Ground Probing Radar

Why use GPR?

A ground probing radar (GPR) survey is often the only geophysical technique that can be used in engineering and environmental applications on busy urban and industrial sites. It is capable of working through a wide variety of surface materials both inside and outside buildings and structures.

Typical locations can range from car parks, highways and waste ground through to floors of factories, basements and sub-structures.

 
 
GPR survey of a leaking swimming pool

How it works


The radar system works by transmitting a pulse of radiowaves at a set central frequency into the material being examined. A receiver is set up to scan for returns from the transmitter. The receiver is set up to scan for returns from the transmitter, normally 40 scans per metre. These scans produce a scrolling image on a video display as the antenna is moved across the search area.

The success of GPR relies on variations in the velocity of tranmission of radio waves in different sub-surface materials. These variations are determined by the dielectric constant of each material. The greater the velocity change at each sub-surface interface, the greater the amplitude of the response.


Reconnaissance or Detail


If the area of interest is a fairly large site with little information available, it may be more cost effective to undertake a reconnaissance survey with survey lines every 5m or 10m. Areas of potential interest can be targeted for intensive orthogonal follow-up surveys.

If however only a small area is to be inspected eg 25m x 25m, it would be possible to survey this intensively using a 1m orthogonal grid. This could be carried out within one days fieldwork which is normally the minimum charge for a survey team. The relative close centres for the transects allows interpretation of the data to be made with confidence so providing a real understanding of the sub-surface. A further advantage of using 1m centres is that time slice plots can be generated.

Sites that are known or are suspected to contain small and extremely complex features such as a reinforcing within a slab and environmental applications such as mapping a badger set, may require an even more detailed survey. Typically for sites like these we would deploy the radar over an orthogonal 0.5m or 0.25m grid.

 

Uses for GPR
 

Air voids

  • Vaults, chambers
  • Tunnels
  • Caves
  • Mineshafts
  • Subsidence investigations
  • Solution features

 

 

 

  
 
An example radar section over a brick lined culvert
 

Service Locations (see also the section on utilities)

  • Pipes
  • Cables
  • Trenches
  • Culverts
 
 
A radar section of 5 services
 

Structures

  • Foundations
  • Wall inspections
  • Floor examinations
  • Pile obstructions
  • Landfill materials
  • Reinforced concrete inspection
  
 
The remains of a buried brick wall at a depth of 0.8m
 

Geological

  • Stratigraphy determinations
  • Mapping of solution features
  • Material thicknesses
  • Bedrock determination
  • Subsidence mapping

 

  
 
A section displaying 3 stratigraphic horizons.
 
General characterisation of features
  • Depths, sizes and extents
  • Metal obstructions
  • Drum locations
  • UXO's
  • Piles

 

  
 
A radar section highlighting the position of a concrete pile on the route of a tunnelling device

 

Archaeology

  • Detection of buried foundations
  • Ditches
  • Pits
  • Tombs and vaults
  • Graves
 
 
  A medieval ditch