Seismic Reflection Amplitude
After loading the seismic data on the workstation, one of the first questions an interpreter asks the data processor is: “What’s the polarity?” The processor assures the interpreter that the final seismic has true-amplitude processing with a zero-phase wavelet, so that a peak represents a reflection from a high-velocity bed. Knowing the complexities that mother earth can introduce, and having a little bit of processing experience, seasoned interpreters are often heard mumbling quotes such as, “It’s your dream, tell it the way you want to.”
But even if the processor has produced his dream section, there are amplitude and phase anomalies that are often not associated with rock type or pore-fluid changes. Once these anomalies are reconciled, reflection amplitudes both on the stack and within CDP gathers can be investigated for rock type and pore-fluid predictions. These predictions from the amplitude interpretation are normally conducted using simplified versions of the reflection-amplitude equation. Which equation is employed depends on what the interpreter is trying to emphasize.
A variety of geophysical receivers are used by the oil industry to measure the particle motion of seismic waves. Likewise, these receivers can be planted on land, towed as streamers, deployed in vertical cables, or planted on the ocean bottom. With respect to polarity, two issues arise: type of receiver, and direction of wavefront.
Particle-velocity geophones and pressure-sensitive hydrophones are the two receivers commonly employed in seismic. Starting with a displacement wavelet, the amplitude relationship between a particle-velocity phone and a hydrophone is shown in Fig. 3.A.1.
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.