GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 3.2 433 presence of gas upwelling flows if compared with L1 and L3 model, where this effect is not visible. In Figure 2c, we present the results achieved at the “Capo due Rami” site, where the presence of gas vents was well documented. The electrical layering, already seen on L2 (Fig. 2b) is confirmed also for the L4 line, with the location of the unconformity surface in agreement with the A’-B’ seismic sub-bottom profile. We detect 6 anomalous zones (black arrows), likely due to gas upwelling flows, of which the most significant are located between 280-310, 380- 410 and 460-480 m on the surface. This evidence is confirmed by the SP model that shows the maximum values of current density around the three main anomalies (Fig. 2c), and by previous measurements of gas concentration (Bigi et al., 2014). The H/V spectra (Fig. 3) exhibit only moderate amplification effects, where the main peak is located at 1.8 Hz for HV1-HV3. Given the depth of the unconformity surface from the seismic reflection, we can estimate the shear wave velocity of the overlying sediments, under the hypothesis of a 1-D simple model, between 300 and 370 m/s (Bard et al. , 2004). For the last stations (HV5-7) a secondary peak at 4.5-5 Hz becomes prevalent and for HV5 a clear effect of directionality is visible. Although this effect was already observed in gas emission areas as well as in fault systems (e.g. Pischiutta et al. , 2013), the investigation of the structural causes of this effect is beyond the scope of this work. Conclusions. This work demonstrated the diagnostic potential of integrating active (HR sub-bottom seismic profiler, multibeam bathymetry and ERT) and passive (SP and HVSR) geophysical data for imaging natural gas emissions at the Fiumicino coastal area. The seismic sub-bottom profile in the Tiber River, together with the multibeam bathymetry, give a HR image of the shallow subsoil (DOI ~60 m), locating the unconformity surface between Pleistocene/Holocene clayey sediments and PGS gravels and highlighting several blank zones likely associated with the gas emissions. Through the combination of ERT (DOI ~80 m) and SP (DOI ~20 m) methods on a selected site, we reconstructed a three-layer model, where local increases in resistivity in the middle clayey layer are related to upwelling gas flows from the underlying gravel layer (gas reservoir). The analysis of ambient noise recordings allows a first approximation estimate of shear wave velocity (given the depth of the unconformity surface), while a clear directional effect is seen nearby the main anomaly due to the gas emissions. Fig. 3 - Directionality of ambient noise recording for HV1 (a), HV5 (b) and HV7 (c) stations (for the location of stations see Fig. 1b and 2c).