Geological Surveys

Ground Penetrating Radar (GPR) can be used for geological surveys to detect features such as the bedrock, water table, and stratigraphic layers. In these cases the GPR frequencies in typical use in the UK, (200MHz and higher) are not suitable and a lower frequency system should be used.


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GPR survey to detect bedrock, water table or stratification of the ground

For the detection of the bedrock, water table and the stratification of the ground, it is possible to buy or source commercially available GPR down to frequencies as low as 25MHz: such GPR have very poor surface resolution and are of little use for typical applications, but they can detect very large features and interfaces (layers) which are located deeper in the ground.

Typical examples of a use for a very low frequency GPR include the detection of the bedrock, the water table, stratification of the ground, and karsts.

GPR survey to detect bedrock, Abersoch, Wales

Due to building works necessitating a cut into a hillside, a GPR survey was requested to locate the depth from the surface to the bedrock, this would allow the engineers to decide to what extent the ground needed to be pinned to prevent future landslides.

GPR bedrock survey results

We attended site and collected a series of cross sections which saved for office based processing and compilation into a report and a table of offsets. Because the surface resolution of such a low frequency antenna is poor, the results were challenging to produce, however we identified the bedrock approximately 2m below the surface (shallower than expected) which was sufficient information for the engineers to work from.

GPR survey of disused landfill sites near London Heathrow Airport

There are many disused landfill sites scattered around the UK. At each of these, the thickness of the waste can vary from just a few meters to tens of meters deep. In the past the sites would have been left open and once the land had been filled to capacity the waste was compressed and capped.

Over the years many of those landfill sites have been left unmanaged and become scrub land, whilst the surrounding areas have been developed. As land prices increase, it has become economical to reclaim a disused landfill, clean the waste, and develop it.

Our client had speculatively purchased some of those sites and was looking for a method to cost effectively determine which ones had the most development potential. Sites with a relatively thin layer of waste could be cleaned much more easily and economically than sites with a thicker layer of waste.

GPR survey to detect deep layers at landfill site near Heathrow Airport

We achieved a maximum penetration of 11m but were unable to positively identify the bottom of the landfill. Although we were unable to answer our customers ultimate question of how deep is the landfill, we could inform them that the depth of fill was greater than 11m and using the area of the site, our customer could calculate a minimum volume of fill and therefor a minimum cost to reclaim the land from which they could make an informed decision about further site investigation.

100MHz GPR for the detection of bedrock and layers

100MHz GPR for the detection of bedrock and layers

The bedrock was expected to be found within 3-5m of the surface, which in the UK is usually beyond the penetration of a 200MHz GPR (despite manufacturers advertising otherwise) so KB GPR Surveys proposed the use of a 100MHz GPR to ensure that the required penetration was achieved. In this case the antenna was a large, ground coupled antenna with an external wheel that was pulled on straps.

25MHz Ground Penetrating Radar for detecting deep layers

25MHz Ground Penetrating Radar for detecting deep layers

After consultation with our client, we proposed to use a very low frequency, 25MHz GPR which is designed for detecting layers in geological applications. The GPR itself consisted of a pair of large dipoles mounted on a fiberglass frame. The electronics were transported in a backpack and connected to the dipoles by cables. Instead of a wheel for triggering, a piece of cotton was attached to an object at the beginning of the survey line and unravelled from a reel mounted on the surveyors hip as the GPR moved forwards (carried by hand). This allows the survey to be conducted over scrub without clearing the land first.

What is underneath bedrock?

The Earth is made up of layers. The top layer, the crust, is formed of organic matter, topsoil, subsoil, parent material, and bedrock. Below bedrock is the Mohorovicic Discontinuity, a boundary between the crust and mantle. Below this is the upper mantle, then lower mantle, outer core, and inner core.

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