On conditions for long-wave equivalent medium to be isotropic and on analysis of parameters indicating anisotropy of equivalent TI medium

Abstract

In this paper, we consider a long-wave equivalent medium to a finely parallel-layered inhomogeneous medium, obtained using the Backus average. Following the work of Postma and Backus, we show explicitly the derivations of the conditions to obtain the equivalent isotropic medium. We demonstrate that there cannot exist a transversely isotropic (TI) equivalent medium with the coefficients $c^{\overline{\rm TI}}_{1212} \neq c^{\overline{\rm TI}}_{2323}$, $c^{\overline{\rm TI}}_{1111} = c^{\overline{\rm TI}}_{3333}$ and $c^{\overline{\rm TI}}_{1122} = c^{\overline{\rm TI}}_{1133}$. Moreover, we consider a new parameter, $\varphi$, indicating the anisotropy of the equivalent medium, and we show its range and properties. Subsequently, we compare $\varphi$ to the Thomsen parameters, emphasizing its usefulness as a supportive parameter showing the anisotropy of the equivalent medium or as an alternative parameter to $\delta$. We argue with certain Berryman et al. considerations regarding the properties of the anisotropy parameters $\epsilon$ and $\delta$. Additionally, we show an alternative way---to the one mentioned by Berryman et al.---of indicating changing fluid content in layered Earth.