There are many subsurface voids that might represent engineering concerns – washouts or settlement gaps beneath roadways or floor slabs, ungrouted masonry cells, bubbles in concrete pours, tunnels, mine workings, vaults, natural limestone (karst) caves or solution cavities, and incipient sinkholes or soil pipes. All of these can be detected and delineated using nonintrusive geophysical techniques.

Large volume voids such as significant washouts, mine workings, tunnels, and karst-related cavities are excellent targets for microgravity surveys.  The “missing” mass of the void creates a measurable disturbance in the earth’s gravitational field, with the magnitude of the disturbance directly proportional to the volume of the void.  This relationship has allowed Enviroscan to use microgravity surveys to assist engineers in the effective design and accurate cost estimation of grouting programs. Other examples of Enviroscan’s application of microgravity are mapping of undocumented mine workings, location of abandoned or clandestine tunnels, and delineation of areas of unsupported floor slab. Since gravity surveys are immune to many common sources of electronic or acoustic noise that can impede many geophysical techniques, they are particularly suited to highly developed or industrial sites.

Smaller shallow voids can be detected using ground penetrating radar (GPR) or subsurface interface radar (SIR). Deeper voids are sometimes best detected and measured using seismic reflection.  Deep, water-filled cavities (e.g. caves or tunnels below the below the water table) are particularly good targets for seismic shear wave imaging since water (or any fluid) cannot transmit shear waves - causing water-filled voids to appear as characteristic “blank spots” or shadows on a shear wave record.

Where boreholes are available, Enviroscan can perform crosshole seismic or electrical tomography.  Beneath water, sub-bottom sonar, as well as gravity and sometimes radar can be used to detect voids such as tunnels or bridge pier scour features.

In some situations, electrical imaging may be used to provide cross-sectional imaging of potential voids.  Great care must be taken in interpreting electrical anomalies as voids, however, since a void or competent rock (both resistive) may appear identical on an electrical image, but represent very different engineering concerns.

Detection of voids in or under structures is a specialty of Enviroscan’s.  We have used radar to detect:

• washouts beneath the floors of buildings, vaults, basins, and pools
• ungrouted vs. grouted concrete masonry unit (cmu) cells
• utility tunnels or pipe chases in or below floors
• voids or bridged zones in poured concrete
• hidden rooms or compartments.

Whether they are detected using gravity, radar, sonar, seismic methods or electrical imaging, Enviroscan provides accurate mapping of the footprints of voids, as well as cross-sectional imaging (i.e. depth and dimensions) and estimates of void volumes where required. 

For some voids (e.g. underground storage tanks or karst solution cavities), it is important to know whether the void is filled with air, water, petroleum, mud, etc. Often, by combining two geophysical techniques (e.g. GPR plus gravity, electrical imaging, or seismic shear wave tomography), it is possible to detemine the contents of a void.