Is there a risk of sub-surface voids and subsidence on your site?
- Claire Rose
- 6 days ago
- 2 min read
Updated: 5 days ago
There are numerous geophysical techniques that SUMO can use to locate voids, areas of loose / collapsed material, geological solution features or specific features such as mine workings, basements, tree roots or drains.
The best technique(s) to use will depend on the site specific conditions and survey objects but can include:
Technique | Target feature |
Microgravity | Voids, shafts, subsidence or solution features, deeper mine workings and adits, karst features and caves |
Magnetics and electromagnetics (EM) | Shafts, voids, areas of infill material, buried tanks, near-surface and historic solution features, near-surface karst features, drains / culverts |
Ground penetrating radar (GPR) | Voids, areas of infill material, tree roots, animal burrows, near-surface shafts, drains / culverts, buried tanks, basements |
Resistivity imaging and seismics | Geological features, such as faults and depth to bedrock. Targeted resistivity imaging can also provide depth information on voids, shafts, subsidence or solution features |
Boreholes and excavations can identify all of the above features but only if they are located directly over the feature. It can often take a number of boreholes to broadly define the extent of a feature and in many cases features / hazards can be missed entirely using just intrusive methods.
A geophysical survey will usually cover an entire site and provides a greater level of information so that intrusive work can be targeted. Geophysics minimises risk and reduces costs!
Case study 1
A ground investigation using boreholes had identified several solution features on a site where there was a history of known subsidence. It would have been very expensive (and increased the risk of further subsidence) to try and define the extents of these by further boreholes and so geophysics (microgravity and resistivity) was used.
The microgravity survey defined the extent of the known solution features but also found several additional areas of voiding or loose material. These features were then targeted by resistivity imaging to obtain depth information.
Above : Microgravity survey.
Above : Microgravity data plot showing anomalies caused by solution features (blue responses).
Above : Resistivity profile (bottom) through the centre of microgravity Anomaly A shows an area of low resistance (void / loose material) extending to significant depth associated with the solution feature.
Case study 2
Redevelopment of a brownfield site. Deep excavations during ground works had uncovered an entrance to an adit but the extent and orientation of the adit was not known. Geophysics using microgravity, electromagnetics and magnetics was used across the site to locate the adit and to help identify mine workings.
Above: Entrance to an adit.
Above: Microgravity data over shallow mine workings showing a broadly linear area of low density material (Anomaly A) interpreted as a void associated with mine workings.
Above: Electromagnetic data showing a low conductivity response (the blue area shown as Anomaly B) which corresponded with the line of an adit.
Ready for a geophysical survey?
Contact SUMO Services today to learn how our geophysical surveying expertise can ensure your project’s success. From subsurface investigations to actionable reports, we’re here to support your modern construction needs.
