The Experts below are selected from a list of 11286 Experts worldwide ranked by ideXlab platform
Paul Sava - One of the best experts on this subject based on the ideXlab platform.
-
Geophysical Prospecting
2020Co-Authors: Paul Sava, Ivan VasconcelosAbstract:A B S T R A C T Wavefield-based migration velocity analysis using the semblance principle requires computation of images in an extended space in which we can evaluate the imaging consistency as a function of overlapping experiments. Usual industry practice is to assemble those seismic images in common-image gathers that represent reflectivity as a function of depth and extensions, e.g., reflection angles. We introduce extended common-image point (CIP) gathers constructed only as a function of the spaceand time-lag extensions at sparse and irregularly distributed points in the image. Semblance analysis using CIP's constructed by this procedure is advantageous because we do not need to compute gathers at regular surface locations and we do not need to compute extensions at all depth levels. The CIP's also give us the flexibility to distribute them in the image at irregular locations aligned with the geologic structure. Furthermore, the CIP's remove the depth bias of common-image gathers constructed as a function of the depth axis. An interpretation of the CIP's using the scattering theory shows that they are scattered Wavefields associated with sources and receivers inside the subsurface. Thus, when the surface Wavefields are correctly reconstructed, the extended CIP's are characterized by focused energy at the origin of the spaceand time-lag axes. Otherwise, the energy defocuses from the origin of the lag axes proportionally with the cumulative velocity error in the overburden. This information can be used for Wavefield-based tomographic updates of the velocity model, and if the velocity used for imaging is correct, the coordinate-independent CIP's can be a decomposed as a function of the angles of incidence. Key words: Imaging, Interferometry, Tomography, Velocity analysis, Waveequation. I N T R O D U C T I O N In regions characterized by complex subsurface structure, wave-equation depth migration is a powerful tool for accurately imaging the Earth's interior. The quality of the final image greatly depends on the quality of the velocity model. Thus, constructing accurate velocity is essential for imaging Generally speaking, there are two possible strategies for velocity estimation from surface seismic data in the context of Wavefield depth migration that differ by the implementation domain. One possibility is to formulate an objective function in the data space, prior to migration, by evaluating the match between recorded and simulated data using an approximate (background) velocity model. Techniques in this category are known by the name of tomography (or inversion)
-
isotropic elastic Wavefield imaging using the energy norm
Geophysics, 2016Co-Authors: Daniel Rocha, Nicolay M Tanushev, Paul SavaAbstract:ABSTRACTBased on the energy conservation principle, we derive a scalar imaging condition for anisotropic elastic Wavefield migration. Compared with conventional imaging conditions that correlate displacement components or potentials from source and receiver Wavefields, the proposed imaging condition does not suffer from polarity reversal, which degrades the image quality after stacking over shots. Our imaging condition also accounts for the directionality of the Wavefields in space and time, leading to the attenuation of backscattering artifacts, which commonly appear in elastic reverse time migration images in the presence of strong model contrasts. In addition, our new imaging condition does not require wave-mode decomposition, which demands significant additional cost for elastic Wavefields in anisotropic media. To properly image structures, we rely on the anisotropy parameters used in migration, as one would do for any other imaging condition. Our imaging condition is suitable for arbitrary anisotropy...
-
acoustic Wavefield imaging using the energy norm
Geophysics, 2016Co-Authors: Daniel Rocha, Nicolay M Tanushev, Paul SavaAbstract:ABSTRACTWavefield energy can be measured by the so-called energy norm. We have extended the concept of “norm” to obtain the energy inner product between two related Wavefields. Considering an imaging condition as an inner product between the source and receiver Wavefields at each spatial location, we have developed a new imaging condition that represents the total reflection energy. Investigating this imaging condition further, we have found that it accounts for Wavefield directionality in space time. Based on the directionality discrimination provided by this imaging condition, we have applied it to attenuate backscattering artifacts in reverse time migration (RTM). This imaging condition can be designed not only to attenuate backscattering artifacts, but also to attenuate any selected reflection angle. By exploiting the flexibility of this imaging condition for attenuating certain angles, we have developed a procedure to preserve the type of events that propagate along the same path, i.e., backscattered...
-
CWP-597 Angle-domain elastic reverse-time migration
2012Co-Authors: Jia Yan, Paul SavaAbstract:Multi-component data are not usually processed with specifically designed procedures, but with procedures analogous to the ones used for single-component data. Commonly, the vertical and horizontal components of the data are taken as proxies for P and S wave modes which are imaged independently with the acoustic wave equation. This procedure works only if the vertical and horizontal component accurately represent P and S wave modes, which is not true in general. Therefore, multi-component images constructed with this procedure exhibit artifacts caused by the incorrect wave mode separation at the surface. An alternative procedure for elastic imaging uses the vector fields for Wavefield reconstruction and imaging. The Wavefields are reconstructed using the multi-component data as boundary conditions for a numerical solution to the elastic wave equation. The key component for Wavefield migration is the imaging condition that evaluates the match between Wavefields reconstructed from sources and receivers. For vector Wavefields, a simple component-by-component cross-correlation between two Wavefields leads to artifacts caused by cross-talk between the unseparated wave modes. An alternative method is to separate elastic Wavefields after reconstruction in the subsurface and implement the imaging condition as cross-correlation of pure wave modes instead of the Cartesian components of the displacement Wavefield. This approach leads to images that are easier to interpret, since they describe reflectivity of specified wave modes at interfaces of physical properties. As for imaging with acoustic Wavefields, the elastic imaging condition can be formulated conventionally (cross-correlation with zero lag in space and time), as well as extended to non-zero space and time lags. The elastic images produced by an extended imaging condition can be used for angle decomposition of primary (PP or SS) and converted (PS or SP) reflectivity. Angle gathers constructed with this procedure have applications for migration velocity analysis and amplitude versus angle analysis. Key words: imaging, elastic, reverse time
-
elastic wave mode separation for tilted transverse isotropy media
Geophysical Prospecting, 2012Co-Authors: Paul SavaAbstract:Seismic waves propagate through the earth as a superposition of different wave modes. Seismic imaging in areas characterized by complex geology requires techniques based on accurate reconstruction of the seismic Wavefields. A crucial component of the methods in this category, collectively known as wave-equation migration, is the imaging condition that extracts information about the discontinuities of physical properties from the reconstructed Wavefields at every location in space. Conventional acoustic migration techniques image a scalar Wavefield representing the P-wave mode, in contrast to elastic migration techniques, which image a vector Wavefield representing both the P- and S-waves. For elastic imaging, it is desirable that the reconstructed vector fields are decomposed into pure wave modes, such that the imaging condition produces interpretable images, characterizing, for example, PP or PS reflectivity. In anisotropic media, wave mode separation can be achieved by projection of the reconstructed vector fields on the polarization vectors characterizing various wave modes. For heterogeneous media, because polarization directions change with position, wave mode separation needs to be implemented using space-domain filters. For transversely isotropic media with a tilted symmetry axis, the polarization vectors depend on the elastic material parameters, including the tilt angles. Using these parameters, we separate the wave modes by constructing nine filters corresponding to the nine Cartesian components of the three polarization directions at every grid point. Since the S polarization vectors in transverse isotropic media are not defined in the singular directions, e.g., along the symmetry axes, we construct these vectors by exploiting the orthogonality between the SV and SH polarization vectors, as well as their orthogonality with the P polarization vector. This procedure allows one to separate all three modes, with better preserved P-wave amplitudes than S-wave amplitudes. Realistic synthetic examples show that this wave mode separation is effective for both 2D and 3D models with strong heterogeneity and anisotropy.
Wapenaar C.p.a. - One of the best experts on this subject based on the ideXlab platform.
-
Quantitative imaging of fractures around a borehole using linear slip theory and elastic least-squares migration
'Society of Exploration Geophysicists', 2019Co-Authors: Minato S., Ghose R., Wapenaar C.p.a.Abstract:Single-well reflection imaging using sonic logging data successfully locates fine-scale structures around a borehole including fractures. In order to achieve accurate and quantitative estimation of fracture properties with high resolution, we propose to couple least-squares migration with linear slip theory. The proposed least-squares migration solves linearized waveform inversion where the Wavefield is approximated using a Born operator incorporating a linear slip boundary condition. Representing a fracture as a linear slip interface is advantageous in accurate seismic wave modeling and efficient estimation of fracture properties. We derive conventional elastic least-squares migration for imaging perturbations in elastic constants, and new elastic least-squares migration for imaging fracture compliances. The two formulations are tested using numerical modeling where a dipping fracture is embedded in random background medium. The results show that least-squares migration generally produces higher resolution images for both SH and P-SV Wavefields than using adjoint operators. Furthermore, it shows the potential of quantitative estimation of fracture compliances which can be further used in interpreting fracture properties, e.g., fracture infill material and surface condition. The proposed approach, therefore, will be crucial in fracture characterization around a borehole.Applied Geophysics and PetrophysicsImPhys/Acoustical Wavefield Imagin
-
Quantitative imaging of fractures around a borehole using linear slip theory and elastic least-squares migration
'Society of Exploration Geophysicists', 2019Co-Authors: Minato S., Ghose R., Wapenaar C.p.a.Abstract:Single-well reflection imaging using sonic logging data successfully locates fine-scale structures around a borehole including fractures. In order to achieve accurate and quantitative estimation of fracture properties with high resolution, we propose to couple least-squares migration with linear slip theory. The proposed least-squares migration solves linearized waveform inversion where the Wavefield is approximated using a Born operator incorporating a linear slip boundary condition. Representing a fracture as a linear slip interface is advantageous in accurate seismic wave modeling and efficient estimation of fracture properties. We derive conventional elastic least-squares migration for imaging perturbations in elastic constants, and new elastic least-squares migration for imaging fracture compliances. The two formulations are tested using numerical modeling where a dipping fracture is embedded in random background medium. The results show that least-squares migration generally produces higher resolution images for both SH and P-SV Wavefields than using adjoint operators. Furthermore, it shows the potential of quantitative estimation of fracture compliances which can be further used in interpreting fracture properties, e.g., fracture infill material and surface condition. The proposed approach, therefore, will be crucial in fracture characterization around a borehole.Presentation Date: Wednesday, September 18, 2019Session Start Time: 1:50 PMPresentation Time: 2:40 PMLocation: 217CPresentation Type: OralApplied Geophysics and PetrophysicsImPhys/Acoustical Wavefield Imagin
-
Full-field MDD for body-wave reflections from passive transient-sources under severely limited and irregular illumination conditions
'Yamaguchi University Medical Association', 2016Co-Authors: Hartstra I.e., Almagro Vidal C., Wapenaar C.p.a.Abstract:Seismic interferometry (SI) presents a set of inexpensive and noninvasive methods that can be applied to any array at the surface to retrieve virtual body-wave reflection responses from earthquake recordings. Conventional SI by cross-correlation requires recordings of Wavefields in lossless media generated by a smooth continuous distribution of passive sources with isotropic source radiation patterns and similar power spectra. These conditions are unlikely to be met in the lithosphere: earthquakes are distributed sparsely and generated by complex mechanisms. The resulting anisotropy in the illumination of the receiver array causes the retrieved virtual-source radiation patterns to be irregular, leading to artifacts which can obscure the desired body-wave reflections. SI by multidimensional deconvolution (MDD) can inherently correct for anisotropic illumination of the array and does not rely on the medium being lossless. We propose an alternative formulation of MDD for two-way Wavefields: full-field MDD. Different from previous MDD methods for passive two-way Wavefield recordings, full-field MDD uses multiples in the passive data to construct the reflection response without free-surface interaction. Therefore, this MDD method profits from additional wavenumbers provided by scattering to compensate for sparse earthquake distributions. Besides, this method does not require Wavefield decomposition, which is sensitive to velocity variations at the receiver level. We compare the reflection retrieval by full-field MDD and cross-correlation for a limited passive source distribution in a lithospheric model with a discontinuous Moho at a depth of 50 km. We simulate earthquakes generated by dipole sources along a listric fault-system with power spectra varying within bandwidth 0.2-2.6 Hz. The reflection response retrieved by full-field MDD shows a continuous high-resolution Moho reflection, while cross-correlation yields a very low resolution response obscured by artifacts
-
Full-field MDD for body-wave reflections from passive transient-sources under severely limited and irregular illumination conditions
'Yamaguchi University Medical Association', 2016Co-Authors: Hartstra I.e., Almagro Vidal C., Wapenaar C.p.a.Abstract:Seismic interferometry (SI) presents a set of inexpensive and noninvasive methods that can be applied to any array at the surface to retrieve virtual body-wave reflection responses from earthquake recordings. Conventional SI by cross-correlation requires recordings of Wavefields in lossless media generated by a smooth continuous distribution of passive sources with isotropic source radiation patterns and similar power spectra. These conditions are unlikely to be met in the lithosphere: earthquakes are distributed sparsely and generated by complex mechanisms. The resulting anisotropy in the illumination of the receiver array causes the retrieved virtual-source radiation patterns to be irregular, leading to artifacts which can obscure the desired body-wave reflections. SI by multidimensional deconvolution (MDD) can inherently correct for anisotropic illumination of the array and does not rely on the medium being lossless. We propose an alternative formulation of MDD for two-way Wavefields: full-field MDD. Different from previous MDD methods for passive two-way Wavefield recordings, full-field MDD uses multiples in the passive data to construct the reflection response without free-surface interaction. Therefore, this MDD method profits from additional wavenumbers provided by scattering to compensate for sparse earthquake distributions. Besides, this method does not require Wavefield decomposition, which is sensitive to velocity variations at the receiver level. We compare the reflection retrieval by full-field MDD and cross-correlation for a limited passive source distribution in a lithospheric model with a discontinuous Moho at a depth of 50 km. We simulate earthquakes generated by dipole sources along a listric fault-system with power spectra varying within bandwidth 0.2-2.6 Hz. The reflection response retrieved by full-field MDD shows a continuous high-resolution Moho reflection, while cross-correlation yields a very low resolution response obscured by artifacts.Applied Geophysics and Petrophysic
-
Data-driven Wavefield focusing and imaging with multidimensional deconvolution: Numerical examples for reflection data with internal multiples
'Society of Exploration Geophysicists', 2014Co-Authors: Broggini F., Snieder R., Wapenaar C.p.a.Abstract:Standard imaging techniques rely on the single scattering assumption. This requires that the recorded data do not include internal multiples, i.e., waves that have bounced multiple times between reflectors before reaching the receivers at the acquisition surface. When multiple reflections are present in the data, standard imaging algorithms incorrectly image them as ghost reflectors. These artifacts can mislead interpreters in locating potential hydrocarbon reservoirs. Recently, we introduced a new approach for retrieving the Green’s function recorded at the acquisition surface due to a virtual source located at depth. We refer to this approach as data-driven Wavefield focusing. Additionally, after applying source-receiver reciprocity, this approach allowed us to decompose the Green’s function at a virtual receiver at depth in its downgoing and upgoing components. These Wavefields were then used to create a ghost-free image of the medium with either crosscorrelation or multidimensional deconvolution, presenting an advantage over standard prestack migration. We tested the robustness of our approach when an erroneous background velocity model is used to estimate the first-arriving waves, which are a required input for the data-driven Wavefield focusing process. We tested the new method with a numerical example based on a modification of the Amoco model.Geoscience & EngineeringCivil Engineering and Geoscience
George A. Mcmechan - One of the best experts on this subject based on the ideXlab platform.
-
true amplitude recovery in reverse time extrapolation of plane and spherical waves
Geophysics, 2018Co-Authors: George A. McmechanAbstract:One of the challenging outstanding problems in reverse-time extrapolation is recovering accurate amplitudes at reflectors from the receiver Wavefield. Various migrations have been developed to produce accurate image locations rather than correct amplitude information, because of inadequate compensation of attenuation, dispersion, and transmission losses. We first evaluated the requirements, and demonstrated the theoretical feasibility, of true amplitude recovery of 2D acoustic and elastic seismic data by using the analytic Zoeppritz equations for plane wave reflection and transmission coefficients. Then, we used synthetic acoustic and elastic Wavefield data generated by elastodynamic finite differences to verify the recovery, in the reverse-time propagation, of spherical waves and illustrated the salient differences between the incident Wavefields reconstructed from reflection data only and from the combination of reflection and transmission data. These examples quantitatively verified that recovering an ...
-
common image gathers in the incident phase angle domain from reverse time migration in 2d elastic vti media
Geophysics, 2011Co-Authors: Qunshan Zhang, George A. McmechanAbstract:Reverse time migration (RTM) was implemented with a modified crosscorrelation imaging condition for data from 2D elastic vertically transversely isotropy (VTI) media. The computation cost was reduced because scalar qP- and qS-Wavefield separations are performed in VTI media, for the source and receiver Wavefields only at the RTM imaging time, to calculate the migrated qP and qS images. Angle-domain common-image gathers (CIGs) were extracted from qPqP and qPqS common-source RTM images. The local incident angle was produced as the difference between the qP-wave phase angle, obtained directly from the source Wavefield polarization, and the normal to the reflector, calculated as the instantaneous wavenumber direction via a directional Hilbert transform of the stacked image. Angle-domain CIGs were extracted by reordering the prestack-migrated images by local incident phase angle, source by source. Vector decomposition of the source qP-Wavefield was required to calculate the qP-wave phase polarization direction...
-
2D and 3D elastic Wavefield vector decomposition in the wavenumber domain for VTI media
GEOPHYSICS, 2010Co-Authors: Qunshan Zhang, George A. McmechanAbstract:A pragmatic decomposition of a vector Wavefield into P- and S-waves is based on the Helmholtz theory and the Christoffel equation. It is applicable to VTI media when the plane-wave polarization is continuous in the vicinity of a given wavenumber and is uniquely defined by that wavenumber, except for the kiss singularities on the VTI symmetry axis. Unlike divergence and curl, which separate the Wavefield into a scalar and a vector field, the decomposed P- and S-Wavefields are both vector fields, with correct amplitude, phase, and physical units. If the vector components of decomposed Wavefields are added, they reconstruct those of the original input Wavefield. Wavefield propagation in any portions of a VTI medium that have the same polarization distribution (i.e., the same eigenvector) in the wavenumber domain have the same decomposition operators and can be recon-structed with a single 3D Fourier transform for each operator (e.g., one for P-waves and one for S-waves).This applies to isotropic Wavefields a...
Johan O A Robertsson - One of the best experts on this subject based on the ideXlab platform.
-
source signature encoding of a vertical source array to separate the emitted direct wave from its ghost
Geophysics, 2018Co-Authors: Moritz B Mueller, David Halliday, Dirkjan Van Manen, Johan O A RobertssonAbstract:Marine air-gun sources can be sequence-encoded by ring their individual elements independently over a short period of time. Using near-orthogonal ring sequences, whose crosscorrelation is minimal, as encoding sequences for multiple sets of air-gun sources enables us to exploit their orthogonality as a separation feature. We show that, by distributing air guns over depths from 5 to 30 m, ring sequences can be designed whose direct, downgoing Wavefield is close to orthogonal to its source ghost Wavefield. The fundamentally new aspect of this approach is that the source ghost signal is no longer just a time-delayed, opposite polarity version of the downgoing Wavefield, but due to the different air gun depths results in a different source sequence. This enables the consideration of the ghost Wavefield as a separate source. We generate a set of such ring sequences by minimizing the crosscorrelation of these Wavefields and optimizing their respective autocorrelations to achieve sharp peaks. The obtained, optimi...
-
wave equation processing using finite difference propagators part 1 Wavefield dissection and imaging of marine multicomponent seismic data
Geophysics, 2014Co-Authors: Lasse Amundsen, Johan O A RobertssonAbstract:ABSTRACTMethods for Wavefield injection are used in, for instance, reverse time extrapolation of shot gathers in reverse time migration. For correct injection of recorded data without any ambiguity of the propagation direction, the Wavefield-injection methodology requires pressure and particle velocity data such as multicomponent towed marine or seabed seismic recordings. We discovered that by carefully considering the models (medium parameters and boundary conditions) for injection, Wavefield injection of multicomponent data can also be used to solve several long-standing challenges in marine seismic data processing by means of conventional time-space-domain finite-difference propagators. We outlined and demonstrated several of these important applications including up-down separation of Wavefields (deghosting), direct-wave removal, source-signature estimation, multiple removal, and imaging using primaries and multiples. Only acoustic models are considered, but the concepts are straightforward to general...
-
interferometric modeling of wave propagation in inhomogeneous elastic media using time reversal and reciprocity
Geophysics, 2006Co-Authors: Dirkjan Van Manen, Andrew Curtis, Johan O A RobertssonAbstract:Time reversal of arbitrary, elastodynamic Wavefields in partially open media can be achieved by measuring the Wavefield on a surface surrounding the medium and applying the time reverse of those measurements as a boundary condition. We use a representation theorem to derive an expression for the time-reversed Wavefield at arbitrary points in the interior. When this expression is used to compute, in a second point, the time-reversed Wavefield originating from a point source, the time-reversed Green’s function between the two points is observed. By invoking reciprocity, we obtain an expression that is suitable for modeling of wave propagation through the medium. From this we develop an efficient and flexible two-stage modeling scheme. In the initial phase, the model is illuminated systematically from a surface surrounding the medium using a sequence of conventional forward-modeling runs. Full waveforms are stored for as many points in the interior as possible. In the second phase, Green’s functions between ...
-
volumetric Wavefield recording and near receiver group velocity estimation for land seismic data
Geophysics, 2002Co-Authors: Andrew Curtis, Johan O A RobertssonAbstract:“Volumetric recording” of the seismic Wavefield implies that the local receiver group or array approximately encloses a volume of the earth. We show how volumetric recording can be used to measure several spatial derivatives of the Wavefield. By making use of the full elastic wave equation, the free surface condition on elastic Wavefields, and derivative centering techniques analagous to Lax-Wendroff corrections used in synthetic finite-difference modeling, these derivative estimates can be inverted for P- and S-velocities in the near surface directly beneath the receiver group. The quantities estimated are the effective velocities of the P- and S-components experienced by the Wavefield at any point in time. Hence, the velocity estimates may vary with both wave type and wavelength. The estimates may be useful to aid statics estimation and are exactly the effective velocities required for separation of the Wavefield into P- and S-, and up- and down-going components.
Jia Yan - One of the best experts on this subject based on the ideXlab platform.
-
CWP-597 Angle-domain elastic reverse-time migration
2012Co-Authors: Jia Yan, Paul SavaAbstract:Multi-component data are not usually processed with specifically designed procedures, but with procedures analogous to the ones used for single-component data. Commonly, the vertical and horizontal components of the data are taken as proxies for P and S wave modes which are imaged independently with the acoustic wave equation. This procedure works only if the vertical and horizontal component accurately represent P and S wave modes, which is not true in general. Therefore, multi-component images constructed with this procedure exhibit artifacts caused by the incorrect wave mode separation at the surface. An alternative procedure for elastic imaging uses the vector fields for Wavefield reconstruction and imaging. The Wavefields are reconstructed using the multi-component data as boundary conditions for a numerical solution to the elastic wave equation. The key component for Wavefield migration is the imaging condition that evaluates the match between Wavefields reconstructed from sources and receivers. For vector Wavefields, a simple component-by-component cross-correlation between two Wavefields leads to artifacts caused by cross-talk between the unseparated wave modes. An alternative method is to separate elastic Wavefields after reconstruction in the subsurface and implement the imaging condition as cross-correlation of pure wave modes instead of the Cartesian components of the displacement Wavefield. This approach leads to images that are easier to interpret, since they describe reflectivity of specified wave modes at interfaces of physical properties. As for imaging with acoustic Wavefields, the elastic imaging condition can be formulated conventionally (cross-correlation with zero lag in space and time), as well as extended to non-zero space and time lags. The elastic images produced by an extended imaging condition can be used for angle decomposition of primary (PP or SS) and converted (PS or SP) reflectivity. Angle gathers constructed with this procedure have applications for migration velocity analysis and amplitude versus angle analysis. Key words: imaging, elastic, reverse time
-
CWP-628 Elastic wave mode separation for TTI media
2009Co-Authors: Jia Yan, Paul SavaAbstract:The separation of wave modes for isotropic elastic Wavefields is typically done using Helmholtz decomposition. However, Helmholtz decomposition using conventional divergence and curl operators does not give satisfactory results in anisotropic media and leaves the different wave modes only partially separated. The separation of anisotropic Wavefields requires the use of more sophisticated operators which depend on local material parameters. Wavefield separation operators for TI (transverse isotropic) models can be constructed based on the polarization vectors evaluated at each point of the medium by solving the Christoffel equation using local medium parameters. These polarization vectors can be represented in the space domain as localized filters, which resemble conventional derivative operators. The spatially-variable “pseudo ” derivative operators perform well in 2D heterogeneous TI media even at places of rapid variation. Wave separation for 3D TI media can be performed in a similar way. In 3D TI media, P and SV waves are polarized only in symmetry planes, and SH waves are polarized orthogonal to symmetry planes. Using the mutual orthogonality property between these modes, we only need to solve for the P wave polarization vectors from the Christoffel equation, and SV and SH wave polarizations can be constructed using the relationship between these three modes. Synthetic results indicate that the operators can be used to separate Wavefields for TI media with arbitrary strength of anisotropy. Key words: elastic, imaging, TTI, heterogeneous
-
isotropic angle domain elastic reverse time migration
Geophysics, 2008Co-Authors: Jia Yan, Paul SavaAbstract:Multicomponent data usually are not processed with specifically designed procedures but with procedures analogous to those used for single-component data. In isotropic media, the vertical and horizontal components of the data commonly are taken as proxies for the P- and S-wave modes, which are imaged independently with the acoustic wave equations.This procedure works only if the vertical and horizontal components accurately represent P- and S-wave modes, which generally is not true. Therefore, multicomponent images constructed with this procedure exhibit artifacts caused by incorrect wave-mode separation at the surface.An alternative procedure for elastic imaging uses the full vector fields for Wavefield reconstruction and imaging. TheWavefieldsarereconstructedusingthemulticomponentdata as a boundary condition for a numerical solution to the elastic wave equation. The key component for Wavefield migration is theimagingcondition,whichevaluatesthematchbetweenWavefields reconstructed from sources and receivers. For vector wave fields, a simple component-by-component crosscorrelation between two Wavefields leads to artifacts caused by crosstalk between the unseparated wave modes. We can separate elastic Wavefields after reconstruction in the subsurface and implement theimagingconditionascrosscorrelationofpurewavemodesinstead of the Cartesian components of the displacement Wavefield.Thisapproachleadstoimagesthatareeasiertointerpretbecause they describe reflectivity of specified wave modes at interfaces of physical properties.As for imaging with acoustic Wavefields, the elastic imaging condition can be formulated conventionally crosscorrelation with zero lag in space and time and extendedtononzerospaceandtimelags.Theelasticimagesproduced by an extended imaging condition can be used for angle decomposition of primary PP or SS and converted PS or SP reflectivity. Angle gathers constructed with this procedure have applicationsformigrationvelocityanalysisandamplitude-variation-with-angleanalysis.
