Determining thermal rock properties of soils in Canterbury, New Zealand: Comparisons between long-term in-situ temperature profiles and divided bar measurements

This paper presents ground temperature data from a 9.1 m deep borehole located at Lincoln, Canterbury, New Zealand. Thermal properties of the soil are inferred from three years of in-situ temperature measurements, and compared to cored samples later measured in the laboratory using a divided bar thermal properties analysing system. The data show that seasonal changes affect ground temperatures to depths of approximately 7.5 m, beyond which the temperature is constant (±0.2°) year-round. This stable ground temperature is determined to be 11.8 °C, which is equivalent to the average ambient air temperature of the area, recorded at the adjoining weather station. Rain is seen to disturb ground temperatures immediately after the event, affecting depths up to 0.5 m by several degrees depending on the volume of water and ambient air temperature. Thermal diffusivity estimated from in-situ observation suggests soils in the top 9 m at Lincoln range from 3.4 × 10−7 (in shallower soils) to 10.6 × 10−7 m2s−1 (deeper ground). Laboratory measurements measure the diffusivity ranging from 3.8 × 10−7 to 7.9 × 10−7 m2s−1. These values agree with the ranges measured from in situ temperature measurements. The determination of thermal properties and ground temperatures is important for the development and potential utilisation of the low enthalpy geothermal resources.