An engineering firm retained multiVIEW to perform a geophysical survey. The objective of the survey was to map the location of possible voids beneath the concrete floor slabs in four small tunnels situated in the basement of a large office building.
The survey was performed using a 1000 MHz ground penetrating radar system. The ground penetrating radar system collected high resolution profiles of the subsurface using a sampling interval of approximately 2 centimetres.
Profile lines were collected at 30 cm intervals along the long axis of each tunnel. Survey grids were installed in each tunnel to maintain positional control of the survey instruments. Two 4 by 4 foot grid scans were also completed (more detailed scans with profile lines collected at intervals of 10 cm in two directions). The profile lines were combined and processed to create plan view maps of the data from discrete depth intervals (depth slice maps).
Figure 1 shows relatively undisturbed subgrade conditions, with an undulating yet regular reflection pattern from the subgrade material. The center of the white band in the undulating reflector likely represents the bottom of the poured slab.
Figure 2 shows a similar, even reflection pattern near the start and end of the profile but the central sections are obscured by small, tightly spaced diffractions. This pattern may be indicative of void space below the slab, with the radar energy reflecting off an irregular surface of subgrade material that has settled below the slab.