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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 - 50 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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