Seismo-stratigraphy and marine magnetic of the Naples Bay: from old to new technologies

The onset of new technologies in marine data acquisition, processing and interpretation of the Naples Bay (Southern Italy) is herein discussed, focussing on the contribution of seismo-stratigraphic and marine magnetic data. Seismic stratigraphy and marine magnetics of selected case histories of the Somma-Vesuvius offshore, Phlegrean Fields offshore and Ischia-Procida volcanic complex are here shown. Seismic exploration is commonly performed by means of sources that can generate elastic waves from a rapid expansion of underwater gas bubbles. This can generate many pulses that take the form of double exponential spikes of gradually decreasing amplitude (Cole, 1965). Several technologies can be used in order to produce an acoustic pressure wave into water, such as free-falling weights, chemical explosives, piezoelectric or magneto-resistive sources, sparkers, boomers, air-guns and water-guns. Each of these sources has precise signature and wave frequency that can be considered optimal in function of depth, resolution, etc. The main characteristics of a seismic source is to produce a single high-energy spike that is detectable, despite the presence of noise, after crossing the portion of the seabed that we wish to study. A broad range of frequencies can be reproduced, as well as a broad range of waveforms can be generated in function of frequency-dependent absorption of elastic waves and nearby boundaries presence. Seismic sources for offshore investigation may be impulsive, providing a short-lived burst of elastic wave energy and swept-frequency, producing a low-amplitude sinusoidal signal. Impulsive sources such as explosives can cause damages to marine flora and fauna; for this reason towed sources activated for only few seconds must be preferred. The type of source could be chosen depending on the required resolution and signal penetration. Vibration of piezoelectric and magnetic materials, electric pulses, or pressured fluid discharge, often organised into arrays, can be considered good seismic sources whose signature, spectra and energy output can vary considerably. Sparkers (Knott and Hersey, 1956) and Boomers (Edgerton and Hayward, 1964) systems are based respectively on an electrode array powered by high voltage capacitor bank and on an electromagnetic source. Sparkers and boomers can generate seismic energy to explore continental margin when there are near surface or deep-towed (10-50 m off the sea), moreover boomers with pulse length of 0.1-0.2 ms can be used to explore very shallow waters. Sparker system can produce low-frequency acoustic wave (the maximum frequency contained in the spectrum of acoustic signal is approximately 2000 Hz) that can penetrate several hundred meters of sediment. One of the most interesting seismic sources is the Multispot Extended Array Sparker (M.E.A.S.) that consists of sparker electrodes disposed on a square metal cage. This kind of system, patented by Institute of Oceanology of Istituto Universitario Navale of Naples (Italy) allows obtaining a good signal penetration and high resolution seismic data with relative small energy use. The M.E.A.S. signal is a short impulse with a large frequency spectrum content (Fig. 1). Mirabile et al. (1991) tested the acquisition geometry in order to reduce a superimposing of source signal with return echoes that respect the "far field" condition and demonstrated the utility of some techniques for signal de-convolution in order to produce the so-called seismic profiles "deghosting". Seismic reflection data require a complex series of numerical treatments to increase the signal/ noise ratio of a single profile as well as obtaining a high resolution seismic section to improve the geological interpretation. A more recent technology is the Sparker source SA