3-D focused inversion of near-seafloor magnetic data with application to the Brothers volcano hydrothermal system, Southern Pacific Ocean, New Zealand

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): B10102, doi:10.1029/2012JB009349.We describe and apply a new inversion method for 3-D modeling of magnetic anomalies designed for general application but which is particularly useful for the interpretation of near-seafloor magnetic anomalies. The crust subsurface is modeled by a set of prismatic cells, each with uniform magnetization, that together reproduce the observed magnetic field. This problem is linear with respect to the magnetization, and the number of cells is normally greater than the amount of available data. Thus, the solution is obtained by solving an under-determined linear problem. A focused solution, exhibiting sharp boundaries between different magnetization domains, is obtained by allowing the amplitudes of magnetization to vary between a pre-determined range and by minimizing the region of the 3-D space where the source shows large variations, i.e., large gradients. A regularization functional based on a depth-weighting function is also introduced in order to counter-act the natural decay of the magnetic field intensity with depth. The inversion method has been used to explore the characteristics of the submarine hydrothermal system of Brothers volcano in the Kermadec arc, by inverting near-bottom magnetic data acquired by Autonomous Underwater Vehicles (AUVs). Different surface expressions of the hydrothermal vent fields show specific vertical structures in their underlying demagnetization regions that we interpret to represent hydrothermal upflow zones. For example, at focused vent sites the demagnetized conduits are vertical, pipe-like structures extending to depths of ~1000 m below the seafloor, whereas at diffuse vent sites the demagnetization regions are characterized by thin and inclined conduits.This contribution was made possible through funding by the New Zealand Foundation for Research, Science and Technology (FRST contract C05X0406) and by the Royal Society of New Zealand by the Marsden Fund (grant GNS1003).2013-04-1

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....

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

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 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 ....

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

The revised aeromagnetic anomaly map of Italy

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

A realistic inversion algorithm for magnetic anomaly data: the Mt. Amiata volcano test

The aim of this work is the formulation of a 3D model of the Mt. Amiata volcanic complex (Southern Tuscany) by means of geomagnetic data. This work is shown not only as a real test to check the validity of the inversion algorithm, but also to add information about the structure of the volcanic complex. First, we outline briefly the theory of geomagnetic data inversion and we introduce the approach adopted. Then we show the 3D model of the Amiata volcano built from the inversion, and we compare it with the available geological information. The most important consideration regards the surface distribution of the magnetization that is in good agreement with rock samples from this area. Moreover, the recovered model orientation recall the extension of the lava flows, and as a last proof of validity, the source appears to be contained inside of the topographic contour level. The credibility of the inversion procedure drives the interpretation even for the deepest part of the volcano. The geomagnetic signal appears suppressed at a depth of about 2 km, but the most striking consequence is that sub-vertical structures are found even in different positions from the conduits shown in the geologic sections. The results are thus in good agreement with the information obtained from other data, but showing features that had not been identified, stressing the informative power of the geomagnetic signal when a meaningful inversion algorithm is used