Acoustic ping test

A forgotten metal garden trowel was wedged blade-down between two damp stepping stones. The hollow tubular steel handle pointed skyward like an open vertical conduit. I mounted the rim of the stone slab, picked up a rounded granite pebble with both hand clamps, and positioned myself directly above the opening. Dropping the stone down the metal bore produced an immediate, sharp metallic ricochet followed by a resonant ping from the buried spade head below. Acoustics and structural depth verified.