GNGTS 2013 - Atti del 32° Convegno Nazionale

magnetic anomaly characterizes the Astroni-Agnano volcanic area, probably the result of the overlapping of several lava bodies. A new aeromagnetic map supplement the northern sector of the Phlegrean Fields (Paoletti et al. , 2004), allowing for a better geological interpretation of the structural patterns and morpho-structural features of the Volturno Plain and the Gulf of Pozzuoli and its offshore areas. The main magneto-structural features are the caldera rims of the Neapolitan Yellow Tuff and the Torregaveta anomaly. The Patria lake anomaly has a sub-circular shape and a diameter of about 10 km. A complex pattern of magnetic anomalies coincides with the Parete volcanic complex (Aiello et al. , 2011), while another isolated anomaly corresponds to the Volturno river. Data and methods. A Total Earth Magnetic Field (EMF) survey was performed in the Naples Bay during the oceanographic cruise GMS00-05 (Leg II – October November 2000) carried out by the CNR-IAMC of Naples, Italy onboard of the R/V Urania, resulting in a high resolution magnetic anomaly map of the Naples Bay (Aiello et al. , 2004). The collected data have a densely-spaced coverage if compared to previous measurements, such as the airborne magnetic survey (Agip, 1981; Caratori Tontini et al. , 2003). The acquisition of magnetic data at a shorter distance from the source and at a lower velocity has given more precise sampling and an improved field restoration. The magnetic data have been recorded using a G811 proton magnetometer; the sensor was placed in a fish towed at about 200 m from the ship and at an average depth of 15 m below the sea level. The magnetometer position was regularly controlled and recorded and the data were sampled at 3 sec, corresponding to an average spatial sampling of about 6.25 m. The magnetic anomaly map was compiled with a grid cell size of 200 m. A colorimetric scale expressed in nT is shown on the right side of the maps to quantify the intensity of measured magnetic anomalies. An accurate inspection of the magnetic lines recorded contemporaneously to the seismic lines has been preliminary carried out, since they showed spikes regular to the shot frequency. A non-linear filter has been used to remove the peaks due to non-geological sources. A repositioning of the marine lines has been carried out, taking into the account the offset among the fish and the GPS. The elimination of the variation for the diurnal component has been carried out using as reference magnetic station the geomagnetic observatory of L’Aquila. In order to recognize the total field anomaly linked to the geological structures, the values of the main magnetic field must be subtracted to the measured values corrected according to the Italian Geomagnetic Reference Field (Coticchia et al. , 2001). Some factors contribute to explain the shifts in the magnetic field among adjacent navigation lines. They include the differences in height among the navigation lines and the inadequate removal of the variations of magnetic field due to the diurnal component. The elaboration of the magnetic data has been carried out according to a procedure of levelling, consisting in the removal of the short period magnetic variations. The high resolution magnetic anomaly map of the Naples Bay. The main magnetic anomaly fields recognized on the high resolution magnetic anomaly map of the Naples Bay have been constrained by geological features based on the geologic interpretation and on the correlation with high resolution Multibeam bathymetry (Fig. 2; Aiello et al. , 2004). The magnetic anomaly fields have been also correlated with morpho-structural features and volcanic edifices, recognized on high resolution seismic profiles recorded along the same navigation lines. Two main belts of sharp magnetic anomalies have been identified, the first located offshore the Somma-Vesuvius volcanic complex and the second offshore the Phlegrean Fields volcanic complex. The latter one is interpreted as the seaward boundary of the Phlegrean caldera (Pescatore et al. , 1984; Orsi et al. , 2002). The detailed interpretation of the magnetic anomaly field and related seismic structures offshore the Somma-Vesuvius volcanic complex suggests a NNW-SSE structural alignment, while in the Pozzuoli Gulf several complex E-W and NE-SW trending anomalies have been identified, whose values are related to small buried volcanic structures. The shape of magnetic anomalies is not directly related to the submarine topography of large volcanic banks, revealed by Multibeam bathymetry. Main fields are also 88 GNGTS 2013 S essione 3.2