GSSI StructureScan Mini XT Quick Start Manual page 30

StructureScan Mini XT Quick Start Guide Page 26
Potential Void under Slab
This image shows a wire mesh reinforced slab.
Note the heavy black band below the wire mesh
and shown by the orange arrows. The black band
results from a pronounced high to low dielectric
change as in concrete to air. While we do not
know how thick the void might be we can tell the
starting and ending point by locating where the
black band starts and ends. Once we know the
location, the next step is to drill a small hole in
the concrete for confirmation.
When searching for voids behind slabs it is best
to collect a long file so that any change (such as
the black band at the right) will be more readily
visible.