GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 431 the head 30 ° to investigate the riverbanks up to hydrographic zero. The vessel positioning was supplied in real-time by an Applanix Position and Attitude System (POS/MV 320 V5) using RTK corrections received by a GPS master base-station belonging to the GNSS National Dynamic Network (http://www.igmi.org/rdn/) and processing kinematic (PPK) techniques for very highly accurate positioning of the navigation lines. Multibeam data were processed using Caris Hips & Sips 9.1 to generate a high-resolution Digital Elevation Model (DEM) with a 0.2 m cell size. The high-resolution (HR) seismic (sub-bottom) profiling was performed using a Benthos Chirp III source with a sweep between 2 and 7 kHz and a single-channel zero-offset configuration (Bosman and Orlando, 2017). GNSS positioning of the single-channel seismic was conducted in RTK mode with centimetric accuracy. Raw data were processed through Geo Suite All Works software, and we set a value of 1470 m/s for the time-depth conversion. The electrical resistivity tomography (ERT) profiles were acquired using the IRIS Instruments Syscal Pro resistivimeter with 48 stainless steel electrodes spaced 5 or 10mapart, depending on the external limitations. Multiple gradient array is employed for ERT acquisition, as it combines good signal strength in extreme conductive environments with good resolution and depth of investigation. The ERT apparent resistivity dataset was inverted using the VEMI algorithm (De Donno and Cardarelli, 2017), where the 2.5D forward solution is achieved using a finite element approach with quadrangular elements, while a Gauss-Newton iterative procedure is used for data inversion. Self-potential (SP) data were acquired with an offset of 180 m to the first electrode of the L4 line, by a string of 10 non-polarizable electrodes (Cu/CuSO 4 ), spaced 5 m apart, rolled along the investigatedprofilebyoverlapping anelectrodepair for eachbaseline. At each station, a small hole (~10 cm deep) was dug to improve the electrical contact between the electrode and the ground. The electrodes are connected to the Syscal Pro via the same equipment used for ERT survey. For each line, we performed from 5 to 10 repetitions every 2 minutes starting from 20 minutes after the electrode plug-in , to check the robustness and consistency of the measurements. SP data were filtered for outliers and inverted using the SP2DINV software (Ahmed et al. , 2013), achieving a current density model directly related to the underground sources. For ambient noise recordings, we used a three-component Sara seismic sensor with a natural period of 5 s; the duration of the seismic noise records was set to 45 minutes Fig. 1 - (a) Aerial plan of the surveyed areas and map of the geophysical investigations with available boreholes after Sella et al. (2014) and anomalous gas emission points after Bigi et al. (2014). FGV: Fiumicino Gas Vent. (b) Detail of the area within the white rectangle in Fig. 1a, with high-resolution multibeam bathymetry.