Elastic wavefield inversion [microform]
- Bib ID:
- 3282836
- Format:
- Book and Microform
- Author:
- Mora, Peter Ronald
- Description:
- 160 p.
- Summary:
-
The goal of seismology is to obtain Earth properties from seismic data. I derive formulas based on nonlinear least squares iterations to find the elastic properties of the Earth corresponding to the synthetic wavefield that best matches the seismic observations. Primary P- and S-wave reflections, mode converted waves and Rayleigh waves are all theoretically useful in the inversions. Beginning with a starting guess, the Earth properties are iteratively updated using a preconditioned conjugate-gradient algorithm. The gradient direction is cast in terms of two wave propagations so Frechet matrices are not necessary and the calculations are easily implemented on fine grain parallel computers. Although the derivation is in three dimensions and commences from the anisotropic elastic vave equation, I have chosen to concentrate on the special case of an isotropic Earth and solve for the compressional and shear wavespeeds.
This case is important because the wavespeeds are the properties that have the largest influence on seismic waves. The sudden (high-wavenumber) variations in the wavespeeds at layer boundaries determine the amplitudes of reflected waves while the gross (low-wavenumber) variations in the layers determine the traveltimes of reflected and transmitted waves. The method is tested by inverting 2D synthetic reflection seismograms (shot profiles), 2D synthetic transmission seismograms (VSP's) and real reflection seismograms (a shot profile). The wave propagations in the tests are done using elastic finite differences to allow for Earth models of realistic complexity. The synthetic data results indicate that low-wavenumber components in wavespeed converge slowly when only reflection data were inverted. This was not expected considering that traveltime curves of reflections should rapidly resolve the low wavenumbers.
However, by analyzing an acoustic equivalent of the algorithm in detail, I have identified reflection-tomography-like terms and migration-like terms in the inversion formulas. I prove that the low wavenumbers are resolved by the tomography-like terms and suggest how to get an even rate of convergence of the high and low wavenumbers. I conclude that my iterative elastic inversion formulas obtain all wavenumbers of the compressional and shear wavespeeds that are resolvable separately by migration and tomography. (Abstract shortened with permission of author.)
- Notes:
-
- (UnM)AAI8800996
- Source: Dissertation Abstracts International, Volume: 49-02, Section: B, page: 0343.
- Thesis (Ph.D.)--Stanford University, 1987.
- Reproduction:
- Microfiche. Ann Arbor, Mich.: University Microfilms International.
- Subject:
- Geophysics
- Other authors/contributors:
- Stanford University
- Copyright:
-
In Copyright
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Copyright status was determined using the following information:
- Material type:
- Literary Dramatic Musical
- Published status:
- Unpublished
- Creation date:
- 1987
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