Our logging strategy for Leg 209 is designed to directly complement and/or complete our overall cruise objectives, determining the orientation of deformation fabrics with respect to the Mid-Atlantic Ridge axis and the proportions and orientation of melt features. Downhole logging may, in fact, provide our only continuous record because of the potential of low core recovery. Because of its potential impact on achieving cruise objectives, we have scheduled time for downhole logging operations at all seven primary sites. We expect our penetration depth to be limited (<200 mbsf) by single bit holes, BHA configurations, and formation instability, so our specific logging strategy will be dictated by contribution to the overall science objectives. The main objectives of the wireline logging program will be to orient faults, fractures, and deformation features using borehole imaging techniques. Borehole images may then help orient core pieces or sections if the core recovery is sufficiently high. In addition to defining structural features, the logging program will also attempt to establish lithologic boundaries as interpreted from logging tool response characteristics as a function of depth, determine serpentinization and/or alteration patterns in lower crustal and upper mantle rocks that might be encountered, and produce direct correlations with discrete laboratory measurements on the recovered core. As with our drilling strategy, the logging program will be determined on a site-by-site basis, through coordination between co-chief scientists, the logging staff scientist, the operations manager, and the staff scientist.
Potential complications in determining the orientation of structural features using downhole imaging techniques may arise from the effect of highly magnetized formations on the three-component magnetometer of the General Purpose Inclinometry Tool (GPIT), which is the tool used for orienting the images produced by the Formation MicroScanner (FMS). Results from shipboard paleomagnetic studies and the GPIT will be compared in order to assess variations in orientation resulting from high formation magnetization intensities. Hole stability and time constraints will also dictate the amount of wireline logging completed during Leg 209.
If hole stability and/or time constraints are not an issue, single-bit holes will be drilled to bit destruction, the bit will be released at the bottom of the hole, and the deepest hole in each site will be logged. This process will not require hole reentry procedures, and time estimates for logging operations are shown below not including time estimates for hole preparation that are included in the operations plan. Time estimates and planned wireline logging tool strings are listed in Table T2.
A wireline logging program has been designed for logging the seven main targets using the triple combination (triple combo) and FMS/sonic tool strings. The triple combo tool string will be used to determine concentrations of K, U, and Th, obtain formation density, measure photoelectric effect, electrical resistivity, and porosity values, and determine borehole conditions. These measurements will be utilized for the characterization of stratigraphic sequences, the assessment of variations in serpentinization, and the identification of oxide mineral–rich intervals. If lithologies with different proportions of ferromagnesian phases (i.e., dunite and harzburgite) manifest different degrees of serpentinization over short intervals, then combinations of density and other logging data might prove useful in distinguishing between them and determining formation thicknesses.
The FMS will provide high-resolution borehole images of stratigraphic sequences and boundaries, oriented fracture patterns, fracture apertures, fracture densities, and information regarding hole stability. The Dipole Sonic Imager (DSI) will produce a full set of compressional and shear waveforms that can be used to determine the nature of the shallow velocity gradient in this area. Cross-dipole shear wave velocities measured at different azimuths may be used to determine preferred mineral, fracture, and/or fabric orientations that may produce seismic velocity anisotropy.
Historically, wireline logging programs have met with limited success in hard rock coring expeditions (e.g., Gillis, Mével, Allan, et al., 1993; Cannat, Karson, Miller, et al., 1995). In anticipation of challenging coring and wireline operations, the resistivity-at-the-bit tool with coring capabilities (RAB-C) will likely be substituted for conventional coring and/or logging at selected sites. The strategy for using the RAB-C will be determined on the basis of the ability to obtain wireline logs at a particular site, and the frequency of its use will be determined depending on the amount of core recovered.
The RAB-C will provide borehole resistivity logs and images at three different depths of investigation, total gamma ray logs, and coring capabilities. This tool was first used by ODP during Leg 204 and has the capabilities of recovering 2.56-in (6.5 mm) diameter cores. The RAB-C also provides complete azimuthal coverage of the borehole, providing high-quality resistivity images comparable to those obtained with the FMS. These data will provide visual recognition of igneous layers as well as the identification of fracture patterns, structural orientations, and formation thicknesses. In the past, core recovery has been low in the upper 50 m of holes drilled with conventional drilling techniques (i.e., Legs 147 and 153), and wireline logging techniques preclude the acquisition of downhole measurements at shallow depths because of the need to have the BHA several tens of meters inside the hole. Therefore, the RAB-C data will also provide the only means to obtain continuous information in the upper sections of the holes drilled during Leg 209.
If all or most holes can be logged with conventional wireline techniques, the RAB-C will be used for drilling the last hole during the cruise to determine the tool capabilities in a hard rock environment. The conventional RAB tool without coring capability will be on board as an ultimate backup device.