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Downhole logging

Downhole logs are used to determine physical, chemical, and structural properties of the formation penetrated by a borehole. The data are rapidly collected, continuous with depth, and measured in situ. The sampling (vertical sampling intervals ranging from 2.5 mm to 15 cm) is intermediate between laboratory measurements on core samples and geophysical surveys. Downhole logs are useful in calibrating the interpretation of geophysical survey data (e.g., through the use of synthetic seismograms) and provide a necessary link for the integrated understanding of physical properties on all scales.

Downhole logs can be interpreted in terms of the stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is incomplete or disturbed, log data may help to characterize the borehole section. Where core recovery is good, log and core data complement one another and may be interpreted jointly.

Downhole logs also provide information on the condition, shape, and size of the borehole and on possible deformations induced by drilling or formation stress.

Unfortunately, logging attempts were aborted at the first site because of technical problems with the logging tool, and time did not become available for logging during the remainder of the cruise. For detailed descriptions of the logging methodologies, see other IODP Proceedings volumes.

In situ temperature measurements

In situ temperature measurements were made with the advanced piston corer temperature tool (APCT-3) in Holes U1405A and U1408A. The APCT-3 fits directly into the coring shoe of the APC and consists of a battery pack, data logger, and platinum resistance-temperature device calibrated over a temperature range from 0° to 30°C. Before entering the borehole, the tool is first stopped at the mudline for 5 min to thermally equilibrate with bottom water and give a bottom water temperature. After the APC penetrated the sediment, it was held in place for 10 min as the APCT-3 recorded the temperature of the cutting shoe every second. When the APC is plunged into the formation, frictional heating creates an instantaneous rise in the temperature profile. This heat gradually dissipates into the surrounding sediments as the temperature at the APCT-3 equilibrates toward the temperature of the sediments.

The equilibrium temperature of the sediments was estimated by applying a mathematical heat-conduction model to the temperature decay record (Horai and Von Herzen, 1985). The synthetic thermal decay curve for the APCT-3 is a function of the geometry and thermal properties of the probe and the sediment (Bullard, 1954; Horai and Von Herzen, 1985). The equilibrium temperature must be estimated by applying a fitting procedure (TPFIT by Martin Heesemann, 2006). However, when ship heave pulls the APC up from full penetration, the temperature equilibration curve will be disturbed and temperature determination is more difficult. The nominal accuracy of the APCT-3 temperature measurements is ±0.05°C with a total error of 0.1°–0.5°C.

The APCT-3 temperature data were combined with measurements of thermal conductivity (see “Physical properties”) obtained from whole-core samples to obtain heat flow values. Heat flow was calculated according to the Bullard method, to be consistent with the synthesis of ODP heat flow data by Pribnow et al. (2000).