RTM noise attenuation and image enhancement using time-shift gathers

SEG Technical Program Expanded Abstracts 2014

2014

Multiparameter full-waveform inversion (FWI) suffers from complex nonlinearity in the objective function, compounded by the eventual trade-off between the model parameters. A hierarchical approach based on frequency and arrival time data decimation to maneuver the complex nonlinearity associated with this problem usually falls short in anisotropic media. In place of data decimation, I use a model gradient filter approach to access the parts of the gradient more suitable to combat the potential nonlinearity and parameter trade-off. The filter is based on representing the gradient in the time-lag normalized domain, in which small scattering-angles of the gradient update are initially muted out. The model update hierarchical filtering strategy include applying varying degrees of filtering to the different anisotropic parameter updates, a feature not easily accessible to simple data decimation. Using FWI and reflection-based FWI, when the modeled data are obtained with the single-scattering theory, allows access to additional low model wavenumber components. Combining such access to wavenumbers with scattering-angle filters applied to the individual parameter gradients allows for multiple strategies to avoid complex FWI nonlinearity as well as the parameter trade-off.

Section: Filtering the Gradientmentioning

confidence: 99%

Section: Filtering the Gradientmentioning

confidence: 99%

Conditioning the full waveform inversion gradient to welcome anisotropy

SEG Technical Program Expanded Abstracts 2014

2014

“…The involvement of the nonstationary velocity term cðxÞ in the ADCIG construction (equation 15) prevents us from an efficient Fourier-type implementation. This burden can easily be alleviated by replacing τ with the variable ξ ¼ ðτ∕2ÞcðxÞ (Khalil et al, 2013;Alkhalifah, 2015).…”

Section: Excitation Amplitude Imaging Conditionmentioning

confidence: 99%

Common-image gathers using the excitation amplitude imaging condition

Kalita

2016

GEOPHYSICS

Common-image gathers (CIGs) are extensively used in migration velocity analysis. Any defocused events in the subsurface offset domain or equivalently nonflat events in angle-domain CIGs are accounted for revising the migration velocities. However, CIGs from wave-equation methods such as reverse time migration are often expensive to compute, especially in 3D. Using the excitation amplitude imaging condition that simplifies the forward-propagated source wavefield, we have managed to extract extended images for space and time lags in conjunction with prestack reverse time migration. The extended images tend to be cleaner, and the memory cost/disk storage is extensively reduced because we do not need to store the source wavefield. In addition, by avoiding the crosscorrelation calculation, we reduce the computational cost. These features are demonstrated on a linear vðzÞ model, a twolayer velocity model, and the Marmousi model.

“…Using the normalized time-lag representation (Khalil et al, 2013), the process of a gradient or an image extension in the frequency domain is given by the following formula…”

Section: Extension In Velocity Vs Extension In Timementioning

confidence: 99%

“…To remove the low frequency imaging artifacts in RTM, we can use any function w(K) increasing with absolute K values. We take a smoothed version of the characteristic function (Khalil et al, 2013):…”

Section: Scattering Angle Based Filtering the Efficient Filtering Algmentioning

confidence: 99%

Scattering angle-based filtering via extension in velocity

Kazei

Tessmer

SEG Technical Program Expanded Abstracts 2016

2016

SUMMARYThe scattering angle between the source and receiver wavefields can be utilized in full-waveform inversion (FWI) and in reverse-time migration (RTM) for regularization and quality control or to remove low frequency artifacts. The access to the scattering angle information is costly as the relation between local image features and scattering angles has non-stationary nature. For the purpose of a more efficient scattering angle information extraction, we develop techniques that utilize the simplicity of the scattering angle based filters for constantvelocity background models. We split the background velocity model into several domains with different velocity ranges, generating an "extension in velocity". We establish an analytical relation between the proposed extension and conventional RTM images extended in time and show that extending with a few samples in the newly introduced dimension is sufficient to create a scattering angle based filter. Finally the new efficient algorithm is used in a conventional FWI scheme to filter the misfit gradients and improve inversion results.