On the quantitative interpretation of magnetic anomalies by pseudo-gravimetric integration.     

Maurizio Fedi 《地学学报》1989,1(6):564-572

Poisson's theorem relates linearly the derivative of the gravity potential (taken along the total magnetization vector direction) and the magnetic potential due to a common, isolated source with constant density and magnetization distributions.
From that theorem, two very useful functional transformations for magnetic anomalies were formulated by Baranov, that is the 'pseudo-gravimetric integration' and the 'reduction to the pole'.
In this paper, it is shown that the first transformation is, in some sense, more useful than the reduction to the pole and, further, that its utilization presents noticeable advantages as regards the interpretation of magnetic anomalies. In fact, the pseudo-gravimetric integration, in contrast to the reduction to the pole, also provides a smoothing of the anomaly map and, in particular, reduces the degree of non-linearity existing in the mathematical relationship between the data and the source depth.
In order to show this last property of the transformation, spectral expansion inversions by 2_1/2D structures were performed on both magnetic and pseudo-gravimetric synthetic anomalies and the respective speed and precision were compared. The tests clearly indicate that very accurate estimates of the geometric parameters of the known source can be more easily and rapidly calculated by the transformed anomalies. Furthermore, due to its smoothing property, the method seems particularly useful when data are noisy. Tests on real anomalies confirm these results.  相似文献   

Population effects on the red giant clump absolute magnitude, and distance determinations to nearby galaxies     

Léo Girardi  Maurizio Salaris 《Monthly notices of the Royal Astronomical Society》2001,323(1):109-129

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The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil     

Giorgio Rivalenti  Alberto Zanetti  Vicente A.V. Girardi  Maurizio Mazzucchelli  Colombo C.G. Tassinari  Gustavo W. Bertotto 《Lithos》2007,94(1-4):111-131

New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher ¹⁴³Nd/¹⁴⁴Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest ¹⁴³Nd/¹⁴⁴Nd values at any given ⁸⁷Sr/⁸⁶Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.  相似文献   

Decorrugation and removal of directional trends of magnetic fields by the wavelet transform: application to archaeological areas   总被引：1，自引：0，他引：1  

Maurizio Fedi  Giovanni Florio 《Geophysical Prospecting》2003,51(4):261-272

The problem of removing directional trends frequently occurs in the processing of magnetic data and also in the subsequent steps of data interpretation. The so-called corrugations are typical directional trends occurring in levelled data, which may be removed in several ways. Classical techniques are based on high-pass filtering of the data and successively filtering these transformed data with directional cosine filters. Other linear features are due to real sources, such as pipelines in shallow surveys or dike swarms in regional surveys. They should, nevertheless, be considered as noise, due to the fact that their effect is strong and tends to hide the field features related to structures of more interest. We deal with both kinds of problem, presenting the results of a study in an archaeological area of southern Italy. Decorrugation of magnetic field anomalies is performed using a method based on the excellent space–frequency localization properties of wavelet bases, allowing a very sharp filtering of the field along a selected direction. We compare this technique with the classical one in a synthetic case and find that the wavelet decorrugation is simpler and produces low distortion maps. Besides the field decorrugation, the wavelet approach was also shown to be useful in the subsequent enhancement of the measured field. In fact, we show that the wavelet analysis offers a unique framework where various filtering problems (directional, isotropic, global or local as well) may be easily solved. As regards the archaeological case, strong noisy effects from elongated sources (pipelines) were successfully removed in a sharp and local way.  相似文献   

A porous flow model of magma migration within Mt. Etna: The influence of extended sources and permeability anisotropy     

Maurizio Bonafede  Nicola Cenni 《Journal of Volcanology and Geothermal Research》1998,81(1-2)

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Modelling gravity variations consistent with ground deformation in the Campi Flegrei caldera (Italy)     

Maurizio Bonafede  Mariangela Mazzanti 《Journal of Volcanology and Geothermal Research》1998,81(1-2)

In recent years (1970–72 and 1982–84) two inflation episodes took place in the Campi Flegrei caldera (Italy), characterized by significant ground uplift and gravity variations. An elastic half-space model with vertical density stratification is employed to compute the displacement field and the gravity variations produced by the deformation of buried layers, following the inflation of a spherically symmetric deformation source. Contributions to gravity variations are produced by dilation/contraction of the medium, by the displacements of density interfaces (the free surface and subsurface layers) and of source boundaries and, possibly, by new mass input from remote distances into the source volume. Three cases were examined in detail: In case I, the magma chamber is identified as the deformation source and volume and pressure increase in the magma chamber is due to input of new magma from remote distances; in case II deformation is due to magma differentiation within the magma chamber (deformation source with constant mass); in case III the geothermal system is identified as the deformation source and a pressure increase, possibly driven by the exsolution of high temperature and high pressure volatiles in the magma chamber, is assumed to play a dominant role. From the comparison between measured and computed gravity residuals (free-air-corrected gravity variations) we can assess that, in case I, an inflation source with constant density would predict gravity residuals compatible with observations, whereas an expansion at constant mass (case II) would predict gravity residuals much lower than observed. The resolving power of gravity data however prevents accurate assessment of the density of the emplaced material. In case III, the pervasive density increase of the geothermal fluids induced by pressure increase is assumed to be the main source of gravity variations. The average porosity value required for this model to match both the ground deformation and the gravity residuals is found to be ˜10%, a value which is compatible with measured porosity values at Campi Flegrei in deep wells. The subsidence phases following both inflation episodes and the gravity residuals during subsidence lead us to consider case III as more plausible, even if a suitable combination of cases I and III cannot be discarded.  相似文献   

Thermal, seismic and infrasonic evidences of variable degassing rates at Stromboli volcano     

Maurizio Ripepe  Andrew J. L. Harris  Roberto Carniel 《Journal of Volcanology and Geothermal Research》2002,118(3-4)

During June 1999, we measured the amplitude and rate (number of events per second) at which gas exited the vent at Stromboli volcano as discrete gas bursts or puffs. This allowed us to identify two styles of gas burst (puffing) activity. The first is characterized by frequent, rapidly rising puffs, the second by less frequent, slowly rising puffs. Each style persisted over 5–40-min-long durations and was associated with a high and low number of strombolian explosions per hour, respectively. Each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions; the delays were longer during vigorous puffing periods. To explain our observations, we propose a model in which the degassing process cycles between vigorous and weak degassing phases. During vigorous degassing phases, bubble layers ascend the conduit at a frequency of 0.5–1.0 s⁻¹. This high degassing level reflects a gas-rich magma column and leads to an increased rate in the formation of shallow foams and, hence, an increase in puffing and explosive activity, as well as a higher free surface level and/or gas jet velocity. During weak phases, bubble layers ascend the conduit at a reduced frequency of 0.2–0.3 s⁻¹. During such times the magma column is poor in gas. This leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity, as well as a lower free surface level and/or gas jet velocity. Variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh, gas-rich magma. Our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes.  相似文献   

Linear baroclinic instability in the presence of heat inflow from the lower boundary     

Maurizio Fantini 《Annales Geophysicae》1995,13(4):419-426

A linear Eady model with a parameterization of heat influx from the lower boundary is studied analytically in order to obtain the characteristics of baroclinic normal modes modified by this non-adiabatic source. The results display a secondary maximum of growth rate at high wave numbers and a range of absolutely unstable waves, thus suggesting that the property observed among mid-latitude explosive cyclones of being near-stationary in the phase of maximum growth may be captured by this representation of the air-sea energy exchange.  相似文献   

On using Boussinesq-type equations near the shoreline: a note of caution   总被引：1，自引：0，他引：1  

Giorgio Bellotti  Maurizio Brocchini   《Ocean Engineering》2002,29(12):1569-1575

We briefly analyze some characteristics of the behavior in very shallow waters i.e. near the shoreline of high-order (dispersive-nonlinear) Boussinesq-type equations. By using the Carrier and Greenspan (1958) solution as test flow conditions we illustrate the behavior of both purely dispersive and dispersive-nonlinear contributions near the shoreline. It is also shown that Boussinesq-type equations can be more usefully handled in the swash zone if written in terms of the total water depth.  相似文献   

Temporal variations in the spatial distribution of shoot density in a Posidonia oceanica meadow and patterns of genetic diversity     

Valerio Zupo  Maria Cristina Buia  Maria Cristina Gambi  Maurizio Lorenti  & Gabriele Procaccini 《Marine Ecology》2006,27(4):328-338

The meadows of the Mediterranean endemic seagrass Posidonia oceanica exhibit relatively high variations of structural and biometric features at various spatial scales. An investigation performed in 1992 in the meadow off Lacco Ameno (Island of Ischia, Gulf of Naples, Italy) detected peculiar spatial patterns of plant distribution, characterized by nestlike structures with radially increasing or decreasing shoot densities. Eight years later (2000), geo‐referenced collections at selected points were repeated to trace the temporal variations of shoot density and investigate the recurrence of the density cores previously detected. In addition, shoots for molecular analyses were collected to check the hypothesis that nestlike patterns exhibit highest levels of genetic variability, due to the confluence of several genetically distinct stolons. The 2000 survey confirmed the presence of the main density cores detected in 1992, although their spatial distribution was slightly shifted and a general decrease of spatial anisotropy was observed, probably due to an increased disturbance, mainly due to pleasure boat anchoring. Patterns of genetic diversity showed a more complex picture, well related to the shoot density spatial pattern, especially when compared with the previous 1992 survey. Patterns of genetic diversity confirmed our previous hypotheses on the genesis of shoot density cores, suggesting they are produced over long time, due to a slow stolonization process and a convergence of different genotypes. Regression of the meadow and decrease of density may lead, in short periods, to a homogenization of the density patterns, while genetic diversity cores represent a long‐term ‘memory’ of their previous distribution.  相似文献