One of the primary goals of amplitude interpretation is to determine whether a water-saturated rock or a hydrocarbon-saturated rock generated the reflection of interest. In order to accomplish this task, an estimate of the difference in rock properties between the water-saturated and hydrocarbon-saturated states is required. Thus, a few basic relationships of rock physics are necessary. There are both empirical and theoretical relationships between seismic rock properties and elastic constants that will be called upon, as well as wave-propagation models.
The work presented in these notes has drawn heavily from two excellent references. The first is a tutorial article by Castagna et al. (1993): “Rock Physics—The Link Between Rock Properties and AVO Response.” The second is a book for those who want detailed solutions to various rock-property transforms but don’t want to wade through the messy math. The book is written by Mavko et al. (1998): The Rock Physics Handbook—Tools for Seismic Analysis in Porous Media. In addition, the two SEG reprint volumes, Seismic Acoustic Velocities in Reservoir Rocks, compiled by Wang and Nur (1992), provide easy access to classic articles on petrophysics.
Before a theory can be formulated for wave motion in a medium, a relationship between stress and strain is needed. For waves of infinitesimal amplitude, Hooke’s empirical law supplies this relationship. The three most commonly used elastic constants to quantitatively describe the strength of a body are the shear (μ), bulk (K), and Young’s (E) moduli. The cartoon in Fig. 2.A.1 illustrates the hypothetical experiments that measure these elastic constants.
Figures & Tables
During the last 30 years, seismic interpreters have routinely applied bright spot and AVO technology for recognizing prospects and predicting lithology. New amplitude attributes were added to this technology as new exploration problems were defined. R&D continues in the field of amplitude interpretation, especially when E&P costs escalate as more severe environments are explored, such as the ultra-deepwater plays. With the high interest in reducing exploration risk, this course addresses the methodology of an amplitude interpretation and the subsequent benefits and limitations that one can expect in various rock-property settings. This book, originally produced for use with the fourth SEG∕EAGE Distinguished Instructor Short Course, begins with a review of relationships between rock properties and geophysical observations. Practical problems illustrate the assumptions and limitations of commonly used empirical transforms, and procedures for conducting and verifying fluid-substitution techniques are presented. The book identifies components of the seismic response best suited for differentiating pore fluid from lithologic effects. Field examples emphasize what combination of seismic signatures should be expected for different rock-property environments. To help select the best seismic attribute for calibrating amplitude to rock properties, rules of thumb are provided for predicting AVO responses and interpreting lithology from observed responses. A case history is also provided. The last part examines the numerous amplitude attributes that can be extracted from seismic data to quantify an interpretation. Benefits and limitations of these attributes in soft- to hard-rock environments are discussed with model data and in case histories.