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Site investigation Geophysics on the map

Spotlight

The site investigation sector is undergoing a boom thanks to the increase in brownfield development. Here NCE looks at some of the issues facing companies working in this market, and highlights some recent projects

With increased pressure on property developers to reclaim former industrial land, developments in the use of geophysics are proving to be vital and cost effective tools for non-invasively identifying buried hazards. Cardiff-based TerraDat recently completed such as study for Bristol-based Westmark 'Geophysical urveys represent a suite of nondestructive techniques to measure physical properties of the subsurface, such as magnetism, density and electrical conductivity, ' explains TerraDat director Nick Russill.

'Because the instruments are hand carried and do not generally contact with the ground, surveys enable mapping of all accessible areas of a site very rapidly (up to 2ha per day), reducing the need for more expensive trial pitting or drilling and also the chances of missing buried targets.

The results are integrated with site plans and historical information to provide landowners or developers with information that can subsequently be used for follow-up ground truthing or ameliorative works.' Westman's site occupied 3,000m 2, and all surface structures had been removed to ground level.

TerraDat used magnetic mapping, ground conductivity mapping, resistivity tomography and methane gas detection to identify ferrous metal targets such as tanks, services or foundations, map lateral changes in the materials making up the subsurface, create 2D crosssections of the geology beneath the site and analyse soil gas to identify possible hydrocarbon contamination.

Surveys with each technique were carried out on a regular grid, using real-time kinematic GPS to provide accurate positions. Data from each instrument was downloaded to a portable field computer and processed using dedicated mapping software.

As a result of the geophysical survey, the client had a better understanding of what structures lay beneath the site and could relate these to old plans in the historical desk study.

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