Method and means of passive detection of leaks in buried pipes

A method and means for passive detection of a leak in a buried pipe containing fluid under pressure includes a plurality of acoustic detectors that are placed in contact with the pipe. Noise produced by the leak is detected by the detectors, and the detected signals are correlated to locate the leak. In one embodiment of the invention two detectors are placed at different locations to locate a leak between them. In an alternate embodiment two detectors of different waves are placed at substantially the same location to determine the distance of the leak from the location.

BACKGROUND OF THE INVENTION 
This invention relates to the detection of leaks in pipes. In particular, 
this invention is a method and means of detecting the location of a leak 
in a buried pipe containing a fluid by analysis of the acoustical signals 
produced in the pipe or the fluid by the leak. 
When fluids under pressure are contained or carried in buried pipes, a 
small local failure of the pipe causes two problems. One problem is to 
detect the fact that there is a leak; the other is to locate the leak to 
fix it. As a general rule, it may be stated that the smaller the leak, the 
more difficult it is to detect the presence of the leak and the more 
difficult it is to locate such a leak even if its presence is known. In 
some systems of water pipes a principal method of leak detection involves 
noticing the collapse of ground over a buried water pipe as a result of 
subsurface erosion from a leak. Such a method of detection is obviously 
undesirable in the case of expensive fluids that are carried in the pipes 
or of fluids that are corrosive or flammable. For many years natural gas 
has been doped with chemicals having strong odors to assist in the 
location of leaks. Such a method of leak detection, however, is of most 
use in the absence of pavement over the pipe. Pipe that is buried under 
concrete or other paving and that carries a corrosive or flammable 
substance presents a challenge that is not met by any of the detection 
systems just described. 
Various types of active acoustic systems serve to detect leaks by exciting 
acoustic waves in the pipe or in the fluid conveyed in the pipe. Acoustic 
detectors are placed to detect signals produced by the discontinuities at 
the leak, either by responding to reflections generated by the 
discontinuity or by detecting differences produced in transmitted signals 
by the discontinuity in the pipe. Such systems, however, generally require 
substantial breaks in the pipes to generate signals that are large enough 
to be detected in the presence of the exciting signals. 
It is an object of the present invention to provide a better method and 
means of detecting leaks in buried pipes. 
It is a further object of the present invention to provide a method and 
means of locating leaks in buried pipes. 
It is a further object of the present invention to provide a method and 
means of detecting leaks in pipes carrying fluids under pressure. 
Other objects will become apparent in the course of a detailed description 
of the invention. 
SUMMARY OF THE INVENTION 
Leaks in buried pipes carrying or containing fluids are detected by a 
passive system that responds to acoustic signals generated by the leak. In 
one embodiment of the invention a detector of longitudinal or torsional 
acoustic signals is placed at a first location and a second detector of 
longitudinal or torsional acoustic signals is placed at a location on the 
other side of the leak. A radio broadcasting system is used to couple the 
signals detected by the two detectors to a single location for application 
to an apparatus for measuring the correlation between the two signals. The 
cross-correlogram of the two signals provides a measure of the distance of 
the leak from each of the two measuring points and hence of the location 
of the leak. In a second embodiment a detector of longitudinal acoustic 
signals and a detector of transverse acoustic signals are placed at the 
same location. A combination of the cross-correlogram of the two signals 
with the known differences in the velocity of propagation of longitudinal 
and transverse signals provides a measure of the distance of the leak from 
the measuring point. Signals may propagate either in the fluid or in the 
pipe.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a block diagram of an apparatus for the practice of the present 
invention. In FIG. 1 a leak 10 in a buried pipe 12 is located at a 
distance from a manhole 14 and a borehole 16 that has been sunk from the 
surface 18 to the pipe 12 to provide a measuring point. The manhole 14 and 
borehole 16 have been shown for illustration. It is evident that what is 
important is access to the pipe 12. If manholes were located conveniently 
then two manholes could be used or in the absence of conveniently located 
manholes it might be necessary to use two boreholes. 
A transducer 20 is connected and coupled acoustically to pipe 12 in manhole 
14 and another transducer 22 is connected to pipe 12 and coupled 
acoustically to it in borehole 16. The transducers 20 and 22 may be 
coupled to torsional waves or to longitudinal waves in the pipe 12. While 
the preferred mode of operation is to couple both transducers 20 and 22 to 
the same form of wave, it is also possible to couple one of the 
transducers 20 and 22 to a longitudinal wave and the other to a torsional 
wave. The form of the coupling is a matter of choice for the operator and 
will normally be made so as to detect the strongest signal. 
The invention works because leaks generate noise in the pipe, the fluid or 
both. The term "noise" is here taken to refer to a signal that is 
substantially random in time although not necessarily completely random. 
Such a signal is describable by its spectrum. Two detected signals that 
are generated by the same leak will exhibit a cross-correlogram that can 
be interpreted to locate the leak. The cross-correlation is determined by 
the circuit of FIG. 1 in which the measuring equipment 24 is located near 
manhole 14. An acoustical signal that is detected by transducer 22 is 
there converted to an electrical signal that is amplified in preamplifier 
26 and is amplified again as desired in a variable post-amplifier 28. The 
amplified signal from post-amplifier 28 is coupled to a radio transmitter 
30 that must be capable of broadcasting a signal with a bandwidth of 7 kHz 
through a transmitting antenna 31 to a receiving antenna 32, thence to a 
receiver 33. The signal received by receiver 33 is applied to a bandpass 
filter 34 that passes frequencies in the range of 3 to 4 kHz. The output 
of bandpass filter 34 is connected to highpass filter 36 which passes 
frequencies above 150 Hz. At the same time transducer 20 receives a signal 
in manhole 14 and generates an electrical signal that is coupled to 
preamplifier 38. The output of preamplifier 38 is connected to bandpass 
filter 40 which passes frequencies in the range of 3 to 4 kHz. The output 
of bandpass filter is amplified as necessary in variable post-amplifier 
42, and the amplified signal from post-amplifier 42 is applied to highpass 
filter 44 which passes frequencies above 150 Hz. The output signals from 
highpass filters 36 and 44 are applied to cross-correlator 46 to generate 
a cross-correlogram that is made visible on display device 48. The 
cross-correlogram of two signals is defined as a plot of the 
cross-correlation coefficient of the two signals as a function of the time 
delay between the signals. Knowledge of the distance between manhole 14 
and borehole 16 and of the velocity of propagation of the wave detected by 
each of of the transducers 20 and 22 provides information sufficient to 
interpret the correlogram displayed on display device 48 to locate the 
leak 10. 
FIGS. 2 and 3 are views of a coated pipe showing the placement of detectors 
for the practice of the present invention. FIG. 2 is a top view and FIG. 3 
is a side view of the same pipe. In FIGS. 2 and 3, pipe 12 is a carrier of 
a liquid such as fuel oil or a gas such as natural gas, or it may be an 
electrical conduit that includes a power line and an insulating fluid 
under pressure. When such a pipe 12 is buried underground, it is desirable 
to protect the outer surface by some means such as tar coating 54. When it 
is desired to detect or locate leaks in pipe 12, it is necessary to gain 
access to pipe 12 to remove tar to expose the surface 56 which is 
typically of steel. A coupling block 58 is placed against surface 56 in 
acoustical contact with surface 56 and an accelerometer is connected to 
coupling block 58 to convert acoustical signals into electrical signals. 
In FIGS. 2 and 3, accelerometer 60 is connected to coupling block 58 in 
such a way as to detect longitudinal acoustical waves in pipe 12 and 
accelerometer 62 is connected to respond to torsional acoustic waves in 
pipe 12. Reasons for the selection of longitudinal or torsional waves will 
become apparent in the description of the invention. It should be noted 
that the accelerometers 60 and 62 of FIGS. 2 and 3 could both be placed to 
respond either to longitudinal or torsional waves and that when they are 
placed on opposite sides of a suspected leak as shown in FIG. 1 it would 
normally be simpler to orient each of the accelerometers 60 and 62 of 
FIGS. 2 and 3 to respond to the same type of acoustical signal. This 
simplifies calculations in that the velocities of longitudinal and 
torsional signals are typically different, thus requiring an additional 
step of data processing if the arrangement of FIGS. 2 and 3 is used to 
make an actual measurement. 
FIG. 4 is a block diagram of an alternate embodiment of the invention. In 
FIG. 4 a leak 70 produces noise in the fluid in a pipe 72. A 
longitudinal-wave tranducer 74 and a torsional-wave tranducer 76 are 
located together on the pipe 72 with access through a single manhole or 
borehole. The signal from longitudinal-wave transducer 74 is amplified in 
preamplifier 78, filtered in bandpass filter 80 and fed to variable 
post-amplifier 82. The amplified signal from post-amplifier 82 is applied 
through highpass filter 84 to correlator 85. The output of torsional-wave 
tranducer 76 is amplified in preamplifier 86 and applied to bandpass 
filter 88. The output of bandpass filter 88 is connected through variable 
post-amplifier 89 to highpass filter 90, thence to correlator 85. 
Statistical correlation between the two signals is made visible on display 
device 92 in which a knowledge of the differences in the velocity of 
propagation of longitudinal waves and torsional waves in the fluid 
provides a measure of the distance of the leak from the measuring point. 
FIGS. 5 and 6 illustrate a cross-correlation coefficient obtained on a test 
pipe with a known leak. The pipe was carbon steel, type A3, Schedule 40, 
207 feet in length and 8 inches in internal diameter. It contained a 
length of high-voltage transmission cable and was filled with insulating 
oil maintained at a pressure of the order of 125 psi. A hole having a 
diameter of 0.035 inches was drilled in the pipe and was allowed to leak 
into sand to produce the acoustical waves that were detected to produce 
the correlation plots of FIGS. 5 and 6. The hole was located approximately 
midway between two sensors of longitudinal waves. The correlogram of FIG. 
5 is included for completeness to show the repeated locations of 
correlations that result from the interaction of reflected waves. The 
envelope of the set of high peaks near the center of FIG. 5 represents the 
correlation between signals received directly from the leak to each of the 
two sensors. The envelope of the next peaks going outward in either 
direction from the center represent correlations from signals that are 
reflected from the end caps used to terminate the test section of pipe. 
These signals are an artifact of the test setup and have been removed in 
FIG. 6 which is a plot of the center region of FIG. 5, expanded in scale 
to illustrate better the correlation of the direct signals received by 
each of the two sensors. It can be seen from FIG. 6 that the peak of the 
envelope of the correlation is displaced by 0.48 milliseconds from the 
center of the correlation plot. This displacement in time difference when 
multiplied by the known velocity of propagation of longitudinal acoustic 
waves in the fluid indicates that the leak is located a distance of four 
feet from the center of the pipe in the direction of the transducer that 
is connected to the cross-correlator as the negative input. 
The calculation is performed as follows for the embodiment of FIG. 1: 
With a distance l between sensors A and B, denote by X the distance from 
sensor A to the leak. Sensor B is then a distance (l-X) from the leak. The 
propagation time of a signal from the leak to sensor A is .tau.A=X/c, 
where c is the acoustic velocity of the wave that is detected, either 
longitudinal or torsional. The propagation time from the leak to sensor B 
is .tau..sub.B =(l-X)/c. The time difference 
##EQU1## 
Solving, 
##EQU2## 
and the distance of the leak from the midpoint is 
##EQU3## 
measured in the direction of the A sensor. Since the first peak of the 
correlation coefficient occurs at a time difference of (.tau.A-.tau.B), it 
is necessary only to know the distance l and the acoustic velocity c to 
locate the leak from the correlogram. The distance l is available to a 
utility from maps of its system; failing that, it may be measured. 
Acoustic velocity c will normally be measured by obtaining the correlogram 
of a signal applied at one sensor with that detected at another. The time 
delay of the peak, when divided into the distance between sensors, is the 
acoustic velocity c. 
A comparable calculation is performed for the embodiment of FIG. 4, where 
sensors A and B are together and the acoustic velocities differ. In this 
case, it is necessary to know the respective acoustic velocities, here 
denoted c.sub.A and c.sub.B. Calling x the distance from the sensors to 
the leak, it follows that 
EQU x=c.sub.A .tau.A=c.sub.B .tau..sub.B, 
where .tau..sub.A and .tau..sub.B are the respective propagation times of 
the A and B waves. Hence .tau..sub.A =x/C.sub.A and .tau..sub.B s 
x/C.sub.B, and their difference (.tau..sub.A -.tau..sub.B)=x(1/C.sub.A 
-1/C.sub.B). The difference (.tau..sub.A -.tau..sub.B) is determinable 
from the correlogram, so that 
##EQU4## 
The cross-correlation between two time-varying voltages V.sub.1 (t) and 
V.sub.2 (t) is a measure of the similarity of their statistics. In 
particular, if the two voltages represent random processes whose 
statistics do not change in time, then each is said to be stationary. For 
stationary processes the correlation is well known to be a function only 
of the time delay in measurement for the case of autocorrelation and to be 
a function of the time delay in measuring the cross-correlation between 
two signals. The cross-correlation which is determined as a measure of the 
location of the leak in the present invention is obtained by applying an 
appropriately band-limited signal from each of the detectors to a 
multiplier after applying a variable delay to one of the signals. The 
product of one of the signals with the delayed second signal is integrated 
with respect to time to produce a correlation signal that is a function of 
time delay. In the practice of the present invention, it has been 
convenient either to make cross-correlations of signals of the same kind 
(both longitudinal or both torsional) at two different locations or to 
make cross-correlations of different kinds of signals (one longitudinal 
and one torsional) at essentially the same location. Note that if 
different signals are detected at a single location it will be necessary 
to find out in which direction the leak is, either by measuring at another 
manhole or by separating the detectors by several feet at the single 
location. 
Either of these methods produces a correlated output that is substantially 
stationary over the typical period required to make measurements, which is 
of the order of 15 minutes. The time may vary depending on the ratio of 
signal to noise and the degree of certainty required by the operator. In 
general, as the measuring time becomes longer, the peaks in the 
correlogram become more distinct. The correlations of FIGS. 5 and 6 were 
recorded with a 15-minute averaging time. Other leaks producing larger 
acoustical signals can be expected to provide adequate correlation to 
permit their location in less than 15 minutes. The power of the 
correlation technique lies in the fact that a leak-signal may be buried in 
noise, yet the location of the leak may be obtained with excellent 
results. In FIGS. 5 and 6 the leak-signal output from the amplifiers was 
mixed with a random noise signal so that the leak-signal power and the 
noise-signal power were equal and therefore only marginally detectable 
with a passive acoustic device which measures signal power. The 
correlogram shown in FIG. 5 has a 48-dB signal-to-noise ratio. Thus a 
large gain in the signal-to-noise ratio is achieved through the 
correlation technique. This allows one to locate leaks that were 
previously undetectable.