301 research outputs found
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....
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
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....
Metodologie magneto-gradiometriche applicate ad indagini marine di tipo ambientale all’interno del Mar Piccolo (Taranto)
La caratteristica fondamentale dei metodi di potenziale, è quella di aver il pregio di fornire una risposta relativamente rapida e non invasiva dell’oggetto in studio, che siano strutture geologiche (studi di tipo tettonico o minerario) o isolati corpi suscettivi sepolti (studi prettamente ambientali – relitti, discariche sottomarine, fusti metallici sepolti ecc..). Nei rilievi magnetici marini relativi a quest’ultimo caso, per ottenere dei risultati soddisfacenti, sono di fondamentale importanza alcuni requisiti quali un accurato sistema di posizionamento GPS, l’utilizzo di vettori navali amagnetici, una elevata sensibilità della strumentazione impiegata, e la corretta rimozione degli effetti temporali legati alle variazioni del Campo Magnetico terrestre. Quest’ultimo aspetto è assai delicato perché comporta la necessità di avere un osservatorio fisso coerente in misura durante l’esecuzione del rilievo.
Per superare questo problema sono assai efficaci metodologie di tipo gradiometrico, che si basano sullo studio del gradiente orizzontale del Campo Magnetico terrestre, che si ottiene mediante l’acquisizione contemporanea di due misure (due sensori) poste ad una determinata distanza tra loro.
I principali vantaggi introdotti dall’utilizzo di queste tecniche consistono principalmente in una significativa attenuazione dei contributi derivanti dalle sorgenti più profonde, la non necessità delle correzioni temporali ed in una maggiore rapidità nelle procedure di elaborazione dei dati.
In questa ottica è stato condotto uno studio applicando questo tipo di metodologia in un contesto prettamente ambientale ovvero l’individuazione di corpi metallici in un’area ad elevato noise ambientale.
L’area del rilievo è situata all’interno del Mar Piccolo di Taranto, a ridosso delle strutture portuali e si estende per circa 3 X 1,5 Km.
I rilievi sono stati condotti utilizzando una piccola imbarcazione (idrobarca in GRP - Glass Renforced Plastic) trainante l’apparato di misura filato ad una distanza tale da non risentire del disturbo elettromagnetico generato dai motori e dalle apparecchiature di bordo. Problematiche relative ai bassi fondali dell’area rilevata (da 4 a 13 metri di battente d’acqua) hanno comportato la progettazione di un apposito catamarano in vetroresina (completamente amagnetico) a cui è stata fissata la sensoristica, così da poter navigare poco sotto il pelo dell’acqua; per motivi di navigabilità sono stati pianificate rotte principali in direzione Est-Ovest e traverse di controllo nelle aree con segnali ‘interessanti’. La strumentazione magnetometrica utilizzata (fig. 1) è costituita da due sensori a vapori di Cesio che sfruttano la tecnica del pompaggio ottico. La misura della frequenza viene effettuata con 10 letture al secondo, l’operatività è garantita in un range tra 20.000 – 100.000 nT e l’accuracy assoluta dipende da diversi fattori quali: l’orientazione del sensore, lo shift della luce interna e il tempo del frequency counter. L’errore dovuto all’orientazione risulta comunque compreso entro 1 nT. L’operatività viene garantita in un range da –35° e +50° di orientazione dei sensori. la sensibilità è di 0.02 nT con un intervallo di campionamento di 10 misure al secondo.
Nonostante l’elevato noise ambientale causato dalla presenza di navi, bacini e altre sorgenti di noise legate alle attività portuali, sono stati evidenziati 9 punti anomali di interesse nella mappa del segnale gradiometrico (fig. 2). Le successive ispezioni dirette, hanno rilevato la presenza di evidenze (vari materiali di tipo ferroso e spezzoni di battelli) correlate alle anomalie di 7 punti tra quelli individuati a seguito dell’elaborazione dei dati
Integrated MVG and ERT Survey Over a Shallow Cave
An integrated geophysical MVG (Microgravity Vertical Gradient) and ERT
(Electrical Resistivity Tomography) survey was performed over a shallow
cave in the Armetta Mountain karst area, close to the Liguria-Piedmont wa-
tershed (Tanaro valley). The aim of this study is to test the response of a
known shallow karst cave. The cave was developed in the Mesozoic sedimen-
tary cover (dolostones and limestones - CAU : Caprauna Armetta Unit); the
shallowest portion of the cave exhibits narrow passages and, at about 30 m
below the entrance, a fossil meander which links two large chambers, that
represent the target of the geophysical survey....
- …