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doi:10.2204/iodp.sp.318.2008

Downhole measurements

Downhole measurements during Expedition 318 will focus on characterizing in situ formation properties and establishing the link between core, log, and seismic data.

Wireline logging

Wireline logging is planned for the deepest hole at each of the five sites of Expedition 318. Standard IODP tool string configurations will be deployed at each site. Details of the tool strings are available at iodp.ldeo.columbia.edu/TOOLS_LABS/tools.html. The first run will be the triple combination (triple combo) tool string, which logs formation resistivity, density, porosity, natural gamma radiation, and borehole diameter. If available, we intend to include the Schlumberger inline check shot tool on this first run. The second run will utilize the Formation MicroScanner (FMS)-sonic tool string, which provides an oriented 360° resistivity image of the borehole wall and logs of formation acoustic velocity, natural gamma radiation, and borehole diameter. At all sites except proposed Site ADEL-01B, an additional logging run will conduct a check shot survey that will require the use of a seismic sound source.

Downhole logging data provide the link between the borehole and the seismic section, and enable lithostratigraphy to be tied to seismic stratigraphy. The check shot surveys give depth to traveltime conversion, and sonic velocity and density data will be used to generate a synthetic seismic profile at each site.

Downhole log data provide the only in situ formation characterization and the only data where core recovery is incomplete, as is likely to be the case at the two Wilkes Land shelf sites (proposed Sites WLSHE-09B and WLSHE-08A). For example, individual clasts in diamict will be apparent in the FMS resistivity images, and silica-cemented layers will be clear in the resistivity and density logs.

The wireline logging plan may be modified in the following cases:

1. To reduce the risk of incomplete logging resulting from bad hole conditions in the deep-penetration sites (proposed Sites WLRIS-02A and WLRIS-04A), we may elect to log in two stages: the upper 500 m of the section in the first APC/XCB hole and the lower part in the second RCB hole. This, however, would only be practical if extra time became available.
2. The seismic sound source used during the check shot survey will be subject to the IODP marine mammal policy and may have to be postponed or cancelled if certain policy conditions are not met.

Formation temperature measurements

The downhole measurement plan includes reconnaissance temperature measurements at one hole per site on the continental rise and the abyssal plain (proposed Sites WLRIS-02A and WLRIS- 04A) principally using the advanced piston corer temperature tool, supplemented by the Davis-Villinger Temperature Probe or new SET tool (if available) if necessary where sediments are more consolidated. The scientific objective of the temperature measurement plan is to provide sufficient data to reconstruct the thermal gradient at each site. This information will help constrain the history of burial diagenesis of the sediments encountered.

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