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.