Velocity analysis in the presence of amplitude variation

Conventional semblance velocity analysis is equivalent to modeling prestack seismic data with events that have hyperbolic moveout but no amplitude variation with offset (AVO). As a result of its assumption that amplitude is independent of offset, this method might be expected to perform poorly for events with strong AVO—especially for events with polarity reversals at large offset, such as reflections from tops of some class 1 and class 2 sands. We find that substantial amplitude variation and even phase change with offset do not compromise the conventional semblance measure greatly. Polarity reversal, however, causes conventional semblance to fail. The semblance method can be extended to take into account data with events that have amplitude variation, expressed by AVO intercept and gradient (i.e., the Shuey approximation). However, because of the extra degrees of freedom introduced in AVO‐sensitive semblance, resolution of the estimated velocities is decreased. This is because the data can be modeled acceptably with a range of combined erroneous velocity and AVO behavior. To address this problem, in addition to using the Shuey equation to describe the amplitude variation, we constrain the AVO parameters (intercept and gradient) to be related linearly within each semblance window. With this constraint we can preserve velocity resolution and improve the quality of velocity analysis in the presence of amplitude and even polarity variation with offset. Results from numerical tests suggest that the modified semblance is accurate in the presence of polarity reversals. Tests also indicate, however, that in the presence of noise, the signal peak in conventional semblance has better standout than does that in the modified semblance measures.

Download Full-text

Traveltime and relative geometric spreading approximation in elastic orthorhombic medium

Geophysics ◽

10.1190/geo2019-0811.1 ◽

2020 ◽

Vol 85 (5) ◽

pp. C153-C162 ◽

Cited By ~ 1

Author(s):

Shibo Xu ◽

Alexey Stovas ◽

Hitoshi Mikada

Keyword(s):

Seismic Data ◽

Real Data ◽

P Wave ◽

Form Expression ◽

Amplitude Variation ◽

Rational Form ◽

Amplitude Variation With Offset ◽

Practical Applications ◽

Numerical Tests ◽

Geometric Spreading

Wavefield properties such as traveltime and relative geometric spreading (traveltime derivatives) are highly essential in seismic data processing and can be used in stacking, time-domain migration, and amplitude variation with offset analysis. Due to the complexity of an elastic orthorhombic (ORT) medium, analysis of these properties becomes reasonably difficult, where accurate explicit-form approximations are highly recommended. We have defined the shifted hyperbola form, Taylor series (TS), and the rational form (RF) approximations for P-wave traveltime and relative geometric spreading in an elastic ORT model. Because the parametric form expression for the P-wave vertical slowness in the derivation is too complicated, TS (expansion in offset) is applied to facilitate the derivation of approximate coefficients. The same approximation forms computed in the acoustic ORT model also are derived for comparison. In the numerical tests, three ORT models with parameters obtained from real data are used to test the accuracy of each approximation. The numerical examples yield results in which, apart from the error along the y-axis in ORT model 2 for the relative geometric spreading, the RF approximations all are very accurate for all of the tested models in practical applications.

Download Full-text

Amplitude variation with offset-friendly bootstrapped differential semblance

Geophysics ◽

10.1190/geo2016-0395.1 ◽

2017 ◽

Vol 82 (5) ◽

pp. V297-V309 ◽

Cited By ~ 2

Author(s):

Hamish Wilson ◽

Lutz Gross

Keyword(s):

Spectral Resolution ◽

Seismic Data ◽

Velocity Analysis ◽

Apparent Velocity ◽

Seismic Data Processing ◽

Velocity Control ◽

Amplitude Variation ◽

Trade Off ◽

Amplitude Variation With Offset ◽

Velocity Models

Spectral noise, low resolution, and attenuation of semblance peaks due to amplitude variation with offset (AVO) anomalies hamper the reliability of velocity analysis in the semblance spectrum for seismic data processing. Increasing resolution and reducing noise while accounting for AVO has posed a challenge in various semblance schemes due to a trade-off in resolution and AVO detectability. A new semblance scheme is introduced that aims to remove this trade-off. The new scheme uses the concepts of bootstrapped differential semblance with trend-based AB semblance. Results indicate that the new scheme indeed increases spectral resolution, reduces noise, and accounts for AVO anomalies. These improvements facilitate velocity control for automatic and manual picking methods and, hence, provide a means for more reliable apparent velocity models.

Download Full-text

Seismic reservoir characterization of a class-1 amplitude variation with offset turbiditic system located offshore Cote d’Ivoire, West Africa

Interpretation ◽

10.1190/int-2017-0163.1 ◽

2018 ◽

Vol 6 (2) ◽

pp. SD115-SD128

Author(s):

Pedro Alvarez ◽

William Marin ◽

Juan Berrizbeitia ◽

Paola Newton ◽

Michael Barrett ◽

...

Keyword(s):

West Africa ◽

Seismic Data ◽

Reservoir Characterization ◽

Rock Physics ◽

Rock Properties ◽

Well Log ◽

Amplitude Variation ◽

Amplitude Variation With Offset ◽

Fluid Content ◽

Class 1

We have evaluated a case study, in which a class-1 amplitude variation with offset (AVO) turbiditic system located offshore Cote d’Ivoire, West Africa, is characterized in terms of rock properties (lithology, porosity, and fluid content) and stratigraphic elements using well-log and prestack seismic data. The methodology applied involves (1) the conditioning and modeling of well-log data to several plausible geologic scenarios at the prospect location, (2) the conditioning and inversion of prestack seismic data for P- and S-wave impedance estimation, and (3) the quantitative estimation of rock property volumes and their geologic interpretation. The approaches used for the quantitative interpretation of these rock properties were the multiattribute rotation scheme for lithology and porosity characterization and a Bayesian lithofluid facies classification (statistical rock physics) for a probabilistic evaluation of fluid content. The result indicates how the application and integration of these different AVO- and rock-physics-based reservoir characterization workflows help us to understand key geologic stratigraphic elements of the architecture of the turbidite system and its static petrophysical characteristics (e.g., lithology, porosity, and net sand thickness). Furthermore, we found out how to quantify and interpret the risk related to the probability of finding hydrocarbon in a class-1 AVO setting using seismically derived elastic attributes, which are characterized by having a small level of sensitivity to changes in fluid saturation.

Download Full-text

AVO-sensitive semblance analysis for wide-azimuth data

Geophysics ◽

10.1190/1.2834115 ◽

2008 ◽

Vol 73 (2) ◽

pp. U1-U11 ◽

Cited By ~ 10

Author(s):

Jia Yan ◽

Ilya Tsvankin

Keyword(s):

Azimuthal Anisotropy ◽

Polarity Reversal ◽

Inversion Algorithm ◽

Amplitude Variation ◽

Amplitude Variation With Offset ◽

Reflection Data ◽

Long Offset ◽

Class 1 ◽

Polarity Reversals ◽

Analysis Models

Conventional semblance-based moveout analysis models prestack reflection data with events that have hyperbolic move-out and no amplitude variation with offset (AVO). Substantial amplitude variation and even phase change with offset do not significantly compromise the semblance operator. However, polarity reversal associated with a change in the sign of the reflection coefficient may cause conventional semblance to fail. An existing modification of the semblance operator that takes amplitude variations into account (so-called AK semblance) is limited to narrow-azimuth data and cannot handle nonhyperbolic moveout at large offsets. We employ a 3D nonhyperbolic moveout inversion algorithm to extend the AK semblance method to wide-azimuth data recorded on long spreads. To preserve velocity resolu-tion, the ratio [Formula: see text] of the AVO gradient and intercept is kept constant within each semblance window. In the presence of azimuthal anisotropy, however, the parameter [Formula: see text] has to be azimuthally dependent. Synthetic tests confirm that distortions in moveout analysis caused by polarity reversals become more common for long-offset data. Conventional semblance produces substantial errors in the NMO ellipse and azimuthally varying anellipticity parameter [Formula: see text] not just for class 2 AVO response but also for some models with class 1 AVO. In contrast, the AK semblance algorithm gives accurate estimates of the moveout parameters even when the position of the polarity reversal varies with azimuth. The AK method not only helps to flatten wide-azimuth reflection events prior to stacking and azimuthal AVO analysis but also provides input parameters for the anisotropic geometrical-spreading correction.

Download Full-text

AVO and velocity analysis

Geophysics ◽

10.1190/1.1487076 ◽

2001 ◽

Vol 66 (4) ◽

pp. 1284-1293 ◽

Cited By ~ 27

Author(s):

D. Sarkar ◽

J. P. Castagna ◽

W. J. Lamb

Keyword(s):

Large Amplitude ◽

Analysis Procedure ◽

Velocity Analysis ◽

Constant Amplitude ◽

Amplitude Variation ◽

Amplitude Variation With Offset ◽

Analysis Algorithm ◽

Regularization Term ◽

Polarity Reversals

Velocity analysis using semblance estimates velocities based on a constant amplitude model for seismograms and does not take amplitude variation with offset (AVO) into account. In the presence of AVO, the constant amplitude model becomes inaccurate, particularly for events which exhibit polarity reversals. An AVO sensitive velocity analysis procedure, which is a generalization of the traditional semblance method, can be devised by giving an offset dependence to the modeled seismograms. Incorporating AVO into velocity analysis requires additional parameters to describe the reflectivity. This results in reduced velocity precision. By introducing a regularization term which provides a controlled suppression of the contributions due to AVO effects, we describe an AVO sensitive velocity analysis algorithm that properly deals with events exhibiting polarity reversals or large amplitude variation with offset.

Download Full-text

An integrated broadband preprocessing method for towed-streamer seismic data

Geophysics ◽

10.1190/geo2019-0521.1 ◽

2020 ◽

Vol 85 (2) ◽

pp. V201-V221 ◽

Cited By ~ 2

Author(s):

Mehdi Aharchaou ◽

Erik Neumann

Keyword(s):

Seismic Data ◽

High Frequency ◽

Pressure Data ◽

Amplitude Variation ◽

Field Source ◽

Amplitude Variation With Offset ◽

Ringing Artifacts ◽

Joint Application ◽

Amplitude Compensation ◽

Unified Method

Broadband preprocessing has become widely used for marine towed-streamer seismic data. In the standard workflow, far-field source designature, receiver and source-side deghosting, and redatuming to mean sea level are applied in sequence, with amplitude compensation for background [Formula: see text] delayed until the imaging or postmigration stages. Thus, each step is likely to generate its own artifacts, quality checking can be time-consuming, and broadband data are only obtained late in this chained workflow. We have developed a unified method for broadband preprocessing — called integrated broadband preprocessing (IBP) — which enables the joint application of all the above listed steps early in the processing sequence. The amplitude, phase, and amplitude-variation-with-offset fidelity of IBP are demonstrated on pressure data from the shallow, deep, and slanted streamers. The integration allows greater sparsity to emerge in the representation of seismic data, conferring clear benefits over the sequential application. Moreover, time sparsity, full dimensionality, and early amplitude [Formula: see text] compensation all have an impact on broadband data quality, in terms of reduced ringing artifacts, improved wavelet integrity at large crossline angles, and fewer residual high-frequency multiples.

Download Full-text

Meet West Africa deep exploration challenge with geomorphology and targeted amplitude variation with offset inversion

Interpretation ◽

10.1190/int-2019-0045.1 ◽

2020 ◽

Vol 8 (1) ◽

pp. SA25-SA33

Author(s):

Ellen Xiaoxia Xu ◽

Yu Jin ◽

Sarah Coyle ◽

Dileep Tiwary ◽

Henry Posamentier ◽

...

Keyword(s):

West Africa ◽

Critical Role ◽

Reservoir Quality ◽

Quality Prediction ◽

Reservoir Prediction ◽

Amplitude Variation ◽

Amplitude Variation With Offset ◽

Simultaneous Inversion ◽

Seismic Amplitude ◽

Class 1

Seismic amplitude has played a critical role in the exploration and exploitation of hydrocarbon in West Africa. Class 3 and 2 amplitude variation with offset (AVO) was extensively used as a direct hydrocarbon indicator and reservoir prediction tool in Neogene assets. As exploration advanced to deeper targets with class 1 AVO seismic character, the usage of seismic amplitude for reservoir presence and quality prediction became challenged. To overcome this obstacle, (1) we used seismic geomorphology to infer reservoir presence and precisely target geophysical analysis on reservoir prone intervals, (2) we applied rigorous prestack data preparation to ensure the accuracy and precision of AVO simultaneous inversion for reservoir quality prediction, and (3) we used lateral statistic method to sum up AVO behavior in regions of contrasts to infer reservoir quality changes. We have evaluated a case study in which the use of the above three techniques resulted in confident prediction of reservoir presence and quality. Our results reduced the uncertainty around the biggest risk element in reservoir among the source, charge, and trap mechanism in the prospecting area. This work ultimately made a significant contribution toward a confident resource booking.

Download Full-text

Value of information analysis and Bayesian inversion for closed skew-normal distributions: Applications to seismic amplitude variation with offset data

Geophysics ◽

10.1190/geo2013-0048.1 ◽

2014 ◽

Vol 79 (4) ◽

pp. R151-R163 ◽

Cited By ~ 4

Author(s):

Javad Rezaie ◽

Jo Eidsvik ◽

Tapan Mukerji

Keyword(s):

Seismic Data ◽

Value Of Information ◽

Bayesian Inversion ◽

Information Analysis ◽

Amplitude Variation ◽

Mean Values ◽

Amplitude Variation With Offset ◽

Seismic Amplitude ◽

Value Of Information Analysis ◽

Skew Normal

Information analysis can be used in the context of reservoir decisions under uncertainty to evaluate whether additional data (e.g., seismic data) are likely to be useful in impacting the decision. Such evaluation of geophysical information sources depends on input modeling assumptions. We studied results for Bayesian inversion and value of information analysis when the input distributions are skewed and non-Gaussian. Reservoir parameters and seismic amplitudes are often skewed and using models that capture the skewness of distributions, the input assumptions are less restrictive and the results are more reliable. We examined the general methodology for value of information analysis using closed skew normal (SN) distributions. As an example, we found a numerical case with porosity and saturation as reservoir variables and computed the value of information for seismic amplitude variation with offset intercept and gradient, all modeled with closed SN distributions. Sensitivity of the value of information analysis to skewness, mean values, accuracy, and correlation parameters is performed. Simulation results showed that fewer degrees of freedom in the reservoir model results in higher value of information, and seismic data are less valuable when seismic measurements are spatially correlated. In our test, the value of information was approximately eight times larger for a spatial-dependent reservoir variable compared with the independent case.

Download Full-text

Fracture detection via correlating P-wave amplitude variation with offset and azimuth analysis and well data in eastern central Saudi Arabia

Interpretation ◽

10.1190/int-2016-0128.1 ◽

2017 ◽

Vol 5 (4) ◽

pp. T531-T544

Author(s):

Ali H. Al-Gawas ◽

Abdullatif A. Al-Shuhail

Keyword(s):

Saudi Arabia ◽

Seismic Data ◽

Production Data ◽

Fracture Zones ◽

Fracture Detection ◽

Amplitude Variation ◽

Data Set ◽

Amplitude Variation With Offset ◽

Well Data ◽

Patch Geometry

The late Carboniferous clastic Unayzah-C in eastern central Saudi Arabia is a low-porosity, possibly fractured reservoir. Mapping the Unayzah-C is a challenge due to the low signal-to-noise ratio (S/N) and limited bandwidth in the conventional 3D seismic data. A related challenge is delineating and characterizing fracture zones within the Unayzah-C. Full-azimuth 3D broadband seismic data were acquired using point receivers, low-frequency sweeps down to 2 Hz, and 6 km patch geometry. The data indicate significant enhancement in continuity and resolution of the reflection data, leading to improved mapping of the Unayzah-C. Because the data set has a rectangular patch geometry with full inline offsets to 6000 m, using amplitude variation with offset and azimuth (AVOA) may be effective to delineate and characterize fracture zones within Unayzah-A and Unayzah-C. The study was undertaken to determine the improvement of wide-azimuth seismic data in fracture detection in clastic reservoirs. The results were validated with available well data including borehole images, well tests, and production data in the Unayzah-A. There are no production data or borehole images within the Unayzah-C. For validation, we had to refer to a comparison of alternative seismic fracture detection methods, mainly curvature and coherence. Anisotropy was found to be weak, which may be due to noise, clastic lithology, and heterogeneity of the reservoirs, in both reservoirs except for along the western steep flank of the study area. These may correspond to some north–south-trending faults suggested by circulation loss and borehole image data in a few wells. The orientation of the long axis of the anisotropy ellipses is northwest–southeast, and it is not in agreement with the north–south structural trend. No correlation was found among the curvature, coherence, and AVOA in Unayzah-A or Unayzah-C. Some possible explanations for the low correlation between the AVOA ellipticity and the natural fractures are a noisy data set, overburden anisotropy, heterogeneity, granulation seams, and deformation.

Download Full-text

AVO analysis for high amplitude anomalies using 2D pre-stack seismic data

Kuwait Journal of Science ◽

10.48129/kjs.12351 ◽

2022 ◽

Author(s):

Lamees N. Abdulkareem ◽

Keyword(s):

Seismic Data ◽

High Amplitude ◽

Rock Properties ◽

Fluid Substitution ◽

Amplitude Variation ◽

Amplitude Variation With Offset ◽

The Real ◽

North West ◽

Avo Analysis ◽

Controlling Parameter

Amplitude variation with offset (AVO) analysis is an 1 efficient tool for hydrocarbon detection and identification of elastic rock properties and fluid types. It has been applied in the present study using reprocessed pre-stack 2D seismic data (1992, Caulerpa) from north-west of the Bonaparte Basin, Australia. The AVO response along the 2D pre-stack seismic data in the Laminaria High NW shelf of Australia was also investigated. Three hypotheses were suggested to investigate the AVO behaviour of the amplitude anomalies in which three different factors; fluid substitution, porosity and thickness (Wedge model) were tested. The AVO models with the synthetic gathers were analysed using log information to find which of these is the controlling parameter on the AVO analysis. AVO cross plots from the real pre-stack seismic data reveal AVO class IV (showing a negative intercept decreasing with offset). This result matches our modelled result of fluid substitution for the seismic synthetics. It is concluded that fluid substitution is the controlling parameter on the AVO analysis and therefore, the high amplitude anomaly on the seabed and the target horizon 9 is the result of changing the fluid content and the lithology along the target horizons. While changing the porosity has little effect on the amplitude variation with offset within the AVO cross plot. Finally, results from the wedge models show that a small change of thickness causes a change in the amplitude; however, this change in thickness gives a different AVO characteristic and a mismatch with the AVO result of the real 2D pre-stack seismic data. Therefore, a constant thin layer with changing fluids is more likely to be the cause of the high amplitude anomalies.

Download Full-text