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doi:10.2204/iodp.sp.340.2011

Drilling and coring strategy

Drilling strategy

The overall operations plan and time estimates are summarized in Table T2. Alternate sites have been selected and are presented in Table AT1. Time estimates are based on anticipated formation lithologies and depths inferred from seismic and regional geological interpretations. After departing from Antigua we will transit for 4 h to the first site and prepare for drilling operations.

The proposed drilling strategy is to begin by drilling at proposed Site CARI-02C and end drilling at proposed Site CARI-09B following the sequence laid out in the operations plan (Table T2). With the exception of Site CARI-01C, two holes are planned to be cored at the each site. At all sites, the holes will be cored with the advanced piston coring (APC) system to refusal. The coring system will then be changed over to the extended core barrel (XCB) system and will be cored to total depth as determined by the scientific objectives. An estimate of the required depths can be found in Table T2. The exception (Site CARI-01C) will be a single hole piston cored to a depth of ~132 meters below seafloor (mbsf).

All holes will be plugged and abandoned with heavy mud.

Coring strategy

The first hole at each site (Hole A) will be cored with APC/XCB to planned depths (Table T2). The second hole (Hole B) at each site will also be cored with APC/XCB to planned depth. After reaching the planned depth the “B” holes will be conditioned, displaced with logging mud, and logged as per the logging plan (see “Downhole tools and logging strategy”). Should all objectives be fulfilled coring the “A” hole, a second hole will not be drilled and the “A” hole will be logged. While coring, a number of advanced piston coring temperature tool (APCT-3) measurements will be made, as formation conditions permit. Core orientation with the FlexIt tool will also be measured during the APC-cored sections at each site. If depth objectives cannot be achieved with APC/XCB coring system, an option will be to deploy the rotary core barrel (RCB) system.

Downhole tools and logging strategy

Downhole logging will complement coring operations during Expedition 340 and assist in achieving scientific objectives by providing in situ geophysical characterization of drilled volcanic debris avalanche deposits and sedimentary sequences. Logging data will provide a continuous record to aid in identification of boundaries between multiple avalanche deposits and to evaluate the structures and textures of volcanogenic sediments, avalanche deposits, and breccia. Logging data will be used in conjunction with core images and other data to reorient structures and deposit boundaries, which is valuable for characterizing variations in debris avalanche deposits through time. In addition, logs allow direct correlation of wireline measurements with discrete core measurements and offer data coverage where core recovery is poor.

Wireline logging is planned for eight of the nine primary sites of Expedition 340. Logging is not currently planned for proposed Site CARI-01C because of the shallow target depth (132 mbsf). Two standard tool strings will be deployed in each logged hole. The first run will be the triple combination (triple combo) tool string, which will record resistivity, neutron porosity, bulk density, and natural and spectral gamma radiation. The caliper log provided by the density tool will allow assessment of hole diameter, log quality, and the potential for success of the following runs. The second run, with the Formation MicroScanner (FMS)-sonic tool string, will record gamma radiation, sonic velocity (for compressional and shear waves), and oriented high-resolution electrical resistivity images. A third logging run is planned at three sites (CARI-03C, CARI-07C, and CARI-10B) using the Versatile Seismic Imager (VSI) to acquire a zero-offset vertical seismic profile (VSP) for calibrating the integration of borehole and seismic data. The survey is planned with ~25 m spacing of stations over the open hole interval below 80 mbsf. Spacing may be adjusted based on hole condition. The seismic source for the VSP will be a parallel cluster of two 250 in³ Sercel G guns (Table T3), positioned 2–7 m below sea level and offset by ~50 m from the side of the ship. VSP operations will be subject to the IODP marine mammal policy and may be postponed or cancelled if policy conditions are not met. Operational time estimates for each site can be found in Table T2. For more information on specific logging tools, please refer to iodp.ldeo.columbia.edu/​TOOLS_LABS/.

At three of the sites (CARI-02C, CARI-03C, and CARI-04C) we would like to deploy the Magnetic Susceptibility Sonde (MSS) currently being built by the Lamont-Doherty Earth Observatory Borehole Research Group. The MSS, which measures magnetic susceptibility, would be used identify flank collapse deposits from the island of Montserrat, where the volcanic material has a high magnetite content compared to background sediment. Although the MSS is still being developed, the current production and testing timeline for the MSS indicates that it will be available for deployment during Expedition 340. At these sites, the triple combo will be modified to replace the resistivity tool with the Magnetic Susceptibility Sonde.

APC cores at all Expedition 340 sites will be oriented with the FlexIt tool for paleomagnetic studies. Temperature measurements are planned for four sites (CARI-02C, CARI-03C, CARI-04C, and CARI-10B). We plan to deploy the APCT-3 in the interval where cores will be taken by APC to collect sufficient temperature measurements to calculate a thermal gradient at each location.

Risk and contingency

Three principal factors could affect the implementation of the drilling plan:

1. Adverse hole conditions at the principal sites (e.g., encountering thick intervals of loose sediment/rock that can collapse into the hole).

2. Weather conditions that can limit the ability to drill.

3. Time delays (arising from equipment breakdowns, or measures taken to respond to hole conditions).

Hole conditions

Poor hole conditions at all sites will be dealt with in the first instance by using frequent high-viscosity mud sweeps and or heavy mud to condition the holes. The only possible remedial action if hole conditions prove to be insurmountable is to plug and abandon the hole and move to an alternate site.

Weather conditions

Hurricane season at the operation area is between ~1 July and ~30 November; thus, Expedition 340 is scheduled (February–March) outside this window and weather-related risks are negligible and essentially avoided.

Timing

If significant time is spent responding to poor hole conditions, slower than expected penetration rates, and/or weather-related delays, a primary site may be shortened or dropped from the schedule. Such a decision will only be made following consultation with the science party.

Alternate sites

The alternate sites may be cored and logged if poor hole conditions or other operational difficulties are encountered at the respective primary sites and it is judged that better conditions may be meet at these sites. Seismic profiles of all proposed alternate sites (as well as of all primary sites discussed above) are included in the “Site summaries.” The operations plan for the alternate sites can be found in Table AT1.

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