Incorporating geologic information into reflection tomography

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2005Challenges and opportunities in pre‐stack depth imaging of legacy seismic data: an overthrust belt case studyAuthors: Walt RitchieMihai PopoviciMoritz FleidnerWalt Ritchie3DGeo Development Inc; Chris Saxon, ChevronTexacoSearch for more papers by this author, Mihai Popovici3DGeo Development Inc; Chris Saxon, ChevronTexacoSearch for more papers by this author, and Moritz Fleidner3DGeo Development Inc; Chris Saxon, ChevronTexacoSearch for more papers by this authorhttps://doi.org/10.1190/1.2142225 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Over the past five years, the introduction of affordable 3‐D pre‐stack depth migration (PSDM) has changed industry perspective on how to approach seismic data in areas of complex geology. The ability to define an accurate interval velocity model and economically perform 3‐D pre‐stack imaging in depth provides the opportunity to more fully exploit the hydrocarbon potential of these difficult areas. Legacy or vintage data sets are also amenable to reexamination using these new pre‐stack depth imaging tools. However, the PSDM method is more demanding of the data requirements than conventional time techniques to deliver the degree of velocity model accuracy required for optimal spatial positioning of target reflectors. This paper discusses a seismic reprocessing program intended to assess deep gas potential in a mature producing province with very complex overthrust geology using the PSDM method. It highlights the challenges presented by vintage seismic 3‐D acquisition methodologies in an older rock/fast velocity regime and the opportunities from fully incorporating the wealth of available geologic knowledge into the modelbuilding process. A well‐constrained velocity modelbuilding solution is presented that overcame the limited sensitivity of the available 3‐D seismic data to velocity change due to acquisition geometry in this fast rock regime.Permalink: https://doi.org/10.1190/1.2142225FiguresReferencesRelatedDetailsCited byTomographic velocity analysis and wave-equation depth migration in an overthrust terrain: A case study from the Tuha Basin, ChinaBin Lyu, Qin Su, and Kurt J. Marfurt14 November 2017 | Interpretation, Vol. 6, No. 1Reprocessamento de dados sísmicos da Bacia do Solimões - Migração Prestack x PoststackBruno M. Silva, Leo R. Nascimento, Ronan Ávila, and Roberto R. Baldanza23 November 2007 SEG Technical Program Expanded Abstracts 2005ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2005 Pages: 2668 publication data© 2005 Copyright © 2005 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 07 Dec 2005 CITATION INFORMATION Walt Ritchie, Mihai Popovici, and Moritz Fleidner, (2005), "Challenges and opportunities in pre‐stack depth imaging of legacy seismic data: an overthrust belt case study," SEG Technical Program Expanded Abstracts : 400-403. https://doi.org/10.1190/1.2142225 Plain-Language Summary PDF DownloadLoading ...

The Guando field is in an area of rough topography and complex geology. In order to better site wells in such fields it is of value to have a sound understanding of this geological complexity. Obtaining a satisfactory seismic image to enable such interpretation requires pre-stack imaging. The geological complexity is such that post stack imaging fails due to the failure of the common mid point assumption. Is it sufficient to perform pre-stack time migration or is it necessary to perform pre-stack depth migration? In this instance both have been performed on this field and results for each of them will be shown. The basic theory of both pre-stack time and pre-stack depth migration will be described without using mathematics. Model data will then be used to illustrate the shortcomings of pre-stack time migration using a model of similar geological complexity to the Guando field. Both pre-stack migrations require a velocity field. These differ and the manner of their derivation also differs. Velocity model building for both techniques will be described. Particular emphasis will be placed on the use of tomographic and scanning techniques for pre-stack depth migration. Examples will be shown of the velocity models from both migrations. It will be shown that the pre-stack depth migration velocity model is also a valuable interpretation asset. Data from both migrations will be shown. These will demonstrate the superiority of the pre-stack depth migrated data and the reasons why it is superior.

Reflection tomography and interval velocity analysis (IVA) is applied to a 2D seismic line in the Surat Basin - an area where the primary goal is achieving reliable depth conversion of low relief structures, in the presence of near surface velocity anomalies and subtle lateral velocity gradients.The technique, using commercially available velocity analysis and Pre-Stack-Depth-Migration (PSDM) software, represents a potential solution to problems which have challenged previous workers. We demonstrate reflection tomography can resolve shallow velocity anomalies by using as input the moveout errors of deeper horizons from an initial pass of PSDM. Significantly, reflection energy from the near-surface layer is not required to resolve its velocity. When the shallow velocity model is determined, the interval velocity analysis of subsequent layers becomes stable and a more refined velocity model may be constructed. Several iterations of PSDM and reflection tomography are generally required before running a final pass of PSDM. The example seismic line contains three wells which are used to judge the quality of the final depth migration.The results demonstrate that it is possible to extract accurate lateral velocity information inherent in high fold CMP gathers from land data containing shallow velocity anomalies or residual static errors. A lateral velocity gradient causing an apparent pull up of 26 metres between Wells B and C is successfully resolved. The example demonstrates the predicted formation tops of Well C would have been accurate to around +/- 5m if reflection tomography and interval velocity analysis had been applied in place of a regional velocity function derived from nearby well data.

Time migration algorithms prove to be inadequate when trying to correctly image seismic sections in areas of complex geology. In such media, velocities vary both vertically and laterally giving rise to non-hyperbolicity in the observed moveout of the seismic reflections. To deal with this situation, a depth migration must be employed. However, a dilemma arises in that the performance of the pre-stack depth migration is sensitive to errors in the velocity model supplied by the user, that is the more accurate the underlying velocity model, the better the migrated image. This sensitivity provides an excellent tool for the iterative building of depth models (Figures 1 & 2). We present a method that combines pre-stack depth migration with velocity analysis and mapmigration.

In the mountainous terrain of west China, seismic data quality may be very poor due to severe statics problem and heavy noise especially in presence of complicated surface topography and complex subsurface structures. Moreover the complexity of subsurface structures maks the wave field much more complicated and the velocity distribution laterally varies significantly. Under this unfavorable context, it is very challenging to build a reasonable velocity model for the pre-stack depth migration to delineate the explicit structure contact relationship under complex geological settings. Here one enhanced velocity model building solution is presented. We innovatively make use of diving wave refraction tomography to build shallow velocity model and high-resolution reflection tomography to update the whole velocity model especially deep data. By tightly constraining the seismically derived velocity with available well control, one reliable velocity model can eventually obtained. After that we performed the pre-stack Kirchhoff depth migration and achieved geologically reasonable depth imaging results to help the geologists better understand the complex subsurface structures. This project is the first application of diving wave tomography on land seismic data in China. Its success has provided one robust and effective velocity model building solution and has significant meaning to other complex depth imaging land projects in China.

Prestack depth migration (PSDM) heavily depends on the accuracy of the migration velocity model. Reflection tomography on the base of common-image gathers (CIGs) is an important tool to build such models in depth domain. Unlike the conventional offset domain PSDM, angle domain PSDM can avoid migration artifacts when the velocity models are complex. It is easier to build the back-projection matrix for tomography according to the residual moveout (RMO) on the angle domain common image gathers (ADCIGs). In this abstract, we propose a novel nonlinear post-migration tomography approach to build velocity model using ADCIGs. The picked angle domain RMO is converted to prestack domain as invariants for data-fitting and model updating in the tomography iteration. Synthetic example proves the validity of this approach.

Imaging the subsurface, the ultimate goal of the seismic method, can be done in different ways and in different domains: time or depth. Although depth migration has become almost mandatory in areas of complex geology— because it accounts for traveltime nonhyperbolic moveout, it has, in fact, quite a limited purpose—to convert seismic data from one form to another for a given velocity model. Time imaging provides sufficient information for a subsurface of moderate complexity. Moreover, even for complex areas that require depth migration for correct subsurface imaging, time imaging usually constitutes a key first step that facilitates the estimation of a velocity model for depth imaging. For these reasons, improving the quality of time imaging is a focus of intensive research. A recent advance is multifocusing (MF), a method with the potential to greatly improve the quality of time imaging.

SUMMARY Compared to first-arrival traveltime tomography (FATT), first-arrival traveltime and slope tomography (FASTT) integrates both traveltimes and local slopes of first arrivals at sources and/or receivers to construct more accurate subsurface velocity models. Local slopes serve as additional constraints, helping to mitigate the ill-posedness of tomography by better constraining ray directions. This is particularly beneficial in regions with complex topography, where shadow zones arise due to strong velocity contrasts. However, representing complex topography or bathymetry can be less accurate when using classical rectangular grid discretization. To address these complexities with greater versatility and accuracy, we compute traveltimes of locally coherent events using a factored topography-dependent eikonal solver on curvilinear grids. Slopes are then estimated by finite differences in the traveltime maps after a back-and-forth coordinate transform from the curvilinear grid to a rectangular computational grid. Additionally, we solve the inverse problem with the matrix-free approach, where the data misfit gradient is computed with the adjoint-state method. This adjoint-state formulation avoids the explicit construction and storage of large Fréchet derivative matrices and does not require a tedious posterior ray tracing on curvilinear grids. A land synthetic example first illustrates the sensitivity of slopes to topography and the more accurate velocity models with FASTT than with FATT in the presence of topography. We then perform a first application of the topography-dependent FASTT method on a real redatumed ocean-bottom node data set, where the bathymetry exhibits a steep scarp. We show that the topography-dependent FASTT generates a velocity model that matches more closely a legacy reflection tomography model than conventional FATT. We conclude that the topography-dependent FASTT provides a versatile approach for handling complex surfaces during velocity model building in both marine and land environments.

Trapped shallow gas severely impacts signal to noise ratio and makes accurate overburden velocity model estimation difficult, since surface P‐wave data may be heavily attenuated and further contaminated with both surface related and internal multiples. Additionally, when the seabed is very shallow (<100m), reflection tomography is ill conditioned for reliably estimating near surface velocities due to the lack of short offset traces and the impact of multiples on deeper reflection events. The problems associated with these issues require an integrated approach to imaging ranging from improving data quality to estimating a reliable shallow velocity model, and integration into a robust global velocity model that will adequately image below gas. Building upon the technical strengths of both companies, BP and WesternGeco (WG) undertook a technical collaboration project to evaluate the fundamental causes of data quality degradation in Greater Cassia, offshore Trinidad and to implement appropriate existing solutions to improve imaging below shallow gas. As a product of this collaborative effort, in this paper we present an integrated workflow on real data showing that a targeted demultiple method combined with multiple passes of refraction and reflection tomography is an effective way to deal with data quality and imaging problems below shallow gas.

SUMMARY This paper demonstrates recent advances in grid-based reflection tomography model building in conjunction with anisotropic pre-stack depth migration. These advances significantly benefit the imaging and resolution of 3D seismic data in a number of structurally complex basins. Using 3D seismic imaging examples from Petroleum Exploration Permit PEP 51558 in New Zealand’s Taranaki Basin, this paper demonstrate how reflection tomography techniques can utilize implicit geological constraints to resolve complex velocity variations and, thereby, reduce structural and imaging uncertainty. One challenge associated with velocity modelling in the Taranaki Basin is to adequately resolve the large lateral velocity contrasts across major faults. This work shows how the use of steering filters during the reflection tomography process significantly improves the resolution and delineation of these fault-constrained velocity contrasts. This approach is compared with the conventional gridded tomography approach of velocity model building. Using a detailed velocity model from the Taranaki Basin, comparisons were made between time and depth migration, as well as alternate depth migration techniques.

Electrical and electromagnetic methods in areas of complex geology

Reflection tomography can be implemented as an algorithm that does not required manual traveltime picking and leads to reliable interval velocity models. Traveltime information can be automatically picked for any locally coherent reflection event, without the need for explicitly defined reflectors. Entropy constraints and staged smoothing help to reduce spurious velocity anomalies and to avoid local minima. A field data example shows that this algorithm can significantly improve an initial interval velocity model without any manual picking or interpretation. The velocity model obtained from this reflection tomography algorithm resulted in a much improved prestack depth migration.

For contributing to precise depth delineation in the Ichthys giant gas field offshore northwest Australia, we investigated shallow velocity heterogeneity effects using forward modelling and seismic data review. We discovered that their removal through applications of high-resolution tomographic velocity modelling and prestack depth migration (PSDM) enabled correct representation of the target reservoir structure. Analysis of synthetic seismic pre-stack gathers generated from forward modelling demonstrated that small velocity anomalies, such as channels, in shallow overburdens could give rise to apparent seismic RMS velocity artefacts at deeper target levels. Delineation of ``true' velocity anomalies and implementation of PSDM using precise shallow velocity model were required for solving these problems. In accordance with the phenomenon predicted by forward modelling, examination of coinciding patterns of time-thickness, amplitude and deep prestack time migration (PSTM) velocity allowed determination of the shallow heterogeneous layers that caused target velocity undulation. We employed two iterations of tomographic velocity model updating for PSDM velocity model building. First, the shallow heterogeneous velocity patterns were successfully identified by utilising the dense residual moveout picking and the layer-based 3D high-resolution finite-offset tomography. Subsequent grid-based global tomography with constraints was used for updating the entire velocity field and delivered stable velocity pattern at the deep. This two-step approach successfully eliminated deep velocity artefacts.

ABSTRACTLeast‐squares inversion of seismic reflection waveform data can reconstruct remarkably detailed models of subsurface structure and take into account essentially any physics of seismic wave propagation that can be modelled. However, the waveform inversion objective has many spurious local minima, hence convergence of descent methods (mandatory because of problem size) to useful Earth models requires accurate initial estimates of long‐scale velocity structure. Migration velocity analysis, on the other hand, is capable of correcting substantially erroneous initial estimates of velocity at long scales. Migration velocity analysis is based on prestack depth migration, which is in turn based on linearized acoustic modelling (Born or single‐scattering approximation). Two major variants of prestack depth migration, using binning of surface data and Claerbout's survey‐sinking concept respectively, are in widespread use. Each type of prestack migration produces an image volume depending on redundant parameters and supplies a condition on the image volume, which expresses consistency between data and velocity model and is hence a basis for velocity analysis. The survey‐sinking (depth‐oriented) approach to prestack migration is less subject to kinematic artefacts than is the binning‐based (surface‐oriented) approach. Because kinematic artefacts strongly violate the consistency or semblance conditions, this observation suggests that velocity analysis based on depth‐oriented prestack migration may be more appropriate in kinematically complex areas. Appropriate choice of objective (differential semblance) turns either form of migration velocity analysis into an optimization problem, for which Newton‐like methods exhibit little tendency to stagnate at nonglobal minima. The extended modelling concept links migration velocity analysis to the apparently unrelated waveform inversion approach to estimation of Earth structure: from this point of view, migration velocity analysis is a solution method for the linearized waveform inversion problem. Extended modelling also provides a basis for a nonlinear generalization of migration velocity analysis. Preliminary numerical evidence suggests a new approach to nonlinear waveform inversion, which may combine the global convergence of velocity analysis with the physical fidelity of model‐based data fitting.

&lt;p&gt;We use continuous seismic data from permanent and temporary, broadband and short-period stations that were operating during 2001 and 2008 to investigate the subsurface velocity structure of the Tongariro Volcanic Centre (TgVC) of New Zealand, particularly the highly active but poorly understood Ruapehu and Tongariro Volcanoes. Stacks of cross-correlation of two concurrent ambient noise seismograms can be used to estimate the interstation Green's Function, i.e., the impulse response of the earth between the two receivers. The Green's Functions are used to retrieve the dispersion relation (frequency-dependent velocity) of surface waves at different periods, which reflects the shear-wave velocity structure in the Fresnel volume of the propagating surface waves. Several studies have used dispersion measurements from ambient noise cross-correlations to investigate the shallow subsurface shear-wave velocity structure of active volcanoes around the world. Most use vertical components to retrieve the Rayleigh waves, but it is becoming increasingly common to use the horizontal seismogram components in addition to the vertical, giving further constraints to Rayleigh-wave measurements and introducing data relating to Love waves. We compute 1,048,968 daily cross-correlations for 955 viable station pairs across the two periods, including all nine-components of the cross-correlation tensor where possible. These daily functions are then stacked into 7458 full-stacks, of which we make group velocity dispersion measurements for 2641 RR-, RZ-, TT-, ZR- and ZZ-component stacks. Cross-correlation quality varies across the networks, with some station pairs possibly contaminated with timing errors. We observe both the fundamental and rst higher-order modes within our database of dispersion measurements. However, correctly identifying the mode of some measurements is challenging as the range of group velocities measured reflects both presence of multiple modes and heterogeneity of the local velocity structure. We assign modes to over 1900 measurements, of which we consider 1373 to be high quality. We invert fundamental mode Rayleigh- and Love-wave dispersion curves independently and jointly for one dimensional shear-wave velocity profiles at Ruapehu and Tongariro Volcanoes, using dispersion measurements from two individual station pairs and average dispersion curves from measurements within specifi c areas on/around the volcanoes. Our Ruapehu profiles show little velocity variation with depth, suggesting that volcanic edifice is made of material that is compacting and being hydrothermally altered with depth. At Tongariro, we observe larger increases in velocity with depth, which we interpret as different layers within Tongariro's volcanic system. Slow shear-wave velocities, on the order of 1-2 km/s, are consistent with both P-wave velocities from existing velocity pro files of areas within the TgVC, and the observations of worldwide studies of shallow volcanic systems that used ambient noise cross-correlation. A persistent observation across the majority of our dispersion measurements is that group velocities of the fundamental mode Love-wave group velocity measurements are slower than those of fundamental mode Rayleigh-waves, particularly in the frequency range of 0.25-1 Hz. Similarly, first higher-order mode Love-wave group velocities are slower than first higher-mode Rayleigh-wave velocities. This is inconsistent with the differences between synthetic dispersion curves that were calculated using isotropic, layered velocity models appropriate for Ruapehu and Tongariro. We think the Love-Rayleigh discrepancy is due to structures such as dykes or cracks in the vertical plane having greater influence than horizontal layering on surface-wave propagation. However, several measurements where Love-wave group velocities are faster than Rayleigh-wave group velocities suggests that in some places horizontal layering is the stronger influence. We also observe that the differences between the Love- and Rayleigh-wave dispersion curves vary with the azimuth of the interstation path across Ruapehu and Tongariro Volcanoes. Some significant differences between Rayleigh-wave velocities of measurements with different interstation orientations are also observed, as are differences between Love-wave velocities. This suggests a component of azimuthal anisotropy within the volcanic structures, which coupled with the radial anistropy makes the shear-wave velocity structures of Ruapehu and Tongariro Volcanoes anisotropic with orthorhombic symmetry. We suggest that further work to determine three-dimensional structure should include provisions for anisotropy with orthorhombic or lower symmetry.&lt;/p&gt;