A New, Small, Wireless Instrument to Determine Ground Thermal Conductivity In-Situ for Borehole Heat Exchanger Design

A small, light, wireless borehole probe has been developed and built which consists of pressure and temperature sensors and a mini-datalogger/programmed microprocessor in a closed metal tube, water-tight up to 100 bar. The probe (235mm long, 23mm dia, 99.8 g) sinks in completed but not yet working borehole heat exchangers (BHEs) through its own weight to the bottom of the BHE U-tube and records pressure (=depth) and temperature at pre-selected time intervals during descent. After completion of the logging the probe is flushed back to the surface by a small pump where the probe is connected to a laptop computer for data retrieval. The measurement run for a 300 m deep BHE takes less than 60 minutes. The wireless temperature logging has numerous advantages over cable-type logging. Our instrument has a temperature resolution of ± 0.003 °C. In the data processing the λ profile of the logged BHE is calculated, with a regional heat flow value at hand, from the temperature gradient along the BHE to be derived from the measured temperature log. Besides thermal conductivity determinations for the design of BHE arrays there are numerous other applications for the wireless probe: 1) lithological subdivision of the borehole profile, 2) data base for paleoclimatic studies, 3) identification of groundwater flow.