Measuring and analysing the underground temperature and heat flux changes for the local assessment of the shallow geothermal potential

Abstract

The surface temperature changes due to meteorological reasons induce downward propagating temperature signal in the soil. The two major components of the underground temperature signal are the daily and annual changes, but other long or short term components can also be present. While a temperature signal propagates downward, its amplitude is attenuating and its phase shifts is increasing. By continuously measuring the natural temperature and/or heat flux variations at different depths, we can derive relevant information about the medium's thermal property and some information about the heat transport processes. Such measurements can be considered as a thermal test method for shallow depths that is similar but not identical to the more frequently used active test measurements in shallow and deeper depths, employing known heat sources.
In this paper we discuss the shallow temperature and heat flux measurements and analysis as a potential tool for the local assessment of the shallow geothermal potential. We share some practical aspects of the application of the method. We notice the importance of the calculated apparent thermal diffusivity. We present our measurements done with common temperature sensors and with special high sensitivity heat flux sensors. We processed the temperature and heat flux recordings with different methods to find the phase shift of daily and annual signals measured at different depths and then to get the apparent thermal diffusivity for the sections between the depths. Through a presented example we show the results got from a shallow temperate and a deeper heat flux measurements from the same station. Finally we make suggestion to include the intensity of the atmosphere-ground thermal interactions into the value of shallow geothermal potential.