GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 3.3 GNGTS 2024 Modelling temperature efect in tme-lapse DC monitoring experiments through inversion of thermal difusivity A. Signora 1 , G. Fiandaca 1 1 The EEM Team for Hydro & eXploraton Dep. Of Earth Sciences A. Desio, Università degli Studi di Milano, Via Botcelli 23, Milano (Italy) 1. Motvaton In the recent years, tme-lapse surveys have been performed widely to monitor, for instance, hydrogeological tracer experiments (Cassiani et al., 2006), groundwater watershed characterizaton (Miller et al., 2008; Deiana et al., 2018), seasonal variatons (Hiblich et al., 2011; Musgrave and Binley 2011), landslide behaviour and evoluton (Cassiani et al., 2009, Wilkinson et al., 2010), and so on. One of the main concerns, when resistvity surveys are performed, is to be sure to impute the variatons to the right phenomena, distnguishing the electrical changes of interest from all the others, which are assumable as noise. Temperature variatons might represent the main noise source in the tme-lapse conductvity surveys since temperature has a strong impact on the resistvity parameters, hence the inversion results. For example, seasonal temperature trends could mask the conductvity variatons, and thus lead to misleading interpretatons, up to the depths from the surface that can be reached by external fuctuatons. Haley et al. (2007; 2009; 2010) have pointed out the importance of considering the temperature variatons in tme-lapse geoelectrical surveys, including in the inversion procedure a correcton for this efect. In this study we intend to disentangle the temperature efect from resistvity variatons invertng for the thermal difusivity of the medium in a simultaneous tme-lapse inversion that does not require direct temperature measurements below ground, both on a synthetc dataset and on-feld experiments. 2. Inversion scheme The temperature efect on electrical resistvity is modelled through the equaton proposed by Haley (2007): (1) where: i) σ T and T are efectve electrical conductvity and temperature. i) σ 25 is the reference conductvity of the material at 25°degrees. ii) T 25 is the conventonal temperature of 25 °C. iii) m is the fractonal change in electrical conductvity per degree Celsius. The temperature is defned in the entre medium solving for the het equaton: (2) σ T = (1 + m ( T − T 25 )) σ 25 ∂ T ∂ t = k ∂ 2 T ∂ z 2