Jonathan E. Downton

Three Term AVO Waveform Inversion

Summary Density reflectivity is a useful AVO attribute to infer fluid saturation. However, accurate density reflectivity estimates are difficult to obtain due to the ill-conditioned nature of the inverse problem. A small amount of noise will lead to large errors in the estimates. To improve the stability of the inversion, large angles and offsets are required, but these bring their own problems. NMO stretch and offset dependent tuning are two of these. This paper develops and demonstrates an AVO waveform inversion that incorporates into its forward model these factors allowing for accurate estimates even in their presence. Well constraints and various regularization strategies are employed to further enhance the reliability of the solution.

Constrained three-parameter AVO inversion and uncertainty analysis

SUMMARY Bayes’ theorem is used to derive a three-parameter non-linear AVO inversion. Geologic constraints based on available well-control or rock-physical relationships are incorporated to help stabilize the solution. Parameter uncertainty estimates arise naturally as part of the derivation and provide estimates of the reliability of the different parameters. The resulting parameter and uncertainty estimates may be transformed to a variety of elastic and rock-physical AVO attributes popular in the literature using a transform matrix.

An efficient method for AVO modeling of reflected spherical waves

A method is presented for efficiently and accurately calculating the spherical-wave generalization of the Zoeppritz P-wave reflection coefficients. The main assumptions are that the wavelet is an exponential form that allows for analytic integration over frequency, and the direction of propagation and arrival time are as dictated by ray theory. These assumptions result in calculations sufficiently rapid to be carried out interactively on the computer. Results for an AVO Class I model show that this method quantitatively reproduces exact spherical-wave reflection coefficients obtained using a Ricker wavelet.

The Applications of Amplitude Versus Offset In Carbonate Reservoirs: Re-examining the Potential

Summary The application of amplitude versus offset (AVO) in carbonate reservoirs is re-examined based on the increasing knowledge in the physical properties of carbonate rocks and advances in AVO technologies. The rock physics analysis indicated that porosity plays the dominating role in influencing carbonate rock properties and suggests that gas effect may be determined. Using derived relations between velocities, porosity and gas effect, The amplitude vs. offset is calculated as a function of these effects in commonly encountered reservoirs. It is demonstrated that carbonate reservoirs have unique characteristics in amplitude variation with offset. This is compared with the AVO in clastic reservoirs. The carbonate rock properties, impedances and elastic modulii, are studied because of their direct link to pre-stack seismic inversion. Their relationships with porosity were used for predicting the porosity of carbonate reservoirs.

High‐resolution AVO analysis before NMO

This paper demonstrates a methodology to produce highresolution AVO reflectivity attribute estimates similar to sparse spike deconvolution. The AVO estimate is performed prior to NMO avoiding the distortions and loss of frequency associated with this process. Long tailed a priori distributions are used to constrain the problem. The resulting sparse reflectivity is able to resolve thin layers and is more reliable than the estimates provided by the traditional AVO analysis that is performed on a sample-bysample basis on NMO corrected gathers. This greater reliability is due to the classic trade-off between resolution and reliability. With the new method a few sparse reflectivity values are estimated with greater certainty than the dense reflectivity at every time sample as in the traditional AVO analysis.

Linearized AVO inversion with supercritical angles

Contrary to popular belief, a linearized approximation of the Zoeppritz equations may be used to accurately estimate the reflection coefficient for angles of incidence up to and beyond the critical angle. These supercritical reflection coefficients are complex implying a phase variation with offset in addition to the amplitude variation with offset. This linearized approximation is used as the basis for a new AVO waveform inversion capable of truly incorporating wide angle information.

Recent Advances In Application of AVO In Carbonate Reservoirs: Calibration And Interpretation

Summary The application of amplitude versus offset (AVO) in the analysis of carbonate reservoirs is on the increase in the Western Canadian Sedimentary Basin (WCSB). This has contributed to a rapidly increasing knowledge of the physical properties of carbonate rocks, and in the active exploration and delineation of carbonate reservoirs. It has been found that not only porosity but also fluid effects may be determined through AVO. Information from a carbonate reservoir, especially that of a fluid effect can be obtained from basic AVO gather and attribute analysis as well as elastic parameter inversion. Recent advances in these areas are reviewed. Furthermore this paper includes new studies in carbonate rock properties, carbonate AVO characteristics due to fluid effects, and elastic rock properties that form the basis for calibration and interpretation, as key issues to be considered.

Sensitivity of rock properties in AVO analysis and prospect evaluation

Sensitivity of rocks properties responding to pore fluid and sensitivity of seismic attributes to a given lithology contrast are two important issues for defining a reservoir. This study attempts to quantitatively determine these sensitivities. The evaluation of the sensitivities was used to predict seismic response and amplitude variation with offset. It is demonstrated that the analysis of these sensitivities is useful for seismic interpretation and prospect evaluation. A methodology is suggested to evaluate these sensitivities by using absolute and relative variations of rock properties with respect to porosity, pore fluid and rock composition. It indicates that the orthogonality of rock properties with given a pair of lithologic units is a measure of lithologic contrast. It could be enhanced through mathematical manipulations.

AVO before NMO

Summary This paper demonstrates a methodology to perform AVO and NMO inversion simultaneously using suitable constraints from the external well control or empirical rock physical relationships. Using synthetic data, it is shown that this new methodology generates superior reflectivity estimates compared to the traditional approach of performing NMO followed by AVO inversion.