Eureka5 User Manual
CXD-930-UM-044
Version: 1.3
2Principles of Leak Noise Correlation
Leak noise correlation is used to detect leaks in pipelines where there is a positive internal pressure, such that a leak causes a
loss of fluid to the outside. The movement of fluid through the leak causes rapid pressure changes around the site of the leak
such that there is a randomly varying noise source located at the leak. This noise travels in both directions, at the same
velocity, from the leak.
(Noise is not a constant frequency like a musical note but is a random and continually changing mixture of different
frequencies. It is this characteristic which enables the correlator to operate.)
2.1 Methods of Location
Previous methods of leak location relied on detecting a noise, then tracing it at ground level to find the point of maximum
noise, which was assumed to be directly above the position of the leak. The equipment used included listening sticks and
stethoscopes, but these were difficult to use if the noise was faint or too loud to be accurately located. Background noise
was another problem; electronically- filtered amplifiers were introduced in an attempt to overcome such problems.
A correlator operates by comparing the noise detected at two different points in the pipeline. Noise travels from the leak in
both directions along the pipeline at a constant velocity (which depends on various factors), so that if the leak is equidistant
between two sensors then these sensors will detect the noise at the same time. Conversely, if the leak is not equidistant
then the sensors will detect the same noise at different times, and the difference in time (the time delay) is measured by the
correlator. The following diagram illustrates this:
The sensors are located on the valves A and B (convenient access points for underground pipes), and as shown the leak site
is closer to A.
By the time a particular noise from the leak has reached A, the same noise heading towards B has only travelled as far as X.
The distance from X to B causes a delay t before the noise arrives at B, thus the correlator detects the delay (t) between the
arrival of the noise at A and its arrival at B. If the velocity of sound is V and the distance between the sensors is D, then as the
distance from X to B = V * t, (velocity x time), then D = (2 * L) + (V * t). This equation may be rearranged to give L, the
distance from the nearer sensor to the leak site:
