Three wireline tool string combinations will be available for deployment during Leg 174A: the Triple-Combo, FMS-Sonic, and the WST (Well-Seismic Tool). The measurements obtained with each of these tool strings are as follows.
Well-Seismic Tool (WST)
The main goals of wireline logging are to provide acoustic data, to provide in situ physical properties, and to complement lithologic characterization of the drilled section. For these purposes, we will attempt to acquire a full suite of both wireline and LWD measurements at a minimum of one site (probably Site MAT-8B). Cross-correlation of LWD measurements with wireline sonic data will provide empirical transforms of sonic velocity from LWD data at the other site (Site MAT-9B). Calibration of the acoustic traveltime will be performed at both sites with a check-shot survey/VSP using the WST.
The WST is a single-component geophone that is clamped to the borehole wall. Shots fired from the surface are recorded downhole, providing traveltime vs. borehole depth information for accurate calibration of drilling data to seismic reflection profiles. For these VSPs, we plan to deploy two acoustic sources, an air-gun and a GI-gun (a modified airgun, the same source used in the high-resolution Oceanus 270 seismic surveys over the proposed sites), shooting each alternately, and to clamp the WST at 50-m intervals or less. The traveltime versus depth curve at each site will serve to calibrate the interval velocities measured with the WST, or estimated through log transforms. Synthetic seismograms can then be generated using calibrated velocity and bulk density measurements.
FMS images provide a high-vertical resolution (~1 cm), azimuthally-oriented picture of the borehole wall that can be important for interpreting sedimentary and structural features (bed thicknesses, grading, cross-bedding, faults, and fractures). Where both FMS images and cores are available, the image interpretation can be calibrated to the lithology, thus providing an important complement for geologic characterization where cores are not available due to incomplete recovery.
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