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doi:10.14379/iodp.sp.355.2014

Downhole measurements strategy

Wireline logging

The downhole measurements plan for Expedition 355 aims to provide continuous stratigraphic coverage of in situ formation properties at all three primary drilling sites over both the sedimentary and basement sections. Downhole logging data will provide the only stratigraphic data where core recovery is incomplete, which is likely when sites are single-cored with XCB and RCB coring. The logging thus represents an essential data set for the fulfillment of the scientific objectives.

The two standard IODP tool strings will be deployed at each logged site, with an additional tool string deployed at proposed Site IND-03C if conditions and time permit (Table T2). The first run will be the triple combination (triple combo) tool string, which logs formation resistivity, density, porosity, natural gamma radiation (NGR), and borehole diameter. The General Purpose Inclinometry Tool (GPIT) will be added to the triple combo tool string because it includes a fluxgate magnetometer that can provide information on the magnetization of the basement rocks, which may be significant for constraining the age of formation. If available, the Lamont-Doherty magnetic susceptibility sonde will also be added to the triple combo tool string to provide magnetic field and susceptibility information for both the sedimentary and basement sections. The borehole diameter log provided by the caliper on the density tool will allow assessment of hole conditions (e.g., wash-outs of sandy beds), log quality, and the potential for success of the following runs.

The second logging run will be the Formation MicroScanner (FMS)-sonic tool string, which provides an oriented resistivity image of the borehole wall and logs of formation acoustic velocity, NGR, GPIT magnetometry, and borehole diameter. To provide a 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. If hole conditions allow, a third run at proposed Site IND-03C will consist of a check shot survey using the Versatile Seismic Imager (VSI), with a station spacing of ~100 m where the borehole diameter is narrow enough to give good coupling of the tool’s geophone with the borehole wall. The objective would be to directly establish the link between lithostratigraphic depths in the borehole and reflectors in the seismic profiles. The seismic source for the check shots will be a generator-injector air gun, and its deployment is subject to the IODP marine mammal policy; the check shot survey would have to be postponed or canceled if policy conditions are not met. Details of the logging tools are available at iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html.

Downhole temperature measurements

Temperature measurements are planned for all sites with APC coring to reconstruct the thermal gradient at each location. Typically, ~3–5 measurements are made at one hole per site using the advanced piston corer temperature tool, potentially supplemented by the sediment temperature tool if necessary where sediments are more consolidated.

Risk and contingency

Any logging operations involve a number of potential risks. First, the upper parts of the holes will have been open longer before logging, and high levels of fluid circulation will have been used to raise the cuttings and clear the hole, particularly in the sites that penetrate to dense basement material. Therefore, the hole could be washed out (wide) over intervals through unlithified sediment, and log quality will be reduced for those tools that need good contact with the borehole wall (density, porosity, FMS resistivity images, and VSI check shots). Secondly, there is a risk of bridging where the hole closes up. This would mean either not reaching the total depth of the hole or, in the worst case scenario, getting a tool string stuck in the hole. A good guide for this will be the conditions encountered during drilling and a wiper trip before logging. If the risk is considered to be significant, the radioactive source will be left out of the density tool.

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