Elastic reverse time migration of marine walkaway vertical seismic profiling data

Geophysics ◽  
1998 ◽  
Vol 63 (5) ◽  
pp. 1685-1695 ◽  
Author(s):  
Ketil Hokstad ◽  
Rune Mittet ◽  
Martin Landrø
Keyword(s):  
Vertical Component ◽  
Horizontal Component ◽  
Reciprocal Relationship ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
Depth Images ◽  
Time Migration ◽  
Walkaway Vsp

Walkaway vertical seismic profiling (VSP) acquisition with three‐component geophones allows for direct measurement of compressional as well as shear energy. This makes full elastic reverse time migration an attractive alternative for imaging data. We present results from elastic reverse time migration of a marine walkaway VSP acquired offshore Norway. The reverse time migration scheme is based on a high‐order finite‐difference solution to the two‐way elastic wave equation. Depth images of the subsurface are constructed by correlation of forward‐ and back‐propagated elastic wavefields. In the walkaway VSP configuration, the number of shots is much larger than the number of geophone levels. Using processing methods operating in the shot/receiver domain, it is advantageous to use the reciprocal relationship between the walkaway VSP and the reverse VSP configurations. We do this by imaging each component of each geophone level as a reverse VSP common shot gather. The final images are constructed by stacking partial images from each level. The depth images obtained from the vertical components reveal the major characteristics of the geological structure below geophone depth. A graben in the base Cretaceous unconformity and a faulted coal layer can be identified. The horizontal components are more difficult to image. Compared to the vertical components, the horizontal component images are more corrupted by migration artifacts. This is because the horizontal component images are more sensitive to aperture effects and to the shear‐wave velocity macromodel. When converted to two‐way time, the migration results tie well with the surface seismic section. Comparison of fully elastic and acoustic reverse time migration shows that the vertical component is dominantly PP-reflected events, whereas the horizontal components get important contributions from PS-converted energy. The horizontal components also provide higher resolution because of the shorter wavelength of the shear waves.

Local vertical seismic profiling (VSP) elastic reverse-time migration and migration resolution: Salt-flank imaging with transmitted P-to-S waves

Geophysics ◽  
2010 ◽  
Vol 75 (2) ◽  
pp. S35-S49 ◽  
Author(s):  
Xiang Xiao ◽  
W. Scott Leaney
Keyword(s):  
Velocity Model ◽  
Staggered Grid ◽  
Image Point ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
S Wave ◽  
S Waves ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
Time Migration

To avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model, we introduce a vertical seismic profiling (VSP) local elastic reverse-time-migration (RTM) method for salt-flank imaging by transmitted P-to-S waves. This method back-projects the transmitted PS waves using a local velocity model around the well until they are in phase with the back-projected PP waves at the salt boundaries. The merits of this method are that it does not require the complex overburden and salt-body velocities and it automatically accounts for source-side statics. In addition, the method accounts for kinematic and dynamic effects, including anisotropy, absorption, and all other unknown rock effects outside of this lo-cal subsalt velocity model. Numerical tests on an elastic salt model and offset 2D VSP data in the Gulf of Mexico, using a finite-difference time-domain staggered-grid RTM scheme, partly demonstrate the effectiveness of this method over interferometry PS-PP transmission migration and local acoustic RTM. Our method separates elastic wavefields to vector P- and S-wave velocity components at the trial image point and achieves better resolution than local acoustic RTM and interferometric transmission migration. The analytical formulas of migration resolution for local acoustic and elastic RTM show that the migration illumination is limited by data frequency and receiver aperture, and the spatial resolution is lower than standard poststack and prestack migration. This new method can image salt flanks as well as subsalt reflectors.

Elastic reverse‐time migration

Geophysics ◽  
1987 ◽  
Vol 52 (10) ◽  
pp. 1365-1375 ◽  
Author(s):  
Wen‐Fong Chang ◽  
George A. McMechan
Keyword(s):  
Finite Difference ◽  
Vertical Component ◽  
Common Source ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
Imaging Condition ◽  
Time Migration ◽  
Two Component ◽  
Synthetic Test ◽  
Data Extrapolation

Elastic, prestack, reverse‐time, finite‐difference migration of two‐component seismic surface data requires data extrapolation and application of an imaging condition. Data extrapolation involves synchronous driving of the vertical‐component and horizontal‐component finite‐difference meshes with the time reverse of the recorded vertical and horizontal traces, respectively. Extrapolation uses the coupled elastic wave equation for variable velocity solved with a second‐order, explicit finite‐difference scheme. The imaging condition at any point in the grid is the one‐way traveltime from the source to that point. Elastic migrations of both synthetic test data and real two‐component common‐source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections (compressional waves and shear waves) into single loci.

Up/down separation of seismic depth images

Geophysics ◽  
2018 ◽  
Vol 83 (5) ◽  
pp. S375-S385 ◽  
Author(s):  
Eric Duveneck
Keyword(s):  
Hilbert Transform ◽  
Real Data ◽  
Depth Image ◽  
Time Shift ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
Hilbert Transforms ◽  
Depth Images ◽  
Time Migration ◽  
The Individual

Reverse time migration (RTM) is normally based on wavefield modeling that allows wave propagation in all spatial directions. While this is one of the strengths of RTM, it can also lead to undesired effects, including partial image amplitude cancellation for reflectors that are illuminated and imaged from two sides, as well as the appearance of ghost-reflection artifacts if the modeled source- and receiver-side wavefields are both scattered back from a hard model contrast. Both issues can be addressed by separating the seismic depth image into components imaged from above and components imaged from below. I derive and compare two methods that directly achieve such an image up/down separation, without relying on an up/down separation of the individual simulated wavefields used for imaging. The first method is based on the formation of time-shift gathers during imaging and filtering out events with either positive or negative slopes in these gathers, corresponding to imaging from below or above, respectively. The second method is a new image up/down separation approach inspired by previously published wavefield up/down separation approaches. It involves combining a migrated image with an additional version of the image, obtained by applying a temporal Hilbert transform to the input data and a subsequent Hilbert transform in depth on the migrated image. Compared with approaches based on up/down separation of the individual wavefields, the presented direct image up/down separation methods have distinctive advantages. While the approach based on filtering of time-shift gathers is efficient and offers considerable flexibility, the approach based on Hilbert transforms is attractive because of its simplicity since no changes to the migration algorithm itself are required. I demonstrate both image up/down separation methods on synthetic and real data and show that both methods lead to very similar results.

Improved quality of depth images using reverse time migration

2007 ◽  
Author(s):  
André Bulcão ◽  
Djalma Manoel Soares Filho ◽  
Webe João Mansur
Keyword(s):  
Reverse Time ◽  
Reverse Time Migration ◽  
Depth Images ◽  
Time Migration
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