Analysis of converted‐wave extended images for migration velocity analysis

Abstract

Converted-wave data have been recognized to have potential in complementing conventional compressional data. However, imaging for converted-waves is more difficult mainly due to the need for estimating shear-wave velocities, in addition to compressional-wave velocities. The common practice is to obtain the shear-wave velocity by registering PS and PP images. Despite its low cost, this procedure is prone to error due to the assumption of imaging simple structure and due to the high potential for cycle skipping. On the other hand, we could obtain S velocities by adapting wave-equation migration velocity analysis (MVA) tools for shear waves. Our assumption is that we can update the S velocities while keeping the known P velocity fixed. We assume that the P-wave velocity is known from MVA of the PP component of the data, then we estimate the S-wave velocity from MVA on the PS component of the data. If P-and S-wave velocities are all correctly estimated, corresponding PP and PS events match in the migrated depth sections. This velocity analysis can make use of both common image gathers (CIGs) and common image point gathers (CIPs). We derive the moveout function for CIPs of converted-wave images and find that they present more complicated moveout than their pure-mode counterparts. We explore the applicability of differential semblance optimization (DSO) to the PP and PS gathers (CIPs and CIGs) to obtain objective functions, based which we can construct optimal velocity models. We find that the objective functions for both PP and PS data are convex, which warrants their use for migration velocity analysis using efficient gradient-descent numerical optimization schemes.

Elastic imaging with exact wavefield extrapolation for application to ocean-bottom 4C seismic data

“…Using our new extrapolation method, we would therefore expect similar improvements in elastic media. Moreover, because the penalty function usually defined for migration velocity analysis of elastic waves (Yan and Sava, 2010), which exploits the information contained in the extended images to invert for the velocity model, does not take into account the artificial events resulting from the vectorial extrapolation step, the inversion process could also benefit significantly from this new extrapolation procedure. However, although our new wavefield extrapolation procedure improves the elastic image, the receiver wavefields still show some nonphysical waves at early times (e.g., before a scattering event occurs -see Figure 5c -time 1) and partial wavefronts at later times (e.g., after a scattering event occurs -see Figure 5c times 3-5).…”

Joint PP and PS plane-wave wave-equation migration-velocity analysis

“…In practice, migration and the velocity model building are more robust and easier in the depth domain for structurally complex environments (Liu et al, 2007). Recently, Yan and Sava (2010) conducted MVA in the depth domain for inverting the S-wave velocity model using PS data only. Rosales et al (2008) suggested to make use of PS CIGs in the depth domain to update both the P-and S-wave velocity models.…”

Image registration guided wavefield tomography for shear-wave velocity model building

“…However, they use a Kirchoffbased migration, which is likely to break down in complex structures. Yan and Sava (2010) present a wave-equation migration velocity analysis (WEMVA) method that finds the S-wave velocities and P-S depth-migration images simultaneously, but the computation of elastic angle gathers is expensive. Moreover, the method does not impose any constraint on the proximity of P-S to P-P images.…”