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doi:10.2204/iodp.sp.349.2013

Downhole measurements strategy

Wireline logging

The downhole measurement plan aims to provide continuous stratigraphic coverage of in situ formation properties at all three Expedition 349 sites. Both sedimentary and basement intervals will be logged, but the main focus is characterizing the basement section. 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 two standard Integrated Ocean Drilling Program tool strings will be deployed at each logged site, and additional tool strings may be deployed at Site SCS-6A (or alternate Site SCS-6B) if conditions and time permit. The first run will be the triple combo tool string, which logs formation resistivity, density, porosity, NGR, and borehole diameter. The GPIT will be added to the triple combo because it includes a fluxgate magnetometer that can provide information on the magnetization of the basement rocks. The borehole diameter log provided by the caliper on the density tool will allow assessment of hole conditions (e.g., 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 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. Details of the logging tools are available at iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html.

In the primary operations plan, the upper ~900 m of Site SCS-6A/6B would not be cored or logged, apart from NGR logs, which obtain a good but attenuated signal through casing and pipe. However, in the backup operations plan, the upper 900 m of this site would be logged with the triple combo and FMS-sonic tool strings. Additional tool strings may be run in the lower half of Site SCS-6A/6B, depending on the condition of the borehole and the time available. A check shot survey may be run using the Versatile Seismic Imager (VSI), with a station spacing of ~25 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 (GI) air gun, and its deployment is subject to the Integrated Ocean Drilling Program marine mammal policy; the check shot survey would have to be postponed or cancelled if policy conditions are not met. Another possible additional tool string is the Ultrasonic Borehole Imager (UBI), which logs 360° ultrasonic amplitude and radius images of the borehole wall. This would be useful to map the features, fractures, and dip and strike directions, particularly in the basement section. Finally, the Lamont-Doherty magnetic logging tools, the magnetometer tool (MMM) and the magnetic susceptibility tool (MSS), would provide accurate magnetic field and susceptibility information for both the basement and sediment sections. Both tools should be available during the expedition, although temperatures at the base of the Site SCS-6A/6B borehole will probably exceed their temperature ratings (~70°C). These additional tool strings will also be available to run at the other expedition sites, if time and hole conditions permit.

Formation temperature measurements

Temperature measurements are planned for all sites with APC coring to reconstruct the thermal gradient at each location; the results will be compared with temperature measurements of nearby ODP Leg 184 sites (Clift et al., 2001). Typically, ~3–5 measurements are made at one hole per site using the advanced piston corer temperature tool (APCT-3), potentially supplemented by the Sediment Temperature Tool (SET) if necessary where sediments are more consolidated.

Risks and contingency

There are several risks involved in logging the very deep holes that are planned for Expedition 349. First, the upper parts of the holes will have been open for >7 days before logging, and high levels of fluid circulation will have been used to raise the cuttings and clear the hole. Therefore, the hole will likely be washed out (wide) where there is unlithified sediment, and log quality will be reduced for those tools that need good contact with the borehole wall (density, porosity, FMS resistivity images, VSI check shots).

Secondly, there will be 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 to this will be the conditions during drilling and a wiper trip. If the risk is considered to be significant, the radioactive source will be left out of the density tool.

Thirdly, there is a high geothermal gradient in the SCS (e.g., 84°C/km at nearby Site 1148; Wang, Prell, Blum, et al., 2000; Clift et al., 2001). If the same gradient exists at Site SCS-6A/6B, the temperature at the bottom of the hole would reach ~160°C. Most of the Schlumberger logging tools are rated to 175°C, and because the borehole fluid temperature would take time to warm up to the formation temperature, most tools should be able to be deployed. However, temperatures will very likely be too hot for magnetic logging tools. The APCT-3 temperature measurements will provide a valuable guide to the temperature gradients for at least the shallow depths at the Expedition 349 sites prior to logging.

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