during Leg 194), the mbrf depth of the seafloor is determined with a mudline core, assuming 100% recovery for the cored interval in the first partly filled core barrel. Water depth is calculated by subtracting the distance from the rig floor to sea level from the mudline measurement in mbrf. This water depth usually differs from precision depth recorder measurements by a few to several meters.

Sea Trials with the Advanced Diamond Core Barrel

The diamond core barrel (DCB) was developed in 1990 as a spin-off project of the diamond coring system's drill-in bottom-hole assembly seafloor hardware project. The DCB provided an alternative method to the RCB system for obtaining hard rock cores. The DCB used the same inner barrel as the RCB but was packaged inside 6-in drill collars in place of the larger 8-in drill collars used for the rest of the Ocean Drilling Program (ODP) coring systems. Even though the 6-in drill collar connection is slightly underbalanced, it does not present any strength problems as long as it is not used for bare-rock spudding. It can be operated inside an existing hole or casing.

The ADCB takes the DCB one step further by using a different inner barrel while maintaining the same size 6-in outer barrel. These dimensions allow the bit to have a thinner kerf width (1.95 in), similar to mining-style core barrels (typically 0.96 in), compared to that of the RCB (3.78 in) (Table T1). The ADCB must be considered a hybrid, thick kerf system even though it more closely approximates a conventional mining-style DCB.

The rational for the smaller inner barrel diameter is not necessarily a larger core diameter, but increased bit life. Thick kerf diamond bits have shown excessive wear because the inner portion of the bit cuts at a significantly slower rate than the outer portion of the bit (known as "ringing"). Mining systems can operate with a much thinner kerf than the ADCB because they can operate with drill rods, which do not have the upset tool joints. These types of thin rods must be laterally supported; otherwise the connections will fail. In an offshore environment, this type of rod can only be used within a riser pipe. ODP open hole operations must use strong enough connections to allow for vessel offsets due to currents and surface sea states. Based on the combination of connection strength and economic and operational constraints aboard the JOIDES Resolution, a hole diameter was set at 7 in, maintaining a packed hole condition with 6-in drill collars.

Earlier attempts at using diamond bits resulted in only marginal success. This was primarily due to the inability of the passive heave compensator to keep the bit on the bottom of the hole. Diamond bits generally require less weight than roller cone bits, and the breakaway seal friction in the JR's passive cylinders alone was sometimes more than the weight-on-bit requirements, particularly in a shallow borehole. Therefore, the timing of the ADCB project was intended to dovetail the installation of the new active heave compensator (AHC), which should eliminate much of the weight fluctuation on the bit. By the time of Legs 193 and 194, installation of the AHC and training of personnel had been completed. Unfortunately, the AHC acted erratically at moderate to high sea states, when it was needed most, and had to be turned off during some of the ADCB drilling on Leg 194.

A proven and robust inner core barrel was selected "off the shelf" (Boart Longyear's PQ inner barrel hardware). This was primarily done to keep the project expenses within the allocated budget and to avoid reinventing or redesigning hardware until it was determined that enhancements were indeed needed. A few modifications were made including a new positive indicator latch.

The core obtained from the new ADCB inner barrel produces over twice the core volume per unit length compared with the RCB. The ADCB core is available in two sizes (Table T1). The double tube core barrel (outer barrel and inner barrel; referred to as PQ barrel) is run without a liner and cuts a 3.345-in cores. The triple tube system (PQ3 barrel) is run with or without metal split liners (wall thickness of 0.065 in or 1.65 mm) or lexan liners (wall thickness of 0.05 in) and cuts a 3.27-in core. The PQ3 barrel with lexan liners was used during Leg 194.

The ADCB offers the following advantages over the RCB in certain applications:

  1. Improved core quality,
  2. Better hole stability,
  3. Increased core recovery,
  4. Less hole disturbance,
  5. Smaller hole diameter,
  6. Less susceptibility to becoming stuck,
  7. Easier hole cleaning because of smaller cuttings, and
  8. Potentially better logging results.

The ADCB also can be configured in either a 15 ft (4.75 m) or a 30 ft (9.5 m) version. The shorter version, used during Leg 194, is the preferred version in which to operate the ADCB.

The ADCB will require pulling short cores more often than with the RCB. Once a core jams it is unlikely that further advancement of the outer barrel will occur, and continued advancement will lead to faster bit destruction and loss of time for round trips to change bits. While interval drilling may be possible in soft, friable formations such as the reefal limestone drilled on Leg 194, it is doubtful that it can be performed in hard crystalline rock where the more sensitive ADCB inner barrel components, as well as the bit, might be damaged.

Operating flow rates for the ADCB are 35 to 150 gal/min, which is significantly less than the rates used for roller cone bits. Cuttings produced by a diamond bit are much smaller and the amount per length of core is less than half that of the RCB. With the ADCB, a drop in pressure will notify the driller immediately of a core block, and a threshold pressure that must first be overcome will notify the driller that the core barrel has landed and successfully latched in place. This is accomplished by forcing all the flow through a regulated valve within the latch once the core barrel has landed.

Curatorial Procedures and Sample Depth Calculations

Numbering of sites, holes, cores, and samples follows the standard ODP procedure. A full curatorial identifier for a sample consists of the 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 194-1192A-1H-1, 10-12 cm, would represent a sample removed from the interval between 10 and 12 cm below the top of Section 1, Core 1 of Hole 1192A during Leg 194 (H designates that this core was taken with the APC system; R stands for RCB, X for XCB, and Z for ADCB cores). Cored intervals are also referred to in "curatorial" mbsf. The mbsf depth 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 sediment core from less than a few hundred mbsf may, in some cases, expand upon recovery (typically 10% in the upper 300 m), in which case its length will not match the drilled interval. In addition, a coring gap typically occurs between cores, as shown by composite depth construction (see the Initial Reports volumes for Legs 138, 177, and 184 [Shipboard Scientific Party 1992, 1999a, 2000]). Thus, a discrepancy may exist between the drilling mbsf and the curatorial mbsf. For instance, the curatorial depth (mbsf) of a sample taken from the bottom of a core may be larger than that of a sample from the top of the subsequent core. During Leg 194, multiple APC/XCB holes were not cored and continuous composite sections, therefore, could not be constructed.

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 not known, resulting in an uncertainty, for instance, in age-depth analysis and correlation of core facies with downhole log signals.

Core Handling and Analysis

General core handling procedures are described in previous Initial Reports volumes and the Shipboard Scientist's Handbook and are only summarized here. As soon as cores arrived on deck, gas void 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 obtained for biostratigraphic analysis. When the core was cut in sections, whole-round samples were taken for shipboard interstitial water analysis. In addition, headspace-gas samples were immediately extracted 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. Additional whole-round samples were taken at that point for postcruise research. 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. When short pieces of sedimentary rock were recovered, the individual pieces were split with the rock saw and placed in split liner compartments created by sealing spacers into the liners with acetone.

Curation methods had to be improvised for the larger diameter ADCB cores. The lexan liners used to retrieve the cores imposed two problems: (1) when split, the edges warped inside, and (2) the plastic spacers could not be sealed onto that material with acetone. No long intact core sections were recovered with the ADCB, so the procedure was limited to rock pieces of maximum 30-cm length. The core pieces were moved into a "liner patch," which is a liner with a diameter between those of the regular core liner and the ADCB lexan liner. Liner patch is usually used to bandage broken regular liners. It was divided into compartments of appropriate size using slightly modified plastic spacers and placed in a D-shaped cross section tube that had the top cut off, which provided the additional strength needed. The top of the core section was always measured from the top of the blue end cap glued into the liner patch and not from the top of the D-tube. ADCB core pieces were then placed in the new customized liner and, after sampling was completed, shrink-wrapped for safe transport.

Coherent and reasonably long archive-half sections were measured for color reflectance using the archive-half multisensor track. All archive-half sections were run through the cryogenic magnetometer, described visually and by means of smear slides and thin sections, and photographed with both black-and-white and color film. Close-up photographs were taken of particular features for illustrations in site summaries as requested by individual scientists. During Leg 194, a digital close-up imaging system was installed for the first time and used extensively. All images are available from the ODP database (TIF or JPEG format; ~0.5- to ~20-MB file sizes). Compressed JPEG versions (~0.5 MB) were written to CD-ROM on the ship and distributed to shipboard scientists.

The working half was sampled both for shipboard analysis including physical properties, further sedimentologic and biostratigraphic analyses, carbonate, and bulk X-ray diffraction (XRD) mineralogy, and for shore-based studies. Both halves of the core were then put into labeled plastic D-tubes, sealed, and placed in cold storage space aboard the ship. At the end of the leg, the cores were transferred from the ship into refrigerated containers and shipped to the ODP Gulf Coast Core Repository in College Station, Texas.

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 194IR-102