AVO attributes of compressional and converted shear waves

In this article we investigate AVO effects of large offset P-wave data (up to ) and converted waves (up to ). In addition to conventional P-wave AVO attributes ( and ) a third term ( ) is examined. For P to S converted waves we introduce two AVO attributes and . Background trends for certain cross plots are related to rock property relationships. These new attributes are used to discriminate commercial gas, from noncommercial concentrations. AVO analysis on long offset P-wave data, and converted waves, based on real log data, shows encouraging results. Introduction A problem of AVO analysis is its ambiguous nature. Several phenomena leads to similar AVO anomalies. A specific problem in hydrocarbon exploration is discriminating commercial amounts of gas (pay-gas), from non-commercial gas (fizz-gas). Using the usual gradient/intercept ( / ) approach, e.q. Shuey (1985), commercial gas cannot be distinquished from non-commercial gas. Hence it is desirable to find a technique that is sensitive to the gas saturation. In recent years most oil-related seismic surveys have been 3D surveys. But there has also been an increasing interest in obtaining more sophisticated 2D profiles, i.e. longer offset P-wave data. Shuey (1985) and Aki and Richards (1980) approximations are based on the assumption that a linear relation between the reflection coefficient and sin( ) exists for angles of incidence less than . There is an additional assumption that rock property contrast are relatively small (first order approximation). Using long streamers results in angles of incidence up to , so clearly the small angle approximation is not valid. Furthermore, converted S-waves have been recorded using sea bottom cables. New types of data is thus available, and here we will propose how to use these types of data in AVO analysis. Foster, Keys, and Schmitt (1997a) showed a new approach to obtain the intercept/gradient AVO-attributes, and . This approach avoids the small scale contrast approximation and allows any rock property contrast. In this paper we will extend that work to include longer offset P-wave data, introducing a new AVO attribute, , and converted waves, by introducing the AVO attributes and . Defining , and Contrary to the 1st order approximation to Zoeppritz equations used by Aki and Richards (1980) and Shuey (1985), Foster, Keys, and Schmitt (1997a) linearized the Knott Zoeppritz equation with respect to . Using the first two terms of the expansion of the reflection coefficient for pressure data ( ) around they obtained and . This approach allows arbitrarily large changes in the elastic parameters, , , and , across interfaces, and gives exact expression of A and B as function of the elastic parameters itself. We extend this to incorporate the third term, , of the expansion of , and the first two terms, ( , ), of the expansion of the reflection coefficient for converted waves, . Taylor expansions of Reflection coefficients At the interface between two elastic solids the amplitudes of the scattered waves can be expressed as (following Achenbach (1973))