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LEAKY water mains can be pinpointed quickly and cheaply with a technique that
builds up a three-dimensional picture of the ground beneath our feet. The
technique should make it easier to reduce the amount of water lost through
leaks鈥攚hich can waste more than a third of the water that should reach our
taps.

Leaks from underground mains are usually not discovered until the water
pressure drops, or water spurts from the ground. Before digging up the road,
engineers locate the leak with a metal rod or 鈥渓istening stick鈥� that picks up
the hiss of escaping water when placed on a valve.

But the technique is fraught with difficulties, says Alf Pearce, a leakage
manager for Thames Water in south-east England. Most leaks can only be
pinpointed at night with this technique, when the background noise from traffic
and other sources dies down. The problem is severe in major cities like London,
where the streets are quiet for only a few hours each week. 鈥淎nd even then the
noise and pressure drop might be caused by an all-night laundrette using water
constantly,鈥� says Pearce.

In order to 鈥渟ee鈥� underground, scientists can place electrodes at various
points in the ground and pass a current between them to measure its resistance.
This is partly a function of the ground鈥檚 water content and partly its
chemistry. From these measurements a computer can build up a picture of how much
water there is in the ground. It can even spot non-conducting objects such as
plastic pipes.

The technique has been used in the past to build up pictures of contaminants
leaking from underground tanks, such as those at the nuclear waste storage dump
at Hanford in Washington state. But in order to get a 3D image of the ground
beneath your feet you need to drill boreholes for the electrodes鈥攁n
expensive process.

Now Ramon Pall谩s-Areny and colleagues at the Polytechnic University of
Catalonia in Barcelona have developed a program that builds up a 3D picture of
the resistance from electrodes placed on the surface.

They start by putting an array of electrodes on the ground and sending
currents between each pair. The depth at which the technique sees is
proportional to four times the distance between each pair of electrodes. This
gives a picture of a vertical cross-section up to 1 metre down. Moving the array
gives another vertical slice, and by combining these individual slices the
program can build up a 3D picture of the resistance over a wide area.

The software Pall谩s-Areny uses is sensitive enough to pick out any
leaks from the 3D image it creates. The researchers have already tested the
method on buried pipes near the university. 鈥淭he simulated leak was unmistakably
detected,鈥� they say.

The new technique 鈥渟ounds interesting and we鈥檇 be happy to test it for them鈥�,
says Pearce. But he points out that to be useful it must work through road
surfaces and when the ground is saturated with water. 鈥淏ut if it works in the
centre of London it鈥檒l work anywhere,鈥� he says.