Constrained Migration Velocity Analysis - A Case Study in the German Molasse Basin

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.

Relevance of the work. The paper considers challenging problems related with outlining of intervals with oil and gas presence in the mature Khylly field by use of latest 3D seismic survey techniques in order to gain larger crude resources base. The purpose of this research is to discover the most promising intervals of target horizons with relatively high reservoir properties outlined by 3D seismic data. The subjects of research are 3D seismic survey data, downhole seismic survey – Vertical Seismic Profiling (VSP) and well logging diagrams. The object of research is the Khylly deposit. The paper describes in brief geological and geophysical characteristics, stratigraphic and lithological features of rocks making the section. It is noted that despite repeated surveys by use of various geological and geophysical techniques, the field setting is not thoroughly studied and it has been covered by 3D seismic survey in 2012. Research results. 3D seismic survey applied across Khylly area is resulted in drawing of 4 structural maps for III and I horizons of Productive Series (PS), Akchagyl and Lower Absheron suites. Taking into account the relevance of structural planes of various stratigraphic levels and III horizon of PS being one of the major reference horizons the paper gives description of structural map drawn for this horizon. The detailed velocity model is designed based on VSP data with wide use of velocity analysis data. It has been made clear that Khylly area has block structure and each block has been described in detail. Based on acquired data it has been recommended to drill exploratory well R-1. Conclusion. Processing and interpretation of seismic data are aimed at solving some geological problems; the main task was to obtain results that ensure the study of the geological structure in the seismic survey area, including tracing of seismic horizons, faults and outlining the areas and section intervals which may be of interest due to possible oil and gas presence. VSP data acquired in well 2012 and velocity analysis made it possible to design velocity model of the section under the study, with the use of which the temporary 3D cube was transformed into a depth cube. The quality of seismic data is good and made it possible to solve the tasks set for this research.

An integrated procedure for migration velocity analysis in complex structures of thrust belts

ABSTRACTWave‐equation migration velocity analysis is a technique designed to extract and update velocity information from migrated images. The velocity model is updated through the process of optimizing the coherence of images migrated with the known background velocity model. The capacity for handling multi‐pathing of the technique makes it appropriate in complex subsurface regions characterized by strong velocity variation. Wave‐equation migration velocity analysis operates by establishing a linear relation between a slowness perturbation and a corresponding image perturbation. The linear relationship and the corresponding linearized operator are derived from conventional extrapolation operators and the linearized operator inherits the main properties of frequency‐domain wavefield extrapolation. A key step in the implementation is to design an appropriate procedure for constructing an image perturbation relative to a reference image that represents the difference between the current image and a true, or more correct image of the subsurface geology. The target of the inversion is to minimize such an image perturbation by optimizing the velocity model. Using time‐shift common‐image gathers, one can characterize the imperfections of migrated images by defining the focusing error as the shift of the focus of reflections along the time‐shift axis. The focusing error is then transformed into an image perturbation by focusing analysis under the linear approximation. As the focusing error is caused by the incorrect velocity model, the resulting image perturbation can be considered as a mapping of the velocity model error in the image space. Such an approach for constructing the image perturbation is computationally efficient and simple to implement. The technique also provides a new alternative for using focusing information in wavefield‐based velocity model building. Synthetic examples demonstrate the successful application of our method to a layered model and a subsalt velocity update problem.

Seismic wave attenuation has strong, negative effect on the seismic data resolution. The high influence of this phenomenon was observed during realization of seismic survey Duza Wolka 3D (Geofizyka Krakow S.A. 2010a). The resolution of recorded seismic data was very low due to high attenuation caused by the Lower Triassic Buntsandstein sandstones. In consequence, interpretation of the thin-layered Zechstein formation is ambiguous. The aim of this paper is to present the multi-stage algorithm for the purpose of minimizing the attenuation effect, hence increasing the resolution of the seismic data. In order to achieve this, the inverse Q filtering process was used. The Q factor (quality factor) was estimated from the vertical seismic profiling data (Geofizyka Krakow S.A. 2010c). To verify the effectiveness of inverse Q filtering, seismic modelling was performed. Results received from seismic modelling gave basis for application the inverse Q filtering on real data. For this operation the seismic time profile inline 112 was chosen from the seismic data set Duza Wolka 3D. Application of inverse Q filtering significantly increased the seismic resolution, which enabled the interpretation of the Zechstein formation with more accuracy.

The idea of path-integral imaging is to sum over the migrated images obtained for a set of migration velocity models. Velocities where common-image gathers align horizontally are stationary, thus favoring these images in the overall stack. The overall image forms with no knowledge of the true velocity model. However, the velocity information associated with the final image can be determined in the process. By executing the path-integral imaging twice and weighting one of the stacks with the velocity value, the stationary velocities that produce the final image can then be extracted by a division of the two images. The velocity extraction, interpolation, and smoothing can be done fully automatically, without the need for human interpretation or other intervention. A numerical example demonstrated that quantitative information about the migration velocity model can be determined by double path-integral migration. The so-obtained velocity model can then be used as a starting model for subsequent velocity analysis tools like migration velocity analysis or tomographic methods.

Thin (<20 m), sand injectites, too small to resolve in conventional seismic reflection data, can constitute a large part of the net-to-gross volume and affect fluid flow in the reservoir. However, they may also cause challenges for well placement and reservoir development because they are too small to be reliably imaged in seismic reflection data. It is therefore important to better understand how small-scale injectites influence seismic images, and how they may be recognized and characterized above reservoirs. The Grane Field (North Sea) hosts numerous small-scale sand injectites above the main reservoir unit, challengin well placement, volume estimates and seismic interpretation. Here, we investigate how such small-scale sand injectites influence seismic images and may be characterized by (1) using well, 3D seismic and outcrop data to investigate geometries of small-scale sand injectites (0–15 m thick), (2) performing seismic convolution modelling to investigate how these would be imaged in seismic data, and (3) comparing these synthetic seismic images with actual 3D seismic data from the well-investigated Grane Field. Our results show that despite injectites being below seismic resolution, small-scale sand injectites can be detected in seismic data. They are more likely to be detected if they have high thickness (>5 m), steep dip (>30°), and when densely spaced. Furthermore, as the fraction of sand injectites increases the top reservoir amplitude will decrease. Moreover, comparison of the synthetic seismic images with real seismic data from the Grane Field indicates that the low-amplitude anomalies and irregularities observed above the reservoir may be a result of the overlying sand injectites. Additionally, the comparison strongly suggests that the Grane Field hosts sand injectites that are thicker and located further away from the top reservoir than is indicated by well observations. These results may be used to improve well planning and develop reservoirs with overlying sand injectites. Supplementary material: A PDF file containing all the seismic modelling results allowing the reader to flip back and forth between the models is available at https://www.doi.org/10.6084/m9.figshare.14333102 and well logs from well 25/11-18T2 are available at https://factpages.npd.no/pbl/wellbore_documents/2358_25_1_18_COMPLETION_REPORT_AND_LOG.pdf

Summary The waveform inversion method based on full wave field is limited by the quality of seismic data. In practical application, due to the shortcomings of low signal-to-noise ratio at low frequency, short offset and large spatial variation of wavelet in onshore seismic data, the application of full waveform inversion in onshore data will generally face great challenges. This paper relies on the onshore data of Bohai Bay Basin to carry out acoustic full waveform inversion processing, including seismic data pre-processing, wavelet estimation and full waveform inversion iterative processing, and finally updates the velocity model of the subsurface shallow layer to obtain a velocity model that well matches the logging velocity, effectively improving the imaging quality of pre-stack depth migration.

Subsurface injection of carbon dioxide (CO2) is a technique to enhance oil recovery and so the economic value of depleting fields. It complements carbon capture and storage, which is a key technology to mitigate greenhouse gas emissions. In this work, an integrated method developed by the British Geological Survey and Cardiff University uses high-resolution 3D seismic and borehole data from the Jæren High to analyse potential seal breaches and fluid flow paths in a frontier area of the North Sea, ultimately assessing the risk of a possible carbon capture and storage site. We integrate the spatial analysis of subsurface fluid flow features with borehole and geochemical data to model the burial and thermal history of potential storage sites, estimating the timing of fluid expulsion. On seismic data, fluid pipes connect reservoir intervals of different ages. Spatial analysis reveals clustering of fluid flow features above strata grounded onto deep reservoirs intervals. Our integrated method shows that gas matured from Dinantian coal and migrated up-dip during the Triassic-Jurassic into the lower sandstone reservoir of the Rotliegend Group. The containing seal rock was breached once sufficiently large volumes of gas generated high overpressures in the reservoir. Some of these fluid flow features may still be active conduits, as indicated by bright amplitude anomalies within the pipes. This study shows how integrated analyses may enhance our understanding of fluid-flow pathways, de-risking prospective sites for carbon capture and storage. The method proposed in this work is particularly important to assess the suitability of area with trapped gas pockets and understand tertiary migration in areas proposed for geological storage of CO2.

Summary PETRONAS recently conducted the world’s first 3D broadband full azimuth marine seismic survey by an operator in East Malaysia. The survey seamlessly deployed two of the most cutting edge acquisition technologies concurrently i.e. the circular shooting technique for extending the azimuth sampling and the constant gradient slant cable towing technique to extend the bandwidth sampling. The legacy narrow azimuth 3D streamer data, acquired in the year 2001, suffers from poor imaging in the zone of interest. Though there were two successful wells with hydrocarbon discovery in the areas which have good seismic data quality, the area in between these wells is characterized by poor quality seismic data not amenable for interpretation and delineation of the reservoir. This poor data zone is understood to be caused by the inadequate illumination of the target horizon and low energy penetration through the monotonous shale in the overburden. Prior to the data acquisition, a survey design and modeling study was performed to select the optimum acquisition technique. A 3D earth model was created using the surface seismic and borehole data. 3D illumination ray tracing was performed at the target horizon using different acquisition geometries such as conventional narrow azimuth, multi azimuth and full azimuth circular shooting. A pilot broadband circular shooting survey was conducted over a subset of the study area based on the parameters derived from the survey design and modeling study. The time imaging processing result shows significant improvement in the seismic image at the target interval as well as the shallow level as compared to the legacy data. The extension of azimuth sampling and broader bandwidth enabled by the slant streamer circular shooting acquisition technique provides a superior image compared to the conventional narrow azimuth, flat streamer legacy data. These advanced acquisition techniques have the potential to offer solutions for other areas with similar imaging challenges in South-East Asia.

A three-dimensional inversion technique is developed to investigate the structure of the oceanic crust, using high quality offshore bathymetry, gravity and seismic data. The gravity signatures associated with variations in the thickness of the oceanic crust are isolated from the observed free-air anomaly by subtracting the gravitational effects of seafloor topography and the upper mantle thermal structure, downward continued to the mean depth of the crust/mantle interface and converted onto the relief on that surface. The thickness of the oceanic crust is then calculated by subtracting sea water depth from the depth of the gravity-inferred crust/mantle interface. Seismic refraction data was introduced directly as a constraint in the construction of the initial model for the configuration of the crust/mantle interface and the iterative process of the 3- D joint inversion to reduce the ambiguity in gravityinterpretation. This technique can be easily applied to the off-shore areas to interpret bathymetry, gravity and seismic data that have been routinely collected for the purpose of geophysical exploration. Compared to the unconstrained gravity inversion, this technique can predict a 3-D crustal model that fits better both gravity and seismic observation data of the study area. Introduction The non-uniqueness is an inherent problem in gravity inversion: the observed gravity field cannot be inverted to obtain a unique subsurface structure, unless some additional restrictions are imposed'. The ambiguity in gravity interpretation may he reduced by placing bounds on the density contrast and depth of the subsurface structure, which can be determined by applying certain physical principles. In the present study we propose a method to use seismic data to constrain the depth and configuration of a subsurface density discontinuity in a 3-D gravity modeling. The results of the seismically constrained gravity interpretation are in good agreement with seismic results, and the misfit of the predicted gravity field to the observed does not increase. Methodology The method is simple in principle: in a study area where both seismic and gravity data are available, we use seismic data to construct a framework for gravity modeling, and use gravity inversion technique to resolve the details of the configuration and depth of the subsurface density discontinuity. The implementation of the method may include the following steps. In the initial model for a subsurface interface, those points (nodes) that follow the projections of the seismic profiles are given the depths determined in seismic refraction (or reflection) surveys. Other points on the interface may be chosen at a constant depth (e.g., the average depth of the interface in the area, or the mean value of !he seismic depths). Doing so may introduce discontinuity, or the artificial impulse-shaped variations, to the configuration of the interface. To solve this problem we further CU1several narrow bands on that interface, These narrow hands are centered by the projections of the seismic profiles.

SUMMARY The accurate and fast estimation of velocity models is crucial in seismic imaging. Conventional methods, such as tomography, stereotomography, migration velocity analysis and full-waveform inversion (FWI), obtain appropriate velocity models; however, they require intense and specialized human supervision and consume much time and computational resources. In recent years, some works investigated deep learning (DL) algorithms to obtain the velocity model directly from shots or migrated angle panels, obtaining encouraging predictions of synthetic models. This paper proposes a new flow to recover structurally complex velocity models with DL. Inspired by the conventional geophysical velocity model building methods, instead of predicting the entire model in one step, we predict the velocity model iteratively. We implement the iterative nature of the process when, at each iteration, we train the DL algorithm to determine the velocity model with a certain level of precision/resolution for the next iteration; we name this process as ‘Deep-Tomography’. Starting from an initial model, that is an ultrasmooth version of the true model, Deep-Tomography is able to predict an appropriate final model, even in complete unseen during the training data, like the Marmousi model. When used as the initial model for FWI, the models estimated by Deep-Tomography can also improve substantially the final results obtained with FWI.

Nonlinear reverse-time migration is a modified reverse-time migration that accounts for the nonlinear relation between seismic data and model in order to image multiply scattered waves including multiples. The illumination of multiply scattered waves yields a representation of the Earth's subsurface that is more sensitive to model parameters, which allows for advanced seismic interpretation. The gain in sensitivity that multiply scattered waves provide also applies to velocity model building. I present a strategy for using the illumination and sensitivity of internal multiples for migration velocity analysis and illustrate this method with numerical examples.

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.

The collection of a range of different seismic data types has greatly improved our understanding of the crustal architecture of Australia's Archaean Yilgarn Craton over the last few years. These seismic data include broadband seismic studies, seismic receiver functions, wide-angle recordings and mine-scale to deep seismic reflection transects. Each data set provides information on the three-dimensional (3D) tectonic model of the Yilgarn Craton from the craton scale through to the mine scale. This paper demonstrates that the integration and rationalisation of these different seismic data sets into a multi-scale 3D geological/seismic model, that can be visualised at once in a single software package, and incorporating all available data sets, significantly enhances this understanding. This enhanced understanding occurred because the integrated 3D model allowed easy and accurate comparison of one result against another, and facilitated the integrated questioning and interrogation across scales and seismic method. As a result, there are feedback questions regarding understanding of the individual seismic data sets themselves, as well as the Yilgarn Craton as a whole. The methodology used, including all the data sets in the model range, had to allow for the wide range of data sets, frequencies and seismic modes. At the craton scale, P-wave, S-wave and surface wave variations constrained the 3D lithospheric velocity model, revealing noticeable large-scale velocity variations within and across the craton. An interesting feature of the data, easily identified in 3D, is the presence of a fast S-wave velocity anomaly (> 4.8 km s − 1 ) within the upper mantle. This velocity anomaly dips east and has a series of step-down offsets that coincide approximately with province and terrane boundaries of the Yilgarn Craton. One-dimensional receiver function profiles show variations in their crustal velocity across the craton. These crustal velocity variations are consistent with the larger-scale geological subdivision of the craton, and provide characteristic profiles for provinces and terranes. The receiver function results and the deep seismic reflection data both agree on the depth to the Moho, and both indicate an increase in Moho depth to the east. The 2D seismic refraction results in the south-west of the craton provide crustal thickness information, an indication of middle and lower crustal compositions, and information regarding the broad-scale architectural framework. At the province- and terrane-scale, the deep seismic reflection data and the mine-scale seismic data provide geometric constraints on crustal architecture, in particular the orientation of the region's fault systems as well as variations in the thickness of the granite–greenstone succession. Integration of the results from wide-angle seismic refraction data coincident with the deep seismic reflection data provided additional constraints on likely upper crustal lithologies. The integrated 3D seismic model implies the dominant geodynamic process involved the development of an orogenic belt that developed with a series of contractional (folding and thrusting) events, separated by equally important extensional events. The seismic reflection data in particular suggests that extensional movement on many shear zones was more common than previously thought. The seismic reflection data suggest that the dominant mineral systems involved deeply sourced fluid flowing up crustal-penetrating shear zones. These deeply sourced fluids were further focussed into sites located above fault-breached domal regions in the upper crust.