525 ANNALS OF GEOPHYSICS, VOL. 46, N. 3, June 2003 A realistic inversion algorithm for magnetic anomaly data: the Mt. Amiata volcano test Osvaldo Faggioni ( 1 )( 2 ), Nicolò Beverini ( 1 )( 3 ), Fabio Caratori Tontini ( 1 )( 3 ), Cosmo Carmisciano ( 1 ) and Iacopo Nicolosi ( 4 ) ( 1 ) Istituto di Geofisica Marina, Università di Pisa, Consorzio Universitario della Spezia, Italy ( 2 ) Dipartimento di Scienze della Terra, Università di Pisa, Italy ( 3 ) Dipartimento di Fisica and INFM, Università di Pisa, Italy ( 4 ) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Abstract 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. Mailing address: Dr. Osvaldo Faggioni, Istituto di Geofisica Marina, Università di Pisa, Consorzio Universitario della Spezia, Via Vailunga 1, 19125 La Spezia, Italy; e-mail: of@castagna.it Key words magnetic methods - Mt. Amiata volcano - potential field inversion 1. Introduction Geophysical data, and above all geomagnetic data, are particularly useful in volcanology to define the characteristics of a volcanic complex. A geomagnetic data set is very informative, in fact, if connected to magmatic rocks or lytotypes that show a particular intense value of the mag- netic susceptibility. Using adequate inversion software, one can find solutions that seem to correspond to the tectonic-structural model of the subsurface, but additional information can be obtained over areas that cannot be investigated directly. Practically, the geomagnetic data are used to find solution about an equivalent source that should be responsible for the observed data, through the inversion process. The transfer from the observed data to a meaningful set of information on the generating source is not obvious, and has a fun- damental relevance. This is particularly true for the geomagnetic field, that because of its di-