Beyond AVO: Examples of Elastic Impedance Inversion
Authors: Cosban Terry , Helgesen Jan , Cook Doug 
Published: October 2008

Elastic inversion provides an estimate of elastic parameters by the use of Amplitude Variations with Angle data. This information gives the geoscientist additional parameters for the interpretation of lithology, porosity and fluid type. By applying a stratigraphic inversion method to the angle volumes the seismic-wavelet effect is minimized thus reducing the likelihood of wavelet variations within the gathers causing false AVAs. Additionally inversion unravels both the amplitude shape and strength into an interval attribute as opposed to reflectivity that is an interface attribute. This allows a volumetric interpretation of the data producing more accurate understanding of the reservoir volume.

Introduction

Seismic attributes play a critical role in the efforts to reduce risk associate with reservoir prediction and description. Post stack inversion has proven to be a significant tool that allows geologic calibration and a volumetric understanding of the impedance data. This method allows the interpreter to go beyond contact event interpretation and begin understands the amplitude strength and wavelet shape and its relationship to the vertical and lateral impedance profiles. This begins the understanding of the spatial geologic parameters.

Acoustic impedance alone may not always be enough to describe the geologic parameters in complex environments. Acoustic impedance is the result of the Pwave velocity and density. The Pwave velocity is a function of the mineral constituents, the manner in which they are arranged and the pore volume fluid. In cases where two of these unknowns are constant the third parameter may be interpreted. However in cases where these are not constant ambiguous results will occur. When attempting to relate geologic parameters such aslithology, porosity, and fluid type in complex environments additional parameters related to the rock physics must be utilized. To provide these attributes elastic impedance inversion is used. When these additional elastic parameters are used with acoustic impedance they offer a strong tool for increasing the quality of the interpretation of lithology, porosity, and fluid type. Fig.1 shows the crossplot relationships of common lithologies and fluid types.

Fig. 1 Crossplot of P-wave velocity and Vp/Vs for various lithologies and fluids.(Available in full paper)

Method Overview

A common practice of the AVO analysis is to obtain the intercept and gradient from prestack data. These correspond to A and B terms in Shueys equation.

R(?)= A + Bsin2 ? + C sin2?tan2?

After these two parameters are calculated they may be transformed into parameters such as P reflectivity, S reflectivity, and Poisson's reflectivity. Although valuable attributes to assist in the interpretation of fluid type this method does not address the variations in wavelet and NMO stretching as a function of offset. In a previous paper by Cambois1 it was shown that variations in the wavelets may produce a false avo phenomenon referred to as "intercept leakage".

To address the wavelet variations Connolly2 developed a method called "elastic impedance". This method involves inverting the angle volumes for the angle dependant elastic impedance. These angle dependant elastic impedances can then be converted to P and S impedance.