How to Find Leaks

Understanding Acoustic Leak Detection
What are the Sounds of Water Leaks?

Water leaks in underground, pressurized pipes may make many different sounds:

• " Hiss" or "Whoosh" from pipe vibration and orifice pressure reduction
• " Splashing" or " Babbling Brook" sounds from water flowing around the pipe
• Rapid "beating/thumping" sounds from water spray striking the wall of the soil cavity
• Small "clinking" sounds of stones and pebbles bouncing off the pipe

The "Hiss" or "Whoosh" sound, which often sounds like constant static noise, is the only one which is always present for leaks in pipes with 30 psi or higher water pressure. The other sounds may or may not be present, and usually they are not as loud. So, we decide " Is there a leak?" by listening for the " Hiss" or " Whoosh" .

Small Leak on Cast Iron Water Main

What Factors Affect These Sounds?

There are several factors that affect the loudness and the frequency range of the sounds made by water leaks transmitted on the pipes and transmitted to the surface of the ground:

• Water pressure in the pipe
• Pipe material and pipe diameter
• Soil type and soil compaction
• Depth of soil over the pipe
• Surface cover: grass, loose soil, asphalt, concrete slab, etc.

The loudness or intensity of the leak sound is directly proportional to the water pressure inside the pipe (up to a limit).
Metal pipes, such as iron mains, copper services, and steel pipes, transmit water leak sounds that are louder and higher frequency than do PVC pipes or asbestos-cement pipes. Thus, knowledge of the pipe material is important. Large diameter pipes, whether they are PVC, concrete, steel, or iron, transmit much less sound from water leaks than small diameter pipes. And, large diameter pipes transmit lower frequency sounds than small diameter pipes.
Sandy soil and very loose soils, particularly over a freshly buried pipe line, do not transmit the sounds of water leaks very well, nor do water saturated soils such as bogs and swamps. Hard, compacted soil transmits the sounds of water leaks best. Soil absorbs the sounds of water leaks very quickly. Leaks in water lines that are only 3 or 4 feet deep are much easier to hear at the ground′s surface than leaks in deeper lines. At 7 or 8 feet deep, only very large leaks with good water pressure will produce enough noise to be heard at the surface.
Finally, the ground cover, whether it is an asphalt street, loose dirt, concrete slab, or grass lawn, also makes an important difference. Hard street surfaces and concrete slabs resonate with the sounds of the water leak, and the leak may be heard for 5 to 10 feet or more on either side of the water pipe. Grass lawns and loose dirt surfaces do not offer such a resonating plate-like surface, and their surface variations make firm contact more difficult.

How Do Water Leak Noise Sounds Travel on Pipes?

Metal pipes, particularly iron mains between 6 inches and 12 inches, copper services, and steel pipes transmit the sounds of water leaks for hundreds of feet in every direction. Asbestos-cement pipe and PVC pipe do not transmit the sounds nearly as far.
Distances transmitted for the "Hiss" or "Whoosh" sounds of water leaks are a function of the pipe diameter as well as the pipe material:

Pipe Material and DiameterDistance Sounds Travel for 2 GPM Leak at 60 PSI

6 inch Cast Iron Pipe 600 to 1000 feet
12 inch Cast Iron Pipe 400 to 800 feet
24 inch Cast Iron Pipe 200 to 400 feet
6 inch AC Pipe400 to 800 feet
12 inch AC Pipe 300 to 500 feet
24 inch AC Pipe100 to 300 feet
6 inch PVC Pipe 200 to 300 feet
12 inch PVC Pipe100 to 200 feet
24 inch PVC Pipe50 to 100 feet
Thus knowledge of the pipe material and diameter is important to knowing how far the leak sound may be transmitted along the pipe walls.

How Do Leak Sounds Travel Through Soil?

Soil absorbs water leak sounds very quickly:
Soil absorbs the high frequencies to a greater degree than the low frequencies. For a leak in a pipe 6 ft deep, the "Hiss" or the "Whoosh" sound is weak and "muted," i.e. only the lower frequencies are heard. For a leak in a pipe 3 ft deep, the sound is louder and slightly higher in frequency.

Surveying

"Surveying" is the term applied to listening for water leaks when there is no obvious evidence, like water flowing on the street. Every hydrant, valve, and service line is a possible location to hear the sounds of water leaks:
Since the sounds travel on the pipe walls better than through the soil, always listen at the hydrants, valves, and meters first. As you get closer to the leak, the sound gets louder. Finally, decide which two of these locations are the loudest. Now you are ready for "Water Leak Pinpointing."
Surveying at a Hydrant and a Service Line:

• Listening for Leak Sounds at Hydrant
• Listening for Leak Sounds at Meter

Pinpointing

"Water Leak Pinpointing" is the term applied to the process of pinpointing the exact leak location. For Acoustic Leak Detection, the exact leak location is usually the spot where the leak sounds are the loudest:
To find this spot, the listener must carefully mark the location of the water line on the street after locating it exactly with a pipe and cable locator. Usually, the piping between the valve or hydrant with the loudest sound and the valve or hydrant with the second loudest sound is the section of the line that needs to be marked.
The section must be accurately located and marked on the street in order for the listener to consistently listen directly over the pipe. The listener moves the ground microphone 3 to 4 feet each time in the direction of the water line, listening, and moving closer to the water leak. While the listener is moving, he does not adjust the volume control, since the volume control must be held constant in order to make accurate comparisons. When the listener is very close to the leak, it may be impossible to decide based upon the user′s hearing alone whether the leak is in one spot or in a spot 3 to 4 feet away. When this occurs, the listener must study the visible display (meter) to see if the signal is slightly stronger at one location than at another location.

How to Find Leaks

http://www.subsurfaceleak.com/find_leaks.html

"Leak-noise correlators pinpoint leaks by measuring vibration signals at two points that bracket the location of a suspected leak"

Detecting Leaks in Plastic Pipes
http://irc.nrc-cnrc.gc.ca/fulltext/nrcc43058.pdf

Cross Correlation

The cross correlation function gives a measure of the extent to which two signals correlate with each other as a function of the time displacement between them...
Applications:
1. To determine to what extent a signal measured at one point originates from a particular source, and with what time delay.
2. To detect the existence of a signal in extraneous noise

«Science & Engineering Encyclopedia»
http://www.diracdelta.co.uk/science/source/c/r/cross correlation/source.html

Uses of Cross- and Autocorrelation

Cross-correlation is used to measure the similarity between two signals, to detect a known signal in a noisy one, or to search for cyclic data. One application is finding the time for a known signal to pass through a system if the signal is not grossly distorted during the transient. The lag time at the maximum value of the cross-correlation is the time shift caused by the system. This application of cross-correlation is at the heart of processing vibroseis data. Vibroseis is the most extensively used land seismic exploration technique...

The autocorrelation function is used in geophysics to find hidden periodicities such as occur in multipath reflection seismic records, to compute power spectra, and to perform a "defiltering" operation called deconvolution. It also appears in vibroseis processing

«Digital Geophysical Analysis »
http://130.191.21.201/multimedia/jiracek/dga/filtering/correlation.html
"It is well-known that an acoustic signal is generated when a leak is present in a pressurized pipeline. The sounds or vibrations associated with the leak are propagated in both directions away from the leak, at a constant velocity. Propagation occurs within the medium flowing in the pipeline and along the pipeline itself. Two sensors, placed on the pipeline on opposite sides of the leak, will sense the leak signal at different times, proportional to their distances from the leak. Conventional leak detectors have each sensor connected to an electronic unit which filters, amplifies, and transmits the received signals in analog form to a base station. At the base station, the signals from two sensors are received and bandpass filtered in analog form. The signals are then digitally sampled. The two sampled signals are then cross-correlated. If a leak is present, a peak appears in the cross-correlation function at time lag, T. ... the location of the leak can be determined from the lag, T, a knowledge of the distance between the sensors, and a knowledge of the velocity of sound in the pipe. "...

Method for detecting leaks in pipelines
US Patent No.5,974,862
"When a leak is present in a pipe, a pressure wave emanates from the turbulent source of the leak and travels away from the leak through the wall of the pipe and the fluid in the pipe. This leak signal is attenuated with distance and has a spectral signature (varying energy at different frequencies) that depends on the effective transfer function of the pipe network and the sensor connection. The effective range of the recorder depends on such factors as the pipe pressure, the leak signal strength and the variable background pipe flow and ambient noise levels present at the sensor."

"Water and other utility companies manage capital and operational expenditures, often with capital expenditures being more available than operational expenditures. Leak detection will yield significant savings in the form of reduced requirement for treatment and plant capacity, lost product, mandatory water use (revenue) restriction due to limited water resources, and reduced risk of catastrophic events. The challenge for water companies is to manage their human and capital resources to achieve sustainable network and leakage management. Currently, leak detection is performed in the field using personnel, vehicles and computerized leak detection and pinpointing equipment. The complete system, including recorders, readers, and controllers, provides the information needed to focus this effort with no additional operational expenditures.
In one general aspect, tracking vibrations on a pipeline network includes installing multiple vibration recorders on the pipeline network. Each vibration recorder includes a sensor, a timer, a processor, and a communication device. At each vibration recorder, vibration signals are received from the sensor at programmed times under the control of the processor of the vibration recorder, and the received vibration signals are processed by the processor of the vibration recorder. Processed vibrations signals are communicated from the vibration recorders to one or more reader devices using the communication devices of the vibration recorders. The processed vibration signals are collected from the one or more reader devices at a central computer system that analyzes the collected processed vibration signals to determine abnormal vibration patterns and to obtain measures of any leaks present in the pipeline network.

Tracking vibrations in a pipeline network
US Patent No.6,957,157
"Leaks in pipelines create vibrations that can be sensed at significant distances from the leak site. These vibrations are propagated in both directions away from the leak at a constant velocity. Sensors positioned at different locations on the pipeline will sense the vibrations at different times. The time at which each sensor senses the vibration is proportional to the distance from the sensor to the leak. Leak noise correlation is a well-known technique for measuring this time delay. For example, .... In Lander's system, the location of the leak is determined using remote sensing units having processors in digital radio communication with a computer base station.

"Embodiments may include one or more of the following features. For example, the timers within each monitoring unit may be time-synchronized prior to deployment. The programmed recording times may be at night when pipeline pressure and leak sounds are high and flow and environmental noise are minimal. Received vibration signals are processed individually in each monitor to model, characterize, and trend pipe vibration patterns. Vibration signals may be received regularly or intermittently over periods of hours, days, months or years. Variations in the received signals can identify leak sounds having magnitudes far below the threshold of human hearing. Processed signals may be sent to a base station, either regularly or on demand, in a flexible manner. The timers in two or more monitors may be re-synchronized using a docking station to enable time-alignment of the received signals in any number of monitors. Time-aligned received signals may be analyzed using a correlative method, which enables the detection and localization of the source of coherent sounds, such as those created by leaks in the pipeline network.

"The invention offers a number of advantages. For example, distribution of monitors throughout a pipeline network allows previously unavailable information about the network to be assessed at a central or otherwise convenient location. Each monitor contains a locally-intelligent processor, which obviates the need for continuous manual surveying of the pipeline distribution system. Current and historical processed vibration signals, optimally recorded at night, can be analyzed using graphical displays of pipe sounds, listening, correlation, and other signal processing methods.

"The monitors used by are light-weight, low-power, cost-effective devices which may be safely applied to the pipeline network without intervention for extended periods of time. The monitors offer a high degree of protection from shocks, weather, leaking water, and vandalism. The system is virtually maintenance-free. Monitors can communicate individually or collectively by short or long range digital radio, telephone, or direct connection to a base station.

"The techniques can be used to perform short or long-term serial analysis of pipeline vibrations. Monitors can detect existing leaks, newly emerging leaks, and sudden breaks or ruptures of the pipeline. Areas of particular susceptibility to pipe failure may be monitored indefinitely, preempting potentially serious problems. The techniques also can be used to perform routine surveying of the pipeline network using short-term analysis of vibrations in a particular area of the network. After detection and localization of any leaks present, the monitoring units may be initialized and deployed in a different area of the network. Mobility and short-term analysis capabilities increase the cost-effectiveness and usefulness of the invention, particularly in more sparsely populated areas.

"The techniques can reliably detect leaks significantly below the threshold of human hearing due to the sensitivity to vibrations achieved by the invention's design and signal processing capabilities.

"The techniques also can account for changes in flow and pressure profiles in gas and water distribution systems due to modifications, new construction, and changes in consumption patterns. These physical changes in pipelines lead to changes in vibration characteristics. The techniques adapt to these changing system conditions in two ways. First, because each monitor's processing is data-adaptive, individual monitors automatically adapt to locally-changing conditions. Second, the techniques can adapt to known changes in the distribution system by reprogramming the monitors from a base station using a digital communication device.

"The techniques' data-adaptive, serial analysis of vibrations is effective with all Newtonian fluids, all pipe types, and a wide range of distribution system pressures. This effectiveness is achieved because each vibration monitor acts as its own reference to analyze contemporary vibration signals with respect to trends computed from historical processed data.

"The monitors can transmit alarms at different, pre-programmed levels of urgency. This means that network maintenance personnel can be alerted to the presence of leaks on high pressure gas mains or very large water mains immediately. Problems with smaller water mains and service lines can be monitored and reported at regular intervals, and scheduled for repair as appropriate. p The techniques combine the flexibility of both short and long-term monitoring. Monitors can be deployed for hours, days, months or years and can be programmed both to record at any time, and to communicate with a base station. They are thus able to pinpoint leaks by recording vibration data at the most advantageous times, such as at night.

"The techniques can adapt to the pipeline conditions by unique adaptive processing of serially recorded vibration signals. This permits detection of pre-existing leaks and of emerging leaks that develop either gradually or suddenly.

"The techniques are able to detect very small abnormal vibration signals, inaudible to human hearing, which are significantly below the threshold of existing detection methods. Such abnormal small vibration signals are very important to detect. They often represent the most difficult to find leaks. These leaks are typically long-lived and are the most hazardous and the most expensive in terms of lost product from the pipe. Abnormal small vibration signals may also be the harbinger of a catastrophic pipe failure. Their successful detection is the key to preventing catastrophic pipe failures before they occur.

"The techniques allow deployment for any period of time. They also promise to permit pinpointing of previously undetectable leaks, and to facilitate flexible communication with a base station.

"The systems employed by the techniques are almost maintenance-free and may be produced at low cost. Modes of digital communication are flexible and readily available worldwide. The network of monitors is suitable for utilities with pipeline distribution systems of almost any size and configuration. Any number of monitors can be deployed, either temporarily or permanently, to meet the needs of small rural utilities through to major cities.


Monitoring vibrations in a pipeline network
US Patent No.6,567,006
"A leak detection survey is the first step in finding leaks. This process begins with good maps of your distribution system. Maps help create an efficient leak detection plan and also help to find lines, valves, and other buried parts of the distribution system where leaks might occur.

Hydrophone. This is an inexpensive device that works the same way as a screwdriver or wrench, but is designed for more comfortable and convenient listening

Geophones. Also inexpensive, these are similar to a doctor’s stethoscope.

Electronic leak detectors. These are much more sophisticated and complex than geophones. They amplify sound caused by water vibrations. Operating these devices requires some skill and practice. For assistance, contact ASRWWA or an authorized factory representative.

To use a leak correlator sucessfully, you must know the location and length of the pipe you are checking. This is why you need detailed maps of your system. When accurate data is fed into a leak correlator, the chances of pinpointing leaks are extremely good.

Leak correlators.
This very expensive and complex device is the most elaborate leak detection tool. A microprocessor, it measures the time it takes for sound to travel between two points along a pipe. It is not affected by depths or soil type. Leak correlation services are normally provided by private contractors. To use a leak correlator sucessfully, you must know the location and length of the pipe you are checking. This is why you need detailed maps of your system. When accurate data is fed into a leak correlator, the chances of pinpointing leaks are extremely good.

FALL 2000 WATER LOSS AND LEAK DETECTION
http://www.asrwwa.org/ASRWWA_Leak_Detection_9-00.pdf
GIS Mapping: The process by which a computer generates thematic maps that combine geographic information with demographic information and/or other relevant information.

Internal Database: Database developed from data within the organization.

Leak Detection: A systematic search for water loss in a delivery system or at an end users location. Considered a means of water conservation, repairing leaks found through leak detection controls the loss of water that water agencies have paid to obtain, treat, and pressurize and the loss of water consumers have purchased.

The Colorado Water Conservation Board (CWCB)
http://cwcb.state.co.us/owc/Drought_Water/pdf/Glossary.pdf
The correlation technique is basically the same with all correlators. The Field Sensor Units (FSU’s) are placed at various points on the distribution system . The distance between the FSU’s are recorded to a computer program along with the pipe size and material . The computer program then calculates the velocity of sound on a particular pipe and uses this information to “correlate” the exact location of the leak on the pipe by measuring the time it takes for the leak sound to travel from one FSU to another. The computer is able to tell the difference in time travel between the 2 FSU’s and thus correlate the leak.
What makes the DigiCorr 98 totally unique is that it digitizes the signal at the FSU and sends this digitized signal to the computer program. All other correlators send an analogue signal from the FSU to the computer and digitize in the computer. Sending a digitized signal eliminates interference normally associated with analogue. The DigiCorr is able to correlate over greater distances with the digitized signal and identify leaks in frequency ranges that analogue correlators cannot.

Heath Enters Digital Leak Detection Age
http://www.heathus.com/InfoCenter/Consultant-Winter2001.pdf
The position of the largest peak indicates the time difference between the leak noise arriving at the two sensors, and is the variable in equation (1). Clearly a peak that that is large compared with other spurious peaks is desirable. A peak value of one would indicate perfect correlation and only occurs if there is no background noise, and if the sensors are equidistant from the leak. Provided that the background noise at each sensor is uncorrelated, then increasing the time over which the measurement takes place will increase the signal-to-noise ratio (at the rate of 3 dB per doubling of data acquisition time). The other quantity of interest in the cross-correlation function is the accuracy of the time difference estimate and the width of the peak (discrimination). It has been shown that the signal-to-noise ratio has only a marginal effect on the accuracy of the estimate. However, the positioning of the sensors and the type of sensor used can have a significant effect on the width of the peak. Clearly, as the distance from the sensor to the leak increases, the higher frequency components of the signal will be significantly attenuated, resulting in a much smaller signal to noise ratio; this will reduce the height of the peak in the cross-correlation function. Also, because the higher frequency content of the signals is diminished, the ability to accurately locate a leak will be adversely affected because the width of the peak increases.
The acoustic pressure inside the pipe is related to the radial displacement of the pipe integrated around the circumference, and a sensor that measures this will indirectly measure the pressure. If an accelerometer is used instead of a pressure or displacement sensor then the filter that the leak noise passes through is modified, because acceleration is proportional to the product of displacement and square of frequency. Thus an accelerometer will amplify high frequencies compared to a hydrophone. So, for better discrimination of the leak location, an accelerometer is preferable provided that it is positioned to sense the fluid wave; a hydrophone or integrated displacement sensor is preferable, however if the signal to noise ratio is small.
There is always a limited range of frequencies which contains the information on the location of the leak. It should be noted that the coherence is less for the accelerometer measured signals, but the bandwidth is much greater, because of the reasons discussed above.

Some Recent Research Results on the use of Acoustic Methods to Detect Water Leaks in Buried Plastic water Pipes
ask authors or read PDF file from www.isvr.soton.ac.uk/DG/Water%20Leak%20Detection%20in%20Plastic%20Pipes.pdf