ODP Technical Note 10


A. Introduction

The geothermal gradient is a fundamental property of the crust that influences and reflects the effects of tectonic, hydrologic, and geochemical processes, and accordingly temperature was among the first downhole properties to be measured during DSDP. In fact, Von Herzen and Maxwell (1964) actually measured sediment temperatures during drilling at the preliminary Mohole site, and a strong effort was made to measure downhole temperatures from the beginning of DSDP. Erickson et al. (1975) and Hyndman et al. (1987) reviewed the downhole-temperature measurements made during DSDP and the techniques for determining in-situ temperatures from the measured values.

Two primary methods are presently available in ODP to measure in-situ sediment temperatures - the water-sampling temperature probe (WSTP) and the APC coring-shoe temperature recorder (APC tool). Logging tools operated by LDEO and some independent investigators can also be used to measure temperatures in open boreholes. The use of the WSTP and APC tools in their primary mode, measuring sediment temperatures during the process of coring a hole, are described in detail in this chapter. In addition, the use of these tools to obtain profiles of borehole temperatures in open hole or of sediment temperatures in an uncored "hole" is discussed.

While the WSTP and APC tool can both be used to measure sediment temperatures, they differ in their application in three key ways: (1) The WSTP is used to measure temperatures between cores, and thus requires 1-3 hours for each measurement, whereas the APC tool is used during the process of cutting a hydraulic piston core, and only requires 10-20 minutes additional time; (2) during a measurement, the WSTP is pushed into uncored sediments and will disturb the section to be cored next, whereas the APC tool should not affect the quality of the next core; (3) the WSTP can be used to depths at which the sediments can be penetrated without fracturing, typically several hundred meters below the seafloor. In contrast, the APC tool can be used only to depths at which the force required on pull out is within safe limits (80k-100k lb overpull), typically 50-120 mbsf. Neither the WSTP nor the APC tool is effective at extremely shallow depths (<20 mbsf), where the sediments are not competent enough to hold the probes stable.

Temperatures measured in boreholes do not often reach full equilibrium with in-situ values and must usually be corrected for two major effects, the thermal disturbance to the borehole and surrounding formation by drilling and circulation (e.g., Bullard, 1947; Jaeger, 1961), and the transient approach of probe temperature to sediment temperature (Bullard, 1954). The latter condition is the result of either the decay of frictional heating of the probe on penetration into the sediments (if they are relatively cool) or the rise in probe temperature toward equilibrium (if the sediments are warm). Borehole disturbance is mentioned only briefly, as it usually affects open hole temperature logs measured shortly after drilling, which are not normally attempted using the ODP tools. The gradual approach of probe temperatures to in-situ values is clearly seen in data measured in sediments with the WSTP or APC tool, and the methods and shipboard computer programs required to extrapolate the measured temperatures to in-situ values are briefly summarized. More detailed treatments of these topics can be found in the WSTP and APC tool cookbooks available on board the JOIDES Resolution.

To In-Situ Temperature Measurements, Section B.
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