GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.2 541 Distortions on the temperature depth profiles of wells P8 and P10 can be also due to lateral changes in underground thermal properties. Unfortunately, no detailed stratigraphicinformationisavailable for these wells. Therefore, in order to understand the stratigraphic setting of the subsoil, a passive seismic survey was performed. Microtremor measurements were carried out with a portable digital tomographer, equipped with three velocimeters orthogonal, recording in the 0.1 to 1024Hz band. The noise was acquired at 128 Hz sampling frequency, amplified, digitized to 24-bit equivalents and recorded for 20 minutes at each of the measurement sites. ���� ���� �������� ���� ��� �������� ������ Data were analyzed with the Micromed Grilla software and fitted according to the 1-D layered soil theory (Castellaro and Mulargia, 2009). The seismostratigraphic reconstruction obtained by assuming a shear-wave velocity at the top of 200 m s -1 taking into account the sediment compaction according to Ibs-Von Seht and Wohlenberg (1999) (Fig. 3) The results show the presence of a continuous reflector, all along a cross-section running from P8 to P10. The reflector lies at about 20 m from ground level. Beneath this level, the passive seismic surveys point to a rather homogeneous stratigraphy, likely consisting of sandy succession, arguing that the temperature profile distortions are due to advective perturbations. Quantitative analysis of thermal data. The analytical procedure of thermal profile analysis consists in matching the experimental measurements of temperature with analytical models of advective heat transfer (see for details Verdoya et al. , 2008). For a steady-state thermal and groundwater regime, in permeable horizons with homogeneous thermal and hydraulic properties, the variation of temperature T with depth z can be expressed by Eq. 1, where ρ w and c w are the density and specific heat, k is the thermal conductivity and v x and v z , supposed to be constant, are the horizontal (positive for a cooling flow) and vertical (positive downward) Darcy velocities, respectively. (1) Assuming � 2 T / � z 2 >> � 2 T / � x 2 and a linear horizontal variation temperature gradient Γ x , Eq. 1 simplifies as (2) Far from recharge and discharge areas, the vertical component of velocity can be considered negligible. Therefore, by integrating twice the Eq. 2 with boundary conditions (� T / � z ) z =0 = Γ z 0 and T = T L for z = L , one obtains the following solution (3) where α = ρ w c w v x L /λ, L is the thickness of the aquifer investigated, T L the temperature at the base of the portion of the aquifer and Γ zo the vertical thermal gradient at the depth z = 0 . Eq. Fig. 3 – Seismo-stratigraphic model: the colors indicate impedance contrasts HV1-HV6 are measurement sites, P8 and P10 wells.