Logging and downhole experiments are critically important to the scientific objectives of Leg 169, particularly as core recovery may be poor, based on the previous coring experiences in Middle Valley during Leg 139. One of the first priorities of the leg will be to remove and reinstall CORKs already present in Holes 858G and 857D. Temperature measurements and fluid sampling with minimum disturbance will be part of these two operations. A full logging program will be run at Hole 857D.
Tools planned for possible use in all holes (Table 2) include the full set of Schlumberger tool logs, several downhole temperature tools and a high temperature borehole fluid sampler. In addition, a packer/flowmeter might be used in Hole 857D, depending on borehole temperatures and such constraints as whether or not the side-entry-sub (SES) is required in order to cool the hole during logging. Unless the holes can be kept cool enough by the down-hole flow of ocean bottom water that may be induced by drilling, the SES is expected to be used in most of the sites as the temperatures will presumably exceed the operational limits of the standard logging tools. Following preliminary temperature measurements, the logging suites will include sonic/resistivity, density/porosity and FMS, as well as a geochemical string in high-priority sites ET5 and BH1 to assess compositional variations throughout these massive sulfide deposits and the underlying altered rocks. If the holes cannot be kept cool enough for the standard Schlumberger tools, a hostile environment gamma ray/density/porosity combination tool can be deployed which can be valuable for characterization of the different lithological units.
The main objectives of the downhole program will be to assess the changes in physical properties resulting from hydrothermal alteration and enhanced diagenesis, as well as how they relate to existing hydrological models. In addition to defining structural and lithological boundaries as a function of depth, the logging program will attempt to establish hole-to-hole correlations to determine lateral stratigraphic variations in active hydrothermal systems, as well as direct correlations with discrete laboratory data. Also, sonic velocities and densities will provide the necessary information for the calculation of synthetic seismogram models, and a direct correlation with high resolution seismic data obtained prior to Leg 139.
To 169 Special Sampling Plan/Strategy