Old sewers or subsurface canalised streams may be damaged and hence lose water. Whereas the proper damage normally can be detected by video inspections, the consequential damages like washouts and cavitations often remain undetected until dangerous collapses crop up.

GPR is a superb non-destructive method of detecting those phenomena early enough to avoid dangerous accidents. That applies as well to areas of very low compactness (figure at the right).

200 MHz dataset showing a "void" of about 25 m length