'In-depth' radar: Seeing what lies beneath the surface

A georadar is designed to map underground objects. It sends electromagnetic waves down into the ground, and its antenna captures the return signals from objects and constructs images of them.

In 2007, SINTEF/NTNU bought a 3D georadar in order to find out whether such a device might have other, less obvious, applications. When Frenchwoman Anne Lalagüe of SINTEF Building Research started work on her doctorate at NTNU two years ago, she picked this as her subject: she wanted to evaluate how georadar could be utilised, and perhaps to develop the equipment in order to see whether SINTEF could use it in contract research projects.

Halfway through her research programme, Lalagüe can point to a wide range of applications, all of which have been tested in the course of contract research projects for public-sector organisations and industrial companies. This has given the researchers a wide range of experience, references and verifications to which they can refer. Anne Lalagüe has also written several international papers as well as other publications. Since her work has been financed for the most part by the the Norwegian Public Roads Administration, many of her studies have focused on the use of the radar in roads and tunnels.

Frost, frost-heave and soft road surfaces in spring are serious problems in Norway, and the Public Roads Administration asked SINTEF to find out how georadar can be utilised in this field. Can it "see" how far into the ground the frost reaches, and will findings of heavy sub-surface frost in a particular stretch of road lead to it being relaid in a different way in the future?

"Just now, the Public Roads Administration has to take a core sample from the road surface for analysis. The radar will allow continuous profiles to be constructed, which is cheaper and more efficient," says Lalagüe. "When severe frost and problems on particular stretches of road come to light, it may be necessary to employ a more suitable asphalt technology when the road is due to be maintained."

Every winter, Norway spreads thousands of tonnes of salt on slippery winter roads, a practice that is criticised by both environmental activists and motorists, who say that the salt corrodes their vehicles.

Many people say that it would be a good idea to use less salt, but this is difficult, because no-one knows just how much salt actually lies on Norwegian roads, nor whether less could be used because some of what has already been applied still lies on the surface.

Preliminary trials of georadar on roads produce results that depend on the concentration of salt on the surface. For this reason, Anne Lalagüe believes that it could be developed into a tool that would enable decisions to be made regarding whether or not more salt should be spread on particular stretches of road.

Norway has more than one thousand tunnels. After the Hanekleiv Tunnel disaster in 2006, when the tunnel roof collapsed, the Norwegian Public Roads Administration decided to check the quality of the rock in every tunnel in the country. The problem is that a good many tunnels have been built with a concrete vault that provides protection against water leakages and frost. There are thus few hatches or means of direct access to the rock that would allow it to be inspected, and analyses must be performed via holes drilled more or less at random through the vaulting and into the rock. Since the space between the rock and the concrete vault varies, it can be difficult to know where it would be worthwhile to drill.

"We have been asked by the Public Roads Administration to scan such spaces by georadar in order to produce a profile of their depths. It will be simplest and cheapest to drill where the space between the vault and rockface is greatest," says Lalagüe, who mentions that 13 tunnels have been scanned so far.

When old roads are due to be rehabilitated, up-to-date maps may not be available, but the engineering consultants involved need to know what is lying beneath the surface before digging operations can begin: do invisible water pipes criss-cross the area, or might power cables be buried there?

This is partcularly necessary in suburban areas," says Lalagüe, as she mentions that SINTEF Building Research is currently running a project for Oslo's Department of Traffic. Pipes and cables with a diameter of at least 10 cm and lying at depths of up to three metres are easily revealed by the georadar.

Norsk Enøk and Energi AS wanted to lay a new district heating pipeline through the city of Tønsberg, and since the area involved is well-known for the richness and importance of its archaeological remains, the companies wished to avoid destroying any of them. The georadar soon revealed unknown structures beneath the surface. These were taken into account in the planning process, and the two companies modified the route of the excavations in order to avoid the problem.

The last project that Anne Lalagüe talks about concerns flat roofs and humidity. Some flat-roofed buildings in Trondheim have severe problems of leakages and heat-loss. The scientists have scanned some roofs using georadar and believe that they have identified the sites of such leakages.

"We actually have two radar units," says Lalagüe. "The larger one is two metres wide and needs to be mounted on the front bumper of a large vehicle. The other is only 70 cm wide and is much lighter, and this is the one that we have used on hospital roofs and in tunnels. We have also focused on the accuracy of the equipment, and can guarantee that it is accurate to 90 -- 95 percent."

Lalagüe still has two years of her doctorate ahead of her; two years of testing and trialling exciting applications for the Norwegian georadar.