Geophysical investigation is providing engineers with better knowledge of what is underground, reports Lisa Russell.
The construction industry digs holes every day without knowing what it will find, says Geotec Surveys managing director Nigel Knowles. Many dangers can lurk beneath - utilities, tunnels, unexploded bombs - but all too often the problems only become apparent when the contractor arrives on site, armed with nothing more than a topographical survey.
Forthcoming changes in the Construction (Design & Management) regulations will make it clear that the contractor must not be left to discover such risks, says Knowles. The regulations will put more onus onto the client to ensure project-specific information identifies hazards.
Geophysics can be used to indicate subsurface conditions, including identifying geological and man-made features. The applications of underground mapping are wide-ranging in highlighting obstructions and changes in material across all sectors of civil engineering.
'Typically, you use geophysics to find something you can't find using conventional intrusive methods, ' says STATS Geophysical associate director Dr George Tuckwell.
Studying the ground properties can, for instance, show foundations or the boundaries between soil types.
There are many techniques for picking up the differences, says Met Surveys geophysics manager Mark Whittingham, such as examining the relative magnetic properties. Tiny differences in gravitational pull can also yield invaluable data, like the presence of a void.
'There is an increasing awareness of the benefits of geophysics, ' says Stratascan managing director Peter Barker. 'It's not considered quite such a black art as it used to be.' One popular application is in mapping the position of utilities. Radar and electromagnetic techniques can trace the route of pipes.
Aperio director Simon Brightwell believes that a milestone has been passed in another key field, the management of transport networks. 'Traditionally, engineers have called in the geophysical specialist to answer a specific question, ' he says. 'Now they are being asked to lead largescale, information-gathering contracts, for example, on railway or highway networks.' Standard cable avoidance tools and generator combinations should be used 'every time a shovel is put in the ground' says Knowles, but they can only find metal and power-carrying services. Hitting a fibre-optic line is costly and disruptive, but can be avoided with the use of geophysical techniques. Plastic gas and water pipes can also be found.
Survey penetration depth depends on the technique and the type of instrument used. Radar antennae with a penetration of, say, 200mm would give a high-resolution view of features such as reinforcement. Utility-tracing antennae could penetrate deeper - say 2m - while going down 5m would enable foundations to be picked up.
Much geophysical surveying takes place from ground level. European Geophysical Services specialises in taking measurements from down a hole.
'We are seeing an increase in the acceptance of geophysics, ' says TerraDat director Nick Russill. Conferences and publications often feature a paper or case study with some geophysical investigation. Plus, some local authorities and the Environment Agency stipulate non-destructive techniques.
'Clients phone up and say that they want a radar survey - they tend to lump the geophysics disciplines into the radar method, ' says Russill.
'But it is good that they are considering a non-invasive method - it's our job to put them on the right track and recommend the best approach.' www. metgeo. com www. geotecsurveys. com www. stats. co. uk/geophysics www. terradat. co. uk www. stratascan. co. uk eurogeophys@compuserve.