GNGTS 2013 - Atti del 32° Convegno Nazionale

proposed as a possible cause for local subsidence within the Phlegrean Fields volcanic complex (Dvorak and Mastrolorenzo, 1991). The episodic uplift of some sectors of the Phlegrean Fields is clearly indicated onshore. These sectors include the most recent eruptive vents, including the Monte Nuovo vent, historically erupted during the 1538. Geophysical evidence suggests that the uplift at the Phlegrean Fields is caused by the periodic reactivation of a shallow magmatic body, possibly by intrusion of additional magma or alternatively, hydrothermal mechanisms (Dvorak and Mastrolorenzo, 1991). The Phlegrean Fields are a young explosive caldera having a diameter of about 12 km, created by the volcano-tectonic collapse during two main volcanic events (Rosi and Sbrana, 1987). This caldera was the source of two large volcanic deposits in the Campania Plain, i.e. the Campanian Ignimbrite (35 ky B.P.) and the Neapolitan Yellow Tuff (12 ky B.P.). After the formation of the Neapolitan Yellow Tuff, representing the main bulk of the Naples town, two periods of eruptive activity occurred in the Phlegrean Fields from 10 to 8 ky B.P. and again from 4.5 to 3.7 ky B.P. (Rosi and Sbrana, 1987). Each eruptive period was followed by a period of quiescence that lasted a few thousand years. Some deep well data (Mofete and San Vito geothermal areas: Aiello et al. , 2012) have been also taken into account for a better correlation with the volcanic products of the Phlegrean Fields volcanic complex. A deep stratigraphic section in the Mofete geothermal area (northern Phlegrean Fields: Aiello et al. , 2012) has shown the occurrence of deep trachytic subvolcanic bodies, overlain by siltites, sandstones and shales and then by tuffites, tuffs and lavas interbedded. Pre-calderic products superimpose on these ones and are represented by latitic and trachytic lavas, overlain by the Campanian Ignimbrite. Post-calderic products, overlying the Campanian Ignimbrite are constituted of chaotic tuffites, the Mofete yellow tuffs and the Baia pyroclastites. Another stratigraphic section has been constructed in the San Vito geothermal area (Gulf of Pozzuoli: Aiello et al. , 2012). The deep products are represented by silts and sandstones interbedded overlain by tuffites, tuffs and lavas interbedded, both in a submarine environment. Pre-calderic volcanites consist of trachytic lava domes, overlain by chaotic tuffs. Post-caldera products are represented by the Gauro yellow tuff, overlain by latitic scoria and pumices and then by incoherent pyroclastic rocks. Data and methods. The interpreted Sparker grid consist of seven seismic sections recorded in the Pozzuoli Gulf, both parallel and perpendicular to the shoreline. Seismic acquisition was carried out by using a multielectrode sparker system (SAM96), including shorter pulse lengths for an equivalent energy discharge, increasing the peak pressure and the amplitude of the acoustic wave. The seismic sections were graphically recorded on continuous paper sheets with a vertical recording scale of 0.25. The resolution of the seismic data was about 1 m. The ability of Multitip Sparker systems to record high resolution seismic data on continental shelf and deeper waters has been revealed and their technical characteristics have beenmeasured in laboratory using different arrangements of energy storage and salinity of water (Rutgers and de Jong, 2003). Sparkers can be used for the production of acoustic pulses as seismic source for sub-bottom profiling in the sea. A multi-tip (200-800 tips) sparker has been developed, which makes high-resolution seismic profiling possible in deep water. The characteristics of the multi-tip sparker system have been measured in a basin in the laboratory using different arrangements of the capacitive energy storage and salinity of the water. A model in Matlab has been used to calculate the current waveform in the spark array for different layouts of the power supply (Rutgers and de Jong, 2003). The geological techniques of high resolution seismic stratigraphy have been applied in the geological interpretation of seismic profiles (Mitchum et al. , 1977; Vail et al. , 1977). The stratigraphic interpretation of the seismic data for a particular geologic setting is based on the evaluation of variations in acoustic characteristics reflecting changes in sedimentary processes, tectonic setting and position of sea level. Parallel reflectors, usually continuous and distinct reflectors draping the sea floor typically indicate fine-grained pelagic and 7 GNGTS 2013 S essione 3.1