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EAGE/SEG Research Workshop - Frequency Attenuation and Resolution of Seismic Data 2009
- Conference date: 14 Sep 2009 - 15 Sep 2009
- Location: Barcelona, Spain
- ISBN: 978-90-73781-29-0
- Published: 14 September 2009
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Complex Seismic Trace, Bandwidth and Vertical Resolution
Authors P. Cristini, G. Le Touze and N. Favretto-CristiniResolution is the ability to distinguish separate features. Improving resolution is the key problem to see thinner stratigraphic units, for instance. Horizontal resolution is connected to the size of the Interface Fresnel Zone. Vertical resolution is usually considered as being enhanced by both high frequencies and broadband signals. This common belief is true for zero-phase signals, such as Ricker wavelets, but is incorrect for mixed-phase signals (i.e., non zero-phase signals). For this type of signals, the complex trace, and more specifically the envelope, is a fundamental tool for the analysis of seismic resolution. We propose a criterion based on the separation of two envelopes as a new key element. This criterion is connected to the bandwidth that is consequently the key parameter for enhancing vertical resolution.
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Increasing Resolution with a New Method for Efficient Broadband Marine Acquisition and Processing
Authors E. Kragh, M. Svendsen, D. Kapadia, G. Busanello, R. Goto, E. Muyzert and T. CurtisWe present a new method for broadband marine acquisition and processing. A 3D shallow towed-streamer spread is deployed, designed to optimize the mid- and high-frequency parts of the bandwidth. In addition, data are simultaneously acquired from a small number of deeper towed streamers. The depth of these deeper streamers is optimized for the low frequencies such that the combined overall bandwidth is enhanced. Because the deep streamers only provide the low-frequency part of the bandwidth, we can more sparsely sample these data enabling efficient acquisition scenarios as fewer streamers are required. A 3D case study using this new acquisition method was acquired off the NW Shelf of Australia. The streamer spread consisted of six shallow streamers towed at a depth of 6 m and two deeper streamers (below shallow streamers 2 and 5) towed at a depth of 20 m. The resulting data exhibit both high resolution and deep penetration for subsalt and sub-basalt imaging, for example. In addition, inversion for acoustic impedance, imaging, and velocity model building, also benefit from the broadband result. Data acquired in this way are more robust to poor weather conditions than conventionally acquired data.
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Enhanced Seismic Resolution from Dual Sensor Towed Streamer Acquisition
Authors M.T. Widmaier, A. Day, C. Reiser and A. LongThe benefits from dual sensor towed streamer acquisition comes from collocated pressure and particle velocity sensors. The combination of the sensors allows the separation of the up- and down-going wavefields, and thus the removal of the ghost reflection. The removal of the ghost significantly enhances frequency content and resolution. The presentation will review the key principles of towed dual sensor streamer acquisition and present case studies which highlight the uplift provided by the increase bandwidth.
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Seismic Wavelet Spectral Enhancement – Quantity or Quality?
Authors D. Rappin, T. Castex, B. Patternoster and C. DeplanteFrom the digital communication theory, we present some considerations to quantify signal characteristics and the relation with the theoretical resolution. However, this theory was basically developed on narrow bandwidth signals and transposed to broadband seismic signal. This has some implication and also gives the carrier frequency to be considered as a characteristic parameter. ‘Detection’ is also included in the discussion as the ability to identify and hopefully characterize thinner geological bodies. One can also consider tuning effect to identify the key parameters. We show that these two approaches give the equivalent conclusion about the main role played by the bandwidth and the carrier frequency to be considered together. If a larger bandwidth could be the processing challenge to drive the seismic imaging to higher resolution and/or better detection, it is necessary to spend time on controlling what kind of bandwidth broadening is sought after and how it is obtained. It is useless to reach broader spectrum while amplifying noise. Implicit techniques must involve checking the level of noise to ensure reliability. But when enlarging the bandwidth cannot be the solution, other way can be used to make signal in proper condition to deliver relevant layer properties.
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Intrinsic Attenuation in a Periodically Layered Medium
By A. StovasIn this paper, I introduce the intrinsic attenuation for periodically layered structure to investigate how the intrinsic attenuation interferes with the scattering due to layering. The presence of intrinsic attenuation results in the stretch of reflection and transmission responses in frequency domain. I show that the anomalies in energy loss follow the stop-bands in frequency scale which are wider for higher acoustic contrast between the layers.
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Evaluating Net Sand Thickness on Seismically Thin Reservoirs – An Integrated Approach Applied to Kadanwari Field
Authors A. Amato del Monte, F. Luoni, L. Baruffini and N. AhmadRock properties of seismically thin reservoirs cannot be quantified correctly using either amplitude or impedance data because of tuning effects. An original approach developed at Eni combines forward modeling, tuning analysis and rock physics to deal with such limitations. Band-limited seismic resolution and tuning effects can be turned to the geophysicist’s advantage by using an appropriate workflow combined with an understanding of what reservoir properties can be safely extracted given a certain reservoir body’s spatial configuration. We have applied these concepts to the Kadanwari field (Pakistan), where gas-bearing reservoir sands are below seismic resolution. Rock physics modeling is used to relate rock to elastic properties; then forward modeling of various rock-property scenarios are used to understand the seismic response with decreasing thickness. Finally, tuning charts are built and used to quantify the target reservoir property (net thickness). The net thicknesses output by this workflow are compared with the results of cluster analysis techniques, and the internal consistency of net maps is checked against the sedimentologists' depositional model; they both proved positive, confirming once more the validity of the approach which has already been tested successfully on several other fields worldwide.
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Relaxation Spectra of Wave Attenuation in Single-crystal Quartz and Sandstone
Authors E.I. Mashinskii and D.A. MednykhThe experiments have been conducted to investigate the effect of strain amplitude on the compressional and shear wave velocity, Vp, Vs, attenuation, Qp-1, Qs-1 and dynamic Poisson’s ratio, Vd in quartz samples of three types: the intact quartz, fractured quartz and smoky quartz and sandstone. The measurements were performed using the reflection method on a pulse frequency of 1 MHz with changing strain in the range 0.3 < e < 2.0 m-strain under a confining pressure of 10 MPa and at ambient temperature. The anomalous inelastic behavior occurs owing to the change in value of the strain-amplitude. The evident strain-amplitude effect takes place only on Q-1, Vd - parameters. P- and S- wave velocities are little sensitive (~1%) to changing strain amplitude. The greatest decrease in Q-1 (to 16%) takes place in the smoky quartz whereas in other quartz, effect is practically absent. Amplitude variation leads to the modification of relaxation spectrum. The broadening of peaks width with increasing strain amplitude that takes place for P- wave attenuation in the smoky quartz. Such behavior of attenuation can be explained by a joint action of viscoelastic and microplastic mechanisms in the material with defects.
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Estimation of the Near-surface Q Distribution from Pre-stack Surface Seismic Data
Authors M. Cavalca and R.P. FletcherKnowledge of the near-surface distribution can be helpful in surface seismic data processing to compensate for shallow absorption effects likely to damage our seismic images. In this paper, a tomographic scheme is proposed to derive 3D near-surface distributions from pre-stack surface seismic data. The approach is based on the inversion of the attenuated travel times of the first arrivals (diving waves); where the attenuated travel time of a propagated wavelet is defined as the integration of both velocity and effects along its propagation path. Such seismic attributes are computed from log spectral ratio linear regressions of the first arrivals and decomposed into distributions by a simple linear inversion. This attenuated travel time tomography is integrated into a first-arrival tomography scheme for the derivation of both a near-surface velocity model (travel time inversion) and a near-surface model (attenuated travel time inversion). The approach is illustrated here using synthetic and field data.
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A Robust Prestack Inversion Algorithm for Interval-Q Measurements from Surface Seismic Data
Authors C.A. Reine, M. van der Baan and R.A. ClarkMeasurement of seismic attenuation is affected by spectral interference, which limits the accuracy of standard spectral ratio regressions. Furthermore, heterogeneities and anisotropic effects above the zone of interest complicate the choice of a reference event. We introduce a prestack Q-inversion (PSQI) algorithm which operates in the tau-p domain such that the reference event changes with the slowness of the trace considered. By inverting the spectral ratio data for all of the prestack traces simultaneously, the influence of the interference peaks and notches is reduced. The result of the PSQI approach is a more accurate and precise attenuation measurement. We show the justification for our approach using synthetic data, and then demonstrate the method on a shallow reservoir 3D seismic survey.
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Monitoring Pore Fluid Changes Using Time-lapse Attenuation
Authors T.D. Blanchard, R.A. Clark and M. van der BaanAttenuation is a desirable seismic attribute for monitoring hydrocarbon reservoirs as it is far more sensitive to changes in pore saturation and pressure than P- and S-wave velocities. Here, we demonstrate how a time-lapse estimate of attenuation may be less sensitive to coherent spectral contamination, outline two methods of estimating time-lapse attenuation, and demonstrate their use on a prestack time-migrated dataset. Prestack time migrated data from two vintages (2001 and 2007) situated adjacent to a gas/water injector are analysed. Prestack time-shifts from cross-correlation between vintages are calculated to account for changes in velocity and coefficients from the cross-correlation are used to wieght the data. A single value of 1/Q for each vintage is then calculated and differenced. A value of the change in attenuation is also calculated directly between the vintages of data. Results suggest an increase in attenuation and hence an increase in pressure or gas saturation near to the injection site.
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Estimating in Situ Rock Properties by Multiparameter Full Waveform Inversion – A Case Study
Authors M. Malinowski, A. Ribodetti and S. OpertoThis study deals with the extension of the frequency-domain full-waveform inversion/modelling (FWI) from the acoustic to the viscoacoustic case in application to the wide-aperture seismic data recorded in the Polish Basin. Attenuation was accounted for by introduction of complex velocities. Two inversion strategies were tested: (i) coupled inversion in which Qp and Vp were simultanously inverted and (ii) decoupled inversion in which Vp model was taken from the acoustic inversion and remain fixed during the inversion for Qp. The latter approach produced preferred attenuation model. Together with some a priori information from well-log data and laboratory measurements we interpret recovered high attenuation anomalies in terms of fluid saturation and rock permeabilities.
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The Fracture Attenuation Effect on Reflection Seismic Waveform Tomography
More LessSeismic waveform tomography in reflection geometry is used to reconstruct attenuation images. This paper envisages how these attenuation images can represent subsurface fracture distributions. When the size of fractures is smaller than or equal to a half the wavelength, fractures act as single scatters and produce strong attenuation ellipses in the image. When the size of fractures approaches the wavelength, different fracture orientations have significantly different attenuation effects on the reconstructed attenuation image. Horizontal fractures act as individual interfaces and show strong attenuation in the image, and such attenuation effect increases with increasing fracture size, while vertical fractures have the weakest attenuation effect in the image. Therefore, in order to properly image the attenuation associated with fractures from different orientations, a joint inversion is needed that combines both surface and borehole seismic data, not only for broadening the available frequency band but also for including reflections from different directions in the waveform tomography.
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Azimuthal Variations of Attenuation Analysis Applied on a 3D Multi-azimuth Towed-streamer Dataset
Authors L.C. Moffat, R.A. Clark, M. van der Baan and T. ManningKnowledge of key fracture properties at reservoir level such as their dominant orientation is important for successful hydrocarbon exploration since it can help enhance oil recovery. A common method to determine fracture orientations is by means of the analysis of amplitude variations with offset and azimuth (AVOA). Unfortunately inferred results are not unique and a 90 ambiguity in fracture orientations can exist – even if solely a single major trend is present. Like amplitudes, attenuation also exhibits azimuthal variations in the presence of fractures and/or cracks. The analysis of attenuation variations with offset and azimuth (QVOA) thus holds complementary information to AVOA inversion results. This paper discusses a methodology used to extract variations of attenuation on a 3D multi-azimuth sectored dataset. This methodology is also applicable to both multi-azimuth walkaway VSPs.
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Observations of Seismic Attenuation as Loss of Energy in the Overburden and as a Fracture Indicator
Authors A.V. Varghese, M. Chapman and X.Y. LiWe present an illustration of the two aspects of understanding seismic attenuation: as a means of correcting seismic data to enhance resolution of standard imaging techniques, and as a potentially important quantity which may provide information on fluids and fractures. We study two different datasets for this purpose. In the first case, we show a clear example of anisotropic attenuation in the overburden using a walkaround VSP gather. We suggest that this effect may be important for processing and interpretation of multi-azimuth reflection data. In the second case, we estimate the attenuation between two wells as a function of depth in a crosswell shotgather. The attenuation appears to be sensitive to the density of open fractures, with high attenuation in high fracture density areas. We show that the fracture density and permeability are statistically related to the observed attenuation and these parameters can reproduce the measured Q values with a rock physics model.
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Imaging below Gas Clouds through True Amplitude Redatuming
Authors A.R. Ghazali, D.J. Verschuur and A. GisolfConventional imaging processes for a situation with a gas cloud do not offer satisfactory solutions. Due to the complex wave propagation through the anomaly and the transmission imprint on the reflections from below this area, the image below the anomaly is usually not properly recovered. The main idea is that the reflection response of the complex area - e.g. the gas cloud - including its coda, carries detailed information on the gas cloud properties that can be obtained and translated into a transmission correction operator. With this multi-dimensional deconvolution operator the transmission effect from the overburden can be removed from the reflections below these complexities, after which true amplitude images of the reflections below this overburden can be obtained. For the 1.5D situation the inversion procedure is shown to provide satisfactory results. Furthermore, for the 2D case, the feasibility of a full waveform deconvolution process will be illustrated with synthetic data from a complex gas cloud model.
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Seismic Resolution Enhancement for the Characterization of Tight-sand Gas Reservoirs
By Y.H. WangThis is a case study on the application of inverse-Q filtering to improve the resolution of 3D seismic data, for the characterization of tight-sand gas reservoirs. Stabilized inverse-Q filtering is able to simultaneously compensate the amplitude and correct the phase of seismic waveforms. After processing, the frequency bandwidth has been increased and the width of the wavelet has been narrowed, so that we are able to identify reflections of thin sand layers clearly. Because of the high signal-to-noise ratio with the stabilization scheme, low-amplitude zones of interest corresponding to target high-fracture areas can be easily identified, and the detail within the anomalies can also be observed. Finally, spatial variations of tight-sand layers are depicted in the inversion profile with high resolution.
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Contribution to the Discrimination of Field-specific Seismic Amplitude Attenuation Causes
More LessThe quantitative use of the seismic amplitude information during the interpretation is a key point for many prospect evaluations and almost all reservoir characterization studies. The local amplitude information of interest is always affected by a series of signal attenuators along the propagation of the incident and reflected wavefield which are highly dependent upon the geological context, structural shape, lithologies and fluids. These many causes of attenuation are often pragmatically treated by a combination of a few well-known tools: spherical divergence compensation, surface-consistent or volumic time & frequency-dependent compensations. Cases where severe amplitude attenuation effects cannot be treated using usual approaches are a serious issue, in particular for seismic characterization of reservoirs. In this paper, a methodology adapted to the study of the impact and relevance of specific attenuation processes is presented; Alternative mechanisms and tools for a quantitative assessment of these processes are proposed. This methodology is illustrated through a field case study, using vertical incidence VSP and 3D surface data. The seismic amplitude attenuation analysis uses the VSP data to validate the modeling of measurable effects and to propose possible causes for the remaining strong attenuation, such as diffuse gas.
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