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 SAM that is characterized by a varying number of electrodes that can be disposed as "dual-in line" (SAM96) and "planar array" multi electrode electro-acoustic source (SAM 400/800; Fig. 2). Other seismic sources are the Airguns and Waterguns, recently used in the Naples Bay in submarine geological mapping and basin studies (D'Argenio et al., 2004; Aiello et al., 2005; 2011). The former one produces high-energy seismic pulses short in duration by means of a discharge of compressed air into water (Fig. 3), while the latter one produces the sudden collapse of a cavitation volume into water that is proportional to kinetic energy of the water plug. Airguns produce a wide range of pulse shapes and source spectra. The seismic exploration of Naples Bay has been performed mainly through Sparker systems and Watergun sources. The evolution of sources capability in terms of technological advances together with processing techniques refinement allowed high resolution studies of main intermediate and deep geological structures in the Bay of Naples. Historically some of the first surveys were conducted by R/V Atlantis II (Woods Hole Ocean. Cruse 59) using an Airgun System. Subsequently, in 1970 R/V Dectra owned by Istituto Universitario Navale of Naples (Italy) obtained a densely-spaced seismic survey through SPARKER E.G.G. (8 kjoules) and BOOMER systems in the Naples and Pozzuoli Bays (Latmiral et al., 1971; Bernabini et al., 1973). Since the 70's 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 ones consist of a single electrode hotter than a mass electrode, other ones of more electrodes over distributed mass (eg. Sparker Teledyne and Sparker EGG). 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 multi-electrode) based on the fundamental equations for the design of an "array". Sparker signals are the base band signals, transitory and continuous spectrum. Based on these 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 the deep geological structure of the Phlegrean Fields volcanic complex. It was aimed at improving the knowledge on technologies and sources that are more appropriated for the investigation of the continental margins, particularly in complex volcanic areas like the Gulf of Naples (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 and the Watergun, while the details to study geomorphological data were analyzed through the Surfboom and the Side Scan Sonar. 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 significant spectral content up to 1000 Hz, with maximum energy output around 150 - 200Hz. Each echo corresponds to an acoustic discontinuity (impedance contrast) that can generally be interpreted in geological terms. MEAS system has been largely used in order to acquire a large database of single channel reflection seismics in the Bay of Naples (Mirabile, 1969; Latmiral et al., 1971; Mirabile et al., 1991). Recently, by means of Multi- tip SAM 96 (0.1-1kJ), SAM400 (1-4KJ) transducer it was possible to record high resolution seismic data in the Bay of Naples both in coastal and deep sea research (Corradi et al., 2009). Some evidences on magnetic field anomalies in the Gulf of Pozzuoli come from the magnetic map of Galdi et al. (1988) who reported a NE-SW interruption of main regional trend where some circular local anomalies are related to products of post-calderic volcanic activity (Rosi & Sbrana, 1987). 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. Presently, the most common are: the fluxgate, the proton precession, Zeeman-effect, sensor suspended-magnet, and satellite magnetometers. The fluxgate and the proton precession are effectively the most used for marine surveys, they are both cable drawn. 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 1 proton each 10 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=?pF/2?????? where ?p is the gyromagnetic ratio of the proton uncorrected for the diamagnetic effect, so that knowing its from laboratory measurements, the total field in nanotesla can be calculated as: F=23.4866 x f (2) ? The total field calculated by means of equation (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 Bay of Pozzuoli and Naples is reported in Galdi et al. (1988) and Aiello et al. (2004). As shown by the maps of Galdi et al., 1988 both positive and negative anomalies were detected, using a magnetometer model Geometrics G-856, globally the area shows an interruption of the regional trend from NE-SW where circular anomalies are probably connected to a post-calderic activity of the Phlegrean Fields. Moreover, the internal area of the Pozzuoli Bay is characterized by a negative anomaly that increases towards the south. Conversely, in the external area there is mainly an alternance of positive and negative anomalies with a dominance of positive values near the area of Bagnoli. For a detailed analysis of the magnetic anomaly field of the volcanic district of the bay of Naples see Secomandi et al. (2003). Recently, Aiello et al. (2004) presented a high resolution map of the Bay of Naples based on data acquired during oceanographic cruise GMS2000-05 performed in October-November 2001 on board of the R/V Urania using the EG&G Geometrics proton magnetometer G-811. REFERENCES Aiello G., Angelino A., Marsella E., Ruggieri S., Siniscalchi A. (2004) Carta magnetica di alta risoluzione del Golfo di Napoli (Tirreno meridionale). Bollettino della Società Geologica Italiana, 123, 333-342. Aiello G., Angelino A., D'Argenio B., Marsella E., Pelosi N., Ruggieri S., Siniscalchi A. (2005) Buried volcanic structures in the Gulf of Naples (Southern Tyrrhenian sea, Italy) resulting from high resolution magnetic survey and seismic profiling. Annals of Geophysics, 48 (6), 883-897. Aiello G., Cicchella A.G., Di Fiore V., Marsella E. (2011) New seismo-stratigraphic data of the Volturno Basin (northern Campania, Tyrrhenian margin, southern Italy): implications for tectono-stratigraphy of the Campania and Latium sedimentary basins. Annals of Geophysics, 54 (3), 265-283. Bernabini M., Latmiral, G., Mirabile, L. & Segre, A.G. (1973). Alcune prospezioni sismiche per riflessione nei Golfi di Napoli e Pozzuoli. Rapp. Comm. Int. Mer. Medit., 21, 929-934. Cole R.H. (1965) Underwater Explosions. Dover Publications, New York. Corradi, N.; Ferrari, M.; Giordano, F., Giordano, R., Ivaldi, R. & Sbrana, A. (2009) SAM source and D-Seismic system:The use in Marine Geological Mapping C.A.R.G and P.n.r.a projects. 27th IAS Meeting of Sedimentologists, Alghero (Italy), pp. 85-90. D'Argenio, B.; Aiello, G., de Alteriis. G., Milia, A., Sacchi, M. et al. (2004). Digital Elevation Model of the Naples Bay and adjacent areas, Eastern Tyrrhenian sea, In: Mapping Geology in Italy, E. Pasquarè & G. Venturini (Eds.), APAT, National Geological Survey of Italy, Spec. Vol. SELCA, Florence, 22-28. De Vita S., Esposito B., Mirabile L. (1979) Criteri di Progetto di Sparker a cortina per sismica ad alta risoluzione. Atti del convegno Scientifico Nazionale Progetto Finalizzato Oceanografia e Fondi Marini. Edgerton H.E. and Hayward G.G. (1964). The boomer sonar source for seismic profiling. Journal of Geophysical Research, Vol. 68, pp. 3033-3042. Fusi, N., Mirabile, L., Camerlenghi, A. & Ranieri, G. (1991). Marine geophysical survey of the Gulf of Naples (Italy): relationship between submarine volcanic activity and sedimentation. Memorie della Società Geologica Italiana, 47, pp. 95-114. Galdi, A., Giordano, F., Sposito, A., Vultaggio, M. (1988) Misure geomagnetiche nel Golfo di Pozzuoli: Metodologia e risultati. Atti del 7° Convegno GNGTS-CNR, Vol.3, pp. 1647-1658. Knott, S.T., Hersey J.B. (1956) Interpretation of high resolution echo-soundings techniques and their use in bathymetry, marine geophysics and biology. Deep Sea Research, Vol.4, pp. 36-44. Latmiral, L., Segre, A.G., Bernabini, M. & Mirabile, L. (1971). Prospezioni sismiche per riflessione nei Golfi di Napoli e Pozzuoli ed alcuni risultati geologici. Bollettino della Società Geologica Italiana, Vol.90, pp.163-172. Mirabile L. (1969). Prime esperienze di stratigrafia sottomarina eseguite presso l'Istituto Universitario Navale. Annali IUN, Vol. 38. Mirabile, L., Nicolich, R., Piermattei, R. & Ranieri, G. (1989). Identificazione delle strutture tettonico-vulcaniche dell'area flegrea: sismica multicanale del Golfo di Pozzuoli. Atti del 7° Convegno GNGTS, Vol.2, Roma, Italy, pp. 829-838. Mirabile, L., Fevola, F., Galeotti, F., Ranieri, G.& Tangaro, G. (1991). Sismica monocanale ad alta risoluzione con sorgente multi spot di tipo sparker: applicazione ai dati di tecniche di deconvoluzione. Atti del 10° Convegno GNGTS-CNR, Roma, Italy. Ranieri G. and Mirabile, L. (1991). Ricerca ed applicazione di metodi geofisici al rilievo sperimentale della struttura medio-profonda dell'area flegrea con uso di sorgenti sismiche water-gun. Annali Istituto Universitario Navale di Napoli, Vol.63. Rosi M. and Sbrana A. (1987). Phlegrean Fields. Quaderni De La Ricerca Scientifica, CNR, Vol.114, No9, 175 pp. Secomandi M., Paoletti V., Aiello G., Fedi M., Marsella E., Ruggieri S., D'Argenio B., Rapolla A. (2003) Analysis of the magnetic anomaly field of the volcanic district of the Naples Bay. Marine Geophysical Researches, 24, 207-221.