Thermal diffusivity and transient thermal response of the Sabatini volcanic succession: insights from the shallow geothermal field at the ENEA-Casaccia Research Centre (Roma, Italy)

Understanding transient heat transfer in heterogeneous volcanic formations improves the design of shallow geothermal energy systems (SGES). However, the thermal behaviour of stratified volcanic successions remains poorly constrained due to lithological variability, porosity contrasts, and groundwater driven heat transport. This study investigates the thermal diffusivity and thermal response of the Sabatini volcanic succession (Central Italy) using laboratory measurements, in situ thermal tests and Distributed Temperature Sensing (DTS) monitoring at the ENEA-Casaccia shallow geothermal field. Thermal conductivity (λ), bulk density (ρ), and specific heat capacity (c p ) were determined along a borehole and used to derive thermal diffusivity (α). DTS monitoring enabled characterised transient thermal evolution along the borehole. Thermal diffusivity values range from 4.2 × 10⁻⁷ to 7.1 × 10⁻⁷ m²/s. Highest values are associated with lava and sandy tuffs, whereas the lowest occur in fine-grained, clay-rich deposits. DTS monitoring during a 12 h thermal stimulation test revealed depth-dependent heterogeneity and contrasting cooling behaviours among lithologies. Saturated lava units showed rapid heat uptake and dissipation, while fine-grained tuffs and sandy silts retained thermal anomalies for up to 48 h, indicating higher thermal inertia and heat storage capacity. Results demonstrate that lithology, porosity and saturation control transient thermal behaviour in volcanic successions.