Your numbers are likely off.
I got a good idea of the muck depth in my pond by probing the bottom. I put together a length 1" PVC and lowered down till I felt resistance, then leaned into it, till I felt solid ground.
Not high tech, but it seemed to work repeatably.
The best way to reduce the muck is aeration. It keeps the good bacteria that digest muck alive. A good way to boost the process is adding Muck digesting pellets. This is a better way to go than digging out the pond.
