Leg 208 Walvis Ridge |
Rio de Janiero - Rio de Janiero
Personnel:
Planned Tool Deployments
LOGGING PLAN
Downhole logging will provide physical and chemical proxy data for interpreting some of the fundamental questions Leg 208 is addressing. Where core recovery is poor, downhole logs may present the most reliable source of information; where core recovery is good, log data can be correlated with core data to produce more detailed and emphatic results. All sites deeper than 300 mbsf and all sites using XCB coring in the lower interval will be logged with the two standard logging tool strings, the triple combination (triple combo) and the Formation MicroScanner (FMS)/sonic string. The Well Seismic Tool (WST) will be deployed at a minimum of two sites. The characteristics of these logging tools are summarized below .
The triple combo tool string includes the Dual Induction Tool (DIT), which measures indirect resistivity at three invasion depths, the Accelerator Porosity Sonde (APS), which measures porosity from epithermal neutron measurements, and the Hostile Environment Litho-Density Sonde (HLDS), which measures bulk density from Compton scattering and provides an indication of general lithology from the photoelectric effect. Commonly, the Hostile Environment Gamma Ray Sonde (HNGS) is added to this tool string. Following its past successes, the LDEO Multi-Sensor Spectral Gamma Ray Tool (MGT) will be added to the triple combo tool string, providing a vertical resolution three times higher than the standard gamma ray sonde. Density measurements will be crucial for calculating acoustic impedance values, and neutron porosity data will be particularly valuable for identifying intervals of high-porosity radiolarian ooze. The downhole natural gamma radiation and density values will be correlated with comparable analyses from the MST, which will enable the precise depth matching of cored sections. MGT logs will be useful for cyclostratigraphic analysis of Upper Cretaceous and lower Cenozoic sequences.
The FMS/sonic tool string includes the Formation MicroScanner, which measures resistivity at centimeter resolution on four pads moving along the borehole, the General Purpose Inclinometer Tool (GPIT), and the Dipole Sonic Imager (DSI), which measures compressional and shear wave velocity, as well as cross-dipole and Stoneley waveforms. Chert horizons likely to present in some intervals at Leg 208 sites show up exceptionally well as resistive stripes on FMS images.
The WST is a single-axis checkshot tool used for zero-offset vertical seismic profiles (VSPs). It consists of a single geophone that is used to record acoustic waves generated by a water or air gun located near the sea surface. Downhole sonic data in conjunction with the density results, and checkshot surveys where necessary, allow generation of a velocity profile, a time/depth model, and synthetic seismograms. These results will be compared with the regional seismic sections to interpret the origin and geological significance of the major reflectors at a regional scale. WST checkshot surveys should be measured at 30- to 50-m spacing.
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