The principle objective of deepening Hole 735B, which may reach the lower crust/upper mantle boundary, is to determine the nature of magmatic, metamorphic, and tectonic processes occurring in the lower oceanic crust. During Leg 118, the first phase of drilling Hole 735B included compressional- and shear-wave velocity, resistivity, VSP, borehole televiewer (BHTV), and magnetic susceptibility logs. These were especially useful in delineating structural and stratigraphic features such as magmatic layering and fractures. The downhole measurement tools planned for use during Leg 176 include the full set of Schlumberger tool logs, the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR; German Geological Survey) magnetometer and a Schlumberger vertical seismic profiling tool. The set of Schlumberger logging tools will include the triple-combo with natural gamma-ray sonde (NGS), porosity (accelerator porosity sonde [APS]), density (hostile environment lithodensity sonde [HLDS]), resistivity (dual lateral log [DLL]), caliper and temperature (Lamont temperature tool [TLT]) probes, the Formation MicroScanner (FMS)/sonic combination, and the Prakla Seismos VSP tool.

Depending on time constraints and depth of penetration, the logging program will be divided into two or three parts. At the beginning of the leg, the Triple-combo will be used for obtaining a temperature profile, assessing borehole conditions, and determining possible variations in alteration. The FMS will provide information regarding the ellipticity of the borehole as well as high-resolution images of stratigraphic boundaries and structural features. The second and third logging runs will include the triple-combo, FMS/sonic, magnetometer, and VSP deployments. Pending funding, the sonic probe (digital array sonic tool [SDT]) commonly used in the FMS/Sonic combination will be substituted with the Dipole Shear Sonic Imager (DSI). The DSI will produce a full set of waveforms (P-, S-, and Stoneley waves); the shear-wave velocity and amplitude measured at different azimuths in the borehole may indicate preferred mineral and/or fracture orientations as well as paleostress directions. During Leg 176, the verification of depth/seismic ties by means of a VSP and synthetic seismograms will be essential for identifying deep crustal reflectors. The location and abundance of Fe-Ti oxide intervals will be determined by using the magnetometer.

A core-log integration program will be essential for the reorientation of cored samples recovered from Hole 735B. Correlation of digital core images obtained with a DMT Color Scanner and FMS images potentially will provide the opportunity for the reorientation of structural and stratigraphic features. The reorientation of the core from Hole 735B may prove to be essential for the determination of the lateral extent of structural features as well as the association of local strikes and dips with the regional tectonic environment. In addition, the FMS high-resolution images will vastly improve the determination of the fracture and alteration zone distribution in the crust and correlations between geophysical logs and discrete laboratory data will provide information regarding compositional variations with depth.

Depending on hole conditions and depth of penetration the approximate time estimates for the logging program during Leg 176 will be as follows:

No Hole ConditioningHole Conditioning
Time Estimate121.8 hrs (5.1 days)147.7 hours (6.2 days)

Special Operations
Pending future funding, drilling operations may include the testing and recording of drill-string pilot sensor data for seismic-while-drilling operations in Hole 735B. This technique allows the drill bit signal to travel up the drill string as axial waves, which can be detected by placing sensors on the top drive. These signals are considered to contain both the source signature for the experiment as well as drill pipe reverberations and other extraneous sources of noise. The acquisition and processing of these data may enable the extraction of axial vibrations generated by the bit which can be correlated with the energy radiating into the formation and received by a geophone (ocean bottom seismometers [OBS]) array on the seafloor around the ship. Cross-correlation of drill string data with the OBS signals may also be used to synchronize the waveforms produced at a specific time with depth of the bit in the hole. When merged with drill-string pilot sensor and depth data, seismic-while-drilling can produce a "real-time" zero offset VSP. These operations will require the deployment of eight ocean bottom seismometers.

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