IODP

doi:10.2204/iodp.sp.329.2010

Logging/Downhole measurements plan

The main objectives of the downhole measurement program are to document crustal physical properties, define structural and lithologic boundaries as a function of depth, and identify alteration in the basaltic basement from the passage of fluids. In addition, wireline logging data can be compared to results of laboratory analyses of discrete samples and should help delineate alteration patterns, fracture densities, and structural orientations and determine how these correlate with current and paleostress environments. These measurements will complement core measurements by determining the thickness of lithologic units in intervals where core recovery is poor.

Borehole logging and core-log integration are invaluable for reconstructing recovery gaps and estimating bulk geochemical and structural characteristics of deep basement drill sites. Logging of the South Pacific Gyre will help characterize current physical properties that may set important constraints on the downhole microbial community.

Two wireline tool string deployments are planned for four sites drilled during Expedition 329 (Fig. F7). These tool strings will provide measurements including natural gamma radiation, density, sonic velocities, resistivity, and borehole images. The operational time estimates for all deployments are given in Table T1. Detailed descriptions of all wireline tools and applications are provided at iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html.

The first tool string deployment in each hole will consist of at least total and spectral gamma ray, density, caliper, and resistivity measurements. These measurements will be used for characterization of stratigraphic sequences, assessment of alteration, and reconstruction of the basement stratigraphy. Depending on borehole conditions, the second tool string will consist of a FMS, Dipole Sonic Imager (DSI), and natural gamma ray sensor. The FMS will provide high-resolution borehole images of lithostratigraphic sequences and boundaries, oriented fracture patterns, and structural features that could be related to current and paleostress environments. These images can also be used for reconstruction and reorientation of core pieces.

A check shot survey has not been included in the current operations plan; it is possible based on scientific need and time that a third tool string consisting of the Versatile Seismic Imager (VSI) and a natural gamma ray tool may be deployed.