High-resolution (plus or minus (±) 0.1°C) temperature measurements were accomplished with a platinum resistance temperature (PRT) probe. The PRT probe, suspended from an armored logging cable, was lowered into the well over a sheave with a portable hand-cranked reel. Attached to the sheave were both mechanical and digital depth counters. The PRT output and digital depth counter were connected to an electronic data logger programmed to display and record the temperature and depth values at 5-second intervals.
The temperature logs were conducted from the top of the water column downward at 10 foot increments so that the established thermal equilibrium of the water column was least disturbed by logging. Once the target depth was reached the PRT probe was allowed to equilibrate for approximately 2 minutes. If after approximately 2 minutes the displayed temperature was within 0.1°C, the probe was lowered to the next 10-foot increment. Otherwise, logging was continued until the displayed temperature was stable within 0.1°C. The recorded data file was post-processed for each 10-foot increment measuring point by averaging the last 10 recorded temperature values, or 50 seconds of data collected. Periodic calibration checks of the PRT probe were performed in a temperature-controlled water bath using an ASTM certified mercury thermometer in a USGS laboratory in Menlo Park, California. On the basis of these calibration checks no corrections were applied to the recorded water temperature logs, which are assumed to have a temperature resolution of ± 0.1°C and a depth resolution conservatively estimated at ± 1 foot. Depth measurements were referenced to the top of the wellhead, located at 1103 meters above sea level at the time of drilling in 1973. In March 2019, no measurements could be made below a depth of 1790 feet.