Downhole logging measurements strategy

Because of the high priority of completing the coring operations at all primary sites and the limited operational time of Expedition 346, sites previously logged during ODP Legs 127 and 128 (Sites 794, 795, 797, and 798) are not in the current logging plan (proposed Sites JB-1, JB-3, YB-3, and YB-2, respectively). Wireline logging is planned for the deepest hole at proposed Sites JB-2, YR-1, YB-1, UB-1, and ECS-1B and at alternate proposed Site ECS-1A. Because we expect 100% core recovery with the APC in the upper 200 m of the sediment sections, downhole logging at shallow penetration sites (200 mbsf or less) is not a priority for the expedition.

The downhole measurement plan aims to provide a broad set of parameters that can be used for the characterization of in situ formation properties (lithologies, structures, and petrophysics) and orbital- and millennial-scale cyclicities (Fig. F14). The plan includes the use of as many as four different tool strings (details may be found at

  1. The triple combination (triple combo) tool string, which logs formation resistivity, density, porosity, natural gamma radiation (NGR), and borehole diameter. A diameter log is provided by the caliper on the density tool to allow assessment of hole conditions (e.g., washouts of sandy beds), log quality, and the potential for success of the following runs. NGR data gathered by the triple combo may prove useful to correlate with the ship-based NGR measurements.

  2. The paleo-combo tool string, which is a modified triple combo in which a newly built Magnetic Susceptibility Sonde (MSS) will replace the porosity sonde. Results can be correlated with magnetic susceptibility measurements from whole-round cores. As with the triple combo, NGR data gathered by the paleo-combo may prove useful to correlate with the ship-based NGR instrument.

  3. The Formation MicroScanner (FMS)-sonic tool string, which provides an oriented 360° high-resolution resistivity image of the borehole wall and logs of formation acoustic velocity and borehole diameter. Sonic logs can be combined with the density logs from the triple combo to generate synthetic seismograms and provide high-resolution seismic-well integration. A NGR tool will also be run in the FMS-sonic tool string in order to depth-match the different logging runs, and perhaps correlate to the ship-based NGR instrument.

  4. The Versatile Seismic Imager (VSI) tool string, which is used to acquire a zero-offset vertical seismic profile (VSP) for high-resolution integration of borehole stratigraphy and seismic profile, may be used at some sites, in particular proposed Site ECS-1B, pending time availability as well as permission from the National Science Foundation to conduct the operation in compliance with IODP marine mammal protection policy. A check shot survey planned with ~25 m spacing of stations over the entire open hole interval will give depth to seismic-traveltime conversions. The seismic source for the check shots will be a generator-injector air gun, and its deployment is also subject to the IODP marine mammal policy. The check shot survey would have to be postponed or cancelled if policy conditions are not met.

Two IODP tool string configurations will be deployed in each logging hole. At proposed Sites JB-2, YR-1, and UB-1, the first run will be the triple combo tool string. At proposed Sites YB-1 and ECS-1, the first run will be the paleo-combo tool string. At each logged site, the second run will be the FMS-sonic tool string. At proposed Site ECS-1B, a third run may include using the VSI tool string. The deployment order for the tool strings at this site may be adjusted to ensure that the VSP is acquired during daylight operations. Logging time estimates for each site are given in “Operations plan and drilling strategy.”

Downhole measurements will be the only data available where core recovery is incomplete. Moreover, the data will provide common measurements for core-log integration (density, NGR, and magnetic susceptibility) and to establish the link between borehole and core features and reflectors on seismic profiles by synthetic seismograms and VSP.