Just want to bump the question back to the top, can the pond be drained and then a sheep’s foot be used? Or would have the sheep foot had to be used when creating the pond from the beginning?
That is a difficult question to answer with confidence. (Just because we don't know the actual circumstances under the dirt.)
How deep is your pond? Some small ponds of 10' or less can be sealed by compacting a 12" clay blanket (usually in two lifts) across the bottom and the banks of the pond.
If your core trench is the main seal of your pond, then that cannot be compacted by running over the top now with a sheepsfoot. It would have to be excavated out, and then put back in as 6" layers with compaction on each layer.
Did you try the "clay test" I suggested from your pond banks? 12" is the recommended thickness of a clay blanket. If your banks have good clay content, and you wet them to the proper water content, then you MIGHT be able to seal the pond with a sheepsfoot at this stage which would probably create about a 6" compacted blanket..
