GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.3 GNGTS 2024 Where, as depicted in Fig. 1, the temperature at the botom of the model is considered constant, while the surface temperature varies seasonally. For any given thermal difusivity , the temperature can be estmated through the numerical soluton of eq. 2 at each tme instant and each depth. Time-lapse 2D DC data are then inverted simultaneously in EEMverter (Fiandaca et al., 2024), using as model space the electrical resistvity at 25 °C in each inversion cell of all tme-lapse models and a unique thermal difusivity value for the entre 2D profle. The sensitvity on the thermal difusivity is retrieved enforcing tme-lapse constraints between all tme-lapse models, which favour the inversion models that minimize variatons through the correct evaluaton of the temperature efect. The asymmetric generalized minimum gradient support (AGMS) introduced by Fiandaca et al. (2015) has been used for tme-lapse constraints. Figure 1. Schematzaton of the heat fux equaton solved for the model discretzaton. Surface and botom temperatures of the model need to be fxed as boundary conditon within this inversion scheme. 3. Synthetc simulaton and feldwork case study Synthetc simulatons have been performed mimicking the seasonal variability of subsoil temperature caused by a homogeneous thermal difusivity, constant temperature at depth and seasonal-varying surface temperature. On top of the temperature-induced changes in resistvity, a growing plume had been modelled, with 75-100 Ωm 25 °C resistvity in a homogeneous 150 Ωm halfspace. Nine tme steps have been modelled, with 50 days of tme diference, and all tme-steps have been inverted simultaneously with AGMS tme-lapse constraints. Fig. 2 presents the comparison of the models obtained with: i) the standard resistvity inversion; ii) the simultaneous tme-lapse inversion with temperature correcton. Not only the tme-lapse inversion retrieves the correct resistvity distributon, not altered by the temperature efect, but the simulated thermal difusivity is retrieved correctly by the inversion. As feld example, a 730 days-long real experiment is analyzed. The temperature efect is clearly visible in data space, with apparent resistvity variatons clearly correlated with surface temperature (Fig. 3). The survey has been carried out contnuously on the roofop of an MSW landfll, with stainless-steel electrodes whose positon never changed for the whole duraton of the study. The daily ERT (Electrical Resistvity Tomography) acquisitons were performed through 18 electrodes, spaced 5 meters from each other, with the Wenner-Alpha array confguraton and 5 mA alternate currents injected with the frequency of 5 Hz. The result of this survey clearly shows a k k