Thank you very much for your reply. Understood - I was able to set a solid physics region to a PML-type without causing any obvious errors in ANSYS, hence my (erroneous) assumption that elastic PMLs were available.

In the case of a semi-infinite fluid-fluid interface, is it possible then to have two PML regions which contact/couple with one another?, as in the figure above (both upper and lower mediums being fluids). 

Thank you again for your help.