- Copyright © 2003 Society of Exploration Geophysicists
Azimuthal AVO (AZAVO) refers to the directional variation of P-wave reflection amplitude at a fixed offset. There are several possible causes for this azimuthal amplitude variation: reservoir property, structural change of the reflector, near-surface effects, overburden property variation, acquisition footprint, processing artifacts, source/receiver radiation/reception patterns, etc. While paying close attention to all these causes is necessary for meaningful azimuthal AVO analysis, geophysicists are most interested in the azimuthal amplitude variation caused by a particular reservoir property: fracturing of the reservoir. A decade of theoretical study and some well-documented case histories have brought widespread acceptance that aligned open fractures will introduce azimuthal anisotropy to the host rock and can therefore be detected from analyzing P-wave reflection amplitude variation as a function of azimuth.
Analysis of fracture-induced anisotropy in reservoirs requires 3D seismic data with suitable offset and azimuth distribution. Wide-azimuth data and longer offset will in general increase the chance of success for azimuthal AVO analysis. However, because of the associated cost in collecting/processing/analyzing these “wide” azimuth data, a quantitative assessment of acquisition adequacy becomes necessary to minimize the acquisition effort while achieving the goals of azimuthal AVO analysis for fractures. In this article, a method of quantifying the adequacy of a planned or completed 3D survey for azimuthal AVO analysis for fracture orientation and intensity is described. This new method is actively used by ExxonMobil operating companies for business and technical evaluation of azimuthal AVO potential of their planned 3D surveys and existing 3D data.
The P-wave reflection coefficient at the interface between an isotropic medium and an azimuthally anisotropic medium depends on both the offset and azimuth formed by the source-receiver pairs. The simplest azimuthal anisotropy medium is an isotropic host rock with a single set of open and aligned vertical fractures. This type of medium has been referred …