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81st EAGE Conference and Exhibition 2019 Workshop Programme
- Conference date: June 3-6, 2019
- Location: London, UK
- Published: 03 June 2019
1 - 20 of 93 results
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A Brief Overview on Seismic Attenuation
More LessSummarySeismic waves decay due to geometrical spreading (in 2D and 3D) and scattering (energy is conserved), and anelastic -- or intrinsic -- attenuation (energy is lost to heat). Amplitude decay in the last two cases is accompanied with wave-velocity dispersion, by which each Fourier component of the signal travels with a different phase velocity (Kramers-Kronig relations). Attenuation can be described by a phenomenological (non-predictive) theory, as the Burgers mechanical model -- composed of springs and dashpots --, or with predictive models, such as the scattering theory, and the Biot and related models of poroelasticity (wave-induced fluid-flow attenuation). Another phenomenological approach is the use of temporal or spatial fractional derivatives, e.g., Kjartansson and Cole-Cole models. In the following, I present a brief overview on the various attenuation mechanisms, where most of the material refers to Carcione (2014) .
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Seismic Attenuation Mechanisms in Fractured Fluid Saturated Media – Numerical and Field Examples
Authors E. Caspari, N.D. Barbosa, M. Novikov, V. Lisitsa, J. Hunziker, B. Quintal, G. Rubino and K. HolligerSummaryA number of different mechanisms can cause attenuation of propagating seismic waves in a fractured fluid-saturated porous medium, notably geometrical spreading, displacement of pore fluid relative to the solid frame, and transmission losses and scattering. In this study, we examine these attenuation mechanisms using numerical forward simulations and a field example. The numerical methods include a quasi-static upscaling approach and wave propagation simulations. They are based on Biot's equations of poroelasticity and, hence, fractures are modeled as soft, highly porous and permeable features. The field examples include full-waveform sonic data from the Grimsel Test Site underground laboratory situated in a granodioritic rock mass, which exhibits both brittle and ductile deformation structures at various scales.
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Use of the General Fractional Zener Model to Represent Attenuation in Poro-Viscoelastic Numerical Modeling
Authors X. Liu and S. GreenhalghSummaryTo approximate seismic wave propagation in double porosity media we use the governing equations of effective Biot theory with complex phase velocity and attenuation dispersion characteristics. To upscale them and extend to shear waves we use the poro-viscoelastic modeling approach of multiple fractional Zener elements and apply a frequency-space domain mixed grid finite difference simulation method to calculate wavefields for solid particle velocity, fluid flux and pore pressure.
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Dispersion and Attenuation of Wave Velocity in Fluid-Saturated Rocks : Experimental Investigations
Authors J. Fortin, J. Borgomano and S. ChapmanSummaryFor fluid-saturated rocks, comparing ultrasonic measurements (1 MHz) in the laboratory and seismic (100 Hz) or logging (10 kHz) measurements in the field is not straightforward due to dispersion of the elastic-wave velocities. If there are several theoretical models, there is a lack of data. We developed an apparatus for measurements over a large-frequency range, by the combination of forced oscillations (0.004 to 100 kHz in apparent frequency) and ultrasonic measurements (1 MHz) at various effective pressures. Our experimental results typically show two cut-off frequencies: i) one related to a drained/undrained transition; and ii) one related to squirt flow.
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Attenuation From Passive Seismic Monitoring
More LessSummaryMeasurements of attenuation from passive seismic monitoring datasets increase in number. We provide an overview of methods that are used to determine attenuation factors using detected microseismic events. We discuss applications and advantages of using passive monitoring to characterize medium -in particular hydrocarbon reservoirs.
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Q Compensation: From Nice to Have to Mandatory But…
Authors P. Charron, B. Duquet, C. Agut and A. LaframSummaryAttenuation is a well-known phenomenon in seismic. For tens of years, this has not been compensated or partially compensated with an inaccuracy that would not be accepted for any other processing steps. Most often, only a phase de-absorption with a constant Q value was applied and the amplitude absorption was compensated with spectral enhancement. This was considered as acceptable until today since the frequency bandwidth was limited to around 2 octaves and the effect of this de-absorption was not clearly observed as an improvement or adrawback. Today, the market has moved forward and the broadband seismic has become a standard. The consequence is that the bandwidth may now be spread over up to 5 octaves depending on the area and the burial of investigation. This wider frequency bandwidth is naturally unbalanced according to the magnitude of the absorption. In order to deliver the promises of the broadband (resolution), compensating the absorption is not an option anymore. It has become a processing step of a major importance that will impact the resolution, the phase and the overall “aspect” of the final result that the end-user will interpret or use to extract some attribute through AVO analysis or inversion processes.
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Q-Tomography: Status and Challenges
Authors Fatiha Gamar-Sadat, Alessandro Pintus, Patrice Guillaume and Andrew WrightSummaryAlmost all current time or depth seismic studies need to go through a correction process to recover energy lost by absorption phenomenon. The so-called Q factor is responsible for dissipation of high-frequency seismic energy, which decreases seismic amplitudes and causes velocity dispersion. For general background Q, a post-migration inverse Q filtering ( Wang, 2002 ), using smooth or even constant Q, may be sufficient for data with gentle geology. In areas with more absorptive heterogeneities such as unconsolidated materialor gas, theneed for a morecomplex Q model is necessary for an accurate correction. Brzostowski and McMechan (1992) have been pioneers for addressing this problem, adapting Q-Tomography from fundamental to applied seismology. Over the last decade, it has resulted in an industrial solution ( Cavalca et al., 2011 ; Valenciano and Chemingui, 2013 ; xin et al., 2014 ; Gamar et al., 2015 ) using VSP or surface seismic data.
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Viscoelastic Full Waveform Inversion of Wide-Angle Data: Application to Ultra-Long Offset Data from Mentawai Basin, Indonesia
More LessSummaryFull Waveform Inversion (FWI) is a powerful tool to quantify the Earth's subsurface structure. However, most of the FWI applications have been limited to acoustic media. In geological settings, such as gas clouds, gas sand, melt lens, where the attenuation becomes important, one must use a viscoelastic FWI. Here we present the theory and application of a viscoelastic FWI in the time domain. First, we carried out sensitivity analyses for back-scattered, reflected and transmitted waves. We find that the presence of attenuation has a significant effect on post-critical reflections, but it has little or no effect on near-offset reflection data, suggesting that the inversion of port-critical reflections can help to reduce the cross talks between attenuation and velocity contrast or propagation effects. We have first tested the method on synthetic data and then applied to 15 km long offset data acquired by CGG Offshore central Sumatra, Indonesia. Apart from the recovery of attenuation parameters, the viscoelastic inversion provides sharper P-wave velocity image as compared to the elastic FWI.
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Effect and Reconstruction of Attenuation in Acoustic FWI - Method and Field Data Application
Authors N. Kamath, R. Brossier, L. Metivier and P. YangSummaryOur contribution is divided into two parts: in the first we discuss the mechanism (based on generalised Zener body) used to incorporate attenuation into our forward modelling engine, and the manner we manage the efficient building of the FWI gradient in 3D. The second part deals with the application of mono- and multi-parameter (velocity only in visco-acoustic media, and velocity-attenuation joint inversion, respectively) FWI to a 3D OBC dataset from the Valhall field.
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A Hybrid Inversion Strategy for Visco-Acoustic Full Waveform Inversion: Application to the Marmousi Model
Authors H. Jiang and H. ChaurisSummaryVisco-acoustic full waveform inversion aims at retrieving the velocity and attenuation models, but suffers from cross-talks between parameters. Attenuation dispersion leads to equivalent kinematic velocity models, as different combinations of velocity and attenuation have the same kinematic effects for band-limited seismic waves. We propose a hybrid inversion strategy: we incorporate the kinematic relationship to guide the non-linear inversion. The hybrid inversion strategy includes two steps. It first updates the kinematic velocity, and then retrieves the velocity and attenuation models for a fixed kinematic velocity. This hybrid inversion strategy is tested on the Marmousi model dominated by reflections, and compared with the conventional simultaneous inversion strategy. It proves that the hybrid inversion strategy mitigates the cross-talks without involving the Hessian matrix.
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Joint Estimation of Velocity and Attenuation by Frequency-Domain TV-Regularized Wavefield Reconstruction Inversion
Authors H. Aghamiry, A. Gholami and S. OpertoSummaryFull waveform inversion (FWI) is a nonlinear waveform matching procedure which can provide high-resolution subsurface models. However, viscous effects must be taken into account in attenuating media to exploit the full potential of FWI. In the frequency domain, attenuation is implemented in the time-harmonic wave equation with complex-valued velocities. During the inverse problem, the real and imaginary parts of the velocity are generally processed as two independentreal-valued parameters. In this study, we process instead the velocityas a complex-valued parameter using derivative of real functions of complex variables. Moreover, we implement visco-acoustic frequency FWI with search space extension in the framework of the wavefield reconstruction inversion (WRI) method. We implement WRI with the alternating-direction method of multiplier (ADMM), which makes the parameter-estimation subproblem linear thanks to the bilinearity of the wave equation and provides a suitable framework to cascade nonsmooth regularizations and bound constraints in the objective function. In this study, we review ADMM-based WRI for complex-valued parameters and show preliminary results of joint velocity and attenuation reconstruction when inversion is performed without and with total variation (TV) regularization. We show the key role of TV regularization to decrease the ill-posedness of the velocity-attenuation reconstruction.
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Accounting for Heterogeneous Attenuation in Full-Wavefield Inversion
Authors M. Lacasse, H. Denli, L. White, V. Gudipati, S. Lee and S. TanSummaryThis talk presents a numerical approach for including attenuation in forward seismic viscoacoustic simulators in the time domain using a generalized Maxwell body (GMB). We discuss how to select the proper number of relaxation mechanisms and the values of the parameters of the GMB. We also present a few synthetic case studies determining the feasibility of performing full-wavefield inversion of viscoacoustic media, and the aperture requirements. Finally, field examples are shown where attenuation needs to be accounted for in order to perform a successful inversion.
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Integrated Visco-Acoustic Model Building: a Case Study Illustrating the Challenges and Benefits for Large Scale Exploration
Authors T. Martin, M. Barbaray, G. Venfield and V. ChavdaSummaryThe seismic character of Early and Late Cretaceous plays in deep water Côte d'Ivoire data are affected by Late Cretaceous and Paleocene channel and canyon systems. Unresolved, these create structural uncertainty and impacting amplitude fidelity. Using an integrated visco-acoustic model building sequence we resolve the impact of complex Late Cretaceous and Paleocene channel systems on deeper targets. A full wavefield FWI approach creates an accurate velocity model removing the structural uncertainty when used in the imaging step. Viscoacoustic effects were determined using tomography. Fully integrated into the sequence, this method calculated measures of log spectral ratio in demigrated space, mitigating stretch and tuning effects. The resulting combination of the complete visco-acoustic model building flow compensated for the structural complexity of the area, whilst significantly improving the amplitude fidelity of the dataset.
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Seismic Inversion for Near Surface Applications and the Derivation of Geomechanical Properties
By A. FoggSummarySeismic inversion methods broadly fall in to two categories; conversion of seismic event amplitudes in to reflectivity or the analysis primarily of seismic event arrival times (and waveform shape) to derive a velocity model. These are generically referred to as Acoustic Impedance (AI) inversion and Full Waveform Inversion (FWI) respectively, the former typically working from processed seismic reflectivity data and the latter being derived during the processing phase. Both procedures have application in the characterisation of the rock properties of shallow stratigraphic sections, indeed FWI is specifically designed (and limited to) no deeper than approximately 1500m below the mudline (though this depth is dependent on seismic acquisition parameters; notably cable length, water column height and subsurface velocity). This paper will review several different approaches to AI inversion, which can be calibrated to derive rock mechanical properties, and discuss their application to the near surface. The paper will also demonstrate how FWI can yield a high resolution image of near surface velocity which improves the seismic image and thus enhances AI inversion results. Case studies will be used to demonstrate the procedures and contrast the advantages and disadvantages of different methods.
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A Request for Quantitative Seismic Solutions for Drilling Hazards Assessment
By G. WoodSummaryIn this paper, we focus on the need to provide a quantitative approach to drilling hazard assessment and the benefits such an approach would have to reducing uncertainty and risk while potentially releasing marine real estate for drilling or developments that has previously been considered unuseable
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Towards QI for Site Investigation in Orsted
Authors K.H. Karkov and S. HviidSummaryA recently concluded Ørsted R&D project successfully demonstrated the possibility of extracting quantitative information on soil composition from 2D UHRS data by leveraging the acoustic inversion method.
Our contribution to the workshop will include an introduction to Site Investigation in Ørsted, a presentation and discussion of our identified key challenges related to acquisition, processing, calibration, interpretation and integration as well as our perceived status on these.
Examples from recent advances will be presented providing the status on commercial scale QI for site investigation purposes in Ørsted.
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Seismic Inversion for Geotechnical Problems
SummaryFor foundation design the geotechnical analyses and interpretations often rely on isolated 1D boreholes and the geophysical data is only used to confirm horizontal layering. The great amount of information capture in the geophysical data, not only related to layering but also related to soil parameters, are therefore not used. Geophysical data are collected in 2D lines and/or 3D volumes and therefore provides the natural link to re-populate geotechnical properties found in the 1D boreholes onto a larger area and thereby build a consistent and robust ground model. There is therefore a great potential in using this data in a quantitative way during all phases of a project.
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A Review of Seismic Attenuation Mechanisms, Measurements, and Inversion Strategies
By E. MorganSummaryThis segment of the Seismic Inversion for Marine Overburden Characterization workshop discusses the physical mechanisms responsible for intrinsic and scattering attenuation, rock physics models for intrinsic attenuation, common methods for measuring attenuation from seismic reflection surveys, and inversion strategies to estimate soil and fluid properties from attenuation measurements.
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Using Vs Measurements in Shallow Marine Sediment to Guide Vp Matrix Velocity Assessment for Seismic Inversion
By A. FoleySummaryMeasuring seabed velocities by transmission surveying, as opposed to those measured by reflection surveys, gives key, unambiguous results. By determination of Vs, which only responds to the sediment matrix, a more useful value of Vp can be found. Geotechnical engineers refer to this as the “drained” velocity. This value for Vp allows practitioners to calibrate inversion results and derive meaningful values for several engineering parameters of seabed soils.
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A Deep Learning Approach to Quantitatively Characterising the Marine Near-Surface
Authors M. Vardy and T. DarnellSummaryIn this paper, we present a Deep Learning workflow developed specifically for inverting marine site investigation data, comparing and contrasting it against the results obtained using traditional stochastic inversion algorithms with both synthetic and field data. In particular, we assess its potential for rapidly deriving a range of subsurface parameterisations, including geotechnical engineering properties of direct interest for various site investigation problems.
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