Interpolation issues

Dear FF++ developers and participants,

I have created FF++ code for the 2D wave equation in a square with a hole,
and save the values of the solution for each point of the mesh, and for each time of the forward problem simulation. (The results from the forward simulation are correct).

Then I create a code for the backward problem, (by backward I mean in time),
again for the 2D wave equation in a square but with a larger hole.

Since the number of DOF in the backward problem is lower than in the forward problem, in the backward code I use interpolation to “pass” the values of the forward solution to the backward scheme at specific locations, namely in the locations of the sensors.

Unfortunately, during the backward simulation, the values of the solution are more negative than in the forward simulation, thus I suspect that there is an error in my backward code, but I can’t find it.

I would like to ask please,
are there any issues using interpolation in FF++ on a 2D domain?

I enclose my FF++ codes for the forward and backward simulations.

Forward problem.edp.edp (2.2 KB) Backward TRAC in the right place.edp (4.2 KB)

Thanks in advance,