IODP

doi:10.14379/iodp.sp.359.2014

Logging/downhole measurements strategy

Because of the high priority of seismic-core integration, all sites except Site KK-03B will be wireline logged. 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. The plan includes the use of different tool strings (details at iodp.tamu.edu/tools/logging/index.html).

The triple combination (triple combo) tool string 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 for correlation with ship-based NGR measurements.

The paleocombo tool string is a modified triple combo in which a newly built magnetic susceptibility sonde (MSS) replaces 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 paleocombo may prove useful for correlation with the ship-based NGR instrument.

The Formation MicroScanner (FMS)-sonic tool string 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 triple combo density logs to generate synthetic seismograms and provide high-resolution seismic-well integration. A NGR tool will also be run on the FMS-sonic tool string to depth-match the different logging runs and perhaps correlate to the ship-based NGR instrument.

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 Sites MAL-01A, MAL-04B, MAL-07A, pending time availability and permission from the National Science Foundation to conduct the operation in compliance with IODP marine mammal protection policy. A check shot survey planned with ~50 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 GI air gun, and its deployment is also subject to the IODP marine mammal policy. The check shot survey will be postponed or canceled if policy conditions are not met.

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 establish the link between borehole and core features and reflectors on seismic profiles by synthetic seismograms and VSPs.