PDF AVO Course Notes, Part 4. Three-term AVO Inversion

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High Poisson's ratio is suggestive of gas sand in a sand-shale sequence. The potential hydrocarbon-indicator stack is similar to the restricted gradient stack in interpretation; a large positive anomaly often indicates the presence of gas in a sand-shale sequence. This section displays an anomaly when the absolute amplitude increases with offset. Correlation coefficient or Intercept weights: When the AVO computation method is 'least squares' then this section displays the correlation coefficient of the least squares fit. The correlation coefficient has an absolute value range of 0.

Geophysics: Seismic - AVO, the 3-term Shuey approximation

If the computation method is geo-stack, then this section displays the intercept weights. Statistics or Gradient weights: When the AVO computation is 'least squares' then this section displays the randomness statistics. Large absolute values of the statistic greater than 3 indicate that the data does not trend in a straight line so a straight line fit is not appropriate.

If the computation method is geo-stack, then this section displays the gradient weights. Fluid factor: The fluid factor is computed using Castagna's mud rock equation. Gardner's relationship is used to determine the density. A deviation of the intercept and gradient from a regional trend will show up as a fluid factor anomaly.

The near value is the amplitude on the nearest offset in the CMP that still has a valid non-zero sample. The far value is the amplitude on the farthest offset in the CMP that still has a valid non-zero sample. The corresponding option number will be placed in the location of offset DIST in the header. References Goodway, B. Denham, L. Shuey, R. Castagna, J. Smith, G. Investigations in Geophysics No. Todd, C. Fatti, George C. Smith, Peter J. Vail, Peter J.

Strauss, and Philip R. The polarity gate is determined by a stack of the CMP gather traces. The gate being samples within the same polarity cycle. The amplitude used for each trace is the maximum found using interpolation. The amplitudes are blocked as constant values within the gate times. Options: At each sample, Polarity gate Method for AVO attribute computations: The best-fit line computation is performed using a least squares linear fit or the weighted stacking Geostack method.

When using the least squares linear fit method, a minimum correlation coefficient can be specified. The correlation coefficient is a statistical measurement of the fit of the data to the best-fit straight line. The correlation coefficient can range from 0. Computed correlation coefficients that fall below this minimum value will result in the attributes at that sample being zeroed. A value of 0. Maximum incident angle deg. However, AVOA renders this as a parameter at the user's discretion.

AVOA will mute the data that has an angle of incidence greater than this maximum angle. Minimum incident angle deg. AVOA renders this as a parameter at the user's discretion. AVOA will mute the data that has an angle of incidence that is less than this minimum angle.


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Space variant start and end times? The user may select space variant or non-spatial variant start and stop times. The space variant times are entered through a window file or a spreadsheet. The non-spatial variant times are constant throughout the survey. Start time ms. Space variant start and stop times? Enter the start time in milliseconds. The attribute stack computations will start at this time. End time ms. Enter the end time in milliseconds. The attribute stack computations will end at this time. Start and stop times Space variant start and stop times? This parameter appears if space variant start and stop times are selected.

The user can select a windows file or use the spreadsheet to enter start and stop times. Only one start and stop time is allowed by CMP location.

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If the file contains additional times per CMP location, they will be ignored. These header values may be retrieved and used as a bulk shift of the start and stop times. The user may select a VIP file that was previously built or may enter the time-velocity pairs in a spreadsheet. The velocities enter should be the RMS velocities. The module will compute the interval velocities from the RMS velocities. The filter length should be entered in milli-seconds. Mudrock line slope Geostack and fluid factor only Enter the mudrock line slope to be used in the fluid factor computation.

The fluid factor is computed from the mudrock line trend from Castagna and Hilterman Probability statistics graphs of P- and S-wave velocities and density at 10, and sampling points, respectively.

Description:

Uncertainty analysis graphs of P- and S-wave velocities and density for the logging data red color indicates greater probability or smaller uncertainty. Angle-stack seismic profiles with three different angles. Comparison between the inversion results of near wellbore seismic trace CDP by using the improved MCMC method and the real logging data 10, iterations. Comparison between the inversion results of near wellbore seismic trace CDP by using the conventional MCMC method and the real logging data 1,00, iterations. Compared with the conventional MCMC method, the improved MCMC method, combining the AM algorithm based on the global adaptive strategy and the DR algorithm based on the local adaptive strategy, can adaptively update the proposal distribution and speed up the convergence of Markov chains;.

The method of nonlinear inversion based on the improved MCMC algorithm using the exact Zoeppritz equation is not only suitable for reservoirs with strong-contrast interfaces and long-offset ranges but it is also more stable, accurate, and anti-noise;. Based on the Bayesian framework and the fusion of a priori constraint information such as logging data and seismic data, the improved MCMC method further reduces the non-uniqueness of the solutions and greatly improves the stability of the inversion solutions. Moreover, it can also estimate the uncertainty of the results to assist us in risk assessment of reservoir prediction.

Tests on logging data and seismic data demonstrate the feasibility and robustness of the method, and in order to invert more accurate density parameter, we will further study this method based on long-offset seismic data. We also thank the anonymous reviewers for their constructive suggestions. Skip to main content Skip to sections.

Advertisement Hide. Download PDF. Open Access. First Online: 20 December The DR algorithm is an improved MCMC method, and its basic idea is allowing partial local adaptation of the rejected candidates, where the Markov chains still retain the Markovian property and converge to the second or higher stages. The ultimate targets of the DR algorithm are to improve the accuracy and efficiency of the estimators.

The creation of proposal distributions in higher stages is allowed to depend not only on the current position of the chain but also on the proposal distribution created previously and the rejected candidates in higher stages Green and Mira ; Haario et al. The second stage proposal is accepted with probability. In conclusion, the improved MCMC method, integrating the advantages and overcoming the disadvantages of the AM and DR algorithms, can improve the practicability significantly.

Open image in new window. To make the method proposed in this paper suitable for reservoirs with strong-contrast interfaces and long-offset ranges and avoid calculation errors brought by the approximate Zoeppritz equation, we choose the exact Zoeppritz equation to do the inversion. We do the inversion using the method proposed in this paper and the method of prestack linear inversion based on the damped least square DLS method using the Aki and Richards approximate Zoeppritz equation to invert the P- and S-wave velocities and the density directly showed as Figs.

From Fig. Based on the results of error comparison showed in Fig. The numerical magnitude in Fig. The prestack seismic data used in this paper is from an oil—gas field in the Sichuan Basin of Southwest China. The seismic data was processed to ensure that the final prestack amplitudes should image the reflection strength of the subsurface interfaces as correctly as possible.

A well located at CDP the black ellipse shows a gas reservoir at around 2. To save the time of inversion, we stack the seismic offset gathers to three different angle-stack seismic profiles, showed as Fig.


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The inverted results are shown as Fig. We can see that both the inversion results of the AVO parameters fit the logging data well and they are consistent with the accuracy, but the efficiency of the improved MCMC method and the conventional MCMC method shows a great difference that the former needs 10, iterations while the latter needs 1,00, iterations to receive the results with similar accuracy. Similarly, the inversion results of P- and S-wave velocities are better than those of the density due to the limitation in the offset of the data. An improved MCMC method, combining the AM algorithm based on the global adaptive strategy and the DR algorithm based on the local adaptive strategy, has been proposed to invert the P- and S-wave velocities and the density based on the exact Zoeppritz equation.

The method has the following characteristics: 1. Aki K, Richards PG. Quantitative seismology: theory and method. London: W. Freeman and Co; Google Scholar. Buland A, Omre H. Bayesian linearized AVO inversion. CrossRef Google Scholar. In: Beijing international geophysical conference and exposition. Downton JE, Ursenbach C.

Practical applications of P-wave AVO for unconventional gas Resource Plays

Linearized amplitude variation with offset AVO inversion with supercritical angles. J Appl Geophys. From Bayes to Tarantola: new insights to understand uncertainty in inverse problems. Efficient Metropolis jumping rules. Bayesian Stat. Green PJ, Mira A. Delayed rejection in reversible jump Metropolis-Hastings.

Stat Comput. An adaptive Metropolis algorithm. A new MCMC algorithm for seismic waveform inversion and corresponding uncertainty analysis. Geophys J Int. High precision pre-stack inversion algorithm based on Zoeppritz equations. Jilek P. Joint inversion of PP- and PS-wave reflection coefficients in anisotropic media. CWP Research Report.