An introduction—Rock geophysics/AVO

Abstract

Since the early 1980s, when Ostrander demonstrated in a practical setting how amplitudes could change with offset on gathers due to the presence of gas, the industry has strived to understand, define, redefine, and improve the technololgy we call amplitude variation with offset (AVO). We have known for decades that AVO is controlled by the various rock physics parameters of the earth as we transmit and receive acoustic signals. However, it has only been in the last few years—due to improved aquisition, processing, and interpretation techniques—that we have been able to derive and better understand some of these rock physics parameters. Rutherford and Williams in 1989 defined three classes of AVO (Castagna and Swan adding another class a few years later) and the industry now routinely discusses AVO in terms of these classes 1, 2, 3, and 4. AVO interpretation has evolved from just comparing modeled gathers to real gathers, to include approaches such as AVO inversion, elastic impedance, AVO attribute crossplots, lambda-mu-rho analysis, and …