AVO SEISMIC TECHNOLOGY
BOTTOM LINE
The amplitude variations with offset (AVO) technology was introduced over 20 years ago. In the intervening years, the technique has evolved from a concept to a primary component of seismic exploration. AVO methods can add reliable constraints to quantitative reservoir characterization if the operator understands the underlying concepts and how to apply the technology.
PROBLEM ADDRESSED
The workshop presentation and notes give an overview of AVO technology methodology and help to define some of the complex issues involved with using the technology. Examples and case studies are included to show the relevance of the technology. The basics of rock physics and current trends in inversion technology are included in the discussion to enhance practical application of AVO.
TECHNOLOGY OVERVIEW AVO is a seismic technique that uses pre-stack seismic data, instead of the more normally used post-stack data, to detect the presence of hydrocarbons in the reservoir. Three physical parameters of seismic data are fundamental to seismic interpretation-density, P-wave velocity and S-wave velocity. Understanding these is necessary to application of AVO technology. Density, P- and S-wave velocity data examples were used to demonstrate how to construct a reservoir model and show the AVO response of the model. Part 1—Introductory AVO and Basic Rock Physics In reservoir rock, AVO response is dependent on the velocities of P- and S-waves and on density to define the pore space and fluids within the rock matrix. Density effects can be modeled in a fluid-saturated rock using the relationships of porosity, and water saturation in the matrix and fluids. Seismic velocity involves the deformation of the reservoir rock as a function of time. As a rock is compressed it changes in volume and shape (P-wave). In rock shear (Swave) the rock deforms in shape, but not in volume. The ability of a rock to compress is defined as its bulk modulus K (the reciprocal of compressibility). The ability of a rock to
Based on an Eastern Gulf Region PTTC workshop, October 9, 2003 in Jackson, Mississippi SPEAKERS: Brian Russell, Co-founder of Hampson-Russell Software Services, Ltd (1987-2002), Calgary, Alberta, now a whollyowned subsidiary of Veritas DGC Inc. 529
KEY WORDS: Aki-Richards AVO Hodogram Biot-Gassmann Crossplotting Elastic Impedance Inversion P- and S-Waves Polarization Angle
shear is defined as shear modulus µ. P- and S- wave velocity equations are functions of density, bulk modulus and shear modulus. Other commonly used terms defined and illustrated include Poisson's Ratio, Biot-Gassmann Equations, Biot-Gassmanshear modulus, and Biot-Gassmann-saturated bulk modulus. In a given reservoir rock the equations can be modified to compute the velocities as a function of fluid saturation. The Biot-Gassmann equations and variations are used to define dry rock versus saturated rock. Seismic reflections are interpreted through the velocity of return of the compressional (P-wave), horizontal shear waves (SH) and vertical shear waves (SV). Computation of the seismic response from a sample survey from Alberta is based on the values obtained for velocities and the density as they differ in the sand and shale units. It is important to note that the P-wave response changes polarity in going from a wet to a gas sand, but the S-wave response retains the same polarity. This difference in function allows the operator to predict where the presence of gas will occur in the reservoir. Unfortunately, most seismic data, unlike the previous example do not give S-wave data, but only P-wave data. The recording of P-wave data at various offsets, which is always recorded, can be used to record a component of the S-wave data. The offset recording is the basis of the AVO technique. A number of examples of how to model the AVO curves are provided in the workshop notebook. The value of using AVO to interpret gas sands was first proposed in 1984 using low impedance value for the sands and higher impedance values for shales. The application of
common offset stack method allows the interpreter to pick seismic events to match the model, fitting known and unknown information into a comprehensive interpretation. In 1989 Rutherford and Williams extended the AVO method to anomalies other than low impedance sands. Other anomalies that the method can be applied to are identification of sand to sand boundaries. The Aki-Richards equations were used to perform forward modeling and data analysis in these examples. Part 2—Crossplotting AVO attributes Crossplotting of intercept data against gradient data can be used in the interpretation of AVO anomalies. Linking rocky physics parameters, AVO modeling and crossplotting are used for polarization analysis of AVO anomalies. Crossplotting was developed in the late 1990s and is based on the classification scheme by Rutherford and Williams. It uses acoustic impedance changes in anomalous layers. Differences in impedance and polarity plot in different portions of an intercept/gradient crossplot. The anomalies form an elliptical trend on plot allowing interpretation of gas wet and dry zones. AVO crossplotting is incorporated into AVO modeling using the link of fluid substitution based on the Biot-Gassmann equations creating that has been termed the AVO hodogram. A hodogram enables display of an anomaly in a three dimensional view. Modeling flow is a five-step method for application of AVO offset data. The first step is to edit and prepare the well logs. Step 2 is to create fluid/lithology replacement logs. In Step 3 in-situ and fluid replacement AVO models are generated. Step 4 involves generating the appropriate AVO attributes (intercept and gradient) for the models. Step 5 is the crossplotting of the attributes from each model simultaneously. Changes in seismic bandwidth on the intercept and gradient result in lowered frequency, and there is a loss of definition. The polarization angle is defined as the positive upwards attribute from the horizontal axis. The advantage of using the polarization angle is to make the anomaly data noise free. Examples of polarization analysis were modeled from datasets from Alberta and Indonesia.
being used today in geophysical interpretation. Traditional seismic lithology estimation involves taking the gathered data, stacking it, applying inversion, and calculating the estimate, using only acoustic impedance, which is not sufficient for estimating fluid content. AVO technology allows the operator to simultaneously estimate additional parameters to infer fluid and/or lithology. A combination of methods including Range-limited stacking, elastic impedance, intercept/gradient analysis, Rp/Rs extraction inversion and lambda and mu attribute analysis methods can be used for processing attributes. Case studies from the Colony sand in Alberta, a Gulf Coast sand, offshore eastern Canada Whiterose field, and a channel sand play from the Western Canadian sedimentary basin illustrate the different uses of the methods, and where each may be most applicable. Using elastic impedance, Rp/Rs, inversion and lambda and mu attribute analysis, and the theory behind Biot-Gassmann perspective; the AVO method allows estimation of two or more independent parameters from pre-stack data. Poststack inversion techniques can then be applied to these extracted attributes. The crossplot of the inverted attributes allows the separate fluid and matrix effects of the reservoir rock to be determined. In each of the case examples, the pair of attributes best suited for the particular play needs to be evaluated using both well log and seismic data. Part 5—Practical considerations in AVO The workshop gave an overview of numerous methods for extracting fluid and lithology information from the subsurface using AVO methods. However, there are problems that can reduce the accuracy of AVO application. Methods using intercept/gradient analysis and cross-plotting must consider the effects of: tuning or thin beds, noise on the far offsets (i.e., multiples), misalignment of events at far offsets, neglecting the third term in Aki-Richards technique, neglecting anisotropic effects, and offset variable phase errors.
Part 3—AVO Case Studies Five case studies of AVO technology used in the field were extracted and discussed based on two articles appearing in Geophysics and three from The Leading Edge. The field data is from South Africa, the North Sea, Gulf of Mexico, and on-shore South Texas. The differences in using the conventional approach and the fluid factor approach to AVO analysis were discussed and illustrated. The fluid factor approach gives a much better definition of the reservoir being modeled. The examples illustrate the data used and how the interpretations can be made based on the technology appropriate to specific lithologies and fluid conditions.
A discussion of the processing issues illustrated the many parameters that must be kept in mind when using AVO analysis. Tuning effects may be seen as the moving of tuning thickness down the chart in the visualization. Noise amplitudes can be confused with true amplitudes or random nose attenuation. In an isotropic earth, P- and S-wave velocities are independent of angle. However, anisotropic velocities depend on three different angles. Fine horizontal layering, as in shales, can cause vertical transverse isotropy in which velocity dependence is transverse to the vertical axis. Equations for full anisotropy are quite complex. It is at times difficult to distinguish the effects of anisotropic and other non-hyperbolic NMO events in layered reservoirs. The discussion of anisotropy as it effects seismic interpretation cited a number of studies and the differences in results.
Part 4—Inversion of AVO attributes Elastic Impedance inversion uses a combination of AVO attributes and post-stack inversion to avoid some of the classical problems in post-stack inversion. Elastic impedance inversion is considered one of the newest methods
Summary The theory and practice of amplitude variations with offset (AVO) techniques were discussed and illustrated in detail in the workshop. The notebook provides a brief overview of the workshop and includes all the illustrations. The work-
shop notebook provides a number of methods and shows the equations to derive and implement the techniques. It describes how each new improvement in the technology has built on previous research. The notebook includes references to a number of critical papers in the development of AVO technology and papers using the various methods in case studies.
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For information on PTTC’s Eastern Gulf Region and its activities contact: Ernest A. Mancini, Professor of Geology, University of Alabama, Box 870338, 202 Bevill Building, Tuscaloosa, Alabama 35487 Phone 205-348-4319, Fax 205-348-0818, Email
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