Prestack depth migration and velocity model building pdf

Pdf nonlinear 3d tomographic leastsquares inversion of. After the length of the target angle zone is small enough, we obtain the initial. Prestack depth migration and velocity model building for imaging in the north sea. The success of standard seismic reflection imaging routines, such as prestack depth migration or nmodmostack depends on the required macro velocity model. Use of prestack depth migration for improving the accuracy. The migration algorithms presented are based in the gulf of mexico but are applicable throughout the world. Anisotropic prestack depth migration in the absence of well control. Therefore, the algorithm of choice for prestack migration must cope with irregularly sampled data. Prestack depth migration, velocity model building, seismic tomography, overthrust, carpathians. First, isotropic prestack depth velocity building and migration are performed for both pp and ps data. Just as we use prestack time migration to estimate rms velocities migration velocity analysis and 3d prestack time migration, we often wish to use prestack depth migration to estimate interval velocities. It gives an overview on the goals and techniques utilized in imaging of seismic data. The first iteration of prestack depth migration uses this initial velocity model to obtain depth migrated gathers or stacks for further iterative velocity analysis and model building.

Prestack depth migration and velocity model building for imaging in. In addition to velocity model workflows, it should be noted that this tool is suitable for a final migration for many additional situations, including most marine processing. Geodepth seismic velocity modeling solutions by emerson e. Geodepth 2d depth velocity model building and depth imaging. He started his industrial career as a research geophysicist developing algorithms for velocity estimation, model building and prestack depth migration and held a senior research scientist position for landmarkita 19911994 and a senior research. Results of both migration categories are most often displayed in time as. Resolution of prestack depth migration we start with the speci. This method does therefore require a velocity model, making it resourceintensive because building a seismic velocity model is a long and iterative process. Prestack depth migration an ultimate aspiration for. Prestack depth migration, velocity model building, seismic tomo graphy, overthrust, carpathians. Velocity updating by seismic tomography, in the form of either traveltime tomography or residual curvature analysis rca, has become an important component of the depth imaging process. There are many methods used in industry to construct velocity models for prestack depth migration, including interval. Generic model building loop for raybased tomography.

Each will be discussed in detail in the following sub. Model velocities were assigned from sonic logs from the vsp well. In areas of rapid velocity changes, energy is dispersed in such a way that. Threedimensional prestack depth migration psdm seismic processing using hardlayer model updating was applied to the.

Residual velocity, the measure of residual moveout, is a function of the interval velocity model. In this example, the accurate velocity modelling approach and depth. Although different implementations of velocity model building tools exist, they usually start with an estimation of an initial velocity model using. Prestack kirchhoff depth migration psdm is a robust method for obtaining proper image of complex subsurface. Kirchhoff prestack time migration pstm can accurately image steep or overturned events, provided that turning rays and anisotropy are taken into account. The example shows a dataset with a salt dome, the fast salt reduces traveltimes for the reflections below it, which can be observed at the time migration result, imaging using a correct velocity model in depth and prestack depth migration solves this issue and furthermore improves the. Integrated prestack depth migration of vsp and surface. Management david kessler, president, seismiccity, inc dr. Anisotropic prestack depth migration cseg recorder. Integrated prestack depth migration of vsp and surface seismic data in order to test the new, integrated prestack depth migration algorithm, a numerical model was developed based on the interpretation of the field data. Prestack depth migration an ultimate aspiration for subsurface imaging in geologically complex area. Overview on the fundamentals of imaging cseg recorder. An integrated prestack depth migration workflow using.

Figure 1a and 1b depict the pp image and the isotropic ps image h g0, respectively. Promax depthcharge velocity modeling and depth imaging. The theory of extrapolation of nonzerooffset wavefields is provided in wavefield extrapolation and migration, while the stationaryphase traveltime trajectory associated with nonzerooffset wave propagation is given in nonzerooffset traveltime equation. Geodepth 3d depth velocity model building and depth imaging. Pdf automatic migration velocity estimation for prestack. An innovative approach to automation for velocity model building. Velocity model building for tilted orthorhombic depth migration system by, calculate the residual. Summary over the last decade, it is predominantly perceived that there have been significant evolutions in prestack depth imaging consisting of both velocity depth model building and.

Weihong fei, duryodhan epili, jing xie, and young kim, tgsnopec geophysical company. Initial anisotropic model building about 240 checkshots were analyzed in and around the survey area. Depth migration, tomography, velocity model, depth conversion introduction depth conversion, ideally, is an iterative process that begins with proper seismic. It is intended for those who are unfamiliar with seismic imaging or are not knowledgeable of seismic. In this technique the velocity model is constructed from the top. We have broad experience in prestack depth migration for marine, land and transitionzone environments. Anisotropic depth imaging case studies from australias. Prestack depth migration and velocity model building for. The method was tested on two different datasets acquired in the polish carpathians. Prestack depth migration velocity model 315 low resolution of stacking velocities, fast lateral velocity changes and sparse coverage of well information, strong influence of noise and steep horizons for estimated velocities, problems with assigning a single gradient function for a given zone. Building of the geologic model and the associated velocity fields is the most critical step in psdm processing. Minden prestack time migration section stretched to depth with smoothed prestm velocities left, prestack depth migration using the final velocity model right figure 6 shows a similar inline comparison between time and depth imaging from the keystone survey. Prestack kirchho time migration for complex media tariq alkhalifah1 keywords.

The regional scale the thrust belt and the improved images along various looking the hydrocarbon potential zones. Shallow velocity model building considerations for. This training manual covers basic workflows in geodepth 3d and offers students two alternatives for building the initial model, a structure independent grid based and a layer based model. Depth migration is applied to seismic data in depth regular cartesian coordinates, which must be calculated from seismic data in time coordinates.

Previous attempts to explore this processing route in 2d suffered from the inability to accurately define the complex sedimentsalt interface that determines the travel paths to deeper reflectors. An interpreters guide to successful prestack depth. In this study rms velocities derived by migration velocity analysis mva are. Velocitydepth model building leading to 3d prestack depth. Abstract despite the nowroutine use of prestack depth migration psdm for unconventionals, confusion abounds on the topic of how to best incorporate nearsurface velocity estimates into the psdm shallow model building process. Depth migration, tomography, velocity model, depth conversion introduction depth conversion, ideally, is an.

Using geothrust, you can process 2d and 3d seismic data and produce the final deliverables including rms velocity model, depth velocity model, prestack time migration, prestack depth migration, poststack migration, and demigration results as a seismic data processing service. An integrated prestack depth migration workflow using model based velocity estimation and refinement andy furniss abstract. In this paper, we describe the components of tomography, discussing rca in. When the velocity model is correct, data migrated in different. Velocity inversions over small depth intervals cannot be corrected for and lead. To justify the proposed method, the border value of possible tomographic velocity updates was evaluated. Kirchhoff prestack depth and time migration schlumberger.

Velocity model building for tilted orthorhombic depth imaging. Fast velocity model building using kinematic demigration. Where lateral velocity variation is severe as in many overthrust areas, more computationally intensive depth migration is required. These gathers play a key role in the velocitydepth model building process. An introduction to the common reflection surface stack. The author assumed that correct model can be built when the lateral resolution of. If the interpretation of the karst features is inaccurate, then the geometry of the geobodies inserted into the velocity model will also be incorrect, and hence, depth migration using this model will not resolve the imaging artifacts on the output seismic volume. In section 3, we specify the properties of a general.

Calculation time channel model kirchhoff time migration marmousi model pspi depth migration poststack migrationstack migration 33 s 318 s prestack migration 23. The strength of prestack time migration is that the imaging is kept in the time. Delineation of karsts, a new approach using seismic. Enhanced anisotropic model building methodology and. Tomographic velocity analysis and waveequation depth. Layer stripping is arguably the simplest approach to building velocity models for depth migration. Conventional velocity analysis methods generally assume flatlayered geology and mild lateral velocity variations. The advent of velocity sensitive depth imaging techniques such as prestack depth migration has created a demand for more accuracy in velocity model building. Geotomo has a core group of researchers and processors dedicated to. Seismic data processing and imaging solutions geotomo.

Depth imaging, on the other hand, needs many passes of migration velocity model building and migration before proceeding through post migration processing. We must remind ourselves that an accurate image from depth migration is attainable only when the velocity depth model is correctly defined, independent of the input data type zerooffset, stack, or prestack. Pdf prestack depth migration of ultrashallow seismic data. The p and s velocity models are obtained by the velocity inversion jin and beydoun, 2000 which focuses best the images. Prestack depth migration and velocity model building geophysics. Fast velocity model building using kinematic demigration and migration. Details of the model at the target level are shown in figure 3a. However, the more complex the geology, the more difficult it is to find an accurate velocity model zelt et al.

Methods of velocity estimation include tomography, gatherflattening techniques, and scans on velocity perturbed stacks. We have shown several examples of constraints on psdm velocity model building, from. It is a fast and accurate wave equation prestack depth migration that can provide users with results the next day. Figure e2 shows a sketch of a nonzerooffset raypath in a constant velocity medium. Prestack depth migration psdm is a proven seismic imaging tool. The only way to eliminate the discrepancy between correct depth and residual moveout on image gathers is to use anisotropic prestack depth migration. This twoday training course teaches students how to create an initial interval velocity model, perform depth imaging, and update the velocity model. Nonlinear 3d tomographic leastsquares inversion of residual moveout in kirchhoff prestackdepthmigration common image gathers article pdf available in geophysics 735 september 2008 with. To get this picture we need an interval velocity model whose accuracy depends. Estimating subsurface parameter fields for seismic. Since their kirchhofftype implementations collect all possible measured reflections events belonging to either a point in the time or in the depth domain they cannot account for the correct.

Note that the terms depth and time migration do not relate to whether the migrated results are presented as a function of time or depth. Building velocity models for steepdip prestack depth. Building a depth velocity model is an important step in the prestack kirchoff depth migration workflow. The seismic velocity model from the prior wem migration was calibrated to generate the initial vertical velocity model, vz. Prestack depth migration an ultimate aspiration for subsurface. The image quality of 3d prestack depth migration in areas of complex geology depends strongly on the accuracy of the velocity model. By using the correct velocity model, and all the correct parameters. Prestack depth migration is an effective method for determining velocities because of the sensitivity of the migrated image to the velocity model liu, 1997.