Empirical relationships between transverse isotropy parameters and Vp/Vs: Implications for AVO

Abstract

The transverse isotropy (TI) parameters ε and δ control amplitude variation with offset (AVO) response at most angles of incidence used in exploration, although the value of δ is not usually known and is difficult to measure. Published measurements on TI materials show that there is a useful empirical correlation between [Formula: see text] and δ. The relationship between δ and [Formula: see text] can be simplified by assuming a linear relationship between [Formula: see text] and [Formula: see text]. Anellipticity parameter η also shows a useful empirical correlation with [Formula: see text]. The correlations imply that knowledge of [Formula: see text] is sufficient to make an estimate of the anellipticity of the P‐ and S‐wavefronts in a rock, regardless of the lithology. In this way, the effect of TI on the AVO response of a particular interface may be estimated in the absence of any more accurate data. The empirical relationships indicate that rocks tend to become more anelliptic with increasing [Formula: see text]. Rocks with [Formula: see text] smaller than about 1.8 tend to have zero to small positive values of δ, while rocks with [Formula: see text] larger than around 2 tend to have zero to medium negative δ values. Most previous work has assumed a positive value of δ in shales, but this is not necessarily true. If in fact the δ of a shale is negative ([Formula: see text] is around 2 or higher), and overlies a sandstone, the positive change in δ across the interface could cause a false negative AVO gas indicator. If shale with [Formula: see text] less than 1.8, as measured in organic‐rich and overpressured shales, overlies a water‐filled sandstone, this could cause a false positive AVO gas indicator. However, if the effect of TI can be estimated, then the chances of success for AVO analysis in correctly predicting the presence of hydrocarbons can be increased.