Skip to Main content Skip to Navigation
Theses

Imagerie sismique du Vésuve

Abstract : Reflected waves carry information about the position, shape and physical properties of the geological discontinuities that originate them. The Ray+Born preserved amplitude migration gives, in a single step, an in-depth image of the discontinuities, and an estimate of the associated contrast in seismic properties. From the theoretical point of view, we show that a slight assumption on the S velocity field allows the simultaneous inversion of pp reflected and PS converted waves, and the optimization of the summations in the frequency space involved when anelastic attenuation is accounted for. The strict application of the Ray+Born migration on synthetics associated with a real experiment on Vesuvius leads to imperfect results: The amplitude of reflection events cannot be described by diffraction laws at wide angle when the expected velocity contrasts are large with respect to the available background velocity mode!. A way around this obstacle is an alternative analysis of the wave amplitudes, based on the reflective point, and no more on the scatterer, that also interprets relative changes in amplitude to avoid the estimation of the source amplitude. This technique is applied to investigate a mid-crust reflector under Mt. Vesuvius, with the data of an active seismic experiment performed in the Bay of Naples (Mareves97). With a global velocity model built through the analysis of first arrivaI times, we apply an empirical migration based on diffraction summation to draw a 2D image of the reflector. It appears flat, horizontal, at the depth of 8 Km b.s.l, and extends at least 35 Km laterally. The migration of synthetics confirm that the waves that mainly contribute to the interface image are PS converted at depth, and that their migration is strikingly stable with respect to the imperfections of the velocity model, thereby constraining the minimum lateral extension of the reflector, and its low mean dip. In a second time, with this geometrical model of the interface, we compare the PS and first arrivai amplitudes to estimate the conversion coefficient variations with angle, independently of the source amplitude and local effects in the vicinity of the receivers. These variations are found very slow, and imply that the S-wave velocity is very low in the underlying medium (Vs < 1 Km/s). The migration of synthetics support this conclusion, and further constrain the P wave velocity, bound to be very low as weil (Vp ~ 2.5 Km/s).
Complete list of metadatas

https://tel.archives-ouvertes.fr/tel-00715724
Contributor : Pascale Talour <>
Submitted on : Monday, July 9, 2012 - 10:43:12 AM
Last modification on : Wednesday, December 9, 2020 - 3:08:02 PM
Long-term archiving on: : Thursday, December 15, 2016 - 9:35:12 PM

Identifiers

  • HAL Id : tel-00715724, version 1

Citation

Emmanuel Auger. Imagerie sismique du Vésuve. Géophysique [physics.geo-ph]. Université Nice Sophia Antipolis, 2000. Français. ⟨tel-00715724⟩

Share

Metrics

Record views

640

Files downloads

201