Rapid geophysical reconnaissance techniques such as electromagnetic conductivity and magnetic gradiometery can easily be used to survey areas upwards of 3ha per day. If towed by a vehicle they can cover even more ground.
Met provides the survey and geophysical expertise required to undertaken a landfill characterisation of old sites situated around the UK.
Met Consultancy Group (Met) were called upon to assist in discovering the extents of an old Victorian dump prior to redevelopment of the site. Using a suite of geophysical techniques, the extents of the landfill was successfully mapped and provided information on the depth and internal structure, providing targets for intrusive investigation. Although modern landfill sites are heavily regulated (with landfill gas and contaminating leachates carefully controlled), the situation is not as straightforward for many of the older landfill sites situated around the country. Inadequate records and uncertainty about what is present within these sites means some of them might literally be a disaster waiting to happen. Many of these sites are now grassed over and give little indication for what hazards lie below. Using non-intrusive techniques, Met can:
• Map out the vertical and horizontal extents of landfill
• Obtain vital information on the areas involved, and
• Calculate the depth of deposits and how the landfill is internally structured.
Because these initial techniques are non-intrusive, there is no risk of breaching or harming the landfill, and it avoids any safety issues associated with contamination. Met were asked whether geophysics could help in finding out more information on some land proposed for redevelopment. An old landfill was known to exist on the site, however there were limited details about the landfill as historical maps documenting the evolution of the land from a clay pit to rubbish dump did not provide clear information, and the precise location and extents of the landfill was unknown. Using an integrated approach it was possible to delineate the landfill dimensions. Many geophysical techniques work by detecting contrasts in the physical or chemical properties of materials. Because the deposits within a landfill are generally so different to the surrounding natural material, these changes in ground properties enable the horizontal and vertical extents of the landfill to be identified. An electromagnetic survey located the extents of the landfill in plan, and vertical profiling using electrical resistivity tomography provided depth information. This was then calibrated using ground-truthed information from a borehole and a trial pit to provide accurate depth and deposit information across the whole site.