TWRI 3-A17

45 Pages·2001·3.38 MB·English

by  Antonius Laenen

Techniques of Water-Resources investigations
of the United States Geological Survey
Chapter Al 7
ACOUSTIC VELOCITY METER SYSTEMS
By Antonius Laenen
Book 3
APPLICATIONS OF HYDRAULICS
I
DEPARTMENT OF THE INTERIOR
DONALD PAUL HODEL, Secretary
U.S. GEOLOGICAL SURVEY
Dallas L. Peck, D/rector
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1985
For sale by the Branch of Distributibn, U.S. Geological Survey,
Text Products Section, 604 South Plckett Street,
Alexandria, VA 22304
PREFACE
The series of manuals on techniques describes procedures for planning and
executing specialized work in water-resources investigations. The material is
grouped under major subject headings called “Books” and further subdivided
into sections and chapters; Section A of Book 3 is on techniques as related to
surface water.
The unit of publication, the Chapter, is limited to a narrow field of subject
matter. This format permits flexibility in revision and publication as the need
arises. Chapter Al7of Book 3 (TWRI 3-A17) deals with acoustic velocity meter
systems.
Reference to trade names, commercial products, manufacturers, or distrib-
utors in this manual constitutes neither endorsement by the U.S. Geological
Survey nor recommendation for use.
III
TECHNIQUES OF WATER-RESOURCES
INVESTIGATIONS OF THE U.S. GEOLOGICAL
SURVEY
The U.S. Geological Survey publishes a series of manuals describing proce-
dures for planning and conducting specialized work in water-resources inves-
tigations. The manuals published to date are listed below and may be ordered
by mail from the Branch of Distribution, U.S. Geological Survey, 604
South Pickett Street, Alexandria, VA 22304 (an authorized agent of the
Superintendent of Documents, Government Printing Office).
Prepayment is required. Remittance should be sent by check or money order
payable to U.S. Geological Survey. Prices are not included in the listing below,
as they are subject to change. Current prices can be obtained by writing to
the U.S. Geological Survey Branch of Distribution. Prices include cost of
domestic surface transportation. For transmittal outside the U.S.A. (except to
Canada and Mexico) a surcharge of 25 percent of the net bill should be
included to cover surface transportation.
When ordering any of these publications, please give the title, book number,
chapter number, and ‘U.S. Geological Survey Techniques of Water-Resources
Investigations.”
TWI 1-Dl. Water temperature-influential factors, field measurement, and data presentation,
by H. H. Stevens, Jr., J. F. Ficke, and G. F. Smoot. 1975. 65 pages.
TWI l-D2. Guidelines for collection and field analysis of ground-water samples for selected
unstable constituents, by W. W. Wood. 1976. 24 pages. Reprint.
TWI 2-Dl. Application of surface geophysics to ground-water investigations, by A. A. R.
Zohdy, G. P. Eaton, and D. R. Mabey. 19’74. 116 pages. Reprint.
TWI 2-El. Application of borehole geophysics to water-resources investigations, by W. S.
Keys and L. M. MacCary. 1971. 126 pages. Reprint.
TWI 3-Al. General field and office procedures for indirect discharge measurements, by M.
A. Benson and Tate Dalrymple. 1967. 30 pages.
TWI 3-A2. Measurement of peak discharge by the slope-area method, by Tate Dalrymple and
M.A. Benson. 1967. 12 pages.
TWI 3-A3. Measurement of peak discharge at culverts by indirect methods, by G. L. Bod-
haine. 1968. 60 pages. Reprint.
TWI 3-A4. Measurement of peak discharge at width contractions by indirect methods, by H.
F. Matthai. 1967. 44 pages.
TWI 3-A5. Measurement of peak discharge at dams by indirect methods, by Harry Hulsing.
1967. 29 pages.
TWI 3-A6. General procedure for gaging streams, by R. W. Carter and Jacob Davidian. 1968.
13 pages.
TWI 3-A?‘. Stage measurements at gaging stations, by T. J. Buchanan and W. P. Somers.
1968. 28 pages. Reprint.
TWI 3-A8. Discharge measurements at gaging stations, by T. J. Buchanan and W. P. Somers.
1969. 65 pages. Reprint.
TWI 3-A9. Measurement of time of travel and dispersion in streams by dye tracing, by E. P.
Hubbard, F. A. Kilpatrick, L. A. Martens, and J. R. Wilson, Jr. 1982. 44 pages.
TWI 3-AlO. Discharge ratings at gaging stations, by E. J. Kennedy. (in press).
TWI 3-All. Measurement of discharge by moving-boat method, by G. F. Smoot and C. E.
Novak. 1969. 22 pages.
IV
TWI 3-A13. Computation of continuous records of streamflow, by E. J. Kennedy. 1983. 53
pages.
TWI 3-A14. Use of flumes in measuring discharge, by F. A. Kilpatrick and V. R. Schneider.
1983. 46 pages.
TWI 3-A15. Computation of water-surface profiles in open channels, by Jacob Dividian. 1984.
80 pages.
TWI 3-A16. Tracer discharge measurement techniques, by F. A. Kilpatrick and E. D. Cobb. (in
press).
TWI 3-A17. Acoustic velocity meter systems, by Antonius Laenen. 1985.
TWI 3-Bl. Aquifer-test design, observation, and data analysis, by R. W. Stallman. 1971. 26
pages.
TWI 3-B2. Introduction to ground-water hydraulics, a programed text for self-instruction,
by G. D. Bennett. 1976. 172 pages.
TWI 3-B3. ‘&pe curves for selected problems of flow to wells in confined aquifers, by J. E.
Reed. 1980. 106 pages.
TWI 3-Cl. Fluvial sediment concepts, by H. P. Guy, 1970. 55 pages.
TWI 3X2. Field methods for measurement of fluvial sediment, by H. P. Guy and V. W.
Norman. 1970. 59 pages. Reprint.
TWI 3-C3. Computation of fluvial-sediment discharge, by George Porterfield. 1972.66 pages.
TWI 4-Al. Some statistical tools in hydrology, by H. C. Riggs. 1968. 39 pages.
TWI 4-A2. Frequency curves, by H. C. Riggs. 1968. 15 pages.
TWI 4-Bl. Low-flow investigations, by H. C. Riggs. 1972. 18 pages.
TWI 4-B2. Storage analyses for water supply, by H. C. Riggs and C. H. Hardison. 1973. 20
pages.
TWI 4-B3. Regional analyses of streamflow characteristics, by H. C. Riggs. 1973. 15 pages.
TWI 4-Dl. Computation of rate and volume of stream depletion by wells, by C. T. Jenkins.
1970. 17 pages.
TWI 5-Al. Methods for determination of inorganic substances in water and fluvial sedi-
ments, by M. W. Skougstad and others, editors. 1979. 626 pages.
TWI 5-A2. Determination of minor elements in water by emission spectroscopy, by P. R.
Barnett and E. C. Mallory, Jr. 1971. 31 pages. Reprint.
TWI 5-A3. Methods for analysis of organic substances in water, by D. F. Goerlitz and Eugene
Brown. 1972. 40 pages.
TWI 5-A4. Methods for collection and analysis of aquatic biological and microbiological
samples, edited by P. E. Greeson, T. A. Ehlke, G. A. Irwin, B. W. Lium, and K. V.
Slack. 1977. 332 pages.
TWI 5-A5. Methods for determination of radioactive substances in water and fluvial sedi-
ments, by L. L. Thatcher, V. J. Janzer, and K. W. Edwards. 1977. 95 pages.
TWI 5-A6. Quality assurance and practices for the chemical and biological analyses of water
and fluvial sediments, by L. C. Friedman and D. E. Erdmann. 1982. 181 pages.
TWI 5-Cl. Laboratory theory and methods for sediment analysis, by H. P. Guy. 1969. 58
pages.
TWI 7-Cl. Finite difference model for aquifer simulation in two dimensions with results of
numerical experiments, by P. C. Trescott, G. F. Pinder, and S. P. Larson. 1976.116
pages. Reprint.
TWI 7-C2. Computer model of two-dimensional solute transport and dispersion in ground
water, by L. F. Konikow and J. D. Bredehoeft. 1978. 90 pages.
TWI 7-C3. A model for simulation of flow in singular and interconnected channels, by R. W.
Schaffranek, R. A. Baltzer, and D. E. Goldberg. 1981. 110 pages.
TWI a-Al. Methods of measuring water levels in deep wells, by M. S. Garber and F. C.
Koopman. 1968. 23 pages.
TWI 8-A2. Installation and service manual for U.S. Geological Survey manometers, by J. D.
Craig. 1983. 57 pages.
TWI 8-B2. Calibration and maintenance of vertical-axis type current meters, by G. F. Smoot
and C. E. Novak. 1968. 15 pages.
V
CONTENTS
Page
Page
Preface .................................................................................. III Analysis.. .............................................................................. 18
Abstract .............................................................................. 1 Multipath interference ............................................ 18
Introduction ........................................................................ 1 Ray bending ................................................................ 19
Overview .............................................................................. 1 Example of ray-bending analysis ................ 19
Theory of operation ......................................................... 2 Signal attenuation .................................................... 21
Associated limiting phenomena ................................... 4 Example of attenuation analysis .................. 21
Interference ................................................................ 4 System considerations .............................................. 22
Refraction .................................................................... 5 Hypothetical consideration ............................ 22
Attenuation ................................................................ 8 Calibration .......................................................................... 22
Streamflow orientation ............................................ 11 Computation of discharge .................................. ... 22
Available equipment ........................................................ 11 Relation of line velocity to mean velocity .......... 23
Simple system ............................................................ 11 Operation and maintenance requirements ................ 24
Versatile system ...................................................... 12 Personnel .................................................................... 25
Cross-path adaptation .............................................. 13 Test equipment .......................................................... 25
Responder adaptation .............................................. 13 Spare parts .................................................................. 25
Accuracy.. .................................................................... 13 Maintenance and troubleshooting procedures .... 25
Site selection ...................................................................... 14 References .......................................................................... 25
Field reconnaissance ................................................ 14 Appendix I.
Equipment considerations ...................................... 15 Acoustic ray bending program listing.. .............. 27
Installation considerations ...................................... 15 Appendix II.
K-curve calculation program listing .......... ........ 35
FIGURES
1. Diagram showing velocity component used in developing traveltime equations ......................................................... 3
2. Diagram showing voltage representation of transmit and receive pulses at upstream and downstream
transducers .............................. ............................................ ..........................................................................................................3
3. Diagram showing the radiation pattern from a typical narrow-beam transducer ...................................................... 4
4. Diagram showing direct and reflected paths in defining multipath interference.. ..................................................... 5
5. Diagram showing voltage representation at a transducer of the direct and reflected received pulses .................. 5
6. Graph showing sonic velocity in water .......................... ...........................................................................................................6
7. Diagram showing vector components used to develop Snell’s law of refraction in a layered medium .................... 6
8. Diagram showing vectors used in defining ray bending in a constant density gradient medium ............................ 6
9. Graph showing beam deflection from linear-temperature gradients for selected path lengths.. .............................. 7
10. Graph showing beam deflection from linear specific conductance gradients for selected path lengths ................ 7
11. Sketch showing signal bending caused by different water-density gradients ....................................... ...................... 8
12. Graph showing attenuation by absorption ..............................................................................................................................9..
13. Graph showing attenuation from sediment particles for selected frequencies .... ......................................................... 10
14. Photograph of a simple microprocessor system ....................................................................................................................1. 1
15. Photograph of a versatile minicomputer system ..................................................................................................................1..2
16. Photograph of a versatile microprocessor system ............................................................................ ..................................... 12
17. Diagram showing components necessary for a responder system. .................................................................................... 13
18. Photographs of various transducer mounts.. ............................................................................................................ ............... 16
19. Photograph of a transducer alignment.. ................................................................................................................................1..7.
20. Sketches of typical transducer mounts ..................................................................................................................................1..7..
21. Typical cross sections to define interference from bottom-reflected signals ................................................................. 19
22. Cross section at the Chehalis River, Wash., showing paths of direct and reflected signals for the maximum
expected gradient ..................................................................................................................................................................... ..... 20
23. Cross section at the Chehalis River, Wash., showing paths of direct and reflected signals for a hypothetical
density gradient .................................................................. ................................................................................................ ......... 20
24. Plan view of signal-beam deflection caused by the worst expected temperature gradient.. .................................. 21
VII
VIII CONTENTS
25. Graph showing mean velocity to line velocity relation (K) for various stages on the Sacramento River at
Freeport, Calif. __._ ....................... . ._.___...................................................................................................................................2..3.. ....
26. Graph showing theoretical vertical velocity distribution ._._.___......................................................................................2..4.. ..
TABLES
Page
1. Estimates of tolerable sediment concentrations for AVM system operation, based on attenuation from spherical
spreading and from scattering from the most critical particle size .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2. Possible resolution errors for selected AVM operating frequencies and path lengths ..___............................. 14
ACOUSTIC VELOCITY METER SYSTEMS
By Antonius Laenen
Abstract velocities than can be measured with a current
meter. It can provide acontinuous and reliable
Acoustic velocity meter (AVM) systems operate on the
record of water velocities over a wide range of
principles that the point-to-point upstream traveltime of
conditions, but several constraints <apply.
an acoustic pulse is longer than the downstream travel-
time and that this difference in traveltime can be accu- AVM systems range from a simple velocity
rately measured by electronic devices. An AVM system is meter to complex computerized systems that
capable of recording water velocity (and discharge) under compute and transmit real-time discharges.
a wide range of conditions, but some constraints apply:
At some installations, a single signal path is
1. Accuracy is reduced and performance is degraded if
adequate; at others, multiple signal paths are
the acoustic path is not a continuous straight line. The
path can be bent by reflection if it is too close to astream required.
boundary or by refraction if it passes through density The purpose of this report is to provide a
gradients resulting from variations in either water manual on the installation and operaton of
temperature or salinity. For paths of less than 100 m, a
AVM’s, including (1) theory of operation, (2)
temperature gradient of 0.1” per meter causes signal
selection of equipment and suitable measuring
bending less than 0.6 meter at midchannel, and satis-
factory velocity results can be obtained. Reflection from site, (3) method of installation, (4) calibration,
stream boundaries can cause signal cancellation if (5) operation, and (6) constraints that apply to
boundaries are too close to signal path. their successful use.
2. Signal strength is attenuated by particles or bubbles
that absorb, spread, or scatter sound. The concentration
Overview
of particles or bubbles that can be tolerated is a function
of the path length and frequency of the acoustic signal.
3. Changes in streamline orientation can affect system Summary information under each of the fol-
accuracy if the variability is random. lowing eight topics is intended to answer ques-
4. Errors relating to signal resolution are much larger for tions commonly asked about AVM systems,
a single threshold detection scheme than for multiple
including their suitability for use at a particu-
threshold schemes.
lar site.
This report provides methods for computing the effect of
various conditions on the accuracy of a record obtained Theory-AVM systems work on the princi-
from an AVM. The equipment must be adapted to the site. ples that the point-to-point traveltirne of sound
Field reconnaissance and preinstallation analysis to detect
is longer upstream than downstream and that
possible problems are critical for proper installation and
traveltimes can be measured accurately by
operation of an AVM system.
electronic devices. Measurement of velocity is
Introduction along an acoustic path set 30-45” diagonal to
streamflow. Commercial systems that mea-
An acoustic velocity meter (AVM) measures sure streamflow use the time-of-travel method
the velocity of flowing water by means of a to determine velocity. Fluctuations in the speed
sonic signal, which moves faster downstream of sound due to changes in water density gra-
than upstream. Meters of this type are useful dients are compensated for by methods used to
in measuring discharge at streamflow sites calculate the velocity.
where the relation bet,ween discharge and Limitations-AVM accuracy and perfor-
stage varies with time because of variable mance are limited by four factors: (1) the loca-
backwater conditions and also where stream tion of the acoustic path with respect to water
slopes are too flat to permit measurements surface and streambed, which provide reflec-
accurate enough for slope computations. tive surfaces for multipath interference, (2)
The AVM is a nonmechanical, nonintrusive density gradients (usually caused by different
device that is capable of measuring lower temperatures or salinities), which cause the
1
2 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS
acoustic path to bend, (3) concentrations of sed- required to prevent multipath interference.
iment, air bubbles, organic materials, and The acoustic path should be at least one-half
organisms, all which attenuate the acoustic meter below the water surface in slow moving
signal, and (4) streamflow variability, which water to prevent signal bending from solar
causes the angle between the acoustic path and heating. (2) Signal bending will affect system
the flow to change. accuracy. Avoid situations where the acoustic
Availability-Two types of equipment are signal is bent so drastically that it reflects from
available for use in measuring velocity: (1) a either the water surface or the streambed, or
simple one- or two-path microprocessor-based both. (3) Analyze the acoustic path for signal
preprogrammed system that will output veloc- attenuation. The normal sediment concentra-
ity only and (2) a more complex and versatile tion that can be safely tolerated by most AVM
multipath minicomputer- or microprocessor- systems is approximately 2,000 mg/L, but this
based keyboard-programmable system that is dependent on system frequency, particle
can serve as a complete discharge-data aquisi- size, and path length. Avoid situations where
tion unit. At present all systems require an AC air entrainment can be a problem.
power source. The use of acoustic-crossed paths Calibration-AVM system calibration can
will help define any changes in streamflow be accomplished by current-meter measure-
orientation. A responder adaptation, discussed ment or by theoretical computation based on a
later in the report, will eliminate the need for a logarithmic vertical velocity distribution.
hard-wire cross-channel link. Operation and maintenance-AVM systems
Accuracy-For many streamflow situations, are relatively sophisticated electronic devices
single-path AVM systems can attain accura- that have proven to operate well but require
cies of 3 percent and multipath systems can obtain specialized maintenance. Important in the sys-
accuracy to within 1 percent. AVM system tem’s performance is the availability of prop-
error can be attributed to three sources: (1) erly trained personnel. In-house electronic ex-
Timing error as defined by the resolution of pertise is an invaluable asset. Spare parts and
traveltime. This resolution is approximately proper test equipment are necessary in achiev-
+0.03 m/s for systems using signal-quality ing minimal downtime.
checks and approximately to.1 m/s when
quality checks are not employed. (2) Angular- Theory of Operation
ity error as defined by the orientation of the
streamflow to the acoustic path. For every one
Most commercial AVM systems that mea-
degree of uncertainty in path angle there is
sure streamflow use the time-of-travel method
approximately 1 percent uncertainty in veloc-
to determine velocity along an acoustic path set
ity measurement. (3) Deflection error as de-
diagonal to the flow. This method is described
fined by acoustic ray bending. For paths less
in detail in a report by Laenen and Smith
than 300 m, error is usually less than 3 percent.
(1983), in which the general formula (fig. 1) for
Site selection-A review of system limita-
determining line velocity is defined as:
tions and equipment requirements is neces-
sary before site selection. Find a stream reach B
T/L== (1)
where velocity distribution is nearly regular
and somewhat confined and where the channel
where
is stable or can be measured for cross-section
VL = line velocity, or the average water veloc-
change. Reconnoiter the stream for existing
ity at the depth of the acoustic path,
structures to mount acoustic transducers. Ob-
and
tain cross-section sounding information and
0 = angle of departure between streamflow
note obstructions that might reflect the signal
and the acoustic path.
near the acoustic path. Obtain temperature,
= traveltime from A to C (upstream),
tAC
specific conductance, sediment concentration,
= travn:time from C to A (downstream),
tCA
and air-entrainment information.
Analysis-(l) Analyze the acoustic path for
B = length of the acoustic path from A to C.
multipath interference. For every 100 m of
acoustic path, approximately 1 m of depth is The acoustic propagation rate is not a neces-
ACOUSTIC VELOCITY METER SYSTEMS 3
a
sary element in equation 1 and fluctuations in interface and the received pulse is a represen-
-
the propagation of sound resulting from tation of what has been transmitted acousti-
changes in water temperature or any other cally.
factors affecting water density have no effect In the measurement of traveltime, the time
on the computation of water velocity. between the start of the transmit pulse and the
start of the receive pulse is measur’ed in both
the upstream and downstream direction (fig.
2). Normally, threshold detection of the first
cycle of the arriving pulse is used to determine
the completion of a traveltime measurement.
The simplest form of threshold detection is the
,.reamfloti~vL~
triggering of a counter gate when the incom-
ing pulse reaches a selected voltage level. A
variety of schemes have been used to ensure
that the proper signal, and not random noise
P
(electrical or acoustical background or static),
activates the system timing.
Y
Traveltime (tAC)
(Upstream)
Figure I-Velocity component used in developing travel-
time equations.
lb--
Electronic delays encountered in equipment
?IW pulse
and cables are very small and, except for very
short acoustic paths, are insignificant when
compared to total traveltime. These delays are
normally included in the measurement of up- b------- Traveltime (tcA) e
stream and downstream traveltimes. For long (Downstream)
and unequal runs of connecting cable (between
Ii
transducer and electronic signal processor)
delays in either direction are very close to the
same because transmitted and received sig-
nals travel essentially the same electrical path.
In the measurement of traveltime a sonic
transducer is triggered by a single spike of
excitation voltage and responds by oscillating
for a short time at the frequency of the piezo-
electric crystal encased in its face. The oscilla-
tion is dampened by the physical constraints of Figure 2 -Voltage representation of transmit and receive
the system as the pulse is converted to a pres- pulses at upstream and downstream transducers.
sure wave and transmitted into the water.
When the pulse (now traveling through the In most systems the received signal is moni-
water) is received by the second transducer, it tored by one or more voltage threshold levels,
is converted back to electrical energy. Figure 2 and signals of an amplitude not meeting these
is a voltage representation of what would nor- levels are not used. In addition, various
mally be viewed on an oscilloscope attached to schemes are employed for comparing each
the transducer. The transmit pulse is a repre- measured traveltime with the preceding mea-
sentation of the transmit oscillation which is surement, and data that depart by some fixed
affected by feedback from the crystal-water amount are disregarded. Some systems also