Summary
Traditional amplitude variation with offset and azimuth (AVOAZ) analysis for fracture characterization extracts fracture properties through analyzing the characteristics of the reflection amplitude variation with offset and azimuth. Validity of the method relies on its basic assumption that a fractured unit can be viewed as an equivalent anisotropic medium in the vicinity where reflection occurs when the spacing between adjoining fractures is sufficiently small compared to seismic wavelength ?. As a rule of thumb, this assumption is taken to be valid when the fracture spacing is less than ?/10. In fracture characterization using AVOAZ, spatial variation of the fracture properties, such as fracture orientation and spacing, are estimated from the anisotropic parameters (e.g. Thomsen’s parameters) that are inverted from amplitude variation with offset (AVO) or amplitude variation with azimuth (AVAZ) analysis. Under the effective medium assumption, diffractions from individual fractures are destructively cancelled and only specular reflections from boundaries of a fractured layer can be observed in seismic data. The effective medium theory has been widely used in fracture characterization and its applicability has been validated through many field applications. However, through 3D numerical simulations, we find that diffractions from fracture clusters can significantly distort the AVOAZ signatures when a fracture system has irregular fracture spacing even though the average fracture spacing is much smaller than a wavelength (e.g., <?/10). Contamination of diffractions from irregularly spaced fractures on reflection can substantially bias the fracture properties estimated from AVOAZ analysis and may possibly lead to incorrect estimations of fracture properties.
Introduction
The linear slip model of Schoenberg (1980) can be used to represent individual fractures, whose elastic property is determined by the fracture compliance matrix that is a function of fracture surface geometry and infill material properties (Schoenberg and Sayers 1995; Brown and Fang 2012). We simulate the influence of fractures on seismic wave propagation using the effective medium model (EMM) and the discrete fracture model (DFM) separately (Yang et al., 2005). In EMM, a fractured layer is treated as a homogeneous anisotropic layer whose elastic properties are calculated from the given background rock properties, fracture density and fracture compliance (Schoenberg and Sayers 1995). In DFM, individual fractures are modeled as imperfect slip interfaces embedded in the background formation using a finite-difference approach (Coates and Schoenberg, 1995). We investigate the limitation of EMM in AVOAZ analysis through comparing the results obtained from the two different models.
