GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.2 GNGTS 2023 acquired with a vibrating seismic source and with an unconventional layout which allows a meaningful integration of seismic reflection with refraction tomography. The seismic profile is complemented by gravimetric, magnetic and electrical resistivity profiles overlapping the seismic data, as well as CO 2 exhalation flux and soil concentration and self-potential measurements. The 2-D ERT resistivity section shows a very high resistive uppermost layer, about 50 m thick to the West which drastically thins toward the east. Below this layer, high and lower resistivity layers are embedded down to a depth of about 150 m. These electric properties overlap with prominent shallow features in the preliminary seismic reflection image which shows unexpectedly high quality, considering the harsh environment where it has been obtained. A clear, strong low-amplitude reflector, with complex morphology can be followed with continuity along the entire profile at an average two-way-time of about 300 ms (i.e. 200-250 m from the surface). This reflector can be interpreted as the seismic signature of the buried Caldera related to the Avellino Plinian Eruption (Cioni et al., 1999), for which a large debate exists in terms of the exact position within the Mt. Vesuvius edifice. The magnetic measurements were performed on the same line of the other surveys. Measures have been taken with a spacing of 4.0 m with the sensor placed at an elevation of 2.0 m above the ground. The magnetometric data have been filtered in order to highlight features at longer wavelengths. The resulting magnetic section shows anomalies suggesting lateral variations in the layout of volcanic rocks. The horizontal-to-vertical spectral ratio (HVSR) of ambient noise measurement was used to estimate a site’s resonance frequency (f0). For this purpose, 18 measurements were made using a single station (Nanometrics Trillium Compact Horizon with 20 s bandwidth). The stations recorded the signals for a duration between 7 e 19 hours with a sampling frequency of 200 Hz along 3 lines. The computation of the H/V rations was made by means of the Geopsy tool (www.geopsy.org ) in which we chose 110-s-long windows and an anti-trigger algorithm STA/LTA to remove the transient. In 11 sites, the peak of fundamental frequency was recognized, showing a clear pattern with frequency and amplitude respectively increasing and decreasing moving down the southwestern flank of the volcano The average CO 2 flux from the soil reveals values lower than those commonly considered as the threshold to define background emissions in the typical range of vegetated soil respiration. Soil gas data highlight the presence of an area characterized by higher CO 2 flux and concentration values (between 850-110ms), in agreement with SP values indicating the rising of fluids, suggesting that the enriched gas emissions present in that sector are controlled by tectonic structures. References