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doi:10.2204/iodp.sp.320321.2008

Downhole measurements strategy

Temperature measurements

The downhole measurement plan consists of up to five temperature measurements per site in one hole, tentatively Hole A. We plan on deploying the APCT-3 three times in the hole with two DVTP temperature measurements further downhole where sediments are more consolidated. The scientific objective of the temperature measurement plan is to provide sufficient data to reconstruct the thermal gradient at each site. This information will help constrain the history of burial diagenesis of the sediments encountered.

Wireline logging

Wireline logging objectives are

• To collect high-resolution downhole physical property data and integrate them with core measurements and
• To integrate seismic reflection with drilling by using sonic velocity data in conjunction with the density results and/or check shot surveys to obtain a velocity profile, a time/depth model, and synthetic seismograms.

Synthetic seismograms results will be compared to the regional seismic sections to interpret the origin and geological significance of the major seismic horizons. It is thus important to choose logging tools that are appropriate for achieving both high-resolution core-log mapping as well as measuring the range of sound velocities anticipated for these expeditions—from 1500 to 1650 m/s for the majority of the unconsolidated sediment but increasing to >1700 m/s in the deeper, more compacted sediments.

The operations plan for Expeditions 320 and 321 lists downhole logging at all proposed sites in Hole A, should it reach target depth. The purpose in logging the first hole is that near–real time core-log integration will provide important information about the sediment column for subsequent drilling operations at that site (Holes B and C) and to help ensure complete recovery of the stratigraphic section. The plan includes using a maximum of two logging tool strings that would require a range of 16 to 20 h per site from the shallowest to deepest sites, respectively, including time for rigging up wireline operations.

The logging strings to be used are

• A modified triple combination (triple combo) tool string (Fig. F36B), termed the "paleo-combo," that includes the high-resolution Multisensor Gamma Tool (MGT), the High-Resolution Litho-Density Tool (HLDT), and, potentially, the Magnetic Susceptibility Sonde (MSS) and
• Either the Formation MicroScanner (FMS)-sonic tool string that includes the FMS and the Dipole Sonic Imager (DSI-2) or the Versatile Seismic Imager (VSI) string that includes the VSI if good velocities cannot be achieved by the DSI-2 (Fig. F36C). The VSI is a vertical seismic profiling tool that would require use of a separate seismic source deployed from the ship.

Further details on logging tools and their applications can be found at iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html.

One advantage of a modified triple combo tool string is that it combines the Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG)'s high-resolution tools such as the MGT and MSS, providing up to 10 cm resolution natural gamma ray (NGR) and magnetic susceptibility (MS) data that are more comparable to planned shipboard measurements. Such resolution will be critical for proper core-log integration. In addition, the HLDT provides the density and caliper data necessary for evaluating important physical properties, hole conditions, and construction of synthetic seismograms.

A second advantage is that this tool string is considerably shorter than the triple combo (Fig. F36A) and will be able to provide more hole coverage, especially at sites with thin sediment columns (proposed Sites PEAT-1C through 3C).

At deep-penetrating sites, where tool string length is not as critical, there is an option to include additional tools, such as the Phasor Dual Induction–Spherically Focused Resistivity Tool (DIT). This could provide an additional robust physical property data set. Our preferred approach to achieve good velocities is to use the FMS-sonic tool string, providing high-resolution (centimeter scale) resistivity data from the FMS tool.

Assessment of logging at each site will require consideration of multiple issues. The objectives of these cruises are intimately linked to obtaining complete recovery of the sedimentary section, requiring at least three and possibly four holes at each site. Thus, there is an obvious impact when opting between these two operational activities for these cruises; when logging operations are in progress, core is not being acquired. In addition, because the base of the drill pipe must be securely in the hole during logging operations (typically between 60 and 100 mbsf), the upper part of the sedimentary section is not logged. For this reason, the priority of logging operations can be anticipated to be lower in the shallower (<200 m penetration depth) Sites PEAT-1C, 2C, and 3C. The decision to log these holes will be based on logging data quality and the impact of weather conditions on coring and logging operations, previous logging in the area, depth of coring penetration, and operational time constraints.

Difficulties were encountered during ODP Leg 199 when deploying the DSI-2 to capture quality data in low-velocity unconsolidated sediments. The VSI tool will be available for check shot surveys should the DSI-2 not be capable of recording the desired quality velocity data. The VSI will provide lower vertical resolution, and additional operational and planning time may be necessary to respond to air gun deployment and marine mammal policy issues, respectively.

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