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Brownfield boom s activity boost

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The increase in brownfield development has created a demand for more geophysical surveying - despite continuing scepticism among some clients. Dr George Tuckwell, who runs STATS' geophysical surveying division, says: 'We are often called in when a developer needs to iron out the details of a solution.'

Unfortunately, he says, firms like STATS get that call when the client knows there are buried obstacles or difficult ground, but they cannot find what they are looking for with the drilling pattern they have chosen. 'Usually they're on a tight schedule and know they won't manage without using geophysical techniques, ' he says. 'That partly reflects some clients' attitude to geophysical surveys: they only use them when they have to.

'What we would like to see more is people thinking of geophysical methods earlier in the process - particularly on large sites, ' he continues. 'We can spend a couple of days running over the site with some geophysical techniques and say there are some things here and here and then they can design their intrusive programme around that. The power of geophysical surveying is that it can cover the ground very quickly and give an overall picture.'

Over the last year STATS has used seismic - particularly ground stiffness - methods a lot more, to complement other investigation techniques, often on contaminated land.

'Electrical methods are often used to identify the base of a landfill site, ' explains Tuckwell.

'They work very well, but there is a degree of uncertainty because conductivity affects estimates of depth. If you use a complementary seismic method, you are measuring something totally different, so you can compare the two sets of data to tell you what's going on.

'Continued advances in automated and mobile data collection - combined with the development of data processing and numerical modelling software - are making integrated multi-technique site investigations both more cost effective and more useful, ' he adds.

The company has just completed one such project where, before construction of a major road across the site, the client wanted detailed knowledge of the internal structure of the closed landfill to inform the design and construction phases of the project.

This information included the lateral and vertical boundaries of the waste, the zoning of leachate within the waste, and the heterogeneity of the internal structure of the landfill.

STATS designed and executed a multitechnique investigation of the site using electromagnetic ground conductivity, resistivity imaging, and surface wave ground stiffness instruments to collect three distinct but complementary data sets.

Electromagnetic data was collected using a mobile Multi-Sensor Platform designed and built by Leicester University. The system uses telemetry and GPS positioning technology to record data from a number of instruments simultaneously, registering them to an electronic OS grid in real time.

To aid interpretation the data sets were collated within a single software package where their spatial relationship could be interrogated in a 3D visualisation.

Resistivity and seismic ground stiffness data were interpreted by comparison with software in which accurate models of the subsurface were used to calculate synthetic data for comparison. In particular, numerical models of the seismic ground stiffness data provided profiles of subsurface engineering properties and the location of key horizons, which could then be verified by borehole data.

Individual stiffness profiles were combined to provide contours of the base of the waste across the entire site.

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