Sinkholes are voids in the ground formed by water erosion of the underlying ground—serious damage may result. Sinkhole detection and location is, therefore, very important.

Sinkholes are most common in limestone or other rock areas which water can easily dissolve. The water can be natural water circulating through the ground or from a burst mains water pipe or sewer. As the void gets bigger, a point is reached when the land cannot support itself, and collapse occurs.

In urban areas, sinkholes can damage buildings and roads and cause collapse. Therefore, it is important to detect and locate sinkholes and voids as early as possible before damage occurs.

Detecting sinkholes

GPR is an excellent way of detecting and mapping out the extent of sinkholes and subsurface voids; however, site conditions may affect its effectiveness. For example, the geology may affect the survey, or wet ground conditions may limit the signal penetration depth.

Site survey

GPR is a noninvasive and nondestructive method of surveying the subsurface, ideal for sinkhole detection and location surveys.

We typically use a GSSI dual-frequency GPR system with a digital dual-frequency antenna utilising central frequencies of 300 MHz and 800 MHz. Scanning with several different frequencies enables us to cover a wider depth range at the best resolution.

The following photograph and data are from a survey to locate sinkholes and general voiding in an area of asphalt pavement and carriageway. We scanned the area of interest on a regular orthogonal grid; the “tramlines” in the snow are from the GPR antenna traversing the area on the regular grid; we work in all weather!

'Tramline' markings in the snow from the GPR antenna
'Tramline' markings in the snow from the GPR antenna

Sinkhole location survey results

The GPR survey successfully detected a sinkhole and determined its extent.

GPR radargram showing detected sinkhole
GPR radargram showing detected sinkhole

The GPR radargram extract above shows an annotated radargram confirming the extent of a detected sinkhole.