A geophysical Ground Penetrating Radar (GPR) survey is a non-invasive investigation used to identify and map features beneath the ground surface.

GEOTEC regularly uses GPR to support buried-services and utility-mapping surveys, but the technology can detect much more than utilities.

GPR transmits radar energy into the ground and records reflections created by contrasts in subsurface materials. These contrasts can indicate changes in material, moisture content, ground condition or the presence of buried objects and structures.

GPR surveys can detect and help map a wide range of buried objects, structures and changes in ground conditions.

Depending on the site conditions and survey equipment, potential targets include:

• Buried services, utilities and drainage
• Buried tanks
• Changes in construction material
• Disturbed or unconsolidated ground
• Voids and cavities
• Saturated ground
• Cellars, basements and vaults
• Graves and crypts
• Foundations and former building lines
• Air-raid shelters
• Tunnels and other subsurface structures

GPR results are interpreted by experienced surveyors because the strength and clarity of a response depend on factors including the target material, size, depth and surrounding ground conditions.

The depth achievable with GPR depends on the antenna frequency, survey system, ground conditions, target size and the level of detail required.

GEOTEC selects the antenna, data-collection method and processing software to suit each application. Typical examples include:

• A 1.6GHz StructureScan Mini can investigate approximately the upper 450mm while providing excellent resolution for shallow applications such as concrete analysis and reinforcement scanning.
• 350MHz or 400MHz antennas are commonly used to detect and map buried services and utilities, often to depths of around 2m below ground level where conditions permit.
• A 100MHz antenna fitted to a suitable system may theoretically investigate to approximately 10m. GEOTEC has achieved penetration to 8m under suitable site conditions.

Depth cannot be guaranteed before a survey because conductive soils, moisture, made ground and other site conditions can restrict radar penetration.

A GPR survey is useful whenever more information is needed about features that may be concealed beneath the ground or within a structure.

Common reasons for commissioning a survey include:

• Before breaking ground when the position of buried services and utilities is uncertain
• When records, another survey, a desktop utility study or anecdotal information indicate that a subsurface feature may be present
• Where changes in ground colour, level or vegetation suggest a buried feature, depression or possible sinkhole
• Following a water leak or another event that may have removed soil or created underground voiding
• When a buried tank, foundation or other feature is known to exist but its precise location or depth is uncertain
• Whenever a non-invasive investigation could reduce uncertainty about below-ground conditions

Yes. GPR can detect plastic pipes, drainage runs and other non-metallic services where they create a sufficient contrast with the surrounding ground.

The likelihood of detection depends on factors including the pipe diameter, material, depth, contents, surrounding backfill, soil conditions, surface access and the antenna used. Because results are site-dependent, GPR is normally selected and interpreted as part of a wider survey methodology rather than relied upon in isolation.

Yes. GPR is one of the principal detection methods used during PAS 128 utility mapping surveys.

The appropriate antenna configuration and survey-grid spacing are selected in accordance with the required survey methodology and site conditions. GPR data is normally combined with electromagnetic locating and other evidence to produce a more complete and reliable understanding of buried services during an underground utility survey.