Downhole Temperature

On October 8 during a 30 hour break in drilling, downhole temperatures were measured using a thermistor and bridge circuit on loan from the Physics Department, VUW.

Seven temperature readings at known vertical spacing were obtained in the hole plus two further readings in the water column above.

The thermistor was calibrated prior to the field season with cable and bridge held at room temperature (about 15°C] and only the thermistor temperature lowered. Using this calibration a temperature of −3.3°C was obtained for the seawater above the hole. As it is known that the water temperature in McMurdo Sound is remarkably constant at −1.8°C it appears that either the instrument had drifted since calibration or the bridge circuit was affected by low ambient temperatures. A re-calibration was made on site using Mercury thermometers and a commercial thermistor temperature probe. The new calibration curve was approximately parallel to the old but gave a temperature for seawater closer to the expected value of −1.8°C.

Downhole temperatures derived from both calibrations are plotted against depth in Fig. 5. Absolute values of temperature are known to be in error but the estimate of temperature gradient is not thought to be badly affected. The gradient, 35°C/km, is slightly higher than the value of 31°C/km obtained at DVDP 15.

Figure 5 Downhole temperatures from MSSTS 1.M.