Geological survey

What the survey finds

When you need a geological GPR survey

Boreholes and trial pits tell you exactly what is in the ground, but only at the point where they are dug. A GPR survey shows how the ground changes between them. It is used:

• to find the depth of bedrock before foundation or piling design
• to map soil and rock layers across a site
• to locate the water table in suitable ground
• to profile river and canal beds, with some penetration into the sediment below
• to plan where boreholes will give the most useful information

What the survey finds

GPR records reflections from boundaries between materials with different properties. Bedrock, gravel over clay, and the water table all produce clear reflections in the right conditions. Following these reflections along survey lines builds up a picture of the ground across the site.

Over water, GPR can map the bed of a river or canal and see a short way into the sediment beneath. A sonar system only shows the bed itself. This works in fresh water only, as salt water absorbs the signal.

How it is done

For deep layers we use low-frequency antennas, brought in for the job. These see much further into the ground than the antennas used for services or concrete, but they show less detail near the surface. In the right ground, they can reach tens of metres. They are suited to large features and layer boundaries, not small objects.

We run survey lines across the site, positioned with survey equipment. The data is processed and interpreted in the office and presented as profiles and plans in CAD with a report. Wherever possible we calibrate depths against existing borehole logs.

Examples of our work

• Bedrock under a hillside cutting. GPR showed bedrock about 2 m below the surface, shallower than expected. That told the engineers how much of the slope needed pinning.
• Old landfill sites. For a client deciding which disused landfills were worth reclaiming, the survey reached about 11 m. It could not pick out the base of the landfill, but it gave a minimum depth of fill, so the client could calculate a minimum volume.
• Checking for karst. Fields were scanned with a dual-frequency antenna for dissolution features. There was no void within the survey area, but a sinkhole was located to one side of the site.

Limits you should know about

Depth depends heavily on the ground. Dry sand, gravel and many rocks allow deep penetration. Clay, wet silt and saline ground absorb the signal, and in thick clay GPR may only see a metre or two. GPR shows where layers change but cannot identify the material. Boreholes or trial pits are still needed to confirm what the layers are.

Why use GPR

• Non-intrusive, with no drilling or digging.
• Continuous coverage between boreholes.
• Can reduce the number of boreholes needed.
• Gives a picture of the whole site early in a project.

Questions we're asked