LOGGING PLANDownhole logging provides continuous measurements of geophysical parameters, which can be used to assess the physical, chemical, and structural characteristics of the formation. Where core recovery is poor, downhole logs are often 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.
During Leg 199, a great deal of information will be obtained from the in situ downhole physical property data. Density measurements will be crucial for calculating acoustic impedence values, and neutron porosity data will be particularly valuable for identifying intervals of high-porosity radiolarion ooze. Density and porosity results may also be useful for delineating the extent of hydrothermal activity toward the base of the sedimentary section, which will be characterized by high concentrations of iron and manganese. The downhole natural gamma-ray and density values will be correlated with comparable analyses from the multisensor track (MST), which will enable the precise depth matching of cored sections.
A major objective of logging during Leg 199 will be to use the downhole sonic data in conjunction with the density results, and check shot surveys where necessary, to obtain 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.
Downhole measurements, in particular taken by the Formation MicroScannner (FMS) and the new high-resolution (~0.1 m) Lamont Multisensor Gamma tool (MGT), will be useful for cyclostratigraphic analysis of continuous Paleogene sequences. Logs will also be invaluable for determining the position of chert layers, which are likely to occur in the lower part of the boreholes and will be difficult to recover in the core. Chert horizons show up exceptionally well as resistive stripes on FMS images.
The plan is to run the triple combo, MGT, and FMS-sonic toolstrings at the three deepest penetration sites: PAT-8C, PAT-17C, and PAT-9D. Time permitting (Table 3), these toolstrings will also be run at Site PAT-26 (~0.8 days). There is also the possibility that check shot data will be acquired at some of the deeper logged sites, with the priorities being Sites PAT-8C (~0.6 days) and PAT-17C (~0.6 days). Less extensive logging operations may be performed at some of the shallower sites.
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