What sensor can detect "knots" in the thickness of plywood?

Question

Sensors

They gave me an interesting technical task for reflection:
It is required to determine the coordinates of the "knots" in the plywood sheet: D
plywood 1.5x1.5 meters
exactly in the thickness of the sheet, there were no problems from the outside.
thickness maximum 6 mm.
The density of knots is 2-3 times greater than that of the plywood itself.
The size of knots is from 1x1 cm. In a sheet, a maximum of 5 pieces can be scattered over the entire area.
Do not offer X-ray =)
It's just for playing and thinking, so I'm writing a question here, all of a sudden I'll pick something up.

Answer 1

Something to look at with ultrasound, I think ...