Reverse time migration (RTM) involves zero-lag cross-correlation of forward extrapolated source function wavefields and backward extrapolated receiver wavefields. For a near surface with complex structures and velocity anomalies, forward propagating the source wavelet generates wavefields containing reflections, near-surface multiples, and scattered direct arrivals. The wavefields are recorded as upgoing arrivals contaminated by the same reflections, near-surface multiples, and scattered signals, which can be critical for imaging near-surface structures and scatterers.
Here, we develop a new depth migration, duplex reverse time migration (DRTM) technique to improve imaging of complex near-surface structures. DRTM uses the direct arrival as a source to forward propagate and generate source wavefields, and reversely extrapolated recorded data in a zero-lag cross-correlation imaging condition to generate the final section. The interaction between the data components during cross- correlation can use primaries and multiples to image the near-surface structure correctly. Cross-talk artifacts may exist, but they are comparatively weak.
DRTM is demonstrated on both synthetic and field data examples showing an enhanced image in areas with complex near-surface structures compared to conventional RTM imaging methods. The new algorithm can significantly enhance shallow imaging without additional computation costs compared with conventional RTM. It can produce an image with higher resolution and signal-to-noise (S/N) ratio by replacing the source wavelet with the recorded direct arrivals, which include near-surface information necessary to boost the image in areas with near-surface complexity. Since the direct arrivals are one of the most energetic events recorded, the resultant image is typically of high S/N. The wave can also illuminate shallow zones better than primaries in marine environments.
Summary Full traveltime inversion (FTI) is a newly developed waveequation-based traveltime inversion methodology to automatically invert a kinematically accurate velocity model from traveltime information. The key idea of FTI is to make the inversion fully dependent on traveltime information, and thus prevent amplitude interference during inversion which produces strong artifacts in velocity updates. Here, we apply the image-domain FTI to a 2D marine dataset. The inversion result demonstrates that FTI is able to reconstruct both the top and bottom salt boundaries for data without low frequencies. Introduction Conventional wave-equation migration velocity analysis (WEMVA) is an automatic inversion scheme that attempts to recover the background velocity model from seismic data.
Differential semblance optimization (DSO) is a class of tomographic methods which updates the model by automatically optimizing the stack power of common image gathers (CIGs). Here we discuss the possible pitfalls among a class of DSO variants. Unlike residual moveout-based tomographic methods in which the kinematic information is explicitly measured, the image residual in DSO contains both kinematic and amplitude information. The latter severely hampers the inversion. A few modified DSO methods have been proposed to overcome this problem by constructing a new image residual which honors more kinematic information. However, they may fall into another pitfall that leads to a problematic velocity update in the inversion when the velocity structure produces caustics. In this abstract we provide a new perspective on addressing these issues and conclude that a combination of the traditional and modified DSO is a reasonable inversion strategy.
This paper has been withdrawn from the Technical Program and will not be presented at the 87th SEG Annual Meeting.
This paper presents a workflow for near-surface velocity automatic estimation using the early arrivals of seismic data. This workflow comprises two methods, source-domain full traveltime inversion (FTI) and early-arrival waveform inversion. Source-domain FTI is capable of automatically generating a background velocity that can kinematically match the reconstructed plane-wave sources of early arrivals with true plane-wave sources. This method does not require picking first arrivals for inversion, which is one of the most challenging aspects of ray-based first-arrival tomographic inversion. Moreover, compared with conventional Born-based methods, source-domain FTI can distinguish between slower or faster initial model errors via providing the correct sign of the model gradient. In addition, this method does not need estimation of the source wavelet, which is a requirement for receiver-domain wave-equation velocity inversion. The model derived from source-domain FTI is then used as input to early-arrival waveform inversion to obtain the short-wavelength velocity components. We have tested the workflow on synthetic and field seismic data sets. The results show source-domain FTI can generate reasonable background velocities for early-arrival waveform inversion even when subsurface velocity reversals are present and the workflow can produce a high-resolution near-surface velocity model.
Presentation Date: Wednesday, September 27, 2017
Start Time: 9:20 AM
Presentation Type: ORAL
Liu, Yujin (Aramco Research Center(Beijing), Aramco Asia) | Wu, Yan (Aramco Research Center(Beijing), Aramco Asia) | Wang, Xiongwen (Aramco Research Center(Beijing), Aramco Asia) | Fei, Tong (Saudi Aramco) | Luo, Yi (Saudi Aramco)
Common image gather (CIG) extraction is an important procedure for velocity analysis. Surface offset gathers (SOGs) provide longer and more reliable sampling of residual curvature than other CIGs, such as angle gathers, particularly those from deep events, which is useful for building the deep part of the velocity model. SOGs are available after application of Kirchhoff depth migration to properly sorted seismic data cubes with little extra cost, while it's very costly to compute from wave-equation (WE) migration directly. In this abstract, we apply plane-wave encoding to improve the efficiency of SOG extraction from WE migration. Geometric analysis demonstrates the equivalence of plane-wave SOGs and standard SOGs. Numerical tests show that our proposed method can efficiently extract SOGs with reliable residual moveout (RMO) information. We also use plane-wave SOGs in WE tomography and compare it with WE tomography based on plane-wave domain CIGs. Preliminary inversion results show its potential in improving subsalt velocity building.
Presentation Date: Monday, September 25, 2017
Start Time: 1:50 PM
Presentation Type: ORAL
A significant problem in seismic imaging is seismically
Sen, Vikram (BP Amoco) | Wagner, Donald E. (BP Amoco) | Etgen, John T. (BP Amoco) | Rietveld, Walter E.A. (BP Amoco) | Truxillo, Mark (BP Amoco) | Fei, Tong (BP Amoco) | Harrison, Holly (BP Amoco) | Mozer, Ed (BP Amoco) | Stauber, Doug (BP Amoco)
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