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75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013
- Conference date: 10 Jun 2013 - 13 Jun 2013
- Location: London, UK
- ISBN: 978-90-73834-48-4
- Published: 10 June 2013
61 - 80 of 1113 results
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A Comparison of Triangular Mass-lumped Finite Elements for 2D Wave Propagation
By W.A. MulderMass-lumped continuous finite elements provide accurate solutions of the second-order acoustic or elastic wave equation in complex geological settings, in particular in the presence of topography and large impedance contrasts. Elements of higher polynomial degree have better accuracy than those of lower degree but are also more costly. Here, the performance of elements of degree 1 to 6 is compared for a simple acoustic test problem. The element of degree 6 is new. The numerical test confirm the increase of accuracy with the polynomial degree of the basis functions. In terms of computational cost, the element of degree 4 performs best in the specific example. For higher degrees, the cost of having extra nodes and a more restrictive stability constraint on the time step can no longer be compensated by having a smaller number of larger triangular elements. Only at very high accuracy, the new element of degree 6 wins.
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Fomel-Stovas’ Generalized Approximation - Model Study and Improvement
By E. BliasReducing computation cost for raytracing in multi-layered anisotropic models is considered in this abstract. Fomel and Stovas suggested a five-parameter approximation that they named “generalized” because it reduces to several known three-parameter forms. Model tests, performed by the authors, showed that this generalized approximation provides very high accuracy. These results imply that it can be used in place of the exact moveout function in modeling, migration and traveltime inversion. However, detailed model study shows that for some models, this approximation leads to significant errors. I developed a new eight-parameter approximation that provides higher accuracy and can replace the exact traveltime function that requires numerical ray calculations for each receiver.
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Wide-azimuth PP/PS Depth Imaging at Ekofisk Using Full Waveform Inversion
Authors A. Bertrand, T. Hellmann, C. Henstock, B. Lyngnes, S. Buizard, G. Oexnevad and N. HallerA large part of the Ekofisk field is seismically obscured due to gas-charged overburden layers. This has always represented a significant challenge for seismic imaging and interpretation. With the newly installed Ekofisk permanent seabed seismic system, acquisition of wide-azimuth, long-offset multicomponent data has permitted the use of powerful imaging and processing techniques. We present a state of the art velocity model building workflow using FWI and joint PP-PS tomography. Applied in combination with wide-azimuth multicomponent processing, it has enabled to significantly reduce the extent of the Seismic Obscured Area for both PP and PS data, as well as noticeably improve data quality for interpretation of more subtle geological features.
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Application of Full-waveform Inversion for Salt Sediment Inclusion Inversion
More LessSince the 1990s, subsalt imaging in the Gulf of Mexico (GOM) has been a major focus of oil and gas exploration. Due to the high velocity contrast between salt and sediment, defining salt geometry has been the key to successful subsalt imaging and efforts have been made to implement new methods to correctly determine salt geometry. While salt boundary definition is heavily dependent on manual picking, other methods have been proposed for inversion of sediments inside salt. In this paper, we demonstrate that full-waveform inversion can be a powerful and accurate alternative to unravel the complexity of sediment inclusion contained within the salt.
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Salt Exit Velocity Retrieval Using Full-waveform Inversion
More LessAccurate retrieval of salt exit velocity has been a important part in drilling hazard prediction, and is essential to achieve a clear subsalt image. Subsalt velocity retrieval is in many cases a difficult task to perform as the subsalt signal is often very weak. The common industrial practice to derive the subsalt velocity is to superpose a regional velocity gradient below the salt.However, using regional velocity gradient trend to estimate subsalt velocity ignores the relation between effective stress and velocities, which is extra important in determining the salt exit velocity. Full-waveform inversion(FWI) emerges as a advanced model building tool in recent years; however,the capability of FWI is limited due to lack of low-frequency and large-offset dataset, which are essential to the success of FWI retrieval. In this work, we successfully use time-domain full waveform inversion together with large offset dataset to retrieve the salt exit velocity, which improve the subsalt image and benefit the pore pressure prediction.
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Multi-Azimuth PSDM Processing in the Presence of Orthorhombic Anisotropy - A Case History Offshore North West Australia
Authors J. Sun, S. Birdus, W. Sun, Y. Xie, M. Gazzoli and M. AndreolliIn this paper we present a case history of multi-azimuth 3D PSDM processing. The datasets show strong HTI as well as VTI anisotropy. We show the processing workflow with emphasis on the construction of an imaging velocity model that correctly represents the orthorhombic anisotropy and short-wavelength velocity variations. The PSDM image is improved over earlier processings.
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Improving Gather Picking for Tomography in Complex Velocity Models, a Case Study on Shale Diapirs
Authors C.E. Jones, V. Valler, S. Dean, H. Sherazi-Selby and L. BystoelWe present a case study demonstrating that in a complex geological setting travel time tomography can produce high resolution velocity fields without resorting to the use of simplified blocky velocity models to represent the complex velocity field where there is poor seismic imaging. We developed a strategy to enhance the travel time picking which allows the reliable use of small spatial smoothing in the tomographic inversion and produces a stable complex velocity field which significantly improves the deeper imaging. The study area overlies the Vema Dome (offshore Norway) containing irregular structured shale diapirs in the shallow section. These diapirs are possibly cored by diatomaceous oozes deposited during the Paleocene and Miocene periods which results in rapid large velocity changes.
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Velocity-depth Models from Towed-cable Refraction Data - a Southern North Sea Case-study
Authors N. Jones, D.R. Toomey, S. Lunn, C. Biancardi and N. HallVintage narrow-azimuth towed-cable data-sets from the southern North Sea generally suffer from a lack of near-offset cover, giving rise to poor near-surface velocity control. Near-surface chalk units present a problem for such data-sets because of their high-magnitude and laterally-variant velocity structure. We investigate the use of refracted arrivals (that would normally be filtered or muted in conventional processing flows) to derive shallow 3D velocity-depth models. Specifically, we tomographically invert first-break travel-times and compare the resultant models with well-derived velocities over the Thoresby field.
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Successful Application of Joint Reflection/Refraction Tomographic Inversion in a Shallow Water Marine Environment
Authors S. Birdus, D. Criddle, A. Artyomov, L. Li, Q. Tang and V. LendzionowskiWe show how refraction tomography (also called first arrival travel time tomography) helps to produce more accurate and detailed depth velocity models below a shallow seafloor. We do not use refractions by themselves to build a complete shallow velocity model. In our proposed workflow, refraction tomography complements standard reflection tomography and the priority remains with the reflections to guarantee stability of the solution and to avoid uncertainties associated with refracted or diving waves in complex media. We use wave equation modelling to calculate synthetic gathers and estimate the travel time mismatch between real and synthetic first arrivals. It leads to a robust workflow which can be easily introduced into production depth-velocity processing. We show how this joint reflection/refraction velocity inversion works using a real 1000sq.km 3D marine seismic dataset acquired in an area where the water depth varies from 20m to 1100m.
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Depth Velocity Model Building Starting with Well-Focused Time Migrated Data - A 3D North Sea Case Study
Authors A. Vasseur, H. de Haan, M. Bader, N. Vidal and S. TaylorA new advanced tomographic toolbox provides a step change for time and depth velocity model building in terms of flexibility and robustness. We introduce a new accurate workflow combining benefits from time to depth workflow and multi-layer non-linear slope tomography for improving imaging quality and reducing the turnaround time of a depth imaging project when time migrated data are available. We present an offshore North Sea 3D case study which exhibits typical processing challenges such as interbed multiples removal, channel imaging issues and velocity model building in the presence of high velocity contrasts.
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A New Approach of Velocity Model Building by Using Geological Density Model - Study Case of the Bjřrnřya Basin
Authors M.A. Kusuma and A.W. KayatmoSome challenges of velocity model building in a frontier area are the limitation of available well data and sparse seismic data coverage (often only 2D data are available). In the Bjřrnřya basin, the wide variation of uplift histories (from 0 to approximately 2000 meters) and overburden thickness also added complexity to the velocity prediction. In order to tackle these challenges, we introduce a new iterative approach of velocity model building that incorporates the density prediction from basin modeling. The density model was built by incorporating burial history, uplift and pressure regime prediction of the area. This density model was then used as a primary guidance for velocity prediction away from the well location. Available seismic velocity was used for QC and comparison purposes. The result of this approach is a velocity model that has a better consistency with our geological knowledge of the area.
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Rock Physics Constrained Anisotropic Tomography - Methodology and Applications to the Gulf of Mexico
Authors H.K. Helgesen, R. Bachrach, Y. Yang, Y. Liu, N. Seymour, M. Woodward, K. Osypov and S. LeaneyWe present a new method for anisotropic velocity model building for seismic imaging. In the paper we describe the method and a case study that uses stochastic rock physics modeling of the non-linear relationships between the anisotropic parameters (epsilon, delta and vertical velocity) to estimate non-stationary covariance functions that are used as a priori information in the tomographic equations. This method allows us to constrain the tomographic updates with rock physics and thus stabilizes tomography. We present the method and a Gulf of Mexico (GoM) case study for anisotropic model building.
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Can We Build Geologically Consistent Well-Tied TTI Velocity Models from 3D Seismic and VSP?
By J. ZhuAnisotropy is very important to 3D seismic PSDM for building well-tie velocity models. VSP can serve as one of the critical components in estimating the anisotropic parameters. Zero offset VSP is used to estimate the quality factor Q which is used to correct the phases of the seismic data in prestack gathers. We propose to expand the WVSP recording levels from the first major velocity boundaries of any well paths such as the top Miocene in offshore West Africa, down to the target with 0.5-1.0km intervals in the wellbore. Integrating the WVSP traveltimes in the tomographic inversion of 3D seismic, we are hopeful that the TTI velocity model can be built geologically consistently. The PSDM volume from such TTI models will tie to the wells. We show an example from offshore West Africa that the PSDM results from such an integrated effort for TTI model building correlate the 3D seismic very nicely to the VSP images in extensive depth intervals in the wells.
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Tomographic Inversion of P-wave Data for TTI Media - Application to Field Data
Authors I. Tsvankin and X. WangReconstruction of TTI (transversely isotropic with a tilted symmetry axis) velocity fields requires supplementing P-wave reflections with borehole or other data and applying appropriate regularization operators. Here, we discuss a tomographic algorithm that combines reflection data and VSP traveltimes and apply it to an offshore field data set. The stability and convergence of parameter estimation is improved by implementing a three-stage model-updating procedure that gradually relaxes the constraints on the spatial variations of the Thomsen anisotropy parameters, while the symmetry-direction velocity is updated on a fixed rectangular grid. Structure-guided regularization helps propagate along interfaces reliable parameter updates corresponding to well locations. The algorithm is tested on a 2D line from an ocean-bottom survey acquired at Volve field in the North Sea. Check-shot traveltimes in two nearby wells are used in estimating the initial parameters and constraining the tomographic inversion. The obtained TTI model, despite having a somewhat lower vertical resolution than an existing anisotropic model for the field (which incorporates more detailed borehole information), noticeably improves image quality. The accuracy of the depth scale of the migrated section is verified by comparing prominent imaged reflectors to the available well markers.
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Channel Wave Tomography for Detecting Inhomogeneties in Coal Seam - A Real Dataset Example
Authors J. Wang, W.K. Lu and J.Y. ChengMechanized coal winning systems require thorough knowledge of seam structure, which can be obtained by in-seam seismic method. In this paper, we show an application of channel wave tomography method on a real dataset to obtain the velocity and attenuation estimations. The processing workflow consists of three steps: (1) seismic data enhancement; (2) velocity reconstruction; and (3) attenuation ratio reconstruction. Karstic collapse columns are a kind of vertical structures typically formed at Carboniferous-permian coal fields of north China and widely distributed. In our example, two Karstic collapse columns are detected and their ranges are predicted based on the abnormal velocity and attenuation distribution. This prediction is verified by the following coal mining.
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Imaging in the Time-Stretched Depth Migrated (TSDM) Domain
Authors R.L. Silva, C.P. Page and C.J. RudlingPre-stack depth imaging requires, amongst other information, an interpretation of the seismic horizons for building a depth interval velocity model/volume. This interpretation is usually performed on pre-stack time migrated seismic data. In complex areas, such as in the presence of salt domes, time migration is unable to place reflectors at their position of origin. A time interpretation is an important input to any pre-stack depth migration as the current practice in depth imaging is to determine an interval velocity model/volume in depth, which will satisfy the zero offset time present in the pre-stack time migrated section/volume on which the seismic interpretation has been made. We describe a procedure that will simultaneously reduce the reflector mis-positioning error and improve the pre-stack time image response which in turn will lead to a more suitable interpretation required for pre-stack depth imaging. This improvement is made possible by mapping the seismic data to the Time-Stretched Depth Migrated (TSDM) domain.
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Using Extended Correlation Method in Regional Reflection Surveys - A Case Study from Poland
Authors M. Malinowski and P. BrettwoodIn the effort to provide constraints on the deep crustal structure we have applied extended correlation technique to the ION GXTechnology PolandSPAN seismic reflection data. It allows to extend nominal record length of the survey (12 s in this case) to much longer times (18 s and 22 s tested here), given that raw uncorrelated data are stored and the up-sweep is used. The technique is not novel and has been successfully used, e.g. in Canada, during the LITHOPROBE project to save the time spend on single VP. For the times greater than the nominal record length, data are correlated using self-truncating sweep resulting in the original sweep spectrum kept for the nominal record length and the higher frequencies cut off for the greater times. Given the broad sweep spectrum (2-150 Hz) used in the survey, the high-end frequency at 22 s is 57.5 Hz, which is way above the expected frequency of deep crustal arrivals (usually below 30 Hz). The correlation was performed both using the pilot sweep signal and the mean of the measured ground force recorded for each separate vibrator and VP. Processing of the ground-force correlated data produced clearer reflectivity in the deeper section.
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Traveltime Computation and Imaging from Rugged Topography in 3D TTI media
More LessTraditional poststack/prestack migration procedures fail to work well in mountain areas because of the severe variations of the move-out and the low signal to noise ratio (SNR) data. A 3D dynamic programming approach to first-arrival traveltime computation is extended to anisotropic media with rugged topography, which is a key step of the working flow of PSDM in mountain areas. The traveltime computation method based on Fermat’s principle uses simple calculus techniques and a systematic mapping scheme to determine first arrival time on every uniform grid, which has no limitation on large velocity contrast and spatial variation anisotropic parameters. The numerical results of the over-thrust TTI model demonstrate that the traveltime computation method is correct and effective. The imaging results of 3D field data demonstrate that choosing a smooth datum to remove the high wavenumber move-out components and take consider of anisotropy are necessary in mountain areas.
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Simultaneous Source Acquisition - Achievements and Challenges
By D. EllisAs hydrocarbons become more difficult to discover and produce the role of seismic grows ever more important. This challenges the industry to find ways to acquire seismic data not only more safely and efficiently but also to reach the quality required for sophisticated subsurface characterisation applied to increasingly complex reservoirs. BP has responded to this challenge in a number of ways. In this paper I review one aspect of this response – the development and deployment of techniques that exploit simultaneous sourcing, also known as blended acquisition. BP has deployed two techniques to great advantage in North Africa and the Middle East, where conventional acquisition simply could not have got close to meeting their challenging business, HSSE and technical requirements. BP has also tested and applied blended acquisition for OBS. Future challenges include the feasibility of deploying the technology in non-open desert terrains, where access for seismic crews is more difficult. It remains to be seen what the limits are to developing the technique further for OBS and towed-streamer seismic surveys. But we can be confident that simultaneous source acquisition delivers real value, and is here to stay.
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Continued Development of Simultaneous Source Acquisition for Ocean Bottom Surveys
Authors R.L. Abma, A. Ford, N. Rose-Innes, H. Mannaerts-Drew and J. KommedalBP has acquired co-located 3D simultaneous source and conventional OBC surveys in both the North Sea and Trinidad to compare simultaneous source acquisition with conventional acquisition. A standard independent simultaneous source (ISS®) shooting method was used in Trinidad, while a combination of ISS and self-simultaneous shooting (SSS) was used in the North Sea. Two source boats were used with both simultaneous source surveys. Continuous recording was used to reduce the volume of data and simplify the acquisition. It was found that the quality of the processed images and gathers from both simultaneous source surveys were very similar to the quality of conventional surveys, but the acquisition was significantly more efficient.
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