Ground Penetrating Radar (GPR) can be used to detect the presence and extents of sinkholes, underground voids and subsidence.
Why use GPR to detect underground voids and subsidence
GPR is a fully non-destructive and non-intrusive technology (NDT), it is ideal to survey larger areas and focus attention for remedial works or trial holes in the right places.
GPR works by transmitting an electromagnetic signal into the ground, reflections are caused by changes in the electrical properties of the material the signal is travelling through and those reflections are recorded by the GPR. In GPR, reflections are caused by any change in the below surface environment, GPR is therefor able to detect most types of underground features including voids and subsidence (as long as the voids are large to be detected within the resolution of the GPR being used).
What types of voids can be detected
The minimum size of the void which can be detected will vary depending on the frequency of the GPR being used and the survey requirement.
KB GPR Surveys have had success detecting sink holes as shown in this example, in this case large voids and areas of subsidence were present beneath a surface layer of reinforced concrete. We were able to successfully detect and map them in our post processed GPR survey even though they were not visible in the data on site.
We also have experience detecting shallow voids in concrete and road structures. As well as badger tunnels and chambers, and man-made voids such as underground tanks, basements, culverts, mine shafts, and gas pipes etc.
How a GPR survey to detect underground voids is performed
Although voids can be detected using several different methods, a typical survey would involve attending site and surveying the surface (ground or wall depending on the survey in question) in an orthogonal grid of regularly spaced scans, allowing us to collect data for all of the site with no gaps in the coverage.
Although it can be possible to produce results on-site, in most cases the data will be saved for post processing and interpretation back in the office. This allows us to subject the saved GPR data to different processes and visualise it in different ways to achieve the best results, we also subject all of our results to an internal quality control before issue.
GPR void survey results
Usually results are presented in the form of a drawing, with a supplementary report when required.