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doi:10.2204/iodp.sp.304305.2004
Our logging strategy is designed to complement and complete our overall cruise objectives, determining the lithology, orientation of deformation fabrics, and proportions and orientation of features. Downhole logging may provide our only continuous record because of the potential of low core recovery. A main objective 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 or physical property boundaries, as interpreted from logging tool response characteristics as a function of depth; determine serpentinization and/or alteration patterns in basalts, lower crustal, and upper mantle rocks; and produce direct correlations with discrete laboratory measurements on the recovered core.
Because of its potential impact on achieving cruise objectives, we have scheduled time for downhole logging operations at hanging wall and footwall sites (pilot and cased holes). Five tool strings will be deployed: triple combination (triple combo) tool string, Formation MicroScanner/Dipole Sonic Imager (FMS/sonic) tool string, Ultrasonic Borehole Imager (UBI), borehole magnetometer, and Well Seismic Tool (WST). Additionally, our contingency plan calls for use of the RAB tool to drill to bit destruction at an alternate hanging wall (and possibly footwall) site.
Triple Combination Tool String
Data from the triple combo will provide continuous measurement of the natural radioactivity and Th, U, and K contents (Hostile Environment Gamma Ray Sonde), density (Hostile Environment Litho-Density Sonde), neutron porosity (Accelerator Porosity Sonde), electrical resistivity (Dual Laterolog), and temperature profiles (Temperature/Acceleration/Pressure tool). These measurements will be utilized for the characterization of changes in lithology (gabbros versus peridotites) and variations in alteration (fresh to serpentinized peridotites).
Formation MicroScanner/Dipole Sonic Imager Tool String
The FMS/sonic tool string has two main components:
The UBI measures the amplitude and transit time of an acoustic wave propagated into the formation. It provides high-resolution images with 100% borehole wall coverage, which allows detection of small-scale fractures. The amplitude depends on the reflection coefficient of the borehole fluid/rock interface, the position of the UBI tool in the borehole, the shape of the borehole, and the roughness of the borehole wall. Changes in the borehole wall roughness (e.g., at fractures intersecting the borehole) are responsible for the modulation of the reflected signal; therefore, fractures or lithologic variations can easily be recognized in the amplitude image. The General Purpose Inclinometer Tool (GPIT) is deployed with the UBI and allows orientation of the images; evaluation and orientation of fractures can provide information about the local stress field and borehole geometry. Deployment of the UBI is contingent on availability of funds.
The Göttingen Borehole Magnetometer (GBM) previously deployed during ODP Leg 197 will be available as a third-party tool during Expedition 305. This tool has three fluxgate sensors that measure three orthogonal components of the magnetic field. The tool includes a gyroscope, which measures the tool rotation during data acquisition and allows the orientation of the tool to be determined. The data from the magnetometer will be used to monitor changes in the magnetic properties of the oceanic lithosphere as well as changes in paleomagnetic direction that can aid in determination of the magnetic polarity.
Three-Component Well Seismic Tool
The WST-3 records acoustic waves generated by an air gun or water gun located near the sea surface. It provides a complete checkshot survey and a depth-traveltime plot; synthetic seismograms will be essential for determining in situ velocity profiles.
The RAB has been requested as part of our contingency strategy for Expedition 305. The strategy for using the RAB will be determined on the basis of the ability to obtain wireline logs at the hanging wall site. Deployment of the RAB is contingent on availability of funds.
The RAB will provide borehole resistivity logs and images at three different depths of investigation and total gamma ray logs. The RAB measures oriented resistivity images of the borehole wall, similar to an FMS wireline tool. These fracture orientations and distributions can be observed as resistivity contrasts in the image logs and are critical for recognizing the extent of the deformation structures. These data will provide visual recognition of igneous layers as well as the identification of fracture patterns, structural orientations, and formation thicknesses. These oriented images could be critical for a assessing the structure within the uppermost 80 m of the borehole since the RAB is the only tool able to record such oriented images.