Logarithm based PoW problems?

I’m not a mathematician myself but are there any “interesting” logarithm problems that could be applied to a PoW algorithm? Why not?

E.g. Diffie-Hellman key-exchange provides a “hard to calculate, easy to verify” type of problem.

There is Riecoin, which uses slightly different prime algorithm. I personally wonder if it is possible to use Pi as a PoW algorith? But I have not idea about mathematics.