The first ultra-high resolution Digital Terrain Model of the shallow-water sector around Lipari Island (Aeolian Islands, Italy)

Very high resolution bathymetric map obtained through multibeam echosounders data are crucial to generate accurate Digital Terrain Models from which the morphological setting of active volcanic areas can be analyzed in detail. Here we show and discuss the main results from the first multibeam bathymetric survey performed in shallow-waters around the island of Lipari, the largest and the most densely populated of the Aeolian Islands (southern Italy). Data have been collected in the depth range of 0.1-150 m and complete the already existent high-resolution multibeam bathymetry realized between 100 and 1300 m water depth. The new ultrahigh resolution bathymetric maps at 0.1-0.5 m provide new insights on the shallow seafloor of Lipari, allowing to detail a large spectrum of volcanic, erosive-depositional and anthropic features. Moreover, the presented data allow outlining the recent morphological evolution of the shallow coastal sector of this active volcanic island, indicating the presence of potential geo-hazard factors in shallow waters

Caratterizzazione del rumore magnetico di fondo nel nuovo osservatorio di Varese Ligure

The accuracy of geomagnetic field measurement at a given point on the Earth's surface is limited by the ambient magnetic noise level which is usually determined by the superposition of signals generated from different sources. The noise that dominates in all physical systems, particularly in spectral regions at lowest frequencies is the flicker noise [Vladimirov and Kleimenova 1962]. The analysis of the background noise that exists on the site is essential to design a new magnetic Observatory. Noise measurements allow then to evaluate the magnetic pollution status of the area and constitute a basic cognitive tool for the building of a new magnetic Observatory. Only by comparing signals of natural origin and the noise is possible to characterize the site. The purpose of this note is the separation of local and regional magnetic noise from signals of external origin. In the near field zone gradients are very high, the contribution of these sources depends much on the source-observer distance

Potential-field inversion for a layer with uneven thickness: the Tyrrhenian Sea density model

Inversion of large-scale potential-field anomalies, aimed at determining density or magnetization, is usually made in the Fourier domain. The commonly adopted geometry is based on a layer of constant thickness, characterized by a bottom surface at a fixed distance from the top surface. We propose a new method to overcome this limiting geometry, by inverting in the usual iterating scheme using top and bottom surfaces of differing, but known shapes. Randomly generated synthetic models will be analyzed, and finally performance of this method will be tested on real gravity data describing the isostatic residual anomaly of the Southern Tyrrhenian Sea in Italy. The final result is a density model that shows the distribution of the oceanic crust in this region, which is delimited by known structural elements and appears strongly correlated with the oceanized abyssal basins of Vavilov and Marsili

Detph-to-the-bottom Optimization for Potential-field Data Inversion

We show an algorithm for the linear inversion of 2D surface magnetic data to obtain 3D models of the susceptibility of the source. After showing a novel characterization of the ambiguity domain in the Fourier space, which has a simple geometrical interpretation, we will demonstrate that a depth-weighting function is useful to significantly reduce the ambiguity domain in order to characterize the main source properties. The forward model is discretized by a mesh of prismatic cells with constant magnetization that allows the recovery of a complete 3D generating source. As the number of cells are normally grater than the amount of available data, we are left with an underdetermined linear inverse problem, which can be regularized in order to obtain an unique solution by a depth-weighting function, adapted from Li and Oldenburg (1996) to close the source towards its bottom. The main novelty of this method is a first-stage optimization that gives information about the depth-to-the-bottom (dtb) of the generating source. This parameter permits both the evaluation of the appropriate vertical extension of the mesh, and the definition of the shape of the regularizing depth-weighting distribution. The adopted method is suitable under appropriate changes to deal also with gravity data. After showing which kind of a priori information is introduced by this particular regularization, we will describe its limits and its possible improvements and then we will show the results of some synthetic tests. As a final application we will show the 3D magnetic model of an interesting volcanic region in Italy

Depth-to-the-bottom optimization for potential field inversion

We present an algorithm for the linear inversion of 2D surface magnetic data to obtain 3D models of the susceptibility of the source. The forward model is discretized by a mesh of prismatic cells with constant magnetization that allows the recovery of a complete 3D generating source....

Acquisizione magnetica marina dell\u27alto strutturale del Mt. Vercelli (Mar Tirreno Centrale) e del golfo di Napoli: descrizione dei rilievi e primi risultati

Durante la Campagna Oceanografica Ver2010, effettuata nel mese di maggio 2010, sono stati acquisiti due set di dati geomagnetici marini in alta risoluzione relativi alle aree del Mt. Vercelli (Mar Tirreno Centrale) e del Golfo di Napoli. Tale attività di misura è stata resa possibile grazie alla collaborazione tecnica scientifica tra l’Unità di Progetto “Geofisica e Tecnologie Marine” dell’Istituto Nazionale di Geofisica e Vulcanologia (INGV) e l’Istituto Idrografico della Marina (IIM). La proficua collaborazione è sancita dall’accordo di cooperazione “Co.Na.Ge.M” (Coordinamento Nazionale per la Geofisica Marina). Nello specifico, i due rilievi sono stati svolti mediante l’impiego della Nave Idro-oceanografica “Aretusa”, unità navale dell’Istituto Idrografico della Marina con scafo a catamarano in vetroresina e, pertanto, particolarmente adatta a rilievi di tipo geomagnetico. L’attività di acquisizione geofisica del Monte Vercelli rientra nel Progetto TySec – Prin 2007 che coinvolge l’Università degli Studi di Genova, l’Università Politecnica delle Marche e l’Istituto Nazionale di Geofisica e Vulcanologia. L’obiettivo del progetto è quello di valutare le relazioni tra le caratteristiche morfologiche e idrodinamiche dell’area del seamount e l’ecosistema ad esso correlato. In questo lavoro proponiamo i risultati preliminari del survey geomagnetico e batimetrico della struttura sommersa del Vercelli

Inversion of potential-field data for layers with uneven thickness

AB: Inversion of large-scale potential-field anomalies, aimed at determining density or magnetization, is usually made in the Fourier domain. The commonly adopted geometry is based on a layer of constant thickness, characterized by a bottom surface at a fixed distance from the top surface....

Magnetic and Gravimetric model of Panarea (Aeolian islands )

We show the results of the gravimetric and magnetometric surveys of the Island of Panerea and its archipelago. These geophysical surveys belong to a major multiparametric project for the evaluation of the volcanic and seismic hazard of the island. Panarea is considered a volcanic active area as dimostrated by gas eruption of November 2002 ....

Determining the optimal Bouguer density for a gravity data-set

Two methods are commonly adopted to evaluate the optimal Bouguer density for a given data-set, starting from different data characteristics or geological regime, giving in many cases different results. We propose some simple extension of these methods in order to make their results compatible. To this aim, we have used free-air gravity satellite data from Geosat and ERS-1 missions in order to compile a Bouguer gravity map of the Mediterranean Sea. The complete Bouguer correction has been applied by using the method of Parker (1972), that acts in the Fourier domain and allows for an exact evaluation of the gravity contribution from an highly sampled topographic model of the land. The density used for the Bouguer reduction has been obtained thus from the gravity data-set itself, by using two different optimization methods that have given the same optimal result of 2400 kg/m3. We have studied the radial power spectrum of the data, choosing the optimal Bouguer density as the one that minimizes its slope, i.e. the fractal dimension of the resulting gravity map in the band of wavelength from 45 km to 105 km. The second approach consists of studying the correlation between topography and Bouguer anomaly by spatial crossplots for a significant sub-set of the data. In the past these methodologies were applied alternatively since they gave different optimization values, especially the second method that seems to ignore large-wavelength isostatic effects. The main novelty of our work is represented by the combined application of both the approaches having as common goal the reduction of the short-wavelength effects of topography in the gravity map. Actually we have revisited both the methodologies, proposing slight modifications to make their efforts compatible. Their coincident results confirm their validity of application and give reliability to the recovered value of the Bouguer optimal density. As a first result we have obtained a revised Bouguer map for the Mediterranean Sea, that is useful for large-scale geological studies. Moreover, studying the correlation between Bouguer anomaly and bathymetry, we propose the compilation of a new interpretative tool that may be considered a sort of normalized correlation map defining the 2D isostatic setting of the investigated region, without introducing any lithospheric model. In a direct way we have found that the over-all region seems to be in a complete isostatic equilibrium apart from the young basins of Tyrrhenian Sea and Aegean Sea, confirming previous similar results

Magnetic Base Station Deceptions, a magnetovariational analysis along the Ligurian Sea coast, Italy

Reliability of high resolution airborne and shipborne magnetic surveys depends on accurate removal of temporal variations from the recorded total magnetic field intensity data. At mid latitudes, one or a few base stations are typically located within or near the survey area and are used to monitor and remove time dependent variations. These are usually assumed to be of external origin and uniform throughout the survey area. Here we investigate the influence on the magnetic base station correction of the time varying magnetic field variations generated by internal telluric currents flowing in anomalous regional 2D/3D conductivity structures. The study is based on the statistical analysis of a data set collected by four magnetovariational stations installed in northwestern Italy. The variometer stations were evenly placed with a spacing of about 60 km along a profile roughly parallel to the coastline. They recorded the geomagnetic field from the beginning to the end of April 2005, with a sampling rate of 0.33 Hz. Cross-correlation and coherence analysis applied to a subset of 125 five hours long magnetic events indicates that, for periods longer than 400 s, there is an high correlation between the horizontal magnetic field components at the different stations. This indicates spatial uniformity of the source field and of the induced currents in the 1D Earth. Additionally, the pattern of the induction arrows, estimated from single site transfer functions, reveals a clear electromagnetic signature of the Sestri-Voltaggio line, interpreted as a major regional tectonic boundary. Induced telluric currents flowing through this 2D/3D electrical conductivity discontinuity affect mainly the vertical magnetic component at the closer locations. By comparing this component at near (32 km) and far (70 km) stations, we have found that the mean value of the power spectra ratio, due to the electromagnetic induced field, is about 1.8 in the frequency band ranging from 2.5×10−3 to 5.5×10−5 Hz. This energy, folded in the spatial domain of an hypothetical survey in this region produces unwanted noise in the dataset. Considering a fifth of nyquist frequency the optimal tie-line spacing to assure complete noise removal would be 1 km and 15 km for a rover speed of 6 knots (marine magnetic survey) and 100 knots (aeromagnetic survey) respectively. Similar power spectra analysis can be applied elsewhere to optimise tie-line spacing for levelling and filtering parameters utlilised for microlevelling