Antonio Rapolla

3-D inversion of gravity and magnetic data with depth resolution

Magnetization and density models with depth resolution are obtained by solving an inverse problem based on a 3-D set of potential field data. Such a data set is built from information on vertical and horizontal variations of the magnetic or gravity field. The a priori information consists of delimiting a source region and subdividing it in a set of blocks. In this case, the information related to a set of field data along the vertical direction is not generally redundant and is decisive in giving a depth resolution to the gravity and magnetic methods. Because of this depth resolution, which derives solely from the potential field data, an unconstrained and joint inversion of a multiobservation-level data set is shown to provide surprising results for error-free synthetic data. On the contrary, a single-observation level data inversion produces an incorrect and too shallow model. Hence, a good depth resolution is likely to occur for the gravity and magnetic methods when based on the information along the vertical direction. This is also evidenced by an analysis of the kernel function versus the field altitude level and by a singular value analysis of the inversion operators for both the single and multilevel cases. Errors connected to numerical upward continuation do not affect the quality of the solution, provided that the data set extent is larger than that of the anomaly field. Application of the method to a 3-D magnetic data set relative to Vesuvius indicates that the method may significantly improve interpretation of potential fields.

The Campanian Plain and Phlegrean Fields : structural setting from potential field data

Abstract A boundary analysis of the gravity and magnetic fields of the Phlegrean Fields volcanic area and of the surrounding Campanian Plain reveals a complex structural setting. The Campanian Plain results well defined from sharp density boundaries. A set of major E–W lineaments occurs within the Plain. We derive a framework of the structural control to the volcanological evolution of Campanian Plain, which appears strongly influenced by a set of NE–SW lineaments bordering the Acerra depression. The density and magnetization boundaries of the Phlegrean Fields are significantly consistent, both indicating for the collapsed Phlegrean caldera an area considerably less extended than that based on previous geologically-defined models. Many other structures are evidenced in the Phlegrean area, often with agreement between density and magnetization boundaries. Inside the caldera, a strong consistence between the structures we identified and recent seismicity has been noted.

Structural setting of the Bay of Naples (Italy) seismic reflection data: implications for Campanian volcanism

Abstract This paper focuses on the recent tectonic evolution of the Bay of Naples with the aim of exploring the connection between local tectonics and volcanism. We reprocessed the seismic reflection dataset acquired in the area in the late 1973. The new processing was highly successful in obtaining a decisive strong reduction of random noise, removal of coherent noise and reduction of spatial aliasing. Classical interpretative schemes and complex attributes of seismic traces were used to reconstruct fault kinematics and reflector patterns. The results show that the faults affecting the Bay of Naples exhibit prevailing NE structural strikes, with the exception of the Pozzuoli Caldera where NW patterns are also common. Many faults are subvertical and show seismic evidence of volcanic activity along them. A main alignment of conjugate NE–SW faults, named here as “Magnaghi–Sebeto line”, intersects several submarine volcanic banks and separates the bay into two sectors, characterized by important geological, geophysical and petrochemical differences. The structural configuration of the bay may reflect the occurrence of either oblique extension or a transfer zone of the NW–SE fault system, along which, in the Campanian–Lucanian Apennine chain, great vertical displacements occur.

Study of the sub-surface structure of Somma-Vesuvius (Italy) by seismic reflection data

Abstract Eleven seismic reflection lines recorded by the Italian Petroleum Agency, Azienda Generale Italiana Petroli (AGIP), in 1982–1984 around the flanks of the Somma-Vesuvius complex were utilized for studying its sub-surface structure. A new seismic reflection profile acquired within the Avellino caldera, at the southwestern edge of the Great Cone of Vesuvius, integrates the AGIP seismic dataset. The new reflection profile allows us to delineate two main reflectors. The first reflector may indicate a SW lateral collapse experienced by the volcano, probably between 35 and 11 ka ago. Evidence of this seismic discontinuity was also found on one AGIP line and on some refraction surveys. Other volcanic structures, such as buried adventive craters, dikes and pyroclastic deposits were detected on the eastern side of the volcanic complex. In particular, two shallow undulated reflectors are interpreted as pyroclastic deposits older than the Campanian Ignimbrite (CI). The study confirmed the presence of NW–SE striking faults crosscutting Vesuvius, which probably played a major role in the genesis of the volcano. Finally, evidence of ring faults was found mainly in the northeastern and southeastern sectors of the volcano. These faults yielded a low signal-to-noise area characterized by high lateral heterogeneity and the presence of a large number of fractures, possibly due to the stress field created by collapse of the volcanic structure.

Gravity modelling of the litho-asthenosphere system in the Central Mediterranean

Abstract The results of the 2.5-D gravity modelling of the litho-asthenosphere system in the Central Mediterranean, along two regional geotransects, are presented and their possible implications for the geodynamical evolution of this area are discussed. Particularly, gravity data on the Tyrrhenian and Provencal-Balearic basins were interpreted introducing significant constraints by means of several main petro-physical parameters tied to the litho-asthenospheric evolution during the extensive tectonics. By this approach, based on the model of McKenzie and Bickle (in: J. Petrol., 29. pp. 625–679. 1988) and White and McKenzie (in: J. Geophys. Res., 94, B6, pp. 7685-7729, 1988), the estimation of the vertical density distribution in the lithosphere-asthenosphere system has been possible. Significant differences in the gravity anomaly field occur when different density models of asthenospheric upwelling are considered. This depends on factors like the thermal regime, the stretching of the lithosphere and its thickness before it. The obtained results contributed to the validation of models interpreting the origin of the Tyrrhenian and Ligurian-Provencal basins as related to a passive extensional process rather than to the presence of an active upwelling asthenosphere beneath the Central Mediterranean basins.

2.5D modelling of Somma–Vesuvius structure by aeromagnetic data

Abstract This paper deals with the analysis of aeromagnetic data on the Somma–Vesuvius area. An inverse 2.5D method is used, its main features being a variable strike length and a data set relative to a vertical plane. This last provides a depth resolution, so that the result consists in a magnetic tomography along a vertical section. The magnetic model shows that magnetized rocks extend up to carbonate basement depths (about 2 km). The stronger magnetizations (>6 A/m) are concentrated in the emerged part of the volcano, but high magnetizations are still present down to about 1800 m b.s.l. The magnetization distribution found is then compared to the distribution of the hypocenters of Vesuvian earthquakes and for shallow depth a positive correlation is noted. At depths greater than about 2 km, i.e. at the level where a geothermal well started to encounter dolomites, the model shows no magnetization, while hypocenters form a nearly continuous vertical belt. This pattern leads to the conclusion that this evidence may be considered as being due to a small volume of the feeding system at these depths or eventually as an indication of strong alteration processes of magnetic rocks.

Anisotropic magnetic susceptibility in the continental lower crust and its implications for the shape of magnetic anomalies

Magnetic anisotropy measurements on samples from the lower continental crust were made to test the hypothesis that anisotropy may cause deflection of the peak-to-trough axes of magnetic anomalies caused by mid- to lower crustal sources. Average anisotropy (P′) for these samples is 1.5 but can be as high as 3.4. Felsic granulite facies rocks show the highest anisotropy. Magnetic sources with P′ equal to the maximum determined in this study can cause azimuthal rotations of magnetic anomalies by up to about 25°, but P′ lower than or equal to 1.5 causes no significant rotation. Comparison of the model results to the abnormal shape of some magnetic anomalies in southern Italy indicate that these anomalies cannot be related easily to a strong and coherent AMS of the source rocks. The most probable explanation of such shapes is the existence of a strong remanent magnetization in subsequently rotated source bodies.

Upward continuation of scattered potential field data

Numerous methods have been used for upward continuation, but most of them require data on a regular grid. Gridding can introduce errors that affect the continued data in an unpredictable way. To avoid this problem, we design a continuation operator used for the direct continuation of scattered data on a 3-D basis. In this approach a harmonic function, satisfying the constraints imposed by the measured data, is developed. The continuation is written in the form of a linear combination of the measured data, but it depends on the arbitrary choice of the topographic zero level. However, the coefficients of the linear combination depend only on the position of the data points. This allows the zero level to be estimated on the basis of the continuation of synthetic anomalies calculated between the starting and ending surface. An important feature of the method is its local character, which allows the reduction of computation time. Also, the stability of the method for noisy data is reasonably good. The method is applied to both synthetic and real cases. Synthetic examples show how gridding-related errors may affect the continuation when an irregular distribution of data points and a variable topography are considered.

Shape analysis of aeromagnetic anomalies in the southern Italian region for the evaluation of crustal block rotations

Abstract An extensive analysis of aeromagnetic data from the Italian region is presented, regarding the evaluation of the azimuthal direction of the total-magnetization vector. A method based on the Zietz and Andreasen (1967) correlation between anomaly shape and direction of the total magnetization vector was used. A discussion related to possible traps in using this method is presented, with the aid of several tests on synthetic anomalies. The analysis points out a number of anomalies revealing abnormal directions, either SW-NE or SE-NW, which were used to divide the investigated area into five regions with different trends. Finally, assuming elevated Koenigsberger ratios for the source-rocks, the declinations of the remanent magnetization vector were judged very similar to those estimated for the total-magnetization vector. Thus, by comparing these values with the normal declination of the area, rotation values of the crustal blocks related to the investigated anomalies were proposed. Finally, these were compared with the available geological and palaeomagnetic information and significant correspondences were found.

Some Aspects of the Interpretation of Gravity Data for the Study of Regional and Local Structures

During last years a tremendous improvement has been made in all the fields of geophysical prospecting. While other geophysical techniques have strongly gained efficiency also by the advent of a new generation of equipments and of field techniques, in the gravity prospecting field, this improvement has only been caused by the advent of computers which are more and more used in all the phases of treatment of data, i.e. data reduction, data analysis, data interpretation. Basic concepts in gravity prospecting techniques and instrumentation have in fact practically remained inaltered. The precision and the detailing that have been gained in gravity data interpretation have enormously increased the benefit to cost ratio for this methodology in respect to other geophysical techniques.