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• W144480504 abstract "High-frequency seismograms contain features that reflect the random inhomogeneities of the earth. In this work I use an imaging method to locate the high contrast small-scale heterogeneity respect to the background earth medium. This method was firstintroduced by Nishigami (1991) and than applied to different volcanic and tectonicallyactive areas (Nishigami, 1997, Nishigami, 2000, Nishigami, 2006).The scattering imaging method is applied to two volcanic areas: Campi Flegreiand Mt. Vesuvius. Volcanic and seismological active areas are often characterizedby complex velocity structures, due to the presence of rocks with different elasticproperties. I introduce some modifications to the original method in order to make itsuitable for small and highly complex media. In particular, for very complex mediathe single scattering approximation assumed by Nishigami (1991) is not applicable asthe mean free path becomes short. The multiple scattering or diffusive approximationbecome closer to the reality. In this thesis, differently from the ordinary Nishigami’smethod (Nishigami, 1991), I use the mean of the recorded envelope as referencecurve and calculate the variations from this envelope. In this way I implicitlydo not assume any particular scattering regime for the average scattered radiation,whereas I consider the variations as due to waves that are singularly scattered fromthe strongest heterogeneities. The imaging method is applied to a relatively small area(20 x 20 km), this choice being justified by the small length of the analyzed codas ofthe low magnitude earthquakes.I apply the unmodified Nishigami’s method to the volcanic area of Campi Flegreiand compare the results with the other tomographies done in the same area. Thescattering images, obtained with frequency waves around 18 Hz, show the presenceof high scatterers in correspondence with the submerged caldera rim in the southernpart of the Pozzuoli bay. Strong scattering is also found below the Solfatara crater,characterized by the presence of densely fractured, fluid-filled rocks and by a strongthermal anomaly.The modified Nishigami’s technique is applied to the Mt. Vesuvius area. Resultsshow a low scattering area just below the central cone and a high scattering areaaround it. The high scattering zone seems to be due to the contrast between the highrigidity body located beneath the crater and the low rigidity materials located aroundit. The central low scattering area overlaps the hydrothermal reservoirs located belowthe central cone.An interpretation of the results in terms of geological properties of the mediumis also supplied, aiming to find a correspondence of the scattering properties and thegeological nature of the material.A complementary result reported in this thesis is that the strong heterogeneityof the volcanic medium create a phenomenon called coda localization. It has beenverified that the shape of the seismograms recorded from the stations located at the topof the volcanic edifice of Mt. Vesuvius is different from the shape of the seismogramsrecorded at the bottom. This behavior is justified by the consideration that the codaenergy is not uniformly distributed within a region surrounding the source for greatlapse time." @default.
• W144480504 created "2016-06-24" @default.
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• W144480504 date "2008-06-20" @default.
• W144480504 modified "2023-09-24" @default.
• W144480504 title "Elastic Propagation in random media: applications to the imaging of volcano structures" @default.
• W144480504 doi "https://doi.org/10.6092/unibo/amsdottorato/870" @default.
• W144480504 hasPublicationYear "2008" @default.
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