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doi:10.2204/iodp.sp.342.2011

Downhole measurements strategy

Wireline logging

On the basis of the drilling strategy outlined above, we propose to prioritize logging operations according to the accumulated value of each of the logging-related criteria for expedition science. Hence, we will give less consideration to logging shallow penetration holes where we anticipate complete core recovery and sites that sample the acoustically transparent Eocene drift sediment package where we have no acoustic reflection record to compare with synthetic seismograms. Conversely, sites that have deep penetration, sample acoustically complex sections, and those where the core recovery is incomplete will be higher priorities for logging. Therefore, our priority logging sites are JA-1A, SENR-16A, and SENR-11A. Other sites (JA-14A, JA-5A, and SENR-19B) have lower priority for logging because they have shallow penetration, are proposed for triple coring, and are in acoustically transparent sections.

Two standard IODP tool string configurations will be deployed in each logging hole: the triple combination (triple combo) and the Formation MicroScanner (FMS)-sonic tool strings (Fig. F21). The first run will be the triple combo tool string, which logs formation resistivity, density, porosity, natural gamma radiation (NGR), and borehole diameter. The diameter log provided by the caliper on the density tool will allow assessment of hole conditions (e.g., any washouts of sandy beds), log quality, and the potential for success of the following runs. The second run will be the FMS-sonic tool string, which provides an oriented 360 resistivity image of the borehole wall and logs of formation acoustic velocity, NGR, and borehole diameter.

The Magnetic Susceptibility Sonde (MSS) currently under development at Lamont-Doherty Earth Observatory Borehole Research Group is anticipated to be available for deployment during Expedition 340 Lesser Antilles, and therefore also during Expedition 342. If the MSS is available, the triple combo will be modified to replace the Dual Induction Tool (DIT) resistivity tool with the MSS; this modified tool string will be called the “paleo combo” (Fig. F22). Details of the logging tools are available at iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html.

Downhole logging data will provide the only stratigraphic data where core is not recovered and aims to provide characterization of in situ formation properties and establish the link between features in the borehole and reflectors in the seismic sections. To provide the link between borehole stratigraphy and the seismic section, sonic velocity and density data will be combined to generate synthetic seismograms for detailed well-seismic correlations.

Logging time estimates for each site are given in Tables T1, T2, and T3.

Formation temperature measurements

The downhole measurement plan includes a depth series of reconnaissance temperature measurements for all sites. Typically, ~3–5 measurements are made in one hole per site using the advanced piston corer temperature tool (APCT-3). The scientific objective of the temperature measurement plan is to provide sufficient data to reconstruct the thermal gradient at each site.

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