Use of very low frequency Ground Penetrating Radar (GPR) to determine the depth of the fill at a disused landfill site 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 were 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 scrubland, whilst in many cases the surrounding areas have been developed. As land prices increase, it can 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.
Ground Penetrating Radar
After consultation with our client, we proposed to use a very low frequency, 25MHz GPR by IDS which is designed for geological applications.
The GPR itself had a very old school setup and 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 mounted on the surveyors hip as the GPR moved forwards (carried by hand).
How the survey was performed
The survey was performed by walking the GPR across the site, the dipoles were carried on their frame about 30cm above the surface. In practice this allowed it to travel over most of the weeds and scrub rather than being caught upon them and proved to be a very practical implementation. A series of scans were made to take ‘cross-sections’ of the site.
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.
Very low frequency GPR data
The data produced by a 25MHz GPR has a very low resolution and is suitable for detecting large underground features or layers only. Surface resolution is poor, to detect targets in the first few meters, much better results will be achieved using higher frequency GPR.