GNGTS 2013 - Atti del 32° Convegno Nazionale

Integrated marine magnetics of the Naples Bay – from old to new data: examples from Naples and Pozzuoli Gulfs (Southern Italy) G. Aiello, E. Marsella Istituto per l’Ambiente Marino Costiero (IAMC), Consiglio Nazionale delle Ricerche (CNR), Napoli, Italy Introduction. The integrated marine magnetics of the Naples Bay, coming from the old to the new data is here tentatively resumed and discussed, focussing on some examples from Naples and Pozzuoli Gulfs (Southern Italy). Significant correlations between geophysical data come from the comparative analysis of seismic and magnetometric datasets. A magnetometer usually measures the strength or direction of the Earth’s magnetic field. This last can vary both temporally and spatially for various reasons, including discontinuities between rocks and interaction among charged particles from the sun and the magnetosphere. Most technological advances dedicated to measure the Earth’s magnetic field have taken place during World War II. The most common are: the fluxgate, the proton precession, Zeeman-effect, sensor-suspended magnet and satellite magnetometers. The fluxgate and the proton precession magnetometers are effectively the most used for marine surveys, they are both cable drown. The fluxgate magnetometer was the first ship-towed instrument, and it can measure vector components of the magnetic field. Its sensor consists of two magnetic alloy cores that are mounted in parallel configuration with the windings in opposition. The proton precession magnetometer consists of a sensor containing a liquid rich in protons surrounded by a coil conductor. The sensor is towed from the vessel through an armoured coaxial cable whose length depends on vessel length and seafloor depth. Circulating current within the coil generates a magnetic field of approximately two orders of magnitude the Earth’s field. In this way, one proton each ten will follow the coil positioning. Stopping the induced magnetic field, the protons will align according to the Earth’s magnetic field through a movement of precession. The proton precession magnetometer is one of the most used for offshore surveys and it records the strength of the total field by determining the precessional frequency ( f ) of protons spinning about the total field vector ( F ) as follows: f = γ p F /2π (1) where γ p is the gyromagnetic ratio of the proton uncorrected for the diamagnetic effect, so that knowing it from laboratory measurements, the total field in nanotesla can be calculated as: F =23.4866x f (2) The total magnetic field calculated through the Eq. (2) is stored by magnetometer into a string of data containing position data that is displayed as an x , y chart. The signal frequency is measured on a time span of 0.5 seconds when the signal-noise ratio is highest. To ensure a maximum value of initial value of proton precession the angle between the axis of the coil and the Earth’s field it is necessary to use two orthogonal coils. The measured field must be corrected with respect to the regional field in order to evaluate the anomalies. The proton precession magnetometer was largely used to explore magnetic anomalies in the Bay of Naples. Interesting examples of magnetic data acquisition related in the Naples and Pozzuoli bays are reported in Galdi et al. (1988), Secomandi et al. (2003), Aiello et al. (2004) and will be discussed in the following paragraphs. Geologic and geophysical setting. In the Gulf of Naples one of the most important lineaments is the Somma-Vesuvius volcano. The Vesuvius volcano has been intensively studied with respect to the eruptive events, the recent seismicity, the geochemistry and the ground movements of the volcano and the related volcanic hazard (Cassano and La Torre, 1987; Santacroce et al. , 1987; Castellano et al. , 2002; Esposti Ongaro et al. , 2002; Mastrolorenzo et al. , 2002; Saccorotti et al. , 2002; Scarpa et al. , 2002; Todesco et al. , 2002). The Somma edifice started to grow after the eruption of the Campanian Ignimbrite deposits. Its eruptive 86 GNGTS 2013 S essione 3.2