The aim is calculating the damping ratio by employing the G-reduction curves posited by Seed and Idriss (1970), and Sun et al. (1988) for sand and clay, consecutively.
It is possible for approximate estimation. You can go like this:
If you already have a curve for G-reduction you may try to fit it to a empirical model such as those by Ishibashi & Zhang (a simpler one) or Darendeli (much more complex). For example the Ishibashi & Zhang model constructs the G-reduction curve from two parameters - effective confining pressure and Plasticity index. The damping curve is constructed using the G-reduction. This will give you insight about the best estimated (mean) curves only. If you use Darendeli's model you may also introduce uncertainty.
Thanks a bunch Stoyan. Could you please elucidate a bit more coz I could not get what you were striving to say with due attention to the procedure whereby the damping ratio curve can be plotted based on the G-reduction curve?