|Numeric implementation of wave-equation migration velocity analysis operators|
Accurate wave-equation depth imaging requires accurate knowledge of a velocity model. The velocity model is used in the process of wavefield reconstruction, irrespective of the method used to solve the acoustic wave-equation, e.g. by integral methods (Kirchhoff migration), or by differential/spectral methods (migration by wavefield extrapolation and reverse-time migration).
Generally-speaking, there are two possible strategies for velocity estimation from surface seismic data in the context of wavefield depth migration. The two strategies differ by the domain in which the information used to update the velocity model is collected. The first strategy is formulated in the data space, prior to migration, and it involves matching of recorded and simulated data using an approximate (background) velocity model. Techniques in this category are known by the name of tomography (or inversion). The second strategy is formulated in the image space, after migration, and it involves measuring and correcting image features that indicate model inaccuracies. Techniques in this category are known as migration velocity analysis (MVA), since they involve migrated images and not the recorded data directly.
Tomography and migration velocity analysis can be implemented in various ways that can be classified based on the carrier of information from the data or image to the velocity model. Thus, we can distinguish between ray-based methods and wave-based methods. This terminology is applicable to both tomography and migration velocity analysis. For ray-based methods, the carrier of information are wide-band rays traced using a background velocity model from picked events in the data (or image). Methods in this category are known as traveltime tomography (Bishop et al., 1985) and traveltime MVA, sometimes described as image-space traveltime tomography (Clement et al., 2001; Clapp et al., 2004; Chavent and Jacewitz, 1995; Al-Yahya, 1987; Etgen, 1990; Chauris et al., 2002a; Stork, 1992; Chauris et al., 2002b; Lambare et al., 2004; Billette et al., 2003; Fowler, 1988). For wave-based methods, the carrier of information are band-limited wavefields constructed using a background velocity model. Methods in this category are known as wave-equation tomography (Mora, 1989; Tarantola, 1987; Gauthier et al., 1986; Pratt, 2004; Woodward, 1992; Pratt, 1999), and wave-equation MVA (Shen et al., 2005; Sava and Biondi, 2004a; Maharramov and Albertin, 2007; Albertin et al., 2006b; Biondi and Sava, 1999; Sava and Biondi, 2004b). This paper concentrates on methods from the latter category.
The volume of information used for model updates using wave-based methods is at least one order of magnitude larger than the volume of information used for ray-based methods. Thus, a fundamental question we should ask is what is gained by using wave-based methods over ray-based methods. First, modern imaging applications using wave-based methods (downward continuation or reverse-time extrapolation) require consistent velocity estimation methods which interact with model in the same frequency band as the migration methods. Second, wave-based methods are robust (i.e. stable) for models with large and sharp velocity variations (e.g. salt). Third, wave-based methods describe naturally all propagation paths, thus they can easily handle multi-pathing characterizing wave propagation in media with large velocity variations.
Wave-equation tomography and wave-equation MVA have both similarities and differences. Wave-equation tomography uses the advantage that the residual used for velocity updating is obtained by a direct comparison between recorded and measured data. In contrast, wave-equation MVA uses the property that the residual used for velocity updating is obtained by a comparison between a reference image and an improved version of it. On the other hand, wave-equation tomography has the disadvantage that the kinematics of events in the data domain are more complex than in the image domain. In addition, the dimensionality of the space in which to evaluate the misfit between recorded and simulated data is higher too, potentially making a comparison more complex. Also, wave-equation MVA optimizes directly the desired end product, i.e. the migrated image, which potentially makes this technique more ``interpretive'' and less of a computational ``black-box''.
One important component of MVA methods is what type of measurement on migrated images is used to evaluate its deficiencies. Although strictly related to one-another, we can describe two kinds of information available to quantify image quality. First is focusing analysis, which evaluates whether point-like events, e.g. fault truncations, are focused in migrated images at their correct position. Image enhancement for wave-equation MVA can be formulated purely based on this type of information, which makes the techniques fast since it only operates with zero-offset data (Sava et al., 2005). Second is moveout analysis, which is the case for all conventional MVA techniques, whether using rays or waves. In this case, we can formulate wave-equation MVA based on analysis of moveout of common-image gathers using velocity scans (Sava and Biondi, 2004a; Biondi and Sava, 1999; Soubaras and Gratacos, 2007; Sava and Biondi, 2004b) or based on analysis of differential semblance of nearby traces in similar common-image gathers (Shen et al., 2005).
Moveout analysis requires construction of common-image gathers (CIGs) characterizing the dependence of reflectivity function of various parameters used to parametrize multiple experiments used for imaging. There are two main alternatives for common-image gather construction. First, we can construct offset-domain CIGs (Yilmaz and Chambers, 1984), when reflectivity depends on source-receiver offset on the acquisition surface, which is a data parameter. Second, we can construct angle-domain CIGs (Soubaras, 2003; Rickett and Sava, 2002; Xie and Wu, 2002; Biondi and Symes, 2004; Fomel, 2004; Prucha et al., 1999; Mosher and Foster, 2000; de Bruin et al., 1990; Sava and Fomel, 2003), when reflectivity depends on the angles of incidence at the reflection point, which is an image parameter. For wave-equation migration, offset-domain CIGs are not a practical solution, since the information from multiple offsets (or multiple seismic experiments) mixes in the process of migration. Furthermore, angle-domain CIGs suffer from fewer artifacts than offset-domain CIGs, due to the fact that reflectivity parametrization for angle-gathers occurs after wavefield reconstruction in the subsurface, as oppose to offset-gathers when reflectivity parametrization is related to data parameters (Stolk and Symes, 2004).
Both wave-equation tomography and wave-equation MVA methods are based on a fundamental ``small perturbation'' assumption, which requires a reasonably-good starting model. This requirement represents a drawback which is responsible for the main difficulty of methods in both categories. For wave-equation tomography or inversion, we can update models based on differences between recorded and simulated data. If the starting background model is not accurate enough, we run the risk of subtracting wavefields corresponding to different events. Likewise, for wave-equation MVA, we update the model based on differences between two images, one simulated in the background model and an enhanced version of this image. If the enhanced version of the image goes too far from the reference image, we run the risk of subtracting events corresponding to different reflectors. This phenomenon is usually referred-to in the literature as ``cycle skipping'' and various strategies have been designed to ameliorate this problem, e.g. by optimal selection of frequencies used for velocity analysis (Albertin, 2006; Sirgue and Pratt, 2004). However, alternative methods used to evaluate image accuracy, e.g. differential semblance (Shen et al., 2003; Symes and Carazzone, 1991), have the best potential to ameliorate this situation. In this case wave-equation MVA analyzes difference between image traces within common-image gathers which are likely to be similar enough from one-another such as to avoid the cycle-skipping problem. Even in this case, the assumption made is that the nearby traces in a gather are sampled well-enough, i.e. the seismic events differ by only a fraction of the wavelet, which is a function of image sampling and frequency content. In practice, there is no absolute guarantee that nearby events are closely related to one another, although this is more likely to be true for DSO than it is for direct image differencing.
In this paper, we concentrate on the implementation of the wave-equation migration velocity analysis operators for various wave-equation migration configurations. The main objective of the paper is to derive the linearized operators linking perturbations of the slowness model to the corresponding perturbations of the seismic wavefield and image. All our theoretical development is formulated under the single scattering (Born) approximation applied to acoustic waves. We begin by describing the MVA operators corresponding to zero-offset, survey-sinking and shot-record wave-equation migration frameworks. We describe the theoretical background for each operator and emphasize the similarities and differences between the different operators. Throughout the paper, we use pseudo-code to illustrate implementation strategies and data flow for the various wave-equation MVA operators. Finally, we illustrate the wave-equation MVA operators with impulse responses corresponding to simple and complex models. We leave outside the scope of this paper the procedure in which the discussed operators are used for migration velocity analysis.
|Numeric implementation of wave-equation migration velocity analysis operators|