168 Prospectus


Meeting the major scientific objectives of Leg 168 will require quantitatively accurate documentation of rock properties and of lateral and vertical thermal, pressure, and geochemical gradients in pore waters of the sediments and upper basement. Elucidation of these properties and gradients will require the deployment of several specialized tools during Leg 168, as well as modifications to the standard ODP/JOIDES policies regarding whole round sampling and logging. These modifications are required to achieve the primary objectives of the expedition.

Sediment Temperature Measurements
Temperature measurements during Leg 168 will be made in the sediments overlying basaltic basement with three tools. The APC temperature tool will be deployed starting with Core 4 and continuing with every APC core until excessive overpull occurs at all sites where APC cores are collected. This includes four first priority Sites (Sites HT-1A, HT-2A, PP-1A, and PP-3A). The ODP water-sampling temperature probe or a third-party (Canadian/German) self-contained sediment temperature probe will be run for temperature measurements in all APC/XCB holes at depths where APC temperature measurements are not possible. These tools also will be run in all RCB holes, as often as every other core on occasion. Actual measurement frequency with all temperature tools will depend on local anticipated requirements or on results obtained during previous deployments.

Interstitial-Water Measurements
To document the nature, extent, direction, and intensity of fluid flow within the sediments and underlying basement, closely-spaced samples will be required. Interstitial water will be collected by squeezing 5-10-cm-long whole-round samples from every core. Higher resolution sampling (perhaps as frequent as one per section) will be required near the seafloor and immediately above the sediment/basement interface. Additional samples may be collected from split cores. This should result in the collection of approximately 200-250 pore water samples.

Physical Properties Samples
In order to document the physical nature of alteration and hydrologic properties within the sediment column and the extent of lateral variations in sediment properties, whole-round samples will be collected at several of the APC holes. Four to six samples will be collected at each of the Sites HT-1A, HT-2A, PP-1A, and PP-3A. These samples will be subjected to geotechnical and permeability testing, as well as XRD, index properties, grain-size, and SEM analysis to evaluate the relationships between hydrologic properties and alteration. Whole-round basement cores may also be required for short-term, nondestructive anelastic stress relaxation tests. If these measurements are done, the samples will be returned to the normal core handling routine within a few days.

Wireline Logging
JOIDES policy generally directs that a standard suite of logging instruments be run in all sediment holes deeper than 400 m and in basement holes with greater than 50 m of open hole. This standard suite includes a geophysical string, a Formation Microscanner (FMS) string, and a geochemical string. The Quad combination (Quad-combo) geophysical tool string provides measurements of natural gamma activity, sonic velocity, porosity, density, and electrical resistivity. Data from this tool string will aid in characterizing lithology, sediment fabric, degree of lithification, and diagenetic alteration and will aid in defining the location and distribution of possible fluid pathways. Together with core measurements, the sonic velocity and density logs can be used to generate synthetic seismograms, which can tie seismic information directly to the log and core data. The Formation Microscanner produces oriented, two-dimensional, high-resolution images of the variations in microresistivity around the borehole wall that can be used to distinguish thin beds, sedimentary structures, diagenetic features, and fractures. FMS images can also be used for correlation of coring and logging depth, orientation of cores, and location of cored sections where recovery is less than 100%. The Geochemical Logging Tool (GLT) provides bulk mineralogy and delineates mineralogical changes downhole. The GLT measures relative concentrations of Si, Ca, Fe, S, H, and Cl, as well as wet weight percentages of K, U, Th, and Al. Shore-based processing calculates dry-weight percentages of these major rock-forming elements as well as Gd and Ti.

Within the sediment column, the FMS and geochemical logging tools are expected to provide little information of direct benefit to the scientific objectives of Leg 168. For this reason, and because of time and operational limitations (such as the use of an 11-3/4" APC/XCB bit in one of the logged holes) logging within the holes at Sites PP-1A and PP 4A will most likely be restricted to geophysical tool strings. The FMS may also be run at Site PP-4A, depending on drilling conditions, the time required to core, and the time remaining for additional operations. Basement penetration may be limited to 40 m per site, with total open-hole length of only 30 m, essentially precluding the use of wireline logs. However, additional basement penetration and associated downhole measurements (including a full suite of Schlumberger logs and packer/flowmeter work), is included in the list of alternative operational plans. Options for additional basement work during Leg 168 will be considered along with options for additional sediment coring and sampling.

Long-Term Borehole Observatories (CORKs)
CORKs were deployed during ODP Legs 139, 146, and 156. The conceptual and physical design of the CORK experiment are detailed in Davis et al. (1992). The CORK system comprises a modified reentry cone, a hydrologic seal that fits inside the throat of the cone, a data logger with sufficient power and memory to record data for several years, a valve mechanism by which the sealed hole can be vented to the overlying ocean, and a sensor cable to monitor formation pressure and temperature as a function of depth. A continuous fluid sampler will also be installed as part of each sensor cable. The samplers will provide a time-series sample of basement water as the perturbations associated with drilling dissipate.

The CORK deployments during Leg 168 are intended to quantify accurately the temperatures and pressures in uppermost basement that are responsible for vertical and lateral fluid, heat, and solute transport and any resulting gradients in fluid geochemistry. The Leg 168 operations strategy was designed to maximize the chances for four CORK deployments during a single cruise.

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