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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
1 - 100 of 1113 results
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Modal Elastic Inversion
By F. ErnstRecent years have seen the emergence of full-waveform inversion (FWI) approaches for generating detailed velocity models for imaging. For land seismic data, especially in areas of complex near-surface geology, a proper near-surface model is crucial for imaging. These two observations naturally lead to the question whether FWI approaches can also be used for near-surface model building purposes. As most wave propagation effects in the near surface are inherently elastic, this requires elastic FWI. The combination of small scales involved, and typical large sizes of modern land surveys, may make an approach based on finite differences or finite elements computationally very demanding. We propose a fast approximation to elastic FWI, modal elastic inversion, where wave propagation in the near surface is modeled by means of a (small) number of horizontally propagating waves: ground roll, its higher modes and guided waves. These waves comprise the vast majority of energy in land data and are sensitive to near-surface velocities. We apply modal elastic inversion to a 2D test line and show that both the shallow P-wave and S-wave velocity models can be recovered well.
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Q Estimation from Surface Waves
Authors D. Boiero, M. Werning and P. VermeerDespite the fact that surface waves in land seismic data are often regarded as noise, they can also be used to obtain valuable information about the subsurface. Because their propagation is directly related to the properties of the subsurface, the analysis and inversion of surface waves can provide a characterisation of the near surface. In this paper, surface waves were used to estimate the dimensionless quality factor Q, which is most commonly used to measure the attenuation of seismic waves. To invert for the quality factor Q, the surface-wave attenuation coefficient and the phase velocity, as well as the near-surface S- and P-wave velocities, are needed. A weighted damped least-squares algorithm was chosen for the inversion. The algorithm produced promising results both for synthetic data, used to test different subsurface conditions (not shown here), and for real data sets, providing information about the subsurface that is not yet obtainable with other techniques.
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Use of Simultaneous Joint Inversion as a Maximum Concordance Solver for Statics
Authors M. Mantovani, M. Clementi and F. CeciStatics are primarily derived from seismic refractions or first arrivals, and strongly rely on their quality of gathers. At the initial stage of seismic processing, refractions are the single data input. In common practise, inversions of first breaks are based on user’s interpretation of early arrivals and, therefore, they are subject to systematic errors, especially if picking is automated as for large 3D data sets. In such circumstances, it is desirable to avoid overfitting of observations in inversion. An active criteria counter to systematic mistakes is presented here, based on statistical benchmarking against independent non-seismic measures. The approach is based on Simultanous Joint Inversion for measure integration. While rock physics relations are normally unstable at the near surface, the qualitative concept of localized anomaly can be transported between various geophysical domains, as is normally done in prospect play evaluation. Anomaly distribution consistency between domains is here used as a discriminant of input data through a-posteriori inversion result benchmarking. A static solver can, therefore, weight more the first break data, which are experimentally confirmed by several independent measurements, rather than contradicting data. Concordance is based on concordant anomaly generation in the a-posteriori inverted model.
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Ice Scours as Trapping Mechanism for Shallow Gas
More LessShallow gas is commonly found on the Norwegian Continental Shelf and is a potential geohazard for drilling operations. Examples of shallow gas in Upper Pliocene sediments, in the Central North Sea is presented, and a model for how buried ice scours can act as a trapping mechanism for shallow gas is proposed. The examples are from conventional 3D seismic data.
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Mitigation of the 3D Cross-line Acquisition Footprint Using Separated Wavefield Imaging of Dual-sensor Streamer Seismic
Authors A.S. Long, S. Lu, D. Whitmore, H. LeGleut, R. Jones, N. Chemingui and M. FaroukiA modified one-way equation pre-stack depth migration of up-going and down-going pressure wavefields was applied to two datasets derived from 3D towed dual-sensor streamer data in offshore Australia and Malaysia. The primary objective was to mitigate the well-known cross-line acquisition footprint effects upon shallow data quality and interpretability. The new methodology introduced here exploits the illumination corresponding to surface multiple energy, and thus exploits what has historically been treated by the seismic industry as unwanted noise. Whereas a strong cross-line acquisition footprint affected the very shallow 3D data using conventional processing and imaging, the new results yield spectacular continuous high resolution seismic images, even up to, and including the water bottom. One implication of these results is that very wide-tow survey efficiency can be achieved without compromising shallow data quality if dual-sensor streamer acquisition and processing is used, even in very shallow water areas such as that discussed here. The imaging methodology can account for all degrees of lateral variability in the velocity model, full anisotropy, and angle gathers can be created to assist with velocity model building.
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3D Seismic-gravity Simultaneous Joint Inversion for Near Surface Velocity Estimation
Authors D. Rovetta, D. Colombo, E. Sandoval Curiel, R.E. Ley, W. Wang and C. LiangA novel 3D simultaneous joint inversion scheme for gravity and seismic travel time data is developed to solve for near-surface complex velocity distributions. The method incorporates industry-standard gravity and travel time inversion techniques while the joint inversion problem is solved by the introduction of various regularization functions about the model such as a-priori parameter distribution information, solution-space bounds, structural similarity via cross-gradient constraints and rock physics relations. The effectiveness of our joint inversion is demonstrated against a synthetic model representing a complex pattern of near-surface anomalies incorporating low and high velocity and density bodies. Results demonstrate the superiority of our approach where the shallow anomalies are better reconstructed by the joint inversion rather than that obtained by the single-domain inversions. The developed algorithm is tested with real data from Saudi Arabia acquired over a wadi structure. The results show a significant uplift of the time stack using the seismic-gravity joint inversion velocity model. The developed methodology is part of a multi-geophysics platform for near-surface velocity model building in complex geology scenarios.
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Integrating Resistivity and Seismic Surveys to Identify Shallow Hydrocarbon Reservoirs
Authors C. Badulescu, C. Iacob, M. Paduret, L. Tonita, N. Moga, C. Dutu and M. CohutDirect current resistivity survey is one of the oldest geophysical methods used for prospecting natural resources. When integrated in interpretation with seismic investigations, it usually offers high quality results, even in difficult investigation settings and complicated geology. The purpose of the study was to enhance seismic interpretation of the near-surface geology using high resolution direct current resistivity surveys, in order to confirm structures of potential shallow hydrocarbon reservoirs. Resistivity surveys have been executed in an area in Romania where seismic data gave poor results for the near surface geology delineation. Either by constraining inversion or interpretation, seismic and resistivity surveys proved to be an efficient couple of geophysical methods for delineating near-surface geology even in high noise and complicated geological settings. Geological models could be built for the studied area and structures with potential for hosting hydrocarbons have been identified.
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Guided Waves - Inversion and Attenuation
Authors D. Boiero, C. Strobbia, L. Velasco and P. VermeerGuided waves contain significant information about the near surface in seismic data, but prove difficult to remove through conventional velocity discrimination methods. Here, we analyse guided waves jointly with ground roll to characterise the near-surface properties and then we remove them by modelling methods. We begin by retrieving guided wave and ground-roll phase velocities. Then, we jointly invert them to build S- and P-wave velocity models using a robust multimodal inversion algorithm. Finally, we use the inferred guided wave and ground-roll properties to model the dispersive coherent noise and subtract it while preserving the signal.
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Imaging Unknown Faults in Christchurch, New Zealand, after a M6.2 Earthquake
Authors D.C. Lawton, M.B. Bertram, K.W. Hall, K.L. Bertram and J. Pettingaity of Christchurch, New Zealand, following a devastating Mw 6.3 earthquake on February 22, 2011 that caused the loss of 185 lives. The goal of the seismic program was to map previously unknown faults in and around the city for hazard assessment and to assist in the post-earthquake recovery effort. Seismic data were collected along six 2D lines, two of which were within the Christchurch metropolitan area and four were in rural areas west of the city. Recording conditions were challenging within the city, but good quality images were obtained along all of the seismic lines, with events interpretable to a depth of approximately 1.5 km. Numerous faults were imaged along the lines and these were interpreted in two groups – older faults that showed clear offsets in deep (> 1 km) reflections and younger faults that showed displacement in shallow reflections. Some faults in the latter group were interpreted to be directly associated with hypocentres of shallow after-shocks in the region. These interpretations are now being incorporated into a risk assessment for further possible shallow earthquakes in the region.
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Felt Induced Seismicity Associated with Shale Gas Hydraulic Stimulation in Lancashire, UK
More LessShale Gas exploration in the UK began in 2010 with five stages of hydraulic fracturing of the Bowland Shale. Hydofracturing in Cuadrilla Preese Hall-1 used 5 couplets of mini- and main-fracs with c. 10,000 bbl per stage, hydraulically isolated from each other during injection. An earthquake of 2.3 ML, a 1.5ML and a series of smaller events occurred from 31st March through May 2011 when work ceased for detailed analysis. Seismicity commenced after stages 2, 4 and 5; the largest occurring 10 hours after stage-2 shut-in. Seismological analysis using observations from seismometers emplaced after the 2.3 event, and regional data detected 55 seismic events from ML-2 to ML2, with 14 between ML 0.2 to M2.3, indicating a low B-value of 0.8 (+/- 0.3) suggesting an unusually small number of weaker events. The timing was highly correlated with injection with the largest events (stages 2 and 4) preceded by weaker events (ML0 and ML1.4) all located near the Preese Hall well, c. 500 meters south of the injection interval. The observed seismicity is almost certainly induced by hydraulic fracturing of the Preese Hall well. Future mitigation based on microseismic monitoring during hydrofracturing should reduce the likelihood of inducing further felt earthquakes.
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Induced Seismicity at Preese Hall, UK - A Review
Authors T. O‘Toole, J.P. Verdon, J.H. Woodhouse and J.M. KendallHydraulic fracturing activities during shale gas exploration at Preese Hall, near Blackpool, UK induced a series of anomalously large microseismic events, including two that were felt at the surface. The unexpected nature of this seismicity meant that microseismic monitoring of these operations was limited, with only two surface stations being deployed after a M_l 2.3 event was detected by the British Geological Survey's regional seismic network. The small size of the available dataset means that we must try and extract as much information from it as possible if we wish to fully understand what went on, and how risks might be reduced in future. Towards this end, we have used waveform inversion to determine moment tensors for a number of the induced events; events predominantly occurred with a pure strike-slip mechanism. To investigate the state of stress in the sub-surface from a different direction, we have also performed shear wave splitting analyses of the available dataset. These two techniques yield principle horizontal stress orientations that are in good agreement with each other, and that are also consistent with in situ measurements.
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Correlation Between Spatial and Magnitude Distributions of Microearthquakes during Hydraulic Fracture Stimulation
Authors J.P. Verdon, A. Wuestefeld, J.T. Rutledge, I.G. Main and J.M. KendallSeismic and microseismic event populations can be characterized by their spatial and magnitude distributions. The magnitude distribution is described by the Gutenberg-Richter b value. The spatial distribution can be described by the two-point correlation coefficient, Dc. We observe spatio-temporal variations in both b and Dc during hydraulic fracture stimulation in the Cotton Valley tight gas reservoir. We find that during the initial stages of the fracture stimulations, b is high while Dc is low, implying that deformation is dominated by smaller events clustered at points near the injection well. As injection progresses, b decreases and Dc increases. We investigate correlation between b values and Dc, which can be indicative of the style of fracturing. Initially, b and Dc are negatively correlated, consistent with mechanical weakening during the early stages of fracturing. As the stimulation progresses, b and Dc become positively correlated, consistent with later stabilization of the fractures and mechanical hardening when the induced fracture intersects a pre-existing fault. The mechanism of hardening may be crack blunting while propagating into a pre-existing fractured or damaged zone.
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Monitoring the Evolution of Fracture Compliances During Hydraulic Stimulation Using Passive Seismic Data
Authors A.F. Baird, J.P. Verdon and J.M. KendallSeismic anisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs. While many techniques to estimate anisotropy have been successful in inferring fracture density and orientation, they generally provide little constraint on the ability of the fractures to facilitate fluid flow. A potentially useful property to provide insight into this is the ratio of the normal to tangential fracture compliance (ZN/ZT). ZN/ZT is sensitive to many properties including: the stiffness of the infilling fluid, fracture connectivity and permeability, and the internal architecture of the fracture. Here we demonstrate a method to infer ZN/ZT using shear wave splitting measurements on two microseismic datasets from hydraulic stimulations. Both examples show apparent increases in ZN/ZT during the stimulation process. We suggest that this may be produced by the development of new, clean fractures that have a greater normal compliance than their natural counterparts, combined with increases in fracture connectivity and permeability. The ability to monitor ZN/ZT during stimulations provides a means to gain insight into the evolving flow properties of the induced fracture network, and may be beneficial for assessing the effectiveness of stimulation strategies.
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Beyond the Dots - Microseismic Monitoring of a Fractured Reservoir During Steam Injection in Oman
Authors S. Mahrooqi, S. Busaidi, I. Ismaili, F. Clow, T. Urbancic, A. Baig and A. KassamA Microseismic network of 13 wells over a fractured heavy oilfield in Oman has accurately monitored a steam injection program and has also observed sufficient multi-well events to enable extensive SMTI analyses. The SMTI data has been analysed together with the steam injection progress to identify the source mechanisms and fracture planes in relation to the pre-existing fault network, and has indicated, in different areas, both re-activation of existing faults and new fracture activity.
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Towards Self-consistent Microseismic-based Interpretation of Hydraulic Stimulation
Authors M.J. Williams, J.H. Le Calvez and J. StokesThe primary role of microseismic interpretation in the context of hydraulic fracture monitoring (HFM) is to provide understanding of the geometry of a placed hydraulic fracture treatment to enable better completion design, reliable production predictions, and real-time operational decisions during the treatment itself. One aspect of this interpretation is the identification of microseismic events that are related to the fluid-filled fracture propagation directly as distinct from those events which are not. We present a method to statistically identify clusters of microseismic events whose spatial and temporal separation are consistent with the propagation of a hydraulic fracture system, according to various standard models of fracture propagation. We then apply this interpretation to a forward model of complex fracturing to obtain consistency with the pumped data. We subsequently review that complex fracture model via a finite element geomechanical simulation, interpreting via elastic-brittle failure analysis and plastic deformation, to understand the potential source of microseismicity. Our principal finding is that it is possible to obtain a self-consistent interpretation between both fracture propagation and earth stress simulation by following this approach.
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Pseudo-elastic Impedance Calibrated to Rock Physics Models for Efficient Lithology and Fluid Mapping from AVO Data
Authors P.A. Avseth, T. Veggeland, M. Christiansen, K.J. Ĺrdal and F. HornWe have shown how we can derive attributes similar to elastic impedances, which we refer to as pseudo-elastic impedances, directly from rock physics templates of AI versus Vp/Vs, without knowing the density. This allows us to calibrate the elastic impedances to local rock physics models, and to honour the curvature of rock physics models, normally associated with compaction trends. In areas with large variability in burial depth, it is important to honour the true variability of the background trend. The final regression models are easy to implement in quantitative interpretation workflows, and allow for quick mapping of fluid or lithology anomalies that are consistent with rigorous rock physics models. We have demonstrated the validity of this approach on well log and seismic data from a prospective area in the Norwegian Sea.
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Fluid Identification Based on P-Wave Frequency-Dependent Azimuthal AVO Method in Fractured Media
More LessSeismic anisotropy is relevant to frequency. When fractures in reservoirs are saturated with different fluid types (oil, gas and brine), the variation rates of anisotropy versus seismic frequency (which is called anisotropy dispersion) are different. Thus, anisotropy dispersion can be used to distinguish different fluid types in fracture. In this paper, P-wave frequency-dependent azimuthal AVO inversion algorithm is proposed and applied to real seismic data. The studies focus on the analysis of P-wave anisotropic dispersion response for different fluid saturation. Continuous wavelet transform is employed for spectral decomposition, aiming at well time-frequency localization. Real data application indicates that the magnitude of P-wave anisotropy dispersion of brine is far stronger than that of oil and gas in fractures. Our study firmly proves that the method aimed at calculating the P-wave anisotropic dispersion for different fluids saturation is valid and reliable, and this brings us the confidence to discriminate fluid by the calculated anisotropic dispersion values in fractured reservoir.
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Estimation of Gas Saturation Changes from Frequency-Dependent AVO Analysis
Authors X. Wu, M. Chapman, E. Angerer and X. LiSeismic attenuation is believed to be sensitive to many reservoir parameters of interest, but its routine application is inhibited by the lack of reliable quantitative relationships which can relate observed behaviour directly to rock and fluid properties. This paper calibrates a frequency-dependent rock physics model for a gas field in Austria, through analysis of well log-data and frequency-dependent AVO analysis of seismic reflection data. Analysis of the model reveals the potential to estimate changes in gas saturation which cannot be detected with the standard, single frequency approximation.
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How Seismic Anisotropy Improves the Reliability of Exploration DHI (AVO)
Authors M. Ferla, F. de Finis and R. BacenettiThe role of seismic anisotropy has dramatically increased over the past two decades due to advances in parameter estimation. Many seismic processing and inversion methods take into account anisotropy. The inadequacy of isotropic velocity models has been emphasized in prestack depth migration, which is highly sensitive to the accuracy of the velocity field. At the same time this study demonstrates how amplitude variation with offset analyses are sensitive to anisotropy. The correct integration between different seismic processing steps that incorporate anisotropic models represents a valid support for the identification of explorative targets. A new integrated workflow has been developed in terms of anisotropic Thomsen parameters estimation, velocity and AVO models. We present interesting results of modelling for special cases of exploration interest, highlighting the anisotropic effects for gas sands encased in shales. The analysis has been also performed on a real dataset, confirming the relevance of models that account for the seismic anisotropy. This could explain the inability of elastic synthetics to match the prestack amplitudes of field data in some cases. This considerations lead to more realistic reservoir models and hopefully answer some of the unexplained pitfalls in AVO interpretation.
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Non-Stretch Fourth Order NMO through Iterative Partial Corrections and Deconvolution
Authors E. Biondi, E. Stucchi and A. MazzottiSource to receiver distances employed in seismic data acquisition have been steadily increasing and it is now common to work with data acquired with more than 10 km of offset. Sub-basalt exploration and seismic undershooting are just two applications where long-offset reflections are valuable. However, such reflections are often subjected to muting to avoid NMO stretch artifacts, thus causing a loss of valuable information. It is therefore of interest to find ways to avoid the distortions caused by the standard NMO correction and to retrieve these portions of the recorded wavefield for a better use in the processing. To this end we develop a non-stretch NMO correction based on a wavelet estimation and on a iterative procedure of partial NMO correction and deconvolution. To drive the corrections we make use of 4th order traveltime curves, that further extend the offset range of usable reflections. Then we estimate time and space variant wavelets, by means of SVD along the sought traveltimes, that become the desired output for the deconvolution trying to retrieve the original shapes of the partially stretched wavelets. We test our method on synthetic data and we perform a blind test on real data simulating an undershooting acquisition.
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Stacking on OCO Trajectories
Authors T.A. Coimbra, A. Novais and J. SchleicherWe introduce a data-driven stacking technique that transforms 2D/2.5D prestack multicoverage data into a common-offset (CO) section. Similarly to the CMP and CRS stacks, this new method does not rely on an a-priori velocity model but provides velocity information itself. The original offset-continuation-operation (OCO) method is a seismic configuration transform designed to simulate a seismic section as if obtained with a certain source-receiver offset using the data measured with another offset. Since an OCO depends on the velocity model used in the process, it can be combined with stacking techniques for a set of models, thus allowing for the extraction of velocity information. The algorithm is based on so-called OCO trajectories, which are related to the concepts of image waves and velocity rays. We theoretically derive the OCO trajectories from the kinematic properties of OCO image waves that describe the continuous transformation of the common-offset reflection event from one offset to another. Based on OCO trajectories, we then formulate a horizon-based velocity-analysis method, where root mean square (RMS) velocities and local event slopes are determined by stacking along event horizons. A numerical example demonstrates the feasability of the method.
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Moveout Approximation for Compression Waves in Layered Orthorhombic Medium
More LessOrthorhombic models comprise subsurface anisotropy caused by vertical azimuthally-aligned fractures and layering, or by two orthogonal sets of vertical fractures, with or without layering. In this paper we derive new relations for hyperbolic and non-hyperbolic moveout approximations for pure compression waves, considering a 1D model that consists of a set of orthorhombic layers. The layers have a common vertical axis but different orientations of horizontal orthorhombic axes. For 1D models, the azimuth of the phase velocity is the same for all layers, while the azimuths of the ray velocity are generally different. We extend the existing studies on moveout in an orthorhombic model, accounting for the azimuthal deviation between the phase and ray velocities. We then formulate the azimuthally-dependent NMO velocity for a package of layers. Finally, we compare the derived full quartic moveout term with its acoustic approximation, and verify the accuracy of the approximation vs. exact analytical ray tracing.
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Re-mapping the Njord Field with Ocean-bottom Seismic Data
Authors P.J. McFadzean, G. Pless and S. ØstmoOcean-bottom seismic (OBS) data has recently been acquired over the Njord field with a view to improving the image for the seismic interpreter. Comparisons between OBS and previous generations of streamer data show that faults can be placed with significantly higher confidence on the former dataset. This results from improved definition of rotated Jurassic/Triassic fault blocks, assigned in part to the rich azimuthal illumination associated with the OBS acquisition. Additional structural details contained in the OBS data also emerge using automated trace analysis techniques such as coherency. As such, OBS data acts as a powerful tool to reduce the residual structural uncertainty on the highly compartmentalised Njord field.
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The Significance of Ramps and Flats of Mass Transport Deposits (MTD) as Kinematic Indicators
Authors K. Omosanya and T.M. AlvesRamps and flats of mass transport deposits are commonly studied on seismic profiles as kinematic indicators. In this study, ramps and flats on the margin of salt diapirs are mapped in a high quality 3D seismic volume from SE Brazil (Espírito Santo Basin). Our aim is to test how differently oriented ramps and flats at the basal shear surface of specific MTD interval can be used to infer the direction of mass flow. Statistical analysis of thickness variation of the mass transport deposit within depocentre created by ramps and flats was estimated along slope in order to elucidate the mode of emplacement of the associated MTD. Sliding at the plane of failure is enhanced by seafloor faulting and nearby halokinesis. Ramps significantly contribute to local changes in gradient at the basal shear surfaces, as they are linked to promontories (Type I) and paleo-fault scarps (Type II).
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Preliminary Seismic Interpretation of the Labrador Slope and Deepwater, Offshore Newfoundland and Labrador, Canada
Authors D.E.L. Cameron, I.A. Atkinson, J.E. Carter and R.J. WrightThe Labrador Sea is one of the largest under-explored areas along the eastern Canadian Margin. Prior to 2011, exploration of this region had focused on the continental shelf with numerous vintage 2D seismic surveys executed over the past 50 years and 30 wells drilled during the 1970’s and 1980’s. Most of the wells drilled in the Saglek and Hopedale basins were targeting shelfal structural traps. The acquisition of the new 2011-2012 Labrador Sea long offset regional 2D seismic data (22,000 km) has resulted in significant uplift in image quality and has provided new data for a vast area of the slope and deep-water that was previously un-imaged. The enhanced image quality is leading to identification of geological features and potential leads in regions never before considered prospective.
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A Hybrid and Adaptive Attribute for Noise Reduction in Post-Stack Seismic Data
Authors A.A. Aqrawi, D. Barka and W. WeinzierlWe propose a hybrid and adaptive smoothing method which accounts for attenuation and texture variations inherent in seismic data. Seismic conditioning is a fundamental step in numerous attribute assisted interpretation workflows. Despite its simplicity smoothing is utilized widely when it comes to noise reduction while preserving edges. Attenuation and dispersion of the acoustic wavefield often confines the use of such techniques to a limited texture or interval. We present a spatio-temporal windowing technique with adaptive smoothing. Depth dependant operator sizes are used to derive a variance based neighborhood estimate. This consecutively is used to condition the operator size in addition to defining the smoothing method. We alternate between the mean and median smoothing methods depending on the coherency in the seismic response. The median smoothing method is applicable to regions of high variability, whereas the mean form gives an enhanced continuity of the bedding and is hence preferable in coherent regions. The method proposed in this study is applied to a heavily faulted dataset offshore Norway on the continental shelf. Results obtained indicate that the method proposed for hybrid adaptive filtering can provide increased clarity in the identification and evaluation of subsurface features.
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Provenance of a Blocky Debris Flow Deposit in Mid-continental Slopes (Espírito Santo Basin, SE Brazil)
Authors K.O. Omosanya and T.M. AlvesThe provenance of an Oligocene-Miocene Mass Transport Deposit (MTD) in mid continental slope Espírito Santo Basin, SE Brazil was analysed using seismic facies and measurement of preserved blocks. Our method, applied to a high-quality 3D seismic volume from the Espírito Santo Basin (SE Brazil), provides information on the location of source areas of MTDs, their bulk composition and styles of disaggregation in relation to their transporting distances. Whenever blocks of strata are found, their orthogonal axes are measured to estimate parameters such as Maximum Projection Sphericity Index (MPSI), Oblate Prolate Index (OPI), and ratio of short and intermediate axes (ds/di). Eighty two (82) blocks mapped in the MTD have no preferred orientation: they have compact-bladed, bladed, elongate, very-bladed, very-platy and very-elongate shapes. Average block thickness, area coverage and volume are ~128.82m, 0.802 sq. km and ~0.196 cubic km with mean MPSI, OPI, flatness and elongation ratios of 0.398, 0.991, 0.19989 and 0.59861. In addition, the presence of equant blocks with c/a>0.4 and c/b>0.65 ratios are typical of proximal areas in MTDs, reflecting small transporting distances. Thus, the new method used in this research provides an alternative technique to kinematic indicators for determining the provenance of blocky debris flow deposits.
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High Resolution Seismic Geo-Model for Interpretation across a Complex Structure
Authors S. Mukund, I. Schmidt and D. StanbrookIn this paper we present an integrated approach to seismic interpretation using well data, seismic inversion and innovative geo-modeling technology. Structural complexity and poor signal to noise ratio can limit our ability to interpret seismic data, limiting geological understanding. This case study illustrates a methodology that helps to reduce the uncertainty of the seismic interpretation process and improves our understanding of the stratigraphy. The approach to interpretation demonstrated in this paper has helped to impose strong constraints to guide the seismic interpretation and reduce uncertainty. As a result, depositional features were interpreted from seismic attributes, supporting well and core-based regional understanding.
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Structurally Sharpened Continuous Color Method for Extraction of Structural Features from Seismic Data
By A. LaakeThe extraction of structure from seismic data volumes is a challenge, especially when the structure is confined to thin layers and does not follow horizons. Usually, spatial or spectral seismic attributes are used to extract such structural features. A new method that is based on continuous color rendering in red-green-blue (RGB) color space and hue-saturation value allows the extraction of heterogeneities from seismic data cubes that are representative of geological features such as channels and fracture zones. The analysis in continuous color space enhances the dynamic range of the studied data, which allows for image processing to sharpen the heterogeneities representative of geological structures. Simultaneously, it attenuates unwanted noise, which may be present in the seismic data. The structurally sharpened RGB method does not rely on spatial averaging in any direction, which is conventionally required during the generation of seismic spatial attributes. This results in high-resolution imaging of structures that has been demonstrated on channels and fracture zones in a 40-m thick clastic layer package.
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Differential Compaction and Its Seismic Attribute Expression
Authors S. Chopra and K. MarfurtWith continued burial and overburden, pore sizes are reduced and water is squeezed out of the rocks, reducing the rock volume. Different lithologies have different original porosity, pore shapes, and mineral matrix composition, and thus different response to burial. Lateral changes in lithology give rise to lateral changes in compaction, or simply “differential compaction”. For this reason, easily-mapped flooding and other surfaces that were originally flat can exhibit measureable, and often significant structural relief. These maps give rise to lateral “structural” anomalies. Recognition of differential compaction forms a key component in modern seismic geomorphology based interpretation workflows with excellent publications showing the expression of differential compaction on vertical slices. Mapping the 3D expression of compaction features takes considerable time and is thus less well reported and the use of 3D geometric attributes to map compaction features is underutilized. In this paper we illustrate the attribute expression of the more common differential compaction features over channels and carbonate reefs using examples from the Western Canadian Sedimentary Basin.
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Multi-layer Non-linear Slope Tomography
Authors G. Lambaré, P. Guillaume, X. Zhang, A. Prescott, M. Reinier, J.P. Montel and A. CavaliéTomography algorithms using gridded model description and ray tracing have made continuous progress in terms of resolution and efficiency. However one strong limitation is the difficulty to recover strong velocity contrasts encountered in presence of salt bodies, chalk, basalt, carbonates… The conventional solution is to proceed in a top-down manner from one velocity contrast to the next one. In such a layer stripping approach velocities and horizons are updated layer after layer recursively from top to bottom. Such a workflow is time consuming and prone to velocity errors being propagated into deeper layers as the model building progresses. We present a solution that involves a non-linear tomographic approach combining dense dip and residual move-out picks with horizons describing the main velocity contrasts. While dip and RMO picks are used to update 3D velocity grids inside each layer by non-linear slope tomography, the picked horizons describing layer boundaries are kinematically de-migrated and re-migrated recursively from top to bottom to reposition the major velocity contrasts after each velocity update. We present applications of the method to a marine North Sea dataset and to a land dataset with salt structures and compare the results with the layer stripping approach.
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Incorporating Geologic Information into Reflection Tomography with a Dip Oriented Gaussian Filter
By C. ZhouReflection tomography is non-unique and a regularization term is usually added into its objective function as an additional constraint. The anisotropic Gaussian filter has been successfully employed as such a regularization operator. By orienting the smoothing axes along the local dip directions, the new Gaussian filter helps reflection tomography produce models that conform to the reflector structures. For efficiency purpose, the 3D Gaussian filter is factorized into three 1D filters in a non-orthogonal coordinate system. The dip oriented Gaussian filter also provides the freedom for applying spatially variable smoothing.
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Tilted Orthorhombic Imaging for Full Azimuth Towed Streamer Data in Deep Water Gulf of Mexico
More LessRecently, tilted transverse isotropic (TTI) imaging has become a standard practice in deep water Gulf of Mexico (GOM) to resolve the anisotropic effects of wave propagation in salt-withdrawal mini-basins. This anisotropy is thought to arise from the geometry of sedimentation processes, with the “tilt” applied by subsequent tectonic activity. However, the presence of significant tectonic stress or uneven stress can cause fractures in thin-bed layers, which results in additional directional velocity variation for seismic wave propagation, or azimuthal anisotropy around the bed normals. In these cases, the transverse isotropic assumption is insufficient to explain conflicting residual moveouts among CIGs of different azimuths from TTI imaging. A more general anisotropic model, tilted orthorhombic (TOR), is needed to cope with azimuthal velocity variation in these complex geological settings. In this paper, we use a full azimuth (FAZ) data set from Keathley Canyon, GOM, to derive both TTI and TOR models. With the TOR model, we observe improved gather flatness among azimuths and improved structural imaging. We also demonstrate the advantage of FAZ data in detecting azimuthal anisotropy over WAZ data.
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Tomography with Geological Constraints - An Alternative Solution for Resolving of Carbonates
Authors O. Zdraveva, S. Hydal and M. WoodwardCarbonates are often present in close proximity to salt in the sedimentary basins around the world. They could be highly heterogeneous and in addition are often interspersed with lower-velocity sediments. The occurrence of high-velocity contrast layering in some portion of the lithology section could pose a problem for grid tomography and may result in insufficient resolution and poor delineation of the layer boundaries, unless many iterations of high-resolution tomography are run. We present a method for successful delineation of carbonate layers by introducing implicit and explicit geological constraints during the global common image point (CIP) tomography updates. The use of geological constraints in the CIP tomography yields high-resolution models over large areas of significant complexity with a reduced number of iterations. In addition, it eliminates the need to consider separate geobodies and multilayer representation of the medium. We show examples of successful application of this method to data sets with variable acquisition geometries from the Gulf of Mexico and offshore West Africa.
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A Method for Rapid Generation of Complex 3D Models for Numerical Simulations
Authors M. Urosevic and A. DzunicA method for rapid generation of complex 3D geological models is presented. The method utilizes scaled physical models which are built out of simple materials that provide sufficient density contrast. The model is CAT scanned, assembled into 3D and elastic properties are assigned to each node. Subsequent post and pre-stack numerical simulations allows us then to investigate several issues of high importance to the application of seismic for mineral exploration. Those are: understanding better the response of complex, fractal geology; improve 3D acquisition design by data decimation and imaging analysis; evaluate the effectiveness of various processing algorithms and provide help during the interpretation of real field data. The method proposed will be used to build a catalog of seismic responses over world-known mine sites.
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Volume Based Modeling - Automated Construction of Complex Structural Models
Authors L. Souche, F. Lepage and G. IskenovaA new technology for creating, reliably and automatically, structural models from interpretation data is presented. The main idea behind this technique is to model directly volumes (the geological layers) rather than surfaces (horizons that are bounding these layers). In order to enforce the geological consistency of the created models another key element is built into this technology: it guarantees that the variations of dip and thickness of the created geological layers are minimized, while all seismic and well data are properly honored. The proposed method enables the construction of very complex structural models, independently from the geological settings, and even when such models have to be built from sparse or noisy data. The full automation of the model construction process allows to rapidly update the model, to efficiently identify the most uncertain parameters, to understand their impact, and to iteratively optimize the model until it fits all available data. To demonstrate the advantages of this technique the construction of a complex exploration-scale structural model of a prospect located offshore Australia is detailed.
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3D Interpolation and Extrapolation of Sparse Well Data Using Rock Physics Principles - Applications to Anisotropic VMB
Authors R. Bachrach and K. OsypovMapping sparse well data into 3D volumes is a challenging problem. Specifically, as seismic velocity model buildings (VMB) requires 3D velocity and anisotropy volumes, and as most anisotropy estimates are calibrated to borehole data, generating 3D anisotropic models relies heavily on extrapolating sparse well data. This spatial extrapolation problem is ill posed and many subjective decisions are made during model building. New advancements in basin modeling and analysis enable us to model temperature and compaction history using simple physical principles. These fields can be used as auxiliary fields to solve spatial interpolation problems of sparse well log data using the concept of interpolation in the rock physics domain. The principle suggests that sparse well log measurements in the physical (x,y,z) 3D space may still adequately sample the rock physics space of temperature, porosity, and effective stress to allow proper reconstruction of the anisotropic velocity field in a manner that is consistent with the diagenetic process. We present the basic concepts and give a synthetic example to support this direct link between basin modeling and anisotropic velocity model building.
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Ormen Lange Fan Deposition - Evaluation of Source and Palaeo-Bathymetry Scenarios Using Process-Based Modelling
Authors R. Basani, E.W.M. Hansen, M. Grecula, S. Price and A. CantelliThe deterministic software MassFLOW-3D™ has been developed to construct a 3D model for the simulation of turbidity currents. All hydraulic properties of the flow and its responses to topography can be monitored in 3D. It can fill the gap between small laboratory and large field scale. The presented study is aimed to evaluate the flow behavior and distribution of deposits in the Maastrichtian-Danian sequence of the Ormen Lange system.In order to run a 3D numerical simulation, the palaeo-bathymetry at time of deposition had to be reconstructed. This process, combined to the observation of the sediments thickness in the field, led to the creation of alternative surfaces.In addition, multiple grain sizes were represented in the model. Different realizations observing different possible scenarios aimed to reproduce the stratigraphy observed from the core samples were performed.The simulations produced deposits showing an overall good match with what was observed in the field, providing a range of physically plausible scenarios of reservoir development. Nonetheless the match at a well level was not perfect. This is due to the limited number of flows performed, as well as to the uncertainties inherent in the palaeo-bathymetric reconstruction and the definition of initial flow characteristics.
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Tomographic Q Estimation for Viscoacoustic Imaging
Authors A. Valenciano and N. CheminguiViscoacoustic imaging in anisotropic media is achieved by tomographic estimation of the earth attenuation (Q model), and prestack wave equation migration. The proposed Q tomography algorithm uses spectral ratios computed on surface seismic data as the input. An integral tomographic equation relates the Q model with the measured spectral ratios. The tomography numerical implementation results in a linear inversion scheme that we solve by conjugate gradient methods with 3D regularization. The output Q model is combined with VTI or TTI anisotropic models to perform model-driven attenuation and anisotropy compensation during imaging. To that effect we use a viscoacoustic anisotropic Fourier finite differences one-way wave equation migration. Results from a synthetic example and a VTI field dataset from the North Sea demonstrate the accuracy of our tomographic estimation of Q, and the effectiveness of the viscoacoustic wave equation migration for attenuation compensation.
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Estimation of Shear-wave Attenuation Profile from Scholte Waves Using Ocean Bottom Seismic Data
More Lesschers to analyze the attenuation mechanism of seabed sediments. This paper presents a technique for inversion of shear-wave attenuation profile from interface wave dispersion. By focusing on two issues i.e., to extract attenuation dispersion curve and correct geometrical spreading loss of the interface waves, Prony’s method is used to simultaneously extract the dispersion of the velocity and attenuation while the geometrical spreading loss is simply corrected by product of in the interface waves. The attenuation profile is obtained by a synthetic example and it is shown that the method is efficient. This approach is also applied to one set of OBS data.
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Analysis of Seismic Attenuation and Multiple Scattering with a Version of the Shannon Entropy
By K.A. InnanenMultiple scattering and intrinsic friction both contribute to the attenuation of seismic waves, and are difficult to distinguish in seismic experiments. In this paper we will examine attenuation using theoretical tools that put scattering and friction on more or less an equal footing. Multiple scattering is, after all, a process that increases the disorder of mechanical motions in a continuum. If it occurs well below the resolution of an experiment, the energy it carries is, in a sense, classifiable as thermal. The Shannon entropy, defined on snapshots of a propagating wave, is a measure of disorder in something like the above sense. Its rate of monotonic increase, as a wave experiences either or both of multiple scattering and attenuation, may be a useful means to quantify intrinsic and extrinsic Q. Synthetic experiments and a field VSP data set appear to support this idea.
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Continuous Mapping of P Wave Velocity Dispersion - A Useful Tool for Reservoir Characterization
Authors L.F. Sun, A. Campbell and B. MilkereitFor reservoir rocks, P wave velocity dispersion may provide a potential link to rock physical properties. This requires continuous measurement of P wave velocity in a broad frequency band to determine the critical frequency of the attenuation mechanism. In this study, we measure P wave velocity dispersion continuously in sonic frequency range using broadband full-waveform multi-channel sonic logging data with the beam-forming and cross-correlation techniques. Results of the field data from 5L-38 Mallik gas hydrate research well have demonstrated the robustness of this method. The profile of P wave velocity dispersion matches very well with geological settings. The gas hydrate zones show very strong P wave velocity dispersion and a pronounced critical frequency around 15 kHz; weakly-laminated sediments also have strong P wave velocity dispersion with critical frequency lower than 5 kHz; P wave velocity dispersion in consolidated sediments is mild and gradual. In addition, the total magnitude of P wave velocity dispersion is positively correlated to resistivity and gas hydrate saturation. Therefore, continuous P wave velocity dispersion mapping can be a promising tool for reservoir characterization.
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Inversion by Trace Matching
Authors P.A. Connolly and M.J. HughesWe propose a method to estimate reservoir properties by matching seismic to very large numbers of pseudo-wells. Each pseudo-well consists of a full suite of well-log curves based on a comprehensive rock physics model. The pseudo-wells are calibrated to real wells and constrained by the seismic interpretation with stochastic micro-layering. The input seismic data are colour inverted angle stacks (optionally multiple angle stacks). The inversion algorithm is 1D, Monte-Carlo. All pseudo-wells are generated completely independently of any previous result and no spatial correlation is enforced. No low-frequency model is required. The procedure also provides estimates of uncertainties allowing the results to be integrated with other data types in a Bayesian framework. We outline the process and illustrate showing application to a deep marine turbidite field.
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Amplitude Inversion of Depth-imaged Seismic Data from Areas with Complex Geology
Authors S.H. Archer, X. Du and R.P. FletcherConventional amplitude inversion assumes that the input migrated image has preserved relative amplitude information and is free from the effects of illumination. Under this assumption, stretching a depth migrated image back to time and applying inversion based on 1D convolutional modeling can produce reasonable results. However, illumination effects in complex geological settings (such as shadow zones in subsalt imaging) pose a challenge to even the most advanced imaging algorithms such as reverse-time migration (RTM). Traditional approaches to compensate for illumination effects in migrated images are difficult to regularize in areas of very poor illumination. We address this problem by using the modelled response of the acquisition and imaging process, defined by Point Spread Functions (PSFs), to include these effects in forward modeling for inversion directly in the depth domain. We demonstrate this approach for poststack inversion of synthetic, subsalt data and also apply it to field data from the Gulf of Mexico.
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Data Driven Versus Model Based Inversion - When and Why?
By P. ThoreI present a classification of Seismic Inversions in two categories; Data Driven and Model Based. Based on examples I show that no inversion is purely data driven and that the user has to provide prior information which constrains the solution to a certain behavior. On the other hand, Model Based Inversion can provide very appealing results (an example breaks the traditional view of seismic resolution) provided that the underlying model is sound. I conclude by proposing a workflow which tries to get the best from both approaches.
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Model-Based Inversion of Low-Frequency Seismic Data
Authors P. Gavotti, D.C. Lawton, G. Margrave and J.H. IsaacThe Hussar experiment was carried out in central Alberta, Canada, in September 2011 with the purpose of acquiring low frequency seismic data to be used in inversion methods. Three wells located close to the seismic line and a dynamite-source dataset, acquired with three-component 10 Hz geophones, were used for a post-stack inversion test using commercial software. Several low-frequency cut-off filters applied to the data were tested with the 3-5 Hz model being selected as the optimum. The resultant impedance reflects lateral changes that were not present in the initial model and therefore are derived from the seismic reflections. Impedance changes in the target zone shows the general trend and relative variations, which would allow changes in the reservoir to be monitored as variations in the rock properties occur. A final inversion was performed to simulate traditional approaches when the low-frequency component is absent in the seismic data. Filtered seismic-data (10-15-60-85 Hz) and an initial model with a 10-15 Hz cut-off were used for this test. The results at the well locations show a good match but the lateral variation and character of the events resemble more the initial model character.
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The Accuracy of Spectral Decomposition Methods
More LessIn the context of exploration seismology the resolution of spectral decomposition techniques has been extensively reviewed but the accuracy of these methods has only been discussed qualitatively in regards to the resolution. Using a marginal condition the fundamental accuracy of the matching pursuit decomposition, Stockwell transform, Wigner-Ville distribution and reduced interference distribution are quantified. A cut-off for what can be deemed an acceptable decomposition method is set at 0.3 normalized residual energy, the normalized residual energy for the matching pursuit decomposition is 0.59. Used in conjunction with a PSQI method, the Stockwell transform provided the most accurate determination of the quality factor, Q, of a synthetic shot gather, recovering a Q value of 50.4±3 from a synthetic shot gather with an attenuative layer of quality factor 50. It is recommended that the best spectral decomposition method for a specific task be identified through rigorous testing of spectral decomposition methods with synthetic data.
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Spectral Decomposition by Synchrosqueezing Transform
Authors J. Han, R.H. Herrera and M. van der BaanThe synchrosqueezing transform (SST) is a wavelet-based time-frequency reassignment method, which has a grounded mathematical foundation. It produces a well defined time-frequency representation allowing the identification of instantaneous frequencies to highlight individual components. The field data examples demonstrate the high time-frequency resolution feature of SST, therefore render this technique is promising for seismic processing and interpretation.
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Gabor Deconvolution Based on Hyperbolic Smoothing in Log Spectra
More LessThe choice of t-f smoothing method is viewed as a significant procedure of Gabor deconvolution. Inspired by homomorphic deconvolution, this paper extends Gabor deconvolution into the log spectra, wherein the explicit linear form of nonstationary convolution model brings great facilities for studying its reverse process. More pertinently, we propose a hyperbolic smoothing method there, in which the linear analysis is iteratively employed to approach the factorization task. Since divisions are transformed to subtractions in log spectra, the usage of the pre-whitening factors in the iteration is avoided, thus reducing human intervention and boosting calculation accuracy. Moreover, a simple piecewise linearization is also introduced to enhance this technique's practical value. And our experiences on reef reservoir and carbonate reservoir show that nonstationary deconvolution based on this new hyperbolic smoothing can acquire higher resolution than ordinary hyperbolic smoothing, thus providing more detailed information and revealing more subtle geological phenomena, which enable better reservoir characterizations and definitions.
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Gabor Deconvolution - Hyperbolic Smoothing with Variable-step Sampling
More LessEstimation of nonstationary wavelet from seismic trace is the core problem of the nonstationary deconvolution. The accuracy of estimation wavelet affects the deconvolution result. There are several methods to smooth the Gabor magnitude spectrum of seismic trace, and each of these leads to a distinct method of nonstationary deconvolution. Among all of the methods, hyperbolic smoothing has proven most robust. However, uniform sampling in conventional hyperbolic smoothing cause subsampling in the observable area and oversampling in the noise area. Thus, this lead to an inaccurate wavelet estimation and unreliable deconvolution result. In this paper, we propose a new hyperbolic smoothing method with variable-step sampling (VSS), which can lead to an effective division in the time-frequency plane. Tests on synthetic and real data have shown that more reliable results can be obtained using the new method; moreover, it can also reduce the calculation time.
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The Application of Robust Principal Component Analysis for Weak Seismic Signal Enhancement
Authors X. Gao, W.K. Lu, F.Y. Li and X.D. JiangFor weak seismic signal enhancement, a new application of a signal decomposition method, known as Robust Principal Component Analysis (RPCA) is introduced. The motivation of this work stems from the observation that the interested weak seismic signals are always interfered by strong ones besides noises, causing the loss of some detailed seismic information. Principal Component Analysis (PCA), based on second order statistics, however, requires the data to be white and Gaussian, which seismic data may not satisfy. Being an extension of traditional PCA, RPCA utilizes L1-norm instead as the error function and the iterative algorithm obtains the optimal projections one by one with a greedy strategy. The synthetic data experiment indicates that RPCA outperforms PCA in seismic data processing as RPCA forms less artificial horizontal events. Moreover, an example of a 3-D field data is considered on which there are two wells close to each other. The seismic events are continuous across the wells, whereas the oil and gas production of the wells is distinct. The results demonstrate that RPCA is effective for weak seismic signal enhancement and helpful to improve the reliability of oil and gas detection.
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Seismic Reflectivity Inversion in a Multichannel Manner
More LessIn reflectivity inversion problems, due to complicated structures and low signal-to-noise ratio (SNR) of input seismic traces, the conventional Cauchy constrained deconvolution method is normally not able to provide the results that can clearly characterize the geology structure. One of the reasons is that the inversion process is performed on a trace-by-trace basis, and as a result the continuity along reflectors in seismic images may be deteriorated. In this paper, we develop a multichannel algorithm to perform this inversion process, where a multichannel precondition filter is incorporated into the conventional Cauchy constrained deconvolution method and the information of adjacent traces is applied during the inversion process. Numerical experiments have verified the validity and feasibility of this method by field data, showing that the reflectivity profiles obtained using the proposed method can have an improved lateral continuity and clearer structure.
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2D Spiking Deconvolution Approach to Resolution Enhancement of Prestack Depth Migrated Seismic Images
Authors A.K. Takahata, L.J. Gelius, R.R. Lopes, M. Tygel and I. LecomteComplex velocity models, limitations in acquisition geometry and frequency bandwidth, give rise to distortions in prestack depth migrated (PSDM) images. Such distortions can be modelled as the 2D convolution between the actual reflectivity and a resolution function. In the case of Born scattering, the resolution function is referred to as point spread function (PSF). The PSFs can be calculated with relatively low computational effort by ray tracing. In this work, we review the basic idea of the PSF and its relationship with seismic images generated by PSDM. With the help of the PSF concept, we propose the use of 2D spiking deconvolution with the aim of minimizing these image distortions. Finally, the potential and limitations of the proposed method are explored with applications on controlled synthetic data.
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Matching Pursuit with Enhanced Performance and Adaptability
More LessOne of the advantages which matching pursuit (MP) method has over conventional time-frequency decomposition is its ability to get capture of time-frequency fingerprint with extremely high resolution, which also makes itself quite sensitive to data noises and results in the non-uniqueness of its solution. In order to further diminish the influence of noises and stabilize the performance, two targeted modifications are incorporated in this work. Specifically, the lateral continuity of the underground layers is exploited as a constraint and as a reference for aligning the signal traces in the decomposition process. As a consequence, both the number of decomposed wavelets and the computation time can be significantly reduced. Moreover, choosing the best wavelet in each iteration follows a minimal residual energy criterion, which further results in a smaller number of resultant wavelets and thus makes the decomposition more robust.
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Hybrid Algorithm for Simulation of Seismic Wave Propagation in Complex Media - Anisotropy, Attenuation, Multi-scale
Authors V.V. Lisitsa, V.I. Kostin, G.V. Reshetova, D.M. Vishnevsky and V.A. TcheverdaThis paper presents an original algorithm for simulation of seismic waves in models containing geological formations with complex properties such as anisotropy, attenuation, and small-scale inhomogeneities. Each of these structures require special treatment either small gridding or computationally intense models and algorithms. Meanwhile, these formations typically take as few as 25% of the model, thus computationally expensive approaches can be used locally, while efficient algorithm can be applied elsewhere. Proper combination of several techniques allows improving the performance and efficiency of the parallel algorithm, in particular the speed-up of the hybrid algorithm is up to 10 if the formations with the complex properties take about 25 % of the model.
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Illumination/Reliability in the Local Angle Domain
Authors R. Levy, I. Ravve, L. Korkidi and Z. KorenWe demonstrate how we solve the two-point ray tracing problem in the presence of complex geological structures. For a point diffractor (PD) single-ray ray fan, we define surface-to-subsurface mapping (and vice versa) which maps the subsurface angles (take-off dips and azimuths) of individual traced rays to their surface locations. This mapping provides us with an interpolated ray for any subsurface angle or any surface location. We also introduce the reliability factor, which is based on a high-order interpolation technique in which we integrate several physical parameters calculated along the rays. This factor serves as a way of measuring the quality of the surface-to-subsurface mapping, and allows us to obtain a full ray (wave) field representation at any location, similar to wave equation methods. The same methodology is used for Common Reflection Point (CRP) specular ray pairs which are traced from a given subsurface location, with various opening angles and azimuths and a given reflector directivity, to the surface.
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Efficient Traveltime Solutions of the TI Acoustic Eikonal Equation
Authors U. Waheed and T. AlkhalifahNumerical solutions of the eikonal (Hamilton-Jacobi) equation for transversely isotropic (TI) media are essential for integral imaging and traveltime tomography applications. Such solutions, however, suffer from the inherent higher-order nonlinearity of the TI eikonal equation, which requires solving a quartic polynomial at each computational step. Using perturbation theory, we approximate the first-order discretized form of the TI eikonal equation with a series of simpler equations for the coefficients of a polynomial expansion of the eikonal solution in terms of the anellipticity anisotropy parameter. Such perturbation, applied to the discretized form of the eikonal equation, does not impose any restrictions on the complexity of the perturbed parameter field. Therefore, it provides accurate traveltime solutions even for the anisotropic Marmousi model, with complex distribution of velocity and anellipticity anisotropy parameter. The formulation allows tremendous cost reduction compared to using the exact TI eikonal solver. Furthermore, comparative tests with previously developed approximations illustrate remarkable gain in accuracy of the proposed approximation, without any addition to the computational cost.
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Low-frequency Layer-induced Anisotropy
Authors A. Stovas, Y. Roganov, K. Duffaut and A. CarterWe develop a low-frequency dynamic effective medium for a horizontally layered transversely isotropic medium with vertical symmetry axis (VTI), and approximate this effective medium by a homogeneous VTI medium. This results in frequency-dependent anisotropy parameters valid for low frequencies and quasi-vertical propagation.
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Renormalized Integral-equation Method for 3D Acoustic Full-waveform Modeling
Authors A. Abubakar and T.M. HabashyWe present a frequency-domain renormalized integral-equation formulation for solving a three-dimensional visco-acoustic medium using an iterative solver. Upon applying this special renormalization, the resulting integral-equation operator can be proven to have a contraction property. Hence, solving the linear-system of equations using a Krylov optimization method, will result in a good convergence rate. Furthermore since the matrix-vector multiplication can be done using a Fast-Fourier transform (FFT) technique, its operation is of the order of N Log N, where N is the size of the discretization grid. This technique also allows us to use matrix-free implementation. Hence, the memory usage is about N. Numerical tests show that the computational time and memory usage of this renormalized integral-equation approach can be quite competitive with the frequency-domain finite-difference iterative solver. Further, the numerical examples demonstrate that it is possible to solve a problem with over 100 million unknowns using an integral-equation approach.
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The Optimizied Expansion Method for Wavefield Extrapolation
Authors Z. Wu and T. AlkhalifahSpectral methods are fast becoming an indispensable tool for wave-field extrapolation, especially in anisotropic media, because of its dispersion and artifact free, as well as highly accurate, solutions of the wave equation. However, for inhomogeneous media, we face difficulties in dealing with the mixed space-wavenumber domain operator.In this abstract, we propose an optimized expansion method that can approximate this operator with its low rank representation. The rank defines the number of inverse FFT required per time extrapolation step, and thus, a lower rank admits faster extrapolations. The method uses optimization instead of matrix decomposition to find the optimal wavenumbers and velocities needed to approximate the full operator with its low rank representation.Thus,we obtain more accurate wave-fields using lower rank representation, and thus cheaper extrapolations. The optimization operation to define the low rank representation depends only on the velocity model, and this is done only once, and valid for a full reverse time migration (many shots) or one iteration of full waveform inversion. Applications on the BP model yielded superior results than those obtained using the decomposition approach. For transversely isotopic media, the solutions were free of the shear wave artifacts, and does not require that eta>0.
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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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Simultaneous-source Acquisition and Environmental Noise - Good or Bad?
More LessThe promise of simultaneous-source acquisition in improving acquisition efficiency and/or sampling has been studied by a number of authors, especially over the last few years. However, most of these studies focussed on processing the signal associated with the known shots, rather than on the impact of environmental noise. The presence of environmental noise will clearly degrade the processed product from a simultaneous-source dataset, much as it does a conventional (sequential) dataset. In addition, the impact of the noise on the separation process is a concern for simultaneous-source data. The story is not all bad, however. Simultaneous-source acquisition generally involves increased source effort compared to an equivalent sequential data set, and, therefore, the signal-to-noise ratio of the acquired data is improved. This observation raises the possibility that simultaneous-source data could be acquired in higher-noise environments than the equivalent sequential data. We study this possibility using real simultaneous-source data acquired with varying environmental noise levels. Comparisons between simulated sequential and simultaneous-source products with varying noise levels indicate that simultaneous-source acquisition can indeed produce equivalent results in a significantly noisier environment.
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Deblending of Simulated Simultaneous Sources Using an Iterative Approach - an Experiment with Variable-depth Streamer Data
More LessWe have simulated a simultaneous source experiment with a real variable-depth streamer NAZ survey acquired in Brazil and applied an iterative separation flow using a hybrid of median and f-x projection filters for deblending. The results show little leakage after deblending in both near and far offsets while preserving the AVO characteristics. We have also performed Kirchhoff migration in order to examine the residual cross-talk in common image gathers after deblending. The results reveal little leakage, and the quality of the common image gathers is adequate for velocity model building. We also study how the existence of spatial aliasing when shots are coarsely sampled degrades the deblending quality. Lastly, the impact of firing multiple sources simultaneously to improve shot sampling is investigated.
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Simultaneous Shooting for Marine Acquisition - Could We Catch Up with Land?
Authors N. Moldoveanu, J. Quigley and C. BeasleySampling of sources and receiver is at the core of any seismic survey design as the quality of seismic data, as well the cost of the survey, depend on this. Simultaneous shooting was used successfully for land acquisition to improve the source sampling and acquisition efficiency, and is fully accepted commercially. In this paper we look at the evolution of simultaneous shooting for land and marine acquisition and discuss a survey design for multivessel coil shooting that could improve the source sampling without additional source vessels.
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Decentralized Blended Acquisition - Are Networks the Next Big Step in Seismic Data Collection?
More LessUntil today, seismic acquisition is carried out with a centralized architecture, the collected information being linear in the number of detectors (N). In the last decades N has been continuously increased, making seismic acquisition systems very complex. It is proposed to break this trend by abandoning the traditional centralized acquisition concept. Instead, it is proposed to concentrate future developments on the promises offered by the network architecture, showing a quadratic behavior in seismic information (N^2). By moving from a single complex, centralized system to a network of simple, decentralized subsystems, robotization becomes a very attractive proposition.
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High Productivity Cable Less Acquisition in Libya with the ISSN™ Technique - From Field Trial to a Production Survey
Authors M. Dvorak, D. Howe, T. Allen, D. Buddery, M. Foster, T. Manning and M. PfisterThe ISSN™ technique, the use of a cable-less nodal system deployed on a sparse regular gird in conjunction with the ISS® method of acquisition is introduced. The development of the technique from initial field trial using static spreads of nodes acquired in parallel with a production cabled survey through to a production test of the technique with a dedicated crew and 5 Vibrator trucks is presented. An overview is given of the assurance process followed to introduce this new technology during a Seismic exploration program. We then review the implementation of the technique at a full scale production survey in the BP Ghadames concession. The development of the processing strategy for the ISSN technique is reviewed and compared with the ISS method. We show that in moderate desert conditions where Vibroseis acquisition is possible, using a autonomous cable-less recording system with the ISS method allows for a step change reduction in HSSE exposure for land seismic vibrator crews while still acquiring good quality data at very high production rates.
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The Impact of ISS(R) Blending on Amplitude Fidelity for AVO Analysis
Authors M. Dvorak, K. Ahmad, T. Manning and J. BradleyThe ISS® and ISSN™ techniques are high efficiency simultaneous source acquisition technologies. The high productivity of these techniques allows for the acquisition of high fold, wide azimuth datasets. The improvement in the quality of the stack image achieved with high fold, wide azimuth acquisition design has been well documented. However in order to evaluate the impact on pre stack data quality from blending data with the ISS and ISSN acquisition techniques, a study has been conducted using a non simultaneous source dataset acquired with the same high fold and wide azimuth geometry as an ISSN dataset. In this paper we review the acquisition of the control dataset, during a Libya ISSN survey. The artificial blending of the dataset to create different models of the ISSN acquisition, each with varying numbers of active sources and source separations is described. The processing of the ISSN models and control dataset and the subsequent AVO and pre stack amplitude analysis of the datasets are presented. We show that for the level of blending noise (2 to 3 simultaneous source events per record) typically encountered with BP ISS and ISSN surveys the results of the AVO analysis are consistent with the control dataset.
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3D High-Resolution Imaging Using Separated Wavefields
Authors S. Lu, N.D. Whitmore, H. LeGleut and A. LongConventional wave equation migration extrapolates upcoming boundary data generated from primary reflections to image the subsurface. In marine seismic data processing, sea surface related multiples are a major challenge. Particularly, when the water bottom is shallow, short period multiples are difficult to deal with. Acquisition footprint of seismic imaging is another issue for shallow water bottom geology, where the primary reflections do not have the complete illumination coverage to the shallow targets, including the water bottom. In this paper, we review a method of separated wavefield imaging using sea surface related multiples. We present a data processing work flow of separated wavefield imaging including up and down-going wavefield separation using dual-sensor data, wave equation migration of wavefield extrapolation and imaging condition, and a post processing step of amplitude balancing. A shallow water example from Asia-Pacific is tested, where the separated wavefield imaging generates remarkable high resolution 3D images. The separated wavefield image has greater area of subsurface illumination, which mitigates the strong acquisition footprint. The method and results can be useful to reduce well drilling geohazards.
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Channel Modelling Using Full Waveform Inversion Applied to Variable-depth Streamer Data
Authors A. Ratcliffe, R. Jupp and R. WombellFull Waveform Inversion (FWI) aims to estimate high resolution velocity models by minimizing the difference between observed and modelled seismic waveforms. Good low frequency data play an important role in the successful application of FWI. Variable-depth streamer acquisition delivers high signal-to-noise (S/N) seismic data over a significantly wider frequency bandwidth than standard streamer acquisition. We show that FWI adapts naturally to the variable-depth acquisition geometry and so benefits from the improved low frequencies recorded in this configuration. Application to data from the Central North Sea reveals detailed velocity features associated with shallow channels in the near surface geology. Migration with this updated velocity model improves the subsequent imaging.
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Broadband Extended Images by Joint Deconvolution of Multiple Blended Wavefields
Authors I. Vasconcelos and J.E. RickettWe propose a method for inversion of subsurface reflectivity image gathers that jointly relies on the broadband nature of combining multiple wavefields with diverse temporal and spatial spectra. Our inversion retrieves depth-domain extended images (EIs), which represent the full reflectivity operators within the subsurface. Based on interferometry by multidimensional deconvolution (MDD), we present MDD-based imaging conditions for an extended-image inversion. Our method consists of deconvolving correlation-based EIs with the so-called joint point-spread function (JPSF). This method can account for imaging primaries as well as internal and free-surface multiples. Because it is based on MDD, our JPSF approach can account for blended/simultaneous-source data in imaging with no need to separate the simultaneous-source data prior to imaging. With the example dual-source vector-acoustic seismic data the extended-image JPSF system is constructed by separating source and receiver wavefields from upgoing and ghost data, from both pressure and gradient sources. We demonstrate how the method inverts for EIs representing subsurface reflectivity, while benefiting from the increase in temporal and spatial bandwidth brought on by dual-source multimeasurement data. In addition, our joint wavefield approach provides a framework for jointly imaging data from multiple experiments of any kind (e.g., surface and borehole, active and passive).
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Wave Equation Receiver Deghosting
Authors C. Beasley, R. Coates and Y. JiCurrent solutions to receiver deghosting generally involve making complementary measurements of the wavefield or, alternatively, involve estimation of data not recorded due to ghost interference. Both solutions offer challenges in practice today. For marine streamer data, although multimeasurement streamers are commercially available, it is still on a somewhat limited basis and existing p-wave data and conventional acquisition could benefit from accurate single-measurement deghosting. In the second case, statistical estimates of the unrecorded frequencies near the ghost notches can be confounded by a variety of factors. Our approach to up/down wavefield separation does not require complementary measurements – we demonstrate the method on pressure-only data – nor does it rely on statistical estimation of the missing spectral components. Instead, we use the wave equation to compute directly the up and downgoing wavefields between the receiver and the surface. This migration-like approach only assumes that the upcoming wavefield is causal with respect to the downgoing wavefield, a good assumption if certain types of noise such as the direct arrivals are removed from the recorded wavefield. Moreover, our method exposes the underlying physics of the problem and thereby allows for complexities such as variable water velocity, variable sea surface and complex wavefields.
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Premigration Deghosting for Marine Streamer Data Using a Bootstrap Approach in Tau-P Domain
More LessRemoving the receiver ghost before migration provides better low and high frequency response as well as a higher signal-to-noise ratio. We recognize these benefits for preprocessing steps like multiple suppression and velocity analysis. In this paper, we modify a previously published bootstrap approach that self-determines its own parameters for receiver deghosting in a t-x window. Similarly to the t-x bootstrap method, the recorded data in the shot domain are first used to create mirror data through a 1D ray-tracing-based normal moveout correction method. The recorded and mirror data are then transformed into tau-p domain and used to jointly invert for the receiver-ghost-free data. We apply this new algorithm to a field data set with a streamer depth of 27 m. Our deghosting method effectively removes the receiver ghost and the resulting image has broader bandwidth and a higher signal-to-noise ratio.
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Broadband Processing of Conventional Streamer Data - Optimized De-Ghosting in the Tau-P Domain
Authors H. Masoomzadeh and N. WoodburnConventional marine seismic data is affected by the interference from ghosts in both source and receiver sides. The natural diversity provided by propagation directions, depths variations and imperfect reflections at the sea surface means the notches are not as deep as they often appear after stack. Since the apparent time delay between the main signal and its ghost is angle dependent, a deterministic de-ghosting process in the tau-p domain can reduce the effect of ghosts and retrieve the original wavelet spectrum. The amplitude and phase discrepancies around the notch frequencies caused by the variations in depths and effective refection coefficients can be reduced by using a stochastic search for the optimum set of de-ghosting parameters. A deconvolution process stabilized by averaging over a large number of traces in common–slowness panels may be used to address the remaining spectral defects.
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Crossline Wavefield Reconstruction from Multimeasurement, Towed Marine Seismic Data and Its Implications for Imaging
Authors K. Eggenberger, M. Vassallo, P. Kitchenside, N. El Yadari and D.J. van ManenMeasurements of particle motion bring significant benefits to towed marine seismic data when processed in conjunction with pressure data. We describe how horizontal particle velocity (Vy) measurements can enable dealiased crossline wavefield interpolation and, hence, improve the final image. For this purpose, 3D synthetic data, representative of the Barents Sea area and consisting of a single shot line, are used to simulate two different, realistic acquisition geometries. For each acquisition scenario we focus on those features and areas where enhancement through interpolation is expected. We then apply novel interpolation techniques, based on both single- and multimeasurement seismic data, to reconstruct the seismic wavefield between the streamer positions on a shot-by-shot basis prior to imaging. Finally, for both acquisition geometries, we assess the impact of denser crossline sampling on the wavefield-extrapolated migration image and we show that the multichannel crossline wavefield reconstruction prior to imaging provides better protection against aliasing-related noise in the image. This finding is further corroborated by trials on the SEG advance modeling (SEAM) I model where we use modeled multiple shot lines to investigate the contribution of 3D stacking to the attenuation of migration noise originating in spatial aliasing.
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Ricker-Compliant and Pseudo-Unitary Decon
Authors J.F. Claerbout and A. GuittonPredictive deconvolution does not yield Ricker wavelets as source wavelets. Analytic theory here tells how to fix it. The theory is not inverse theory. It is computable in N.log(N) time. Results here confirm better seismogram polarities.
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Under-Explored Palaeozoic and Mesozoic Petroleum Systems
By P.A. CarterThe possible existence of Palaeozoic oil source rocks in the Barakan Graben is based on analogues from the Goulburn Graben and the Petrel Sub-basin. Such sources could charge either interbedded Palaeozoic reservoirs or overlying and updip Jurassic and younger reservoirs.
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A Global Evaluation of Source Rock Potential Since the Ediacaran
Authors J.C. Wrobel-Daveau, G. Baines and J. EtienneKey factors influencing source rock potential are the extent and nature of primary deposition and post-depositional thermal, tectonic and geodynamic history, which will affect the subsequent preservation and maturity of the source rock. We present a global evaluation of source rock potential and risk from the Ediacaran and Phanerozoic. This evaluation is made by combining 52 palinspastically restored gross depositional environment maps with a global plate model for the Phanerozoic and Ediacaran taken from the Neftex Global Earth Model product suite. The 52 palinspastic gross depositional environment maps were produced by taking reconstructed observations and interpreting them regionally based on both the tectonic setting and a predictive sequence stratigraphic framework. When combined with predictions of paleoclimate and paleo-ocean circulation, this framework allows us to predict zones of organic enrichment and source rock deposition through time. Once deposited, the global plate model allows us to identify regions where geodynamic processes (e.g. collision zones, large igneous provinces, rifts) are likely to have influenced source rock preservation and maturity. By combining these data with the observed geology and sediment thickness at the present day, we generate a global risk map of source rock potential that can be used to identify new exploration opportunities,
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Fracture Patterns in Organic Rich Mudstones - Implications for Primary Petroleum Migration
Authors J. Jahren, B.G. Haile, H. Hellevang, K. Bjřrlykke and S.E. OhmScanning Electron Microscopy (SEM) of thin sections taken from organic rich Jurassic shale core samples from the Norwegian Continental Shelf reveal the existence of what appear to be fine scale fracture patterns filled with migrated petroleum oriented normal to depositional bedding. The suggested fracture patterns forms in shales rich in fine grained clays (smectite, kaolinite) at temperatures around 90 şC (2500m burial depth). Coarser grained shales appear not to be fractured on a thin section scale. Fewer and less extensive micro fractures are found in samples intermediate between the fine and coarse grained samples investigated. The mechanism responsible for the fractures is suggested to be pressure build up during initial maturation around isolated patches of organic material before a functioning migration network has been formed. This indicate that a certain amount of petroleum must be produced before an effective long range organic network functioning as a migration pathway is established in source rocks. The absence of fracture networks in coarser grained source rocks indicate that functioning migration networks are establish in such rocks before local fracture pressure is reached. This is probably due to higher permeability resulting in a more effective displacement of the continuous porewater phase.
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The Phanerozoic Evolution of the South-Tethys Domain from Morocco to Iran - Vertical Movements and Main Geodynamic Events
With the aim of giving an integrated tectonic scenario for the Northern African and Arabian plates, we revisit six key areas, namely, the Anti-Atlas Belt (Morocco), the Bechar Basin (west Algeria), the Hassi R’Mel High (central Algeria), the Talemezane Arch (south Tunisia), the Western Desert (Egypt), and the High Zagros Belt (Iran). Below the so-called “Hercynian unconformity,” which is in reality a highly composite discontinuity, surface and subsurface data display a well-known arch-and-basin geometry, with basement highs and intervening Paleozoic basins. We show that this major feature results mainly from a Late Devonian event and can no longer be interpreted as a far effect of the Variscan Orogeny. From the Jurassic, Pangea breakup has been diachronic and the Alpine-Tethys propagated through the development of two branches which will finally connect at the end of Jurassic times, achieving the development of Africa northern plate boundary. By the Late Cretaceous, convergence between Africa and Eurasia led to the progressive closure of the Tethys realm. Two major geodynamic events appear synchronous and occured during the Campanian-Santonian and the Middle-Late Eocene. These two synchronous events coincides either with a change in plate kinematics or with a period of strong lithospheric coupling.
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Timing and Distribution of Biogenic Gas Generation in the Shallow Gas Play in the Dutch Offshore
Authors J.M. Verweij, S. Nelskamp and M.A. Guaipo SarmientoShallow gas accumulations in the Plio-Pleistocene Eridanos Delta deposits in the Dutch offshore are a valuable additional hydrocarbon source. Ongoing studies focus on better understanding the shallow gas play. The focus of this paper is on increasing the understanding of the origin of the shallow gas. For that purpose we used integrated modeling approaches to assess the timing and spatial distribution of biogenic gas generation. It was found that intra-Eridanos Delta deposits are the most probable source of biogenic gas and that these biogenic sources are at maximum depth and temperature today. Detailed glacial-interglacial temperature fluctuations do not have a large influence on the history of the optimum window for biogenic gas generation, i.e. on start of biogenic gas generation. In the oldest and deepest buried units of the Delta, biogenic gas generation started already at the beginning of the Pleistocene, while the youngest units have not yet reached the optimum window for biogenic gas generation.
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Improving Hydrocarbon Exploration through Mantle Circulation Modelling
Authors J.H. Davies and H. van HeckThe nature of the mantle underlying prospective hydrocarbon basins during their critical history is poorly known. This mantle can affect the basin dynamically and thermally. Knowing these effects through Earth history would be invaluable for assessing basin prospectivity. Mantle geodynamic modelling has advanced dramatically over the past decade such that the outputs are starting to become of interest in assessing the palaeo-environments of frontier regions. We focus on outlining the elements of the basic mantle circulation modelling (MCM) method, including the input plate motion histories, the equations solved, and the boundary and initial conditions. The method can predict dynamic topography and mantle flow back to the Jurassic. We introduce an extension of the method to allow tracing of the flow of magmatic source regions. This could be realised in the data from geochemical signatures of magmatic outputs over time. We also introduce how MCM can be used to test plate motion histories by comparing present day predictions with seismic tomography.
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