GNGTS 2014 - Atti del 33° Convegno Nazionale

Set-up and acquisitions. In order to characterize the HZ in our site, we combined two different methodologies, ERT and DTS. The former has been widely applied for this kind of surveys in many different sites, commonly using cross-well configurations and/or superficial cables (Acworth and Dasey, 2003; Crook et al. , 2008). Even if all these typical deployments lead to the acquisition of excellent data, we attempted to improve them by positioning part of the 72 total electrodes used inside the hyporheic zone (Fig. 1): this was possible thanks to the horizontal directional drilling technology, which allowed the remote-guided drilling of a horizontal borehole under the Vermigliana riverbed. The result consists of an 80 m long perforation under the watercourse that permitted the placement of a multicore cable with 48 brass electrodes, spaced 1m, inside the HZ; these electrodes have a diameter of 30 mm and a length of 60 mm. The choice of brass electrodes is due to both economical and technical issues: on the one hand, brass is less expensive than graphite and stainless steel, but offers a lower resistance to water over time; on the other hand, this instrumentation will not be removed from the Vermigliana site and therefore an amortization of its high costs, with an usage in other case studies, will not be possible. To ensure a higher data reliability, we deployed also a superficial survey line of 24 stainless steel electrodes (12 on each river levee) spaced 1 m, in vertical correspondence to the horizontal borehole (Fig. 1). Given the need of creating a single superficial deployment perpendicular to the river and the availability of a non-waterproof multicore cable for superficial surveys, 4 take- outs allowed the connection of the 4 electrodes nearest to the watercourse to the cable itself, which was partly located over the bridge upstream. Each ERT survey is performed with an IRIS Syscal Pro resistivity meter using all 72 electrodes, with a “skip 0” dipole-dipole scheme and duration of circa 30 minutes. The acquisition of both direct and reciprocal measurements (i.e., exchanging current electrodes with Fig. 1 – Vermigliana creek site (1165 m above sea level). The red solid line represents the superficial ERT deployment on both levee surfaces, the red dotted line represents the ERT borehole deployment (under the Vermigliana creek, inside the hyporheic zone) and the blue dotted line represents the DTS fiber-optic under the watercourse. The DTS borehole is a few meters downstream from the ERT borehole and parallel to it. 130 GNGTS 2014 S essione 3.2