Recording downhole geochemical and physical properties data during Leg 185 is essential to filling recovery gaps in both sediment and basement sections, as well as enabling site-to-site comparisons of the geochemical signatures of the drilled sequences.
Measurements in Hole 801C
Downhole measurements were conducted in the upper 100 m of basement during Leg 144 to begin the characterization of typical old oceanic crust generated at a fast-spreading rate (Larson et al., 1993). The most surprising result from Leg 144 downhole measurements was the extremely high permeability measured below 501 mbsf in a hydrothermal alteration zone. This zone appears to act as an aquifer, an argument supported with the apparent bulk porosity profile. Below the hydrothermal zone and within the tholeiitic basalts, the logs begin to approximate more expected values for old oceanic crust. To further characterize the petrology, hydrogeology, structure, and physical properties of the old oceanic crust, the hole will be logged with the triple combo, Formation MicroScanner (FMS)/Sonic, and Geologic High-resolution Magnetometer Tool (GHMT) tool strings. The triple combo consists of the HNGS (Hostile Environment Natural Gamma Sonde), APS (Accelerator Porosity Sonde), HLDS (Hostile Environment Litho-Density Sonde) and DITE (Dual Induction Tool) with the Lamont Temperature Logging Tool (TLT) attached to the bottom of the string. The FMS/Sonic string will contain a standard four pad FMS tool combined with a DSI (Dipole Shear Sonic Imager) sonic tool. Lastly, to satisfy the time-scale objective (i.e., to determine the age of the basement), the magnetic susceptibility and total magnetic field measurements obtained by the GHMT will be deployed to provide a paleomagnetic reversal sequence of the overlying sediment.The anticipated logged interval will be from casing at 481 mbsf to the new total depth expected to be about 850 mbsf.
In addition, if enough time is available, a packer experiment will be completed for the interval below the hydrothermal deposit drilled on Leg 129 at 501 mbsf and the bottom of the newly drilled basement section. This experiment will complement results on the permeability of the basement obtained on Leg 144 (Larson et al., 1993), and may provide important constraints on the alteration process and fluid flow in old oceanic crust.
Measurements at Site BON-8A
The oceanic crust subducted in the Izu-Bonin Trench has never been sampled or logged. Because determining the geochemical budgets in sediment and basement columns is central to the objectives of Leg 185, geochemical logging tool (GLT) will be extremely valuable. Geochemical logging on Leg 129 served as an excellent proxy for actual recovery of sediments similar to those expected at Site BON-8A (Fisher et al., 1992).
To compare the sedimentary sequence and the upper oceanic crust at Site BON-8A with those in Hole 801C, the triple combo, FMS/Sonic, and GLT will be deployed. The GLT is typically combined with an AACT (Aluminum Activation Tool), which requires a Californium 252 source.
Logging at Site BON-8A will occur in two stages to optimize data quality and logged depth. The first logging operation will be completed through the APC/XCB bottom-hole assembly (BHA) following the cessation of drilling near the sediment/basement contact. The second logging program will be completed through the RCB BHA after completion of the hole to basement. The standard tool-string deployment sequence is triple combo, FMS/Sonic, GLT, and GHMT.
To 185 Sampling Strategy
To 185 Table of Contents