Heath Consultants Aqua-Scop - Manual (Page 21)

The frequency and velocity of the sound induced on the pipe by a
leak will be a function of pipe size, thickness, material and boundary
conditions. Quite important is the rigidity of the pipe. Metal pipe has
a high modulus. Therefore, we will hear the characteristically higher
frequency “pipe” noise generated by a leak. Also, the velocity will be
high or the transit time is short and minimum attenuation of the sound
will result. Plastic pipe has a much lower modulus and, for similar
conditions of pipe size and boundary conditions, the frequency will be
lower with much higher transit time (attenuation).
Other sounds may also be generated by restrictions within the system,
such as: a partially closed valve, a reduction coupler, a booster pump,
etc. These devices will cause turbulence along the pipe wall and will
emit a detectable sound similar to that of a leak. Do not be mislead
by this effect. If these sounds are of sufficient magnitude, you may be
incapable of distinguishing the existence of a leak in the immediate
vicinity.
Soil is basically a non-homogeneous material. Generally the soil will
be layered except directly over man-implanted objects such as pipe.
Fluidic type soils such as clay can create special problems. Any soil
which has a rubber like consistency will greatly attenuate sound.
The actual contact area of the Ground Microphone with a surface is
quite small. If a hard surface (even frozen earth will help) is unavailable,
the use of a resonant plate may help. Any solid structure penetrating or
covering the earth can be used to advantage.
Sound may be induced on a pipe by secondary noises such as leaking
water filling a cavity. Once the cavity fills this source of noise
vanishes.
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