United States
                Environmental Protection
                Agency
 Risk Reduction
 Engineering Laboratory
 Cincinnati, OH 45268
                Research and Development
 EPA/600/SR-92/143 October 1992
EPA       Project  Summary
                Acoustic  Location  of Leaks  in
                Pressurized  Underground
                Petroleum  Pipelines
                Eric G. Eckert and Joseph W. Maresca, Jr.
                 Experiments were conducted at the
                Underground Storage Tank (UST) Test
                Apparatus  Pipeline  in  which three
                acoustic sensors separated by a maxi-
                mum distance of 38.1  m (125 ft) were
                used to monitor signals produced by
                11.4-, 5.7-, and 3.8-L/h (3.0-, 1.5-, and
                1.0-gal/h) leaks in the  wall of a 5-cm-
                (2-in.-) diameter pressurized petroleum
                pipeline. The line pressures and hole
                diameters used in the experiments
                ranged from 69 to 138 kPa (10 to 20
                psi) and 0.4 to 0.7 mm (0.01 to 0.03 in.),
                respectively. Application of a leak lo-
                cation algorithm based  on the tech-
                nique of coherence function analysis
                resulted in mean differences between
                predicted and actual leak locations of
                approximately 10 cm. The standard de-
                viations of the location estimates were
                approximately 30 cm. This is a signifi-
                cant improvement  (i.e., smaller leaks
                over longer distances) over the cross-
                correlation-based techniques currently
                being used.
                 Spectra computed from leak-on and
                leak-off time series indicate that  the
                majority of acoustic energy received in
                the far field of the leak  is concentrated
                in a frequency band from 1 to 4 kHz.
                The strength of the signal within this
                band was proportional  to the leak flow
                rate and line pressure. Energy propa-
                gation from  leak to sensor was ob-
                served via three types of wave motion:
                longitudinal waves  in the product and
                longitudinal and transverse waves in
                the steel. The similarity between the
                measured wave speed and the nominal
                speed of sound in a gasoline suggests
that longitudinal waves in the product
dominate the spectrum of received
acoustic energy. The effects of multiple-
mode wave propagation and the reflec-
tion  of  acoustic signals within the
pipeline were observed as non-random
fluctuations  in the measured phase
difference between sensor pairs.
  Additional experiments with smaller
holes and higher pressures (138 to 345
kPa [20 to 50 psi]) are required to de-
termine the smallest leaks that can be
located over distances of several hun-
dred feet. The current experiments in-
dicate that improved phase-unwrapping
algorithms or lower noise instrumenta-
tion, or both, are required to optimize
system performance.
  This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title  (see Project
Report ordering Information at back).

Introduction
  Underground pressurized pipelines are
frequently used to transfer liquid products
for many industrial applications. Some of
these pipelines are  associated with the
underground storage tanks typically found
at retail gasoline stations and others with
tanks at industrial storage facilities; they
can contain petroleum products or a vari-
ety of other chemicals. The many systems
used to detect leaks in underground pres-
surized pipelines are designed for use on
pipelines that are typically 5 cm (2  in.) in
diameter and generally 15.2 to 61.0 m (50
                                                                Printed on Recycled Paper

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to 200 ft) in length. EPA regulations (40
CFR Part 280 Subpart D) require that the
leak detection  equipment used to test a
pipeline on a monthly basis be capable of
detecting leaks at least as small as 0.76
L/h (0.2 gal/h)  with a probability of detec-
tion (P0) of 95% and a probability of false
alarm  (PFA) of 5%. If the equipment is
used to test the line annually, it must  be
able to detect leaks as small as 0.38 L/h
(0.1 gal/h); in the regulations, this type of
test is  designated as a line tightness test.
   If a leak  is found, remediation must
follow, and the first step is to  locate the
leak. Presently, two  methods  are  used,
but neither is totally acceptable. The first
method is to systematically uncover the
line  and perform a visual inspection for
leaks.  Although this method works, it is
time consuming, disruptive to operations,
and costly. In addition, the line is subject
to damage during the excavation process.
The second method is to use a helium- or
halogen-tracer technique, but both of these
have operational and accuracy problems.
There  is a  need  for a nondestructive
method of leak location that is  accurate,
relatively simple to  use, and applicable to
a  wide variety  of pipelines and pipeline
products.
   One method  of expediting the remedia-
tion process is to apply remote sensing
techniques to the pipeline as a means of
accurately locating the leak.  Passive-
acoustic measurements, combined  with
advanced signal-processing methods, may
provide a means for locating small leaks
in limited-access pipeline delivery systems.
Although passive acoustics has been used
for some time to determine the  spatial
location of  leaks, this concept has  not
been applied to underground petroleum
pipelines. With  the use of cross-correla-
tion  techniques, leaks of  approximately
113 L/h (30 gal/h) have been successfully
located in water-filled  pipelines that are
pressurized to 827 kPa (120 psi) and are
less than 30 m  long.
   Cross-correlation analysis  works  well
when the signal is very strong or the
background noise is not excessive. When
the acoustic signal  is weak in relation to
the level of background  noise or has a
finite frequency bandwidth, more sophisti-
cated signal processing  techniques are
available.  Advanced signal processing is
required if any  of the following objectives
are to  be achieved: (1) the detection  of
leaks smaller than several gallons per hour,
(2) a reduction in  the number of false
alarms and missed  detections due to op-
erational  or ambient  noise, and (3)  an
increase in the  distance between sensors
 bracketing the leak. One such advanced
 technique is coherence function analysis.
   The best way to locate the signal source
 is to apply coherence function analysis to
 signals measured by two or more trans-
 ducers. Coherence function analysis, which
 estimates the correspondence between
 two measurements as  a function of fre-
 quency, is analogous to the squared cor-
 relation coefficient but is a far more pow-
 erful tool in estimating  and locating sig-
 nals. The coherence magnitude measures
 the strength of the correspondence, and
 the coherence phase measures the rela-
 tive time delay. In contrast, the correlation
 coefficient is a measure of correspondence
 that is the result of an integration over all
 frequencies. If the correspondence is fre-
 quency^dependent, or  if the_ phase de^
 pendence  of  the correspondence is a
 nonlinear function of frequency, the corre-
 lation is degraded. By contrast, coherence
 is a direct measure of the  complex fre-
 quency  correspondence between  two
 measurements, and, therefore,  preserves
 the actual correspondence  between the
 two measurements of the signal.
   The last 5 to 10 years have seen sig-
 nificant advances in commercially available
 acoustic sensors,  in powerful computers
 that are both small and inexpensive, and
 in digital signal processing. This  means
 that an acoustic leak location system can
 be made available in a portable package,
 a possibility that makes  it  an attractive
 and viable option. Acoustic systems are
 attractive from an operational standpoint
 because the test is short (a few minutes)
 and the  sensors can be mounted directly
 on the outside of the pipeline. Acoustic
 systems have direct application to the 15.2-
 to 61.0-m (50 to 200 ft) pipelines found at
 retail service stations because the sensors
 can be placed at each end of the line.
   The objective of this work was to esti-
 mate, by means of passive acoustic sen-
 sors mounted on the outside wall of the
 pipeline, the accuracy of locating a leak in
 a pressurized  petroleum pipeline as  a
 function of leak rate and  distance between
 acoustic sensors. There are  no  regulatory
 requirements for leak location.  For rapid
 repair of the pipelines and rapid remedia:
tion of any contamination that might have
occurred from the leak, however, it would
 be highly desirable if the leak could  be
 located within 10% of the length  of the
pipeline in the case of a line longer than
30.5 m (100 ft), or within 3.0 m (10 ft)  in
the case of a line shorter than 30.5  m
 (100 ft). This limits the excavation to only
a  small  fraction of the  line. Theoretical
estimates suggest that  when coherence
analysis  is used, acoustic sensing tech-
 niques can detect a leak within 35 cm of
 its actual location.

 Results
   Experiments were conducted on a pipe-
 line at the UST Test Apparatus in which 3
 acoustic sensors separated by a maximum
 distance of 38.1 m (125 ft) were used to
 monitor  signals produced  by 3.8-, 5.7-,
 and  11.4-L/h (1.0-, 1.5-,  and 3.0-gal/h)
 gasoline  leaks. These flow rates were
 generated through drilled holes 0.4 to 0.7
 mm  in  diameter.  The three-transducer
 system enabled the propagation speed of
 acoustic waves to  be measured  for par-
 ticular combinations of product,  pipeline
 geometry, and analysis frequency band.
 Data recorded at the higher leak flow rates
 (5.7 and 11.4 L/h, [1,5 and,3.0 gal/h])
 correspond to  full  line pressure  (103 to
 138  kPa [15 to 20 psi]); data recorded at
 the lower flow rate (3.8 L/h [1.0 gal/h])
 were obtained under partial line pressure
 (69 kPa [10 psi]) because of the limitation
 imposed by  the minimum  available hole
 diameter (0.4 mm). Application of a leak
 location algorithm based on the technique
 of coherence function analysis resulted in
 mean  differences between predicted and
 actual leak locations of 8.7 cm (at 11.4 L/
 h  [3.0  gal/h]), 3.6 cm (at 5.7  L/h [1.5 gal/
 h]), and  -11.6 cm (at 3.8 L/h [1.0 gal/h]).
 Standard deviations of the location esti-
 mates were 26.1 cm (at 11.4  L/h [3.0 gal/
 h]), 26.3 cm (at 5.7 L/h [1.5  gal/h]),  and
 39.1  cm  (at 3.8 L/h  [1.0 gal/h]). The mean
 propagation  speed  was  915 m/s  with a
 standard deviation of 146 m/s.
   Data recorded in the presence of a 1.9-
 L/h (0.5-gal/h) leak  were obtained as part
 of an investigation of signal strength as a
 function of line pressure for a fixed-diam-
 eter hole (0.4 mm). The 1.9-L/h (0.5-gal/h)
 leak produced  a  detectable signal; how-
 ever, because of the reduced line pressure,
 the algorithm, as applied,'yielded no loca-
 tion estimates.
   Spectra computed from leak-on and
 leak-off time series indicate that the ma-
jority of acoustic  energy received in the
far field of the  leak is concentrated in a
frequency band from  1  to 4 kHz. The
 strength  of the  acoustic signal within this
 band  was proportional to  the leak flow
 rate and line pressure, as  expected.  En-
 ergy propagation from leak to sensor was
via three forms of wave motion: longitudi-
 nal waves in the product and both trans-
verse and longitudinal waves  in the steel.
 Isolation  of  each  of these propagation
 modes was achieved through the  use of
gasoline and CO2 as the product fluids and
through the generation of impulsive cali-
bration signals. Though each of these

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propagation modes is believed to contrib-
ute to the overall received signal, longitu-
dinal  wave motion in the product  was
clearly the dominant propagation  mode
for liquid-filled  pipelines. The effects  of
multiple-mode wave propagation and the
reflection  of  acoustic signals within the
pipeline were observed  'as non-random
fluctuations in the measured phase differ-
ence between sensor pairs.
  Accurate leak  location  requires the
identification  of frequency  bands  within
which a high degree of similarity is main-
tained  between acoustic signals propa-
gated along different  paths from leak  to
sensor. Coherence function analysis pro-
vides the  best means  of gauging this
similarity and, thus, separating useful in-
formation opncerningjhe leak location from
ambient or system noise. While the signal-
to-noise ratio (SNR) was observed to be
generally high within the entire 1 - to 4-kHz
frequency band, continuous regions of high
coherence appropriate for source location
were typically  100 to  500 Hz  in  width.
Several data sets recorded in the  pres-
ence of the 11.4-L/h (3-gal/h) leak  exhib-
ited high coherence over a 2-kHz band-
width.  Location estimates obtained by
means of the cross-correlation technique
showed that  without the detailed knowl-
edge of signal  similarity  provided by the
coherence  function,  cross-correlation
analysis  cannot locate small leaks  with
acceptable accuracy.  The observed cor-
respondence between measured  and pre-
dicted phase shifts within the 1 - to 4-kHz
frequency band demonstrates the need  to
develop a more sophisticated location al-
gorithm such that a greater fraction of the
information contained  in coherent  leak
signals may be processed.
  Buried  pipelines provide a generally
quiet ambient environment in which to per-
form acoustic measurements. Since the
SNR for a given  leak largely determines
the ability of a passive acoustic system to
locate  the leak, the system  noise level
should be determined by ambient acoustic
noise rather than by electronic noise. The
combination of sensors (CTI-SOs*) and pre-
amplifiers (Panametrics 5660-Cs) used in
this work was incapable  of resolving the
low levels of ambient acoustic noise asso-
ciated  with the pipeline  at the UST Test
Apparatus. Improved system performance
may be attained through the use of trans-
ducers with greater sensitivity  in the  low-
frequency range (1 to 10 kHz) and  low-
noise preamplifiers.

Conclusions
  Passive  acoustic measurements, com-
bined  with advanced signal  processing
techniques based on coherence analysis,
offer a promising  method for the location
of small leaks in  pressurized petroleum
pipelines found at retail gasoline service
stations and industrial petroleum storage
facilities. Although the  results presented
in this  work  represent a significant im-
provement over previous  pipeline leak lo-
cation  efforts, additional research and de-
velopment are required before system
performance can  be optimized.  Location
of leaks of several tenths of a gallon per
hour over  distances of  several  hundred
feet should ultimately be possible.


Recommendations
  The  full capability of the  location algo-
rithm  was  not evaluated in these tests.
* Mention of trade names or commercial products does
 not constitute endorsement or recommendation for
 use.
The smallest hole used to generate a leak
in the experiments was 0.4  mm.  A line
pressure of 138 kPa (20 psi) resulted in a
leak rate of 3.8 L/h (1.0 gal/h). It is recom-
mended  that  additional experiments  be
performed  with  smaller holes  at  higher
line pressures (138 to 345 kPa [20 to 50
psi]) to determine the minimum leak rate
that can  be reliably located. The current
work  indicates that further improvement
can be realized through the application of
better phase-unwrapping algorithms and
better instrumentation.  A  better under-
standing of the  underlying  physics of
pipeline acoustics, including the propaga-
tion modes  and source  mechanisms of
the acoustic leak signal, will help optimize
the algorithms and the  hardware. It is
recommended that  the following,,work be
performed to extend the technology:
    •   develop a location  algorithm ca-
       pable of processing the coherence
       phase over an  arbitrarily wide-fre-
       quency  band,
    •   characterize the wave propagation
       modes excited by the acoustic leak
       signal and the degree to which each
       mode enhances or  degrades the
       leak location  estimate,
    •   reduce system noise through trans-
       ducers specifically designed for low-
       frequency,  high-sensitivity applica-
       tions and through low-noise,  audio-
       range preamplifiers, and
    •   automate the data acquisition sys-
       tem  and  signal processing  algo-
       rithm, and evaluate system perfor-
       mance on a variety of actual pipe-
       lines.
  The full report was submitted in fulfill-
ment  of Contract No. 68-03-3409 by Vista
Research, Inc., under the sponsorship of
the U. S. Environmental Protection Agency.
                                                                                     •U.S. Government Printing Office: 1992— 648-060/60133

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  Eric G. Eckert and Joseph W. Maresca, Jr., are with Vista Research, Inc., Mountain
    View, CA 94042
  R. W. Hlltgeris the EPA Project Officer (see below).
  The complete report, entitled "Acoustic Location of Leaks in Pressurized Under-
    ground Petroleum Pipelines," (Order No. PB92- 207 687/AS; Cost: $19.00,
    subject to change) will be available only from:
          National Technical Information Service
          5285 Port Royal Road
          Springfield, VA 22161
          Telephone: 703-487-4650
  The EPA Project Officer can be contacted at:
          Risk Reduction Engineering Laboratory
          U.S. Environmental Protection Agency
          Edison, NJ 08837
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati. OH 45268

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