Dictionary:eta, η

1. Electric susceptibility (q.v.).

2. One of the Thomsen anisotropic parameters (q.v.) that captures the deviation of long-offset P-wave moveout from what it would have been for isotropicity:

$\eta ={\frac {\varepsilon -\delta }{1+2\delta }}$

where $\delta$ (delta) and $\varepsilon$ are the other Thomsen anisotropic parameters.