LOGGING PLAN
Downhole tools deployed during Leg 183 will enable accurate mapping of the volcanostratigraphy, volcanic facies variations, and structural features, as well as interpretation of tectonic stresses and correlation between core data and regional seismic reflection profiles. The downhole measurement tools planned for use at all sites during Leg 183 include the full set of Schlumberger logging tools: the triple combo with natural gamma-ray sonde (NGS); porosity (accelerator porosity sonde [APS]), density (hostile environment lithodensity sonde [HLDS]), resistivity (dual induction tool [DIT-E]), caliper and temperature (Lamont temperature tool [TLT]) probes; the Formation MicroScanner (FMS)/sonic (dipole shear sonic imager [DSI]) combination; and the Dual LateroLog (DLL)/NGS combination. Pending funding, the well seismic tool (WST) will be run at all sites.
The geophysical tool string (triple-combo) provides measurements of the porosity-dependent density, porosity, velocity, and resistivity logs, in addition to variations in natural gamma radiation. These logs are useful for determining petrophysical and lithologic variations in both volcanic and sedimentary intervals (e.g., Broglia and Moos, 1988; Planke, 1994).
The FMS provides a detailed resistivity image of the borehole wall. It has previously been used successfully in a volcanic setting on Leg 152 (Cambray, 1998; Planke and Cambray, 1998). The log provides a detailed volcano stratigraphy and is particularly useful to image altered, fractured, and highly vesiculated zones that may be poorly sampled during coring. It also has the potential to provide important structural and stress field information and can be used for core-log integration.
Seismic reflection data are essential to extrapolate the drilling results away from and between the drill sites. The dipole shear sonic imager (DSI) will produce a full set of waveforms (P-,S-,and Stoneley-waves). Synthetic seismograms constructed from the compressional velocity and density logs provide a link between the core and the reflection data. For accurate time-depth conversions, synthetic seismograms require calibration with downhole seismic data. These calibration data will be obtained by the check-shot survey (WST). The shear-wave velocity provides information in both the relatively slow lava flow margins and relatively fast flow interiors. Such data are important as inputs for modeling seismic wave-propagation in volcanic sequences.
Approximate time estimates for logging operations (hole conditioning not included) during Leg 183 are 148.8 hr (6.2 days), including use of the WST.