173 Scientific Prospectus


Downhole measurements will provide an important contribution to the main objectives of Leg 173 because core recovery in the deepest sections and in basement is likely to be low.

The tectonic and petrologic objectives of this rifted-margin study should particularly benefit from logging data. Two standard logs (Triple Combo (resistivity/porosity/density/natural gamma) and Formation Microscanner (FMS/sonic/natural gamma-ray tools)) will be available onboard; they will be run at the standard speed at each site. The basement intervals may be logged several times with the FMS.

The main operational goal of the leg is to drill the basement. Determining the orientation of the structural planes, the fractures, and the faults in the basement should be achievable by interpreting the FMS images. The continuous FMS record will allow us to map the structural features in the basement and to precisely determine their azimuth and dips.

Two independent studies will attempt to predict the lithology of the borehole wall rocks from the log and core information using statistical and neural network techniques.

Core-log integration will be an important goal of this leg because of the likely poor recovery at great depth. Physical logging data, such as natural gamma ray and density, may be used to correlate core and log data. Moreover the FMS images will be used in collaboration with a CoreScan shipboard system, designed to image the surface of uncut whole round cores, to relate features seen on the FMS images to the cores, and to obtain the correct in situ azimuthal orientation of the cores and the structures within them. Therefore, this leg may provide an excellent opportunity to use core and log data integration as a means to reconstruct continuous, detailed records of lithologic variations and of structural features in the basement.

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