Giori, I.

In questo lavoro vengono presentati i risultati derivanti da analisi dei dati della nuova Carta Aeromagnetica d’Italia (Eni Exploration & Production division e Cunispe, 2002) relativi all’area del Tirreno centrale mediante le tecniche di filtraggio direzionale. L’area in analisi comprende la regione del Tirreno centrale in particolare la fascia geografica del 41° parallelo. Lungo questo parallelo sono allineate diverse strutture geologiche come, seamount, faglie profonde e le strutture vulcaniche dell’area campana.......

This paper shows the results of a detailed reprocessing of aeromagnetic data,obtained by the downward projection to the seabed. The area of interest is centered over the Tyrrhenian Basin,whose bathymetric –topographic lay-out is characterized by a somewhat irregular trend.The origin of the intense depth variations depends on the Tyrrhenian structural setting,that is associated with the presence of several tectonic lineaments,seamounts or volcanic islands.The data were characterized by good quality and dense sam- pling,but they have been reprocessed in order either to solve some problems in the original compilation,and to reduce the distor- tion of the geomagnetic anomaly field caused by the difference of distance between the survey level and the magnetic source.The reprocessed magnetic map is proposed as an e ffective analysis tool for the Tyrrhenian area that is characterized by high susceptibility lithotypes.Downward projection of the aeromagnetic data by BTM algorithm increases the de finition of the anomalous magnetic signal without distortions in the geometric pattern of the field,thus showing a more stable and effective association between the magnetic anomalies and their geological sources.This effect is particularly true for high frequency anomalies that are directly comparable after the topographic projection because the depth filtering effect is attenuated.Moreover,the BTM method has been applied for the first time to a regional scale survey that shows substantial advantages because no fictitious anomalies in the high frequency sector of the spectrum were generated.This has been a typical effect of the traditional downward projection methods widely used before.The final result is a BTM anomaly map that is able to show the structural connections between the geological magnetic sources of the Tyrrhenian Sea area.

This paper presents the revised aeromagnetic anomaly map of Italy and its surrounding seas, projected at reference altitude of 2500 m and geomagnetic epoch 1979.0. The magnetic data set used for the map compilation is composed of the total intensity field data acquired partly during the aeromagnetic surveys performed by the Italian National Oil Company (Agip - Direzione Esplorazione Idrocarburi) between 1971 and 1980, and during the new surveys committed by the Geophysical Corporate Services of Eni Spa - Exploration & Production Division in the years 2001-2002. In both campaigns the recorded data were very dense and uniformly distributed over the examined area. A detailed re-processing of this data set and a re-organization into a new digital database were carried out. The re-processing was done using modern adequate techniques, obtaining a remarkable exploitation of the data information content. The result is a colour shaded relief map that shows on a large scale many of the structural lineaments of the Italian area. The inclusion of a larger number of data and the subtraction of an appropriate magnetic reference field are the main reasons of an enhancement in the anomaly definition. This new map replaces the previous Agip version, and aims to become the reference aeromagnetic cartography of the Italian area. We think this work will be useful both for researchers interested in large scale tectonic studies, and for anyone interested in the investigation of smaller scale structures, such as volcanic complexes or infra-sedimentary magnetic bodies, as well as for mining research.

Airborne gravimetry is an important method for measuring gravity over large un- surveyed areas. This technology has been widely applied in Canada, Antarctica and Greenland to map the gravity fields of these regions and in recent years, in the oil industry. In 2005, two tests in the Italian area were performed by ENI in cooperation with the Politecnico di Milano and the Danish National Space Center. To the knowl- edge of the authors, these were the first experiments of this kind in Italy and were performed over the Ionian coasts of Calabria and the Maiella Mountains. The Cal- abria test field is characterized by strong gravity variations due to the geophysical and topographic structure of the area. The ground gravity coverage is also quite dense. It was thus possible to compare airborne gravity with the ground observed values in order to check the precision of the airborne gravimetry. The second campaign was performed in an unsurveyed area centred on the Maiella Mountains, thus filling the data gap of this zone. Comparisons with existing ground data were also carried out in this area. After smoothing, the collected data have an accuracy of 2–3 mgal, as derived by cross-over analysis. Moreover, the collocation method applied to compare and merge ground-based and airborne data proved to be efficient and reliable. The standard de- viation of the discrepancies between airborne data and collocation upward continued gravity is, in both cases, less than 8 mgal. In the Maiella test, the gravity field obtained by merging airborne and ground data using collocation also provides a more detailed description of the high-frequency pattern of the geopotential field in this area.

We show the results obtained by means of a seabed reduction technique on the intensity of geomagnetic anomaly fields applied to a synthetic case and then to the real case of a geomagnetic survey of eastern Ligurian Sea (Italy). The eastern Ligurian Sea has very intense short waves anomaly fields and a sea bed that varies greatly in depth. As a result the geomagnetic space signal is characterized by a very large spectral content; in these conditions it is not possible to obtain a full sampled marine survey and vertical continuation analytic procedures and standard numerical bottom reduction based on a single vertical incremental parameter, whichever is applicable, fails to give accurate results. The present technique, which has been fine-tuned over 4 years of experimentation in environmental researchs, aims to provide a simple and efficient means to reduce the distortion of geomagnetic anomalies field caused by the variation of distance between survey plane and magnetic outcrop source position. The compensation procedure is based on evaluation, by comparison of two measurements carried out at different altitudes, of the mean vertical increment typical of each anomaly field principal frequency component bands. The component anomaly fields are then corrected by application of the corresponding vertical increments and lastly, the anomaly geomagnetic field reduced to the sea-bed is computed as Inverse Fourier Transform of a spectrum built as synthesis of the component anomaly fields' spectra. The results obtained have shown a notable increase in definition of anomaly field intensity without the production of appreciable distortions or false geomagnetic echoes.

In a cooperation project with Eni Spa Exploration & Production Division, actually owner of the aeromagnetic data, we have updated the original Agip map, through the re-processing of the original data set. These data in fact show a very high potentiality, because of the care applied during their acquisition, their remarkable density and the homogeneous and uniform distribution over the covered areas.