Elastic Propagation in random media: applications to the imaging of volcano structures
Creators: name: Tramelli, Anna <1979>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: FIS/06 Fisica per il sistema terra e il mezzo circumterrestre

description: 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 first
introduced by Nishigami (1991) and than applied to different volcanic and tectonically
active areas (Nishigami, 1997, Nishigami, 2000, Nishigami, 2006).
The scattering imaging method is applied to two volcanic areas: Campi Flegrei
and Mt. Vesuvius. Volcanic and seismological active areas are often characterized
by complex velocity structures, due to the presence of rocks with different elastic
properties. I introduce some modifications to the original method in order to make it
suitable for small and highly complex media. In particular, for very complex media
the single scattering approximation assumed by Nishigami (1991) is not applicable as
the mean free path becomes short. The multiple scattering or diffusive approximation
become closer to the reality. In this thesis, differently from the ordinary Nishigami’s
method (Nishigami, 1991), I use the mean of the recorded coda envelope as reference
curve and calculate the variations from this average envelope. In this way I implicitly
do not assume any particular scattering regime for the "average" scattered radiation,
whereas I consider the variations as due to waves that are singularly scattered from
the 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 of
the low magnitude earthquakes.
I apply the unmodified Nishigami’s method to the volcanic area of Campi Flegrei
and compare the results with the other tomographies done in the same area. The
scattering images, obtained with frequency waves around 18 Hz, show the presence
of high scatterers in correspondence with the submerged caldera rim in the southern
part 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 strong
thermal anomaly.
The modified Nishigami’s technique is applied to the Mt. Vesuvius area. Results
show a low scattering area just below the central cone and a high scattering area
around it. The high scattering zone seems to be due to the contrast between the high
rigidity body located beneath the crater and the low rigidity materials located around
it. The central low scattering area overlaps the hydrothermal reservoirs located below
the central cone.
An interpretation of the results in terms of geological properties of the medium
is also supplied, aiming to find a correspondence of the scattering properties and the
geological nature of the material.
A complementary result reported in this thesis is that the strong heterogeneity
of the volcanic medium create a phenomenon called "coda localization". It has been
verified that the shape of the seismograms recorded from the stations located at the top
of the volcanic edifice of Mt. Vesuvius is different from the shape of the seismograms
recorded at the bottom. This behavior is justified by the consideration that the coda
energy is not uniformly distributed within a region surrounding the source for great
lapse time.