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Summary We present a method based on wavefield decomposition to improve wave equation migration velocity analysis. We decompose the gradient into long wavelength and short wavelength components. At early stage of the inversion, we only use the long wavelength component of the gradient. When the long wavelength part of the velocity is well resolved, we start to add more and more short wavelength components of gradient into the inversion. We demonstrate that wavefield decomposition helps the inversion to converge to the correct velocity model starting from essentially a v(z) initial starting model.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling (1.00)
Summary Seismic wave propagation is significantly affected due to the complexity in the near surface area. Therefore, it is important for subsurface imaging to obtain the near surface information as much as possible. Seismic attenuation, described by the quality or Q factor, has great effect on the seismic waveform. But it is rarely estimated for the near surface area. We develop a pseudo 2D elastic waveform inversion for determining QP factor in the shallow near surface area. The input data are the early arrival waveforms in the CMP domain. For the forward elastic wavefield modeling, we use a discrete wavenumber method for 1-D layer models. For inverting a QP model with fixed velocity structures, we apply a conjugate gradient method to solve a 2D problem. The output QP model is in 2D. We test our method on synthetics and also apply this method to field data from an oil field in China.
Summary We studied the effects of surface multiples on the velocity inversion result of wave-equation migration velocity analysis. Migration velocity analysis is based on the seismic migration (imaging) theory, which generally only takes into account the first-order linear born wave-scattering effect. The presence of multiples will incur error in the velocity estimation process and negatively affect the inversion result. Using the numerical simulation on Marmousi model, we examined the extent to which multiples energy could affect the migration velocity analysis results. We also examined the cases where near-offset multiples and far-offset multiples are tapered respectively. Analysis of these comparisons indicates that wave-equation migration velocity analysis is much less affected by near-offset multiples than by far-offset multiples.
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
- Geophysics > Seismic Surveying > Seismic Modeling > Velocity Modeling > Seismic Inversion (0.36)