![Embarassed :oops:](./images/smilies/econ_embarassed.gif)
It turns out an old fashioned stud finder works wonderfully for this
Probably obvious, but I don't remember it mentioned on TNTTT, so thought I would.
Our bulkhead is 1/8" Baltic birch over 3/4" plywood and blue foam. The stud finder measures electrical capacitance (I think). I clicked on it on an area I was confident was over foam, and the transitions to the plywood regions were crisp and clear.
Tom