A NEW HIGH-PRECISION BOREHOLE-TEMPERATURE LOGGING SYSTEM USED AT GISP2, GREENLAND, AND TAYLOR DOME, ANTARCTICA

We describe a high-precision (0.1-1.0 mK) borehole-temperature (BT) lo gging system developed at the United States Geological Survey (USGS) f or use in remote polar regions. We discuss calibration, operational an d data-processing procedures, and present an analysis of the measureme nt errors. The system is modular to facilitate calibration procedures and field repairs. By interchanging logging cables and temperature sen sors, measurements can be made in either shallow air-filled boreholes or liquid-filled holes up to 7 km deep. Data can be acquired in either incremental or continuous-logging modes. The precision of data collec ted by the new logging system is high enough to detect and quantify va rious thermal effects at the milli-Kelvin level. To illustrate this ca pability, we present sample data from the 3 km deep borehole at GISP2, Greenland, and from a 130 m deep air-filled hole at Taylor Dome, Anta rctica. The precision of the processed GISP2 continuous temperature lo gs is 0.25-0.34 mK. while the accuracy is estimated to be 1.5 mK. The effects of fluid convection and the dissipation of the thermal disturb ance caused by drilling the borehoIe are clearly visible in the data. The precision of the incremental Taylor Dome measurements varies from 0.11 to 0.32 mK depending on the wind strength during the esperiments. With this precision, we found that temperature fluctuations and multi -hour trends in the BT measurements correlate well with atmospheric-pr essure changes.