Bayesian joint residual moveout and amplitude-variation-with-offset inversion

A mplitude-variation-with-offset (AVO) has been used extensively in hydrocarbon exploration over the past two decades. Traditional AVO analysis involves computation of the AVO intercept and gradient from a linear fit of P-wave reflection amplitude to the sine square of the angle-of-incidence. This linear fit is based on the approximate P-wave reflection coefficient formulation in intercept-gradient form, given by Bortfeld (1961) and Shuey (1985) among others. Under the assumption of a background P-S velocity ratio of two, the AVO intercept and gradient values can also be combined to obtain additional AVO attributes such as pseudo S-wave data and Poisson's ratio contrast. AVO intercept and pseudo S-wave data are also used in conjunction with prestack waveform inversion by Mallick et al. (2000) in a hybrid inversion scheme.

Reflection P- and S-wave data were used in an investigation to determine the relative merits and strengths of these two data sets to characterize a naturally fractured gas reservoir in the Tertiary Upper Green River formation. The objective is to evaluate the viability of P-wave seismic to detect the presence of gas‐filled fractures, estimate fracture density and orientation, and compare the results with estimates obtained from the S-wave data. The P-wave response to vertical fractures must be evaluated at different source‐receiver azimuths (travelpaths) relative to fracture strike. Two perpendicular lines of multicomponent reflection data were acquired approximately parallel and normal to the dominant strike of Upper Green River fractures as obtained from outcrop, core analysis, and borehole image logs. The P-wave amplitude response is extracted from prestack amplitude variation with offset (AVO) analysis, which is compared to isotropic‐model AVO responses of gas sand versus brine sand in the Upper Green River. A nine‐component vertical seismic profile (VSP) was also obtained for calibration of S-wave reflections with P-wave reflections, and support of reflection S-wave results. The direction of the fast (S1) shear‐wave component from the reflection data and the VSP coincides with the northwest orientation of Upper Green River fractures, and the direction of maximum horizontal in‐situ stress as determined from borehole ellipticity logs. Significant differences were observed in the P-wave AVO gradient measured parallel and perpendicular to the orientation of Upper Green River fractures. Positive AVO gradients were associated with gas‐producing fractured intervals for propagation normal to fractures. AVO gradients measured normal to fractures at known waterwet zones were near zero or negative. A proportional relationship was observed between the azimuthal variation of the P-wave AVO gradient as measured at the tops of fractured intervals, and the fractional difference between the vertical traveltimes of split S-waves (the “S-wave anisotropy”) of the intervals.

Chapter 12 - Specific Methods

The velocity of near-surface materials is one of the most important for Ground-Penetrating Radar (GPR). In the study, we evaluate the options for determining the GPR velocity to measure the accuracy of velocity approximations from the acquired GPR data at an experimental site in Hangzhou, China. A vertical profile of interval velocities can be estimated from each common mid-point (CMP) gather using velocity spectrum analysis. Firstly, GPR data are acquired and analyzed using the popular method of hyperbola fitting which generated surprisingly high subsurface signal velocity estimates while, for the same profile, the Amplitude variation with offset (AVO) analysis of the GPR data (using the same hyperbola fitting method) generate a more reasonable subsurface signal velocity estimate. Several necessary processing steps are applied both for CMP and AVO analysis. Furthermore, experimental analysis is conducted on the same test site to get velocities of samples based on dielectric constant measurement during the drilling process. Synthetic velocities generated by AVO analysis are validated by the experimental velocities which confirmed the suitability of velocity interpretations.

Feasibility of time-lapse AVO and AVOA analysis to monitor compaction-induced seismic anisotropy

The [Formula: see text] of hydrocarbon‐bearing sediments normally deviates from the [Formula: see text] trend of the background rocks. This causes anomalous reflection amplitude variation with offset (AVO) in the seismic data. The estimation of these AVOs is inevitably affected by wave propagation effects and inversion algorithm limitations, such as thin‐bed tuning and migration stretch. A logical point is to determine the minimum [Formula: see text] change required for an anomalous AVO to be detectable beyond the background tuning and stretching effects. Assuming Ricker wavelet for the seismic data, this study addresses this point by quantifying the errors in the intercept/slope estimate. Using these results, two detectability conditions are derived. Denoting the background [Formula: see text] by γ and its variation by δγ, the thin‐bed parameter (thickness/wavelength) by ξ, the maximum background intercept closest to the AVO by |A|max, and the thin‐bed intercept value by |A|thinthe two conditions are [Formula: see text] [Formula: see text] for detectability against stretching and tuning plus stretching, respectively. Tests on synthetic data confirm their validity and accuracy. These conditions provide a quantitative guideline for evaluating AVO applicability and effectiveness in seismic exploration. They can eliminate some of the subjectivity when interpreting AVO results in different attribute spaces. To improve AVO detectability, a procedure is suggested for removing the tuning and stretching effects.

Exploration programs in deepwater Tertiary basins (e.g., the Gulf of Mexico) typically rely on bright‐spot and amplitude variation with offset (AVO) technology to help identify oil‐ and gas‐charged sands. The reliance on these attributes, along with the high cost of exploration programs in deepwater environments, has driven the need to examine the limitations of these technologies and to build robust models for the conditions under which AVO is useful as a fluid and/or lithology indicator. We build subregional AVO background trends for both brine‐ and gas‐saturated sands from several wells from the eastern deepwater Gulf of Mexico. These trends are built from the depth dependencies of velocities and densities for both shale and sands (brine saturated). Simple models of AVO gradient and intercept are constructed as a function of depth below the mud line. Sand and shale properties show little velocity contrast, justifying the interpretation of these data in the context of linearized AVO models. In addition to the in‐situ brine response, the response to gas is also calculated. These trend models indicate that the AVO response is suppressed (although still positive) below a depth of approximately 10 000 ft below the mud line. Even optimistic porosity modeling (sand porosity >30°) does not substantially change this conclusion. An important corollary is that the absence of a strong AVO anomaly at these deeper depths cannot be used with confidence when ruling out hydrocarbon presence. This observation also highlights the need to crossplot attributes to best predict hydrocarbon presence. Velocity data collected as part of this study are also used to generate a local shear velocity estimator for sands and shale. These shear estimators are similar in form to other published estimators, but minor differences in coefficients may become important in AVO modeling.

Numerous examples exist in the literature of the successes and failures of the application of amplitude variation with offset (AVO) 'gas effect' theory in hydrocarbon exploration. The acquisition of wireline and VSP shear data is considered critical information for accurate calibration, quantification, modeling, and successful application of AVO analysis techniques in a particular geologic environment. Wireline compressional, wireline dipole shear, well vertical seismic profiling (VSP), and surface seismic data are used to investigate the AVO behavior of the major lithologic boundaries and hydrocarbon bearing zones. Within the study area, lithologic boundaries produce significant AVO anomalies that may be easily confused with Class III AVO gas effect. In addition, the principal economic target, oil bearing sands, does not appear to produce AVO gas effect anomalies; although a Class III AVO gas effect response is observed when as little as 2 m of free gas is present at the top of the reservoir. Confirmation of an (Class I, II, or III) AVO gas effect in these Tertiary sands can be significant for detecting and delineating reservoirs; however, excessive emphasis on the importance of AVO gas effect, or high amplitudes on relative amplitude seismic sections, will result in an underestimate of (oil) hydrocarbon potential within the region.

Amplitude variations with offset (AVO) analysis which is primarily applied to the prestack seismic gathers has been a topic of great interests as it is expected to provide us with more confident indication of hydrocarbon accumulation than simple investigation of the specific anomalies in the stacked seismic section does.AVO analysis was evaluated for potential indicator of gas sand reservoirs on the seismic data of an area onshore East Malaysia of which the stratigraphy is dominated by sand-shale sequence under deltaic environment of Tertiary age. There are two prospects of interests of which one has been confirmed that hydrocarbon be accumulated in a number of sandstone reservoirs and the other remains untested. AVO has been modeled at the tops of three gas reservoirs, which indicates significant increases in amplitude with offset (AVO anomalies) to be expected for all gas reservoirs.Seismic data was processed to be suitable for AVO analysis. Specific data conditioning and enhancement techniques to preserve amplitudes of the data were employed. These techniques include true amplitude FK filtering and time and space variant offset dependent residual amplitude correction.AVO was analyzed by investigating restricted angle stack sections and partially stacked angle traces of the processed data. Significant AVO anomalies were correlated with some of the tops of hydrocarbon reservoirs particularly gas reservoirs which exhibited considerably high amplitudes on stacked section. However, any significant AVO anomalies were not observed in the seismic data acquired on the untested structure.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 1992Elastic wave AVO using borehole seismic dataAuthors: Craig A. CoulombeRobert R. StewartMichael J. JonesCraig A. CoulombeUniv. of Calgary, Robert R. StewartUniv. of Calgary, and Michael J. JonesSchlumberger of Canadahttps://doi.org/10.1190/1.1822239 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Permalink: https://doi.org/10.1190/1.1822239FiguresReferencesRelatedDetails SEG Technical Program Expanded Abstracts 1992ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 1992 Pages: 1410 publication data© 1992 Copyright © 1992 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 10 Feb 2005 CITATION INFORMATION Craig A. Coulombe, Robert R. Stewart, and Michael J. Jones, (1992), "Elastic wave AVO using borehole seismic data," SEG Technical Program Expanded Abstracts : 864-866. https://doi.org/10.1190/1.1822239 Plain-Language Summary PDF DownloadLoading ...

PreviousNext No AccessSEG Technical Program Expanded Abstracts 1998AVO detectability against tuning and stretching artifactsAuthors: Wenjie DongWenjie DongMobil E&P Technical Centerhttps://doi.org/10.1190/1.1820387 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Permalink: https://doi.org/10.1190/1.1820387FiguresReferencesRelatedDetailsCited By4. Chapter 4: Development GeophysicsColin MacBeth, Sankar Muhuri, William L. Abriel, William Haworth, Mark Chapin, Peter Swinburn, Raphic van der Weiden, Dieter Skaloud, Sola Adesanya, David Stevens, Chris Varley, Jacob Wilkie, Eric Brentjens, Martijn Blaauw, Liz Ross, Kevin King, Gerard Bodewitz, Hajo van Hasselt, Greg Stone, Wim Twigt, Wim Swinkels, Andrew James, Tony Addis, Chris Parsons, Robert Meij, Sarah Bell, Alexei Trofimov, Patrick Jackson, Valery Cholovsky, Edwin Lamers, Syrie Crouch, Rusty (John) Gilbert, Yuhong Liu, William Abriel, Rachel Preece, Dongjun (Taller) Fu, E. Charlotte Sullivan, Kurt J. Marfurt, Jose Luis Masaferro, Ruth Bourne, Jean-Claude Jauffred, P. Avseth, T. Mukerji, A. Jørstad, G. Mavko, T. Veggeland, M. K. MacLeod, R. A. Hanson, C. R. Bell, S. McHugo, Jan H. Kommedal, Susan Fowler, John McGarrity, C. R. Bates, D. R. Phillips, R. Grimm, H. Lynn, B. Blonk, R. W. Calvert, J. K. Koster, and G. van der Zee21 March 2012Quantitative Seismic Interpretation27 January 2010 | , Vol. 80Tuning effect on fluid properties estimated from AVO inversionMaria Rojas and De‐hua Han7 December 2005Analysis and interpretation of seismic data from thin reservoirs: Northwest Java Basin, IndonesiaA. Adriansyah and George A. McMechan7 February 2012 | GEOPHYSICS, Vol. 67, No. 1Seismic reservoir mapping from 3‐D AVO in a North Sea turbidite systemP. Avseth, T. Mukerji, A. Jørstad, G. Mavko, and T. Veggeland7 February 2012 | GEOPHYSICS, Vol. 66, No. 4 SEG Technical Program Expanded Abstracts 1998ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 1998 Pages: 2092 publication data© 1998 Copyright © 1998 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished: 06 Jan 2005 CITATION INFORMATION Wenjie Dong, (1998), "AVO detectability against tuning and stretching artifacts," SEG Technical Program Expanded Abstracts : 236-239. https://doi.org/10.1190/1.1820387 Plain-Language Summary PDF DownloadLoading ...

Summary There are examples of hydrocarbon accumulations in the Exmouth sub-basin that exhibit anomalous amplitude and AVO behaviour which are related to the presence of hydrocarbons. These include the Scarborough gas field, the Jupiter and Vinck gas discoveries, and the Macedon-Pyrenees oil and gas field. In these cases, the pre-drill recognition of anomalous amplitude behaviour aided the decisions to drill the discovery wells. In the case of the Macedon-Pyrenees oil and gas field, high seismic amplitudes caused by gas were recognised pre-drill within the Windalia Radiolarite unit. This unit overlies the Barrow Group reservoir section at the Macedon-Pyrenees field. The mapping of gas-related seismic 'brightening' of the Windalia Radiolarite unit provided an independent check on the depth conversion of the structure and was a key factor in promoting the structure as a drillable prospect. Since drilling the discovery well, seismic ‘brightening’ of the Windalia Radiolarite has been recognised over a number of other fields. This fact provides an indication that valid structural closure exists at the Barrow Group, and indicates the possibility of the presence of hydrocarbons. Further seismic analysis has concentrated on the recognition of gas-related brightening and amplitude variation with offset (AVO) behaviour of the Barrow Group reservoir sands. This work has been carried out to aid exploration in other parts of the WA-155-P exploration permit and in surrounding areas of the Barrow and Exmouth sub-basins. The results of an acoustic impedance study utilising well data have shown that the Macedon sands sub-unit of the Barrow Group should brighten significantly when charged with gas. AVO modelling using recorded P-wave and S-wave data has shown a change in AVO behaviour between Macedon sands filled with gas, and similar sands filled with brine. Detailed reprocessing of seismic data across the Macedon-Pyrenees field has been carried out to see whether the modelled behaviour of gas-filled Macedon sands could be recognised on the seismic data. Seismic data quality has been improved significantly through reprocessing and a strong relationship can now be seen between seismic brightening and gas-filled Macedon sands.

Seismic shear waves have been used in oil and gas exploration for decades. A 2D seismic shear wave inline was acquired in the Sanhu area, located in the Qaidam Basin in western China. Although the acquired shear wave data showed high resolution and comparable bandwidth to the compressional wave, it was contaminated by various types of noise, including linear noise, single-frequency noise, and especially internal multiples. Internal multiples seriously compromise the primary reflections at both near offset and far offset and are difficult to suppress. This paper presents a case study of noise attenuation for the seismic shear wave. First, single-frequency noise and linear noise are attenuated through filtering methods. Then, two methods (the Radon transform and the frequency-wavenumber (F-K) filtering) are evaluated for their effectiveness in multiple suppression and amplitude preservation. The results indicate that both methods successfully reduce long-period multiples at the far offset, enhancing the signal-to-noise ratio. We show that F-K filtering retains the characteristic ‘strong–weak–strong’ amplitude variation in the SV-SV-wave data gather, making it preferable for subsequent amplitude variation with offset analysis and inversion. Finally, a wave equation-based multiple suppression inversion method is used to suppress near-offset internal multiples. This involves iteratively predicting internal multiples and adaptively subtracting them from the original data. Stacked sections of different offsets are compared to demonstrate the de-multiple result, and the result is also validated by the improvement of well calibration with the seismic shear wave data.

We discuss the application of true-amplitude migration to zero offset (MZO) to amplitude versus offset (AVO) studies on a real marine seismic data. The true-amplitude MZO is achieved by means of a cascaded application of standard normal moveout correction (NMO) plus a constant-velocity, true-amplitude dip moveout (DMO). This process is expected to be precise as long as we have only mild lateral velocity variations. The impact of the compensation of geometrical spreading losses from large offsets to short ones on AVO analysis is discussed. The reliable results obtained for dipping events makes this process interesting for AVO analysis in areas of relatively small structural complexity.

First-order approximate analytical expressions of oblique incident elastic wave at an interface of porous media saturated with a non-viscous fluid