Estimativa de parâmetros elásticos em meios anisotrópicos
Amplitude, polarization and the slowness vector measurements carry information about the medium where wave propagation occurs. This thesis investigates these data aiming at the recovery of elastic properties in anisotropic media. Reflection coefficients can be estimated from amplitude data and dep...
| Autor principal: | GOMES, Ellen de Nazaré Souza |
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| Grau: | Tese |
| Idioma: | por |
| Publicado em: |
Universidade Federal do Pará
2014
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| Assuntos: | |
| Acesso em linha: |
http://repositorio.ufpa.br/jspui/handle/2011/5660 |
| Resumo: |
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Amplitude, polarization and the slowness vector measurements carry information about the
medium where wave propagation occurs. This thesis investigates these data aiming at the
recovery of elastic properties in anisotropic media. Reflection coefficients can be estimated from amplitude data and depend nonlinearly on
elastic and density contrasts across an interface. When the impedance contrast is weak, the linear
approximations for the qP reflectivity are more convenient for inversion of density and elastic
parameters using analysis of amplitude versus the angle of incidence (AVO) and amplitude
versus the direction of the incidence plane (AVD). Partitioning the linear system defined by Zoepprittz equations allows one to write the
solution of these equations in terms of impedance and polarization matrices. Using this solution,
linear approximations for the qP reflectivity are derived for weak impedance contrasts and
arbitrary symmetry classes of anisotropy. The linear approximations are evaluated for different
acquisition geometries and choice of the reference medium. The approximations for the reflection
coefficients of the reflected qP and the converted waves are in good agreement with the exact
solution for incidence angles up to 30° for media that satisfy the weak impedance assumption. If a single oriented set of fractures is represented by a transversely isotropic effective
medium, the linear approximations for qP reflectivity can be used to estimate the fractures
orientation. Under these assumptions this problem is reframed as the estimation of the symmetry
axis orientation from qP reflectivity data. This work shows the requirement of multiple
components and multiple azimuthal data and quantifies the minimum amount of data for stable
estimation. Also it is shown that the reflection coefficients of converted waves qS and qT only
are sensitive to fractures dip. The inversion of polarization and slowness from multiazimutal VSP data are investigated for
the estimation of local anisotropy. We use measurements of the vertical component of the
slowness vector and the qP polarization data of direct and reflected waves. The inversion
algorithm is validated in synthetic data sets for different choices of the wave front normal,
reference medium and acquisition geometries. This analysis shows that only a subset of elastic
parameters is recovered. An important application of this approach is its potential to determine the class of anisotropy. The application of this methodology to the Java Sea data set shows that
isotropy and transversely isotropic models are inadequate to fit the data. |