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Conclusions and future plans

A third-generation temperature tool, the APCT-3, has been developed for use during scientific ocean drilling when piston coring. The design of this new tool builds on the prior experience with previous temperature tools, most recently the APCT and the MTL. Consequently, the prototype described here proved to be very mature and only minor modifications are anticipated for the final production version, which should be available in the summer or fall of 2006. Field tests during Expedition 311 and calibration tests prior to the expedition demonstrate that the APCT-3 is a robust and reliable tool, with high and predictable accuracy. Even with only routine calibration, the absolute accuracy of the APCT-3 is better than 0.05C, surpassing the accuracy of data measured with the previous APCT tool. Because of the excellent signal to noise ratio of the APCT-3 tool, it will be possible to achieve an absolute accuracy that is on the order of its temperature resolution, which is in the range of 0.5–2.5 mK, depending on the temperature. In addition to improved accuracy, the tool provides an order of magnitude increase in the possible sampling rate. Whereas the increased temperature resolution is necessary to resolve the small temperature changes at the end of the decay of frictional heat, the APCT-3's short sampling interval of 1 s is needed to capture the rapid temperature changes that occur while the coring shoe is driven into the formation.

Unfortunately, the order of magnitude increase in resolution and accuracy of the temperature data does not necessarily lead to significant improvement in estimates of undisturbed formation temperature. These estimates depend, in large part, on the algorithm that is used to extrapolate the observed transient temperature decay to steady state. The most important deficiency of the currently used TFit algorithm is the simplified nature of the one-dimensional analytical model used to calculate the reference curves to which the measured data are compared. Considerable modeling and analysis of the tool's thermal response is under way. This information will be used to evaluate uncertainties in extrapolated temperatures and to improve standard processing procedures. Since all tools used to measure formation temperature or pore pressure generate a similar transient disturbance, we hope that the processing procedures of other tools will benefit from the analysis of the APCT-3 data. This will be a valuable contribution because the extrapolation of the decay in temperature or pressure to undisturbed formation state needs standardization to allow comparison of data sets (Flemings et al., 2004).