The correct design of a system of borehole heat exchangers (BHEs) is the primary requirement for attaining high performance with geothermal heat pumps. The design procedure is based on a reliable estimate of ground thermal properties, which can be assessed by a Thermal Response Test (TRT). The TRT analysis is usually performed adopting the Infinite Line Source model and is based on a series of assumptions to which the experiment must comply, including stable initial ground temperatures and a constant heat transfer rate during the experiment. The present paper novelty is related to depth distributed temperature measurements in a series of TRT experiments. The approach is based on the use of special submersible sensors able to record their position inside the pipes. The focus is on the early period of BHE installation, when the grout cement filling the BHE is still chemically reacting, thus releasing extra heat. The comprehensive dataset presented here shows how grout hydration can affect the depth profile of the undisturbed ground temperature and how the temperature evolution in time and space can be used for assessing the correct recovery period for starting the TRT experiment and inferring information on grouting defects along the BHE depth.

<p>Atmospheric temperature variations are characterized by different frequency-modes including yearly and daily fluctuations as well as yearly average temperatures. In a ventilated cave, the thermal amplitude is attenuated with increasing distance from the upper and/or lower cave entrance. Convective heat flux for dry air is exchanged with cave wall and the heat transferred by conduction to the surrounding rock is attenuated within a certain distance. Here, we aim at determining the thermal attenuation length for the different modes along a cave and the surrounding rock. Distribution of amplitude of fluctuations along the cave is specified by using Fourier series expansion for dominant modes. In next steps, the effect of latent heat exchange at the cave wall due to evaporation or condensation will be studied. The main variables controlling thermal perturbations in ventilated caves are recognized and compared with field data by developing a numerical model based on the heat and mass transfer between the rock and the air. Moreover, our model aims to quantify the air and wall temperature profile along the conduit for a one-year. The model calculates water-vapor exchange rate along the cave showing the amount of consumed or produced water by evaporation or condensation along the cave and provides the rock temperature distribution in the surrounding hostrock. Our preliminary results contribute to a better understanding of the long-term cave dynamics and may support a quantitative interpretation of speleothem records.</p>

<p>The network of fractures and conduits crossing a karst massif drains water and air from the atmosphere deep into the massif, exchanging heat at the boundaries between rock, air and water. The thermal characteristic of the rock together with thermal processes including convection, evapo-condensation, radiation and conduction, concur to fix the cave’s temperature. The thermal length, the distance at which the external temperature fluctuations are damped, and the energy balance of the cave system depend on its geometry and the fluxes therein. Comprehensive knowledge of what modifies these thermal characteristics is of interest for e.g. low-enthalpy geothermal exploitation, mineralisation in water supplies and also for paleoclimatic studies on speleothems.</p> <p>In Longeaigue cave (Val-de-Travers, Jura mountains, CH), we deployed several sensors measuring airflow and temperature along the main conduit network. The cave is mainly dry and has a lower and upper entrance leading to an intense airflow controlled by the chimney effect. The temperature oscillations observed throughout the cave are chiefly related to external temperature and airflow variations. Results from 8 monitoring stations reveal that 90% of the energy brought in by the air during ventilated periods is exchanged within the first tens of meters from the cave entrances. However, temporary interruptions of the airflow occur during periods of flooding related to rainfall and snowmelt. This situation can take place several times per year. Our observations demonstrate that the transient nature of this airflow modifies the temperature signals in the cave, affecting the cave energy balance in a differentiated way according to seasonal hydrological conditions. With the increasing winter temperatures, we anticipate a progressive shift towards a summer ventilation regime enhanced by limited summer rainfall. A positive feedback is observed on the energy balance of the cave. It is therefore of crucial importance to consider the presence of subsurface ventilation for the thermal characterisation of karstic environments, which can modify the biochemical, physical and thermal characteristics of seeping water and, in turn, impact on the interaction with the encasing rock.</p>

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