Time series analysis of underground temperature and evaluation of thermal properties in a test site of the Po plain (NW Italy)

A 3-year high-resolution monitoring of shallow subsurface temperature (unsaturated and saturated zones) was performed in the experimental site of Mezzi Po (Po River plain, NW Italy) to investigate the thermal behaviour of subsurface and the dependency with the atmospheric (air) temperature. The unsaturated zone of the test site is mainly constituted by sands and silts, and the unconfined aquifer below is hosted in coarse sediments (mainly sands and gravels) with shallow depth to water table (5 m b.g.l). The monitoring results allowed for identifying two main phenomena correlated with the propagation of the atmospheric heat through the subsurface. First, the amplitude of temperature fluctuation during the year, calculated as the difference between maximum and minimum temperature value, reduces with increasing depth, with a maximum range (31.1 °C) in air and a minimum (2.0 °C) in the saturated zone, at 7.00 m below ground level (b.g.l.). Furthermore, a lag of mean temperatures in the subsurface respect to air occurs. The delay is as much longer as the depth increases. Mainly, the time shift ranges between 1 week in the shallow soil (at 0.60 m b.g.l.) and 17 weeks in the saturated zone (7 m b.g.l.). These results are consistent with a 1-D heat diffusion model, through which the thermal subsoil properties were derived for 0.60–7 m depth interval. More specifically, thermal diffusivity is 8.12 × 10–⁷ m²/s, and thermal conductivity is 1.40 W/mK.