LOGGING PLANDownhole logging will be used during Leg 192 to address issues of volcanic stratigraphy, eruptive morphology, and stress orientation and to create synthetic seismograms for depth-time conversions, which calibrate the interpretation of regional seismic data. Logging data will also be used to measure in situ physical properties and to position and orient cores. Whereas core recovery is often biased and incomplete in variable lithology such as alternating pillows and massive flows, logging data are continuous and therefore provide useful information over intervals of low core recovery.
Logging is scheduled for Sites OJ-3B and OJ-7D. At each, the basalt basement and bottom of the sedimentary column will be logged. The logging time estimate (not including hole conditioning) is 27.7 hr at Site OJ-3B to log 150 m of basalt and the lower 375 m of sediments. At Site OJ-7D, it is estimated that logging will take 23.3 hr to log 100 m of basalt and the lower 100 m of sediments. Because of time constraints, no logging is planned at Sites OJ-6B and OJ 11C. This exception to the normal JOIDES logging policy was approved by OPCOM to provide maximum time for achieving the primary objective of coring in basement.
Three tool strings have been selected to address the scientific objectives of the leg: the triple combination (triple combo), Formation Microscanner (FMS)/sonic, and the well seismic tool (WST). The triple combo (also known as Geophysical) tool string provides a basic suite of downhole physical properties measurements: natural gamma, porosity, density, electrical conductivity, and temperature. These logs are used for determining petrophysical and lithologic variations in both sediments and basalts. The triple combo consists of the following units: the hostile environment natural gamma sonde (HNGS), which is a high sensitivity spectral gamma-ray tool; the accelerator porosity sonde (APS), which uses an electrical neutron source to measure porosity; the hostile environment litho-density tool (HLDT), which measures density and photoelectric absorption through the interaction of gamma rays with electrons in the formation; the dual induction tool (DIT), which measures the electrical conductivity of the formation by electromagnetic induction; and the temperature/ acceleration/pressure (TAP) tool developed by the Lamont Borehole Research Group to collect data for the shipboard heave compensation system that decouples the logging tool from the adverse effects of the ship's motion.
The FMS/Sonic tool string gives oriented electrical resistivity images of borehole walls with millimeter-scale resolution, the downhole magnetic field vector, P- and S- wave interval velocities, and a low-sensitivity measurement of natural gamma that is used for depth matching between logging runs. The FMS/Sonic tool string is comprised of three tools: the FMS, the dipole shear sonic tool (DSI), and the natural gamma-ray tool (NGT).
The WST is a single-axis checkshot tool used for zero offset vertical seismic profiles (VSPs). The WST records the acoustic waves received downhole from an air-gun source located near the sea surface. The source bandwidth approximates that used in seismic surveys, and the velocity measure obtained is an integrated one that is robust with respect to local heterogeneities. WST logs are therefore used to construct high-quality synthetic seismograms that calibrate time-depth conversions for the interpretation of regional seismic data. This enhanced interpretation of seismic data will permit extrapolation of the drilling results away from the drill site.
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