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This paper presents a device used to measure and register temperature for long-term subsoil measurements in boreholes. The borehole of this study is located in Gijón (Asturias, Spain). The measurements were made through two fixed sets of sensors coupled to the geothermal pipe, constituting two independent installations: (a) a commercial device called “Hobo”, which uses TMCx-HD-specific sensors based on resistors with variable resistance; and (b) a device built by this research group, which uses DS12B20 Maxim sensors, a bus 1-wire, and a recording device based on a conventional Arduino board. Temperature was registered every 5 min across several years. These measurements were used to thermally characterize the subsoil, determining the apparent thermal diffusivity, and to study the thermo-hydrogeology of the Lower Jurassic Gijón’s formation made of Liassic limestones and dolomites. This work is part of the Q-Thermie group’s research called “Shallow Thermal Energy”.

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Thermal conductivity is an essential property of the geothermal grout, which in turn is considered the most important element in a vertical closed-loop ground heat exchanger. The main aim of this work is to determine the thermal conductivity of four of the most commonly used geothermal grouting materials using a homemade apparatus, developed by the research team. The tests were also performed with a commercial thermal conductivity meter, the Shotherm QTM-F1. Both the homemade apparatus, called MCT, and the commercial thermal conductivity meter are based on the transient hot wire method. The components used to make the specimens are cement, silica sand, bentonite, water and superplasticizer. A laboratory mortar mixer was used to prepare all mixtures. Later, the samples were cured in a water bath for 7 days and finally the samples were dried at 60 °C for 24 h before testing. Each of the samples tested is formed of two solid rectangular prisms of the same material and of the same dimensions (180 mm × 70 mm × 50 mm). The obtained results with the MCT apparatus are similar to the data reported in the literature and to those obtained using the Shotherm QTM-F1thermal conductivity meter. The deviations of the values measured with the MCT apparatus are, in all cases, less than 10%.