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EAGE Workshop on Seismic Attenuation
- Conference date: 28 Oct 2013 - 30 Oct 2013
- Location: Singapore, Singapore
- ISBN: 978-90-73834-64-4
- Published: 28 October 2013
21 - 37 of 37 results
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Wave-equation Migration Q Analysis (WEMQA)
Authors Y. Shen, B. Biondi, R. Clapp and D. NicholsQ model building, which is conventionally done in the data space using ray-based tomography, is a notoriously challenging problem due to issues like spectral interference, low signal-to-noise ratio, diffractions, and complex subsurface structure. To produce a reliable Q model, we present a new approach with two major features. First, this method is performed in the image-space, which uses downward-continuation imaging with Q to stack out noise, focus and simplify events, and provide a direct link between the model perturbation and the image perturbation. We develop two methods to generate the image perturbation for the following scenarios: the model with sparse reflectors and the model with dense reflectors. Second, this method uses wave-equation Q tomography to handle the complex wave propagation. Two synthetic tests on two different 2-D models with a Q anomaly shows the capability of this method on the model with sparse events. Tests with a modified SEAM model also demonstrate the feasibility of this method for the model with dense events.
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Q-compensation of a Complex Synthetic
Authors M. Cavalca, R.P. Fletcher and M. RiedelWe apply and compare three model-based Q-compensation approaches. The first two approaches rely upon ray-based attenuated traveltime (t*) modelling operators. One is applied prior to migration, the other one is embedded into a Kirchhoff depth migration algorithm (KDM). The third approach is based on wavepath-consistent t* modelling and is embedded into reverse time migration (RTM). Benefits and drawbacks of each of these methods are discussed and illustrated on a visco-acoustic version of the complex BP 2004 model. While ray-based Q-compensation performs well, especially when embedded within KDM, wavepath-consistent Q-compensation within RTM compensates more complex events and seems worthwhile in highly complex media. However, all approaches are prone to noise amplification issues which must be accounted for.
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Compensating Attenuation Due to Gas Cloud through QPSDM: Case Study from Offshore Brunei
Authors K.H. Teng, J. Zhou, X. Wu, Y. Zhou, T. Brothers, S. Bergler and C. GibsonMarine seismic broadband solutions, as e.g. variable-depth streamer acquisitions, have shown great potential in providing high resolution seismic imaging and better low frequency penetration compared to conventional data (Lin et al., 2011). The increased low frequency content as well as signal to noise ratio play an important role in velocity model building, especially in the presence of gas. It benefits the PreSDM with absorption compensation technology (QPSDM) as low frequency energy may penetrate better through gas clouds (Xie et al., 2009). In this paper, we will show the methodology to compensate for Q absorption effects for a case study offshore Brunei. The step change in image quality shows that it is a viable solution to improve imaging through gas clouds by combining broadband acquisition and advanced depth imaging technology (Q tomography and QPSDM).
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Continued Development of Full Waveform Redatuming for Gas Cloud Imaging
Authors A.R. Ghazali, M.H. Mad Zahir, N.B. Darman, Y.B.M. Yusoff and N.A. RadziIn Malaysian Basins, offshore Malaysia, there are many imaging problems related to the so-called ’gas clouds’ or ’gas chimneys’. Conventional imaging processes do not offer satisfactory solutions, because of the complex propagation effects that occur. Due to the complex wave propagation through the anomaly and the transmission imprint on the reflections from below these complexities, the image below the anomaly is usually not properly recovered. We aim at constructing full waveform transmission operators (including the codas) from the gas cloud reflection response via an effective medium representation. True amplitude imaging is achieved via multi-dimensional deconvolution for these full waveform transmission operators. In this paper, we present a full waveform redatuming approach on a field in Malaysia, using a non-linear inversion to invert the gas cloud reflection response for its effective medium parameters to calculate the redatuming transmission operator. This imaging approach resulted in improvement in reflector continuity and amplitude restoration below the gas cloud overburden.
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Visco-acoustic Least-squares Reverse Time Migration of Cross-well Data
Authors K. Lu, W. Dai and G. SchusterThe viscoacoustic wave equation is applied into least squares reverse time migration(LSRTM) algorithm, to compensate the energy attenuation and modify the phase shift caused by the anelastic property of the media. Test is conducted on cross-well seismic data of the Friendswood site in Texas. Compared to standard LSRTM, higher image quality is produced by Visco-acoustic LSRTM with Q compensation, especially in the low Q value area. It suggests that by using the viscoacoustic wave equation, the blurring and distortion in the migration image due to the attenuation effect can be well corrected.
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Estimation of Gas Saturation by Frequency-dependent Amplitude-versus-offset Analysis
Authors X.Y. Wu, M. Chapman and E. AngererWe present a new method for the determination of gas saturation from frequency-dependent amplitude-versus-offset analysis. The method works by combining measurements of attenuation with a “low-frequency” acoustic impedance to resolve gas saturation variations. The concept is validated on synthetic data before being applied to field data from a producing gas reservoir. We present both the pre-drill and post-drill analyses, and conclude that the method shows great potential for improving our ability to estimate rock and fluid properties from seismic data.
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Evaluating Feasibility of Estimating Attenuation Changes from 4D Seismic
Authors V.I. Ryzhkov, M. Rapoport and M. DenisovThe main problem in measuring of the frequency-dependent attenuation is non-flat and variable spectrum of reflections caused by interference of seismic waves in thin-layered medium. 4D seismic observations provide an opportunity to reduce (almost eliminate) this effect using an assumption that the layered medium does not change (changes slightly) over the time interval between repeated seismic surveys (several months). We analysed 4D seismic data obtained on offshore field with the purpose to study the possibility to use seismic attenuation as a parameter which is sensitive to changes of the amount of the hydrocarbon fluid in pores. The effects of different factors such as changes of seabed depth, repeatability of source position, etc., have been studied. The presented method of 4D attenuation estimation allows us to study block structure of oilfield and separate operating and idle blocks. 4D analysis showed a decrease of frequency-dependent attenuation during oil production. The anomalous zones for all 4D parameters (amplitudes, time, attenuation decrement) are aligned.
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Effect of Attenuation on Reflections from Interfaces Showing Phase Changes
Authors M. Chapman, X.Y. Wu and E. AngererPhase changes are a common phenomenon encountered in amplitude-versus-offset analyses for interfaces with a weak contrast in elastic properties. The effect is usually modelled under the assumption of no attenuation. In this paper, we study the effect of introducing attenuation into the analysis. We find that such reflections are very sensitive to the presence of attenuation, and that the amplitude-versus offset (AVO), frequency-dependent amplitude-versus-offset and phase-versus-offset behaviours are all significantly altered by attenuation. We show a field data example of the phenomenon, and indicate how the effect was used to constrain an AVO inversion.
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Characterization of Gas Clouds Using Non-linear Full Waveform Inversion
Authors M.F. Abd Rahim, E.U. Ulugergerli, T. Konuk and A.R. GhazaliDue to complex wave propagation, anelastic losses, multiple scattering and strong contrast, it becomes a belief that gas cloud area especially the overburden is not invertable. In this paper we demonstrate that the so-called overburden gas cloud is invertable using non-linear full waveform inversion; Case study shown are based on synthetic and field data example from Malaysian basin. In our inversion strategy, we aim to invert the gas cloud overburden reflectivity response to achieve an effective medium representation which will be implemented in the full waveform redatuming method (see companion paper, Ghazali 2013). The depth interval to be inverted is shallow and the purpose of the inversion was not to have a full quantitative interpretation of the shallow overburden, but only to describe the overburden in terms of properties in a way sufficient to derive full waveform redatuming operators that remove the overburden imprint on the deeper targets (Ghazali et al., 2009; Ghazali, 2011). The actual derivation of the redatuming operators is not within the scope of this paper
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In Situ Acoustic Wave Attenuation Measurements of Ocean Water
Authors R.A. Clark, S. Osborne and J. CooperP-wave attenuation in water in the industry-seismic bandwidth is usually regarded as so low that it’s ‘ignored’, and, if quantified, given semi-arbitrary frequency-independent Q values (e.g. 103, 104) when standard acoustics models give 108-1011: hence, we measured in-situ effective attenuation from decay of direct P wave amplitudes along conventional towed streamers, and from comparison of primary and multiple seabed reflections. Neither experiment, with any dataset we used, delivered expected values. The direct wave gave Qeff increases from ~80 to 180 over 30-70Hz, and from ~30 to 90 over 20-90Hz. The primary-multiple comparison gave Q-1eff=-0.0015±0.0042 i.e. Q=-663, nominally consistent with the standard model of intrinsic attenuation but nevertheless indicating a large but negative Q. Apparent attenuation dominates for both horizontal (direct-wave) and vertical (reflection) ray-paths, raising questions about Q values to use in quantitative modelling and analysis, and implying a more insightful appreciation is needed of acoustic wave propagation in ocean water. Of various reasons that could produce apparent attenuation, to date we have investigated quantitatively dispersive sea-floor reflectivity. We find that an intrinsic Q of ~15-30 in the sea-floor sediment is a credible explanation, in whole or part, for the apparent attenuation that we observe on the vertical raypaths.
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Reflectivity Dispersion for Gas Detection
More LessSeismic wave energy attenuation and velocity dispersion happen when wave propagating viscoelastic rocks. A well known effect of attenuation is the change in frequency content and amplitude of a pulse propagating through an attenuating medium. Another is the presence of frequency dependent reflection coefficients. In viscoelastic media, frequency dependent reflection coefficients bring seismic energy dispersion of wave propagation, especially in seismic exploration where the attenuation is mainly due to the fluid content of the considered media. For the wave propagation in gas reservoirs, even no contrast in acoustic impedance, there is a reflection coefficient depending on the reflectivity dispersion. But most reflection coefficient equations cannot explain the above phenomena. Based on linear viscoelastic theory of velocity dispersion and the linearization of Zoeppritz equations, a new approximate reflection coefficient Rf is presented, which is pore fluid related and adds the effects of frequencies and quality factors. With Rf, the reflected energy change induced by reflectivity dispersion can be calculated and used to directly detect fluid. In our work, this method has been successfully applied to characterize the distribution of tight gas sands in a Triassic clastic sedimentary basin of southwest China. And the results can offer reliable foundations for well designing.
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Ultrasonic Attenuation in Shale
By A.F. SigginsInterpretation of the elastic wave response in shales is a challenging task due to the difficulty of understanding the roles of various elements in the shale microstructure, including clay platelets, rigid grains, pore structure and saturating fluids. In sandstones, the velocity-effective stress response is pronounced and follows a log-linear relationship. The influence of pore fluids on velocity is well described by Gassmann (1951). In contrast, the velocity-effective pressure behaviour of shales does not exhibit the same sensitivity at ultrasonic frequencies. The role of pore fluids and microcracks on the seismo-acoustic response of sandstones is reasonably well understood whereas similar mechanisms are difficult to identify in shales. Probably, some form of local flow in response to propagating elastic waves occurs but the mechanism is unknown at this stage. One experimental approach is to use an extended spectral ratio method (Toksoz et al, 1975, Siggins and Dewhurst, 2011). This technique was used to process the P-wave waveform data obtained with ultrasonic measurements on a Norwegian sea shale, at a range of effective stresses with pore pressure held constant. A generalised standard linear solid model (GSLS), was then fitted to the observed spectral responses.
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Determination of Seismic Wave Attenuation Profile in Carbonate Rocks
Authors F. Bouchaala, M.Y. Ali and A. FaridThe Abu Dhabi underground medium is composed by carbonate rocks, in such medium the seismic wave is highly attenuated and scattered during its propagation. In this abstract we determine robustly the total quality factor profile from Vertical Seismic Data(VSP) and also the scattering quality factor profile from the sonic log. The comparison between the two profiles shows that the scattering is bigger than the total attenuation, this is explained by the difference between the frequency range of the VSP(6-120Hz) and that of the sonic data(in order of mega Hz). The big values of the scattering indicate the large degree of heterogeneity in the medium.
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Enhanced 4D Imaging in West Africa Using High-resolution Tomography and Q-imaging
Authors M. Cavalca, J. Penwarden, A. King, S. Knapp, X.G. Meng and S. MondzielA high-resolution tomography and imaging approach is used to enhance the 3D and 4D responses of a complex production field, West Africa. The approach relies upon building a high-resolution anisotropic vertical transverse isotropic (VTI) earth model including anisotropic velocity and 1/Q components and applying amplitude and phase Q-compensation within Kirchhoff prestack depth migration. Velocity and absorption effects associated with overburden heterogeneities are nicely mitigated, leading to enhanced amplitudes and resolution in the prestack gathers and 3D/4D images.
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Seismic Attenuation in Partially Saturated Porous Rocks
Authors E. Caspari, Q. Qi, J.G. Rubino, S.C. Lopes, M. Lebedev, T.M. Mueller and B. GurevichWe provide an overview on recent developments on the acoustics of partially saturated porous rocks. The focus is on the mesoscopic flow induced by seismic waves and leading to wave attenuation and dispersion. At the laboratory scale recent core plug imbibition experiments with simultaneous acquisition of X-ray CT and ultrasonic waveforms allow us to retrieve the saturation dependence of velocities and attenation. Fluid patches and their evolution at the milimetre scale are observed. To model these relations in a consistent manner we invoke the concept of fluid patch membrane stiffness. The latter accounts for the net effect of capillary forces at the macroscale. We further extract the velocity saturation relation from time-lapse sonic logs aquired during CO2 injection into a sandstone formation. It is shown that this velocity-saturation relation can be also modelled using the mesoscopic wave-induced flow effect. Simulation results give further support that mescopic fluid patches on the centimetre scale have a first-order effect on seismic amplitudes provided that the fluid bulk modulus contrast across the patch boundaries is suffiently large. This is typically the case in the presence of a gas phase. We conclude that fluid patches on the milimetre-to-centimetre scale have important implications for attenuation estimates.
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Simultaneous Determination of Q and AVO from Surface Seismic Data
Authors J.A. Beckwith and R. ClarkKnowledge of the interval quality factor value, Q, is of great interest in seismic exploration. Interval Q values can be used as a lithology and pore fluid discriminator as well as in processing of seismic data to restore high frequencies to deep reflectors. Q values are generally obtained from VSP data which has a spatial coverage limited to borehole locations therefore a method to derive Q from surface seismic data is desirable due to the much larger spatial coverage of the derived Q values. Pre-Stack Q Inversion is a method whereby not only is interval Q calculated from pre-stack data but as a by product attenuation-free AVO is produced. Here we introduce updates and extensions to the PSQI algorithm including an s-tau -p transform designed to overcome aliasing problems in tau-p domain data as well as inversion for powerlaw frequency dependent Q. The inversion intercepts are also analyzed with a view to deriving estimates of the p-wave, s-wave and density of the region surrounding and including the interval of interest. It is found that it is possible to constrain the highest interval velocity surrounding the region of interest through a Monte Carlo simulation.
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Detection of Low-frequency Shadow Zones using Quantum Swarm Evolutionary Matching Pursuit Decomposition (QSE-MPD)
Authors A. Semnani, M. Nabi Bidhendi and B. Nadjar AraabiLow frequency anomalies associated with the presence of hydrocarbons, plays the role of frequency dependent attenuation that can be considered as a direct hydrocarbon indicator. Matching pursuit decomposition (MPD) is a suitable tool for spectral decomposition of seismic data to be able to detect and visualize low frequency zones. However, this method has high computational cost. In this work a combination of two artificial intelligence method which are quantum-inspired evolutionary algorithm and particle swarm optimization, has been applied to accelerate the performance of MPD. The proposed method called quantum-swarm evolutionary-matching pursuit decomposition (QSE-MPD) has been utilized to detect low-frequency shadow zones of an offshore gas reservoir of Iran.
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