Data was taken over a concrete slab with wire-mesh reinforcement. This data is clear, revealing no void.

The right side of this data reveals a void 6" from the surface. There is a high level of reinforcement above the void. The void is indicated by the horizontal black-white-black presence.

This data shows the early stages of a void. In this case, the void was beginning to occur about 6 feet from the exterior wall and proceeding towards the foundation. The sub-grade material was not properly compacted, causing the void.

Data taken on a pool deck reveals a void immediately below the concrete slab. The rebar is on 18" centers.

The difference between grouted cells and hollow cells in a masonry block wall:

The grouted cells have a rebar about 4 inches from the surface and a grey contrast throughout the cell. The hollow cells show a black-white-black reaction near the top of the profile.