Seismic stratigraphy and marine magnetics in the Naples Bay (Southern Tyrrhenian sea): The onset of new technologies and methodologies for data acquisition, processing and interpretation

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 decreased amplitude (Cole, 1965). Several technologies can be used in order to produce an acoustic pressure wave such as free-falling weights, chemical explosives, piezo-electric or magneto-resistive sources, sparkers, boomers, airguns and waterguns. Each of these sources has a precise signature and wave frequency that can be considered optimal in function of survey requirements (eg. depth, resolution, etc.). The seismic exploration of Naples Bay has been performed mainly through sparker systems and water-gun sources. The Sparker source was discovered in 1956 by J.B. Hersey and S.T. Knott. It can produce low frequency acoutic wave (the maximum frequency contained in the spectrum of acoustic signal is approximately 2000 Hz) that can penetrate several hundred meters of sediment. The evolution of source capacity in terms of technological advances together with processing techniques refinement allowed to study with a very high resolution some important medium-deep geological structures in the Bay of Naples. Historically some of the first surveys were conducted by R/V Atlantis II (Woods Hole Oceanographic Cruise 59) using an Airgun System. Subsequently, in 1970 R/V Dectra owned by Istituto Universitario Navale (IUN) of Naples obtained a continuous seismic profiling survey by SPARKER EG&G (8 kjoules) and BOOMER systems in the Naples and Pozzuoli Bays (Latmiral et al., 1971; Bernabini et al., 1973). Since the '70 until now many attempts have been carried out in order to improve seismic technologies performance, data acquisition and processing. In the practice of seismic prospecting Sparker systems technologies were widely analyzed using different acquisition systems. Some consist of a single electrode hotter than a mass electrode, other by more electrodes over distributed mass (eg. Sparker Teledyne and Sparker EG&G). De Vita et al. (1979) tried to identify, also based on experimental data, which one is more appropriate than the two configurations (single electrode or multielectrode) based on the fundamental equations for the design of an array. Based on their measurements it has been demonstrated that energy should never exceed 400 joules/electrode to achieve the best compromise between resolution and electro-acoustic performance. Ranieri and Mirabile (1991) reported technical and scientific results obtained through the geophysical survey of deep structure of Phlegrean Fields volcanic complex. This last one was aimed to improve the knowledge on technologies and sources that are more appropriate for the investigation of the continental margins, particularly in complex volcanic areas like the Gulf of Naples (see also Fusi et al., 1991). Among the sources tested in studies of the Gulf of Naples there are the explosives (Mirabile et al., 1989), the sparker, the water-gun, while the details to study geomorphological data were analyzed through the Surfboom and the Sidescan Sonar. Thorough MEAS (Multispot Extended Array Sparker; Mirabile et al., 1991) seismic source (12 and 16 kJ) consists of an array of 36 (6x6) electrodes placed inside a metal cage in a square size 4.5x4.5 m, spaced 0.75 m and fed in phase. The energy used by the MEAS has a pulse of short duration, the order of 10 milliseconds and a significan spectral content up to 1000 Hz, with maximum energy ouput around 150-200 Hz. Each echo corresponds to an acoustic discontinuity (impedance contrast) that can be generally interpreted in geological terms. MEAS systems has been largely used in order to acquire a large database of single channel reflect