Graph of GPR Survey Data of Buried Air Ducts
The
arrows on the image above point to two very distinct hyperbolic curves.
These curves indicate the locations of the two air ducts below the concrete
slab. The data was collected in a straight line pass perpendicular to
the direction of the air ducts. The radar wave reflects off of the objects
and is plotted in time, where time is correlated to depth. As the GPR
unit approaches the target, the radar wave travel time decreases until
the unit is directly over the object. This is where the shortest time
interval is recorded indicating the shallowest depth. Then as the unit
moves away from the object the travel time increases producing the curves
you see in the image. Through interpretation of the data, the
object centre is located along with its depth below the surface.
Ferroscan
The Ferroscan
can locate ferrous materials(i.e. structural steel such as re-bar and
post-tension cables) in concrete to an effective depth of 150 mm or 6
inches. This instrument can give a good approximation of the depth of
an object as long as it is within the top 100 mm of concrete. The diameter
of re-bar or cable can be ascertained accurately within the top 50 mm.
Metal conduit can also be detected as long as it is near the surface and
there is not too much interference from structural steel. The display
is in plan view and can be printed through a printer or downloaded to
a floppy disc.