CA1254649A - Acoustic scintillation liquid flow measurement - Google Patents

Abstract

ACOUSTIC SCINTILLATION LIQUID FLOW MEASUREMENT
Abstract of the Disclosure A system and method are disclosed for acoustic scintillation liquid flow measurement. Measurement is carried out by acoustic beams transmitted perpendicular to the direction the component of liquid flow to be measured, with both phase and amplitude measurements being made to enable generation of accurate speed of flow indications, which are indicative of liquid flow through a broad area being monitored as opposed to a single point therein. In one realization of this invention, a pair of projectors are mounted on one side of the flow area, such as a channel to be monitored, and a pair of receivers are mounted on the other side of the flow area with pulsed acoustic signals from each of the projectors being transmitted in separate parallel paths through the liquid, such as water, to the receivers. At the receivers, electrical signals indicative of received acoustic signals in each path are complex demodulated after which the demodulated outputs are shaped, converted and then coupled to a microcomputed for evaluation and, responsive thereto, providing the indication of liquid flow speed.

Description

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Eield of the Invention This inverltion rela~es to fluid flow measurement, and, rnore partlcularly, relates to acoustic scintillation fluid flow measurement.
Background of the Invention Measurement devices are now widely known for measuring the flow of various fluids, and such devices have been heretofore suggested for measurement of flow of at least some fluid utilizil~g acoustic signals.
There is no satisfactory technique now ~nown, however, for obtai.ning real time rneasurements of the speed of flow of liquid, and, more ~articularly, of such measurernents in heavily used traffic channels on waterways. While it is presently possi.~l.e -t.o use arl upwarc3 l.ooking Doppler system to obtairl a vertical proiile o:L- tlle current at olle .locatior i.n Li li.qui(3, this wi.11 Ilo~ produce a measllrelnen-t representative of oLher l~oints across a broad area sucll as a water chdr~rle]..
~ tt~:r (~Il.lr~ lcl~ clc3llr(~ llt~ ~ol~ r~Lc,~or~
hLtve re-~uired cal)les and other equilllnerlt on the sea floor, which is nc~t only ex~:)ensi.~e, but also i.s vunerable to brec-~age, such as can occur, for example, due to dragginc~
anchor~.3 and the like. ~50rieover, measuretnents that ~night be :~?.54649 ~b~c~ ^o~ r~ ers l"c~r~ a ~ l)o i l l ~ s acro~s a chc~ lt~l Usi.ll~; radi~ OL- otl~er links c~rl~ot ~cJrl~,ally be used because of the traffic ha~ards created.
An alterl~ative tecl~nique usi.ng horizol~tally projected back-scattered sound i5 also known. For example, sound transmitted from one side of a channel of water whose flow is to be measured is scattered back to a receiver or receivers co-located with the projector. Doppler shifts or horizontâl translation of the patterns in the back-scattered sourld, or some re]ate~ effect, rnay then be detected and the resultinc~ f]ow ir-ferred ~see, for exampl.e, A. I.aenerl ancl ~.
Smith, Acoustic systellls for t.he measure~slent of streamflow, ~aper 2213, U.S. Geol.o~i.c~l Survey, W~ter-Supply, ~ayes 7 and 2~, ].983).
A serious difficulty associated with all such acoustic back-scatter systems however, especially when oriented hori~ontally, is the fundamental inefficiency of the ~ack-scatter process. Only a minute fraction of the projec~ed sound is returned to t.he hydrophones. Thus, it is verlf difficult t.o achieve measurel~lents over subs~c~ iaL ra~l~Jes with this techrli.que, irl c~orltrast to the forward propa~ation technique described in this invention in which the projectors point directly toward the hydrophones.

:a~54~4~3 o~ ifL~ Ly ~S~ tt(~ ~itl ~,ac~ Jtt~r s is ~c ~o scct~ g ~ t~ c~ r.~ s~lrf~l~e ~jr floor, since scattering fLom these boundaries ten~s to be rnucll stronger than scattering frorn particles or inllomogeneities in the ~ater colunm, and thus imposes severe demallds on the acceptable beam patterns of the projectors and receivers. Back-scatter systems are also sensitive to the presence or absence of acoustic scatterers in the water column, and, moreover, it is difficult to tell the depth at which the scattering is taking place.
Normal current measurements in a water channel or the like have therefore been now comrnorlly made over a short time period, and predictions rnade therefrom using harmonic analysis of the data. Such predictions, however, are subject to significant error from several sources, including meteorological effects such as wind, atmospheric pressure chan~es, and river run-off, as well as limitations of the current meter rneasurenlents themselves.
Measurement of gas flow perpendicular to a transmitted electrormagne~lc ~ea~l, is also knowl-, and hcls l~eell u~3ed extensively in connection with the atmosphere. In addition, measurernent of gas flow perpendicular to the transmitted beam of an optical arrangement is also known (see, for example, U.S. Patent Nur~ers 3,623,361 and 4,201,467).

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i~eclsul~e~,lcll~, such as fl~w r~surellellt o~ wcl~er across a channel, ca~lno~, however, normally be made Usilig an electromagnetic beam or an optical arrangement. For measurement of such flow, acoustic waves have therefore been heretofore utilized. When utilizing acoustic signals for measurernent of water flow, however, such acoustic devices have heretofore J~easured the speed of the current in a section oblique to the direction of flow of the water (see, for example, ~.S. Patent Numbers 4,094,193 and 4,446,542).
In addition, while a reciprocal transmission approach has been previously used for water flow measurements in chanr-els, such an approach has been in connection with amplitude rather than phase. This technique can, therefore, only be used to infer the flow component along an acoustic beam as opposed to measurement perpendicular to the acoustic paths, which is considered essential in this invention.
The reciprocal transmission approach has been used to measure flow along water channels by the rather complex procedure of setting up acoustic pa-ths at anyles to the flow, and the componel-lts oE Elow inEerred Erol,l the reciprocal travel times may then be combined to ded~ce the component of flow along the axis.
Aside frorn the additional complexity of set-up, this technique is subject to serious error due to the required ~5~

assu~ iolls r~3~,rdln~3 si~ i lari~y of tlle flo~ fie~d alol~y tlie sel~arate acoustic I)aths. ln~en flow Ineasurel.lents alor.g eacl path are combined to form the meall component alony the channel access, the resulting combination will only be an accurate representation of the flow if the co-nponents from which it is derived are based on similar flow fields. Since the components are derived froln paths that are quite different, this assumption cannot be generally valid.
Acoustic technique~ involvillg back scatter to determine a flow speed profile by scattering sound back to a receiver from sound speed inhomogeneities in the water column are also known, which the mean flow speed along the axis of the SOUIId path is found by the difference in travel time between two points for sound yoing in different directions. Back scdtter teel~lliyues are ir,effective, however, for horizcil-,tal rneasurernellts across a charlnel, but could be bottom lilounted to provi~e a profile at a sinyle locdtion as indicated herei~above.
Measurelnellt oE flow U5il19 a sin~31e transmitter and two receiver6 ~lave al~o l~r~vio-ls~y ~eell suy~;t:!te(~ irl an ar~icle by the named inventors hereirl (C.S. Clifford arld D. E'armer, "Ocean E'low Measurements U5i.ny Acoustic Scintillation", J.
ACOUS. SOC. AM., Volume 74 (6), pages 1826-1832, Dec. 1983).
In tlle experilllellt set fortll in tllis article, paral~el ~5~ g s~ ]~ lt~ s ~ lv.~ r~ c,r~ r~
re;~uire~l wi-l~ Le~r~ t~, tlle di~Lriblitiorl o~ tl-,e flo~J
profiles along the beam due to tlle use of a single transnlitter .
Surlmla~ of the_Inventlorl This invention provides a system and method for monitoring liquid flow across an area (such as a water channel and the like) to thereby provide accurate information with respect to the speed of such liquid flow, with such flow information being provided in real time and not being restricted to any particular spot in the broad area being monitored. Spaced acoustic beams are transmitted perpendicular to the direction of the flow of the liquid being monitored, and both phase and amp]itude measurernents are made with the receiver processing clrcuitry including complex demodulation with the demodulated output being shaped, converted, and then evaluated at a microcomputer to provide an indication of the liquid flow speed.
It is therefore ~n ol~ject oE this invelltLon to provide a system and method for measuring the flow speed of a liquid.

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It i.s another object of this invention to provi.de a system and method for measuring flow sE~eed across a broad area such as a water channel or the like.
It is another object of this invention to provide a system and method for measuring flow speed utilizing acoustic scintillation.
It is still another object of this invention to provide a system and method for measuring flow speed utilizing spaced acoustic beams transmitted perpendicular to the direction of liquid flow.
It is still another object of this invention to provide a system and method for measuring flow speed utilizing complex demodu1ati.on.
It is yet another object of this invention to provide a system and method for measuring flow speed utiliYi.ng signal shaping, conversion and eval.uation using a microcomputer.
With these and other objects in view, which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination, arrangement of parts and method substanti.ally as hereinafter describc(l at1d more particular defined by the appended claims, it being understood that char)ges are meant.
to be included as corne within the scope of the claims.

~4~i~g Ues~ri~tio~ of t'le irawin~s Tl~e accoJnl)allyillg dl-dwirl~s illu~trdte a coll~plete embodiment of the invention according to the best mode so devised for the practical application of the principles thereof, and in which;
FIGURE 1 is a cross-section schematic view of a water channel showiny positioning of the system of this invention with re~spect thereto;
FIGURE 2 is a top plan view of the channel as shown in FIGURE 1, illustrating positioning of the components of each array;
FIGURE 3 is a schematic electrical block diagram of the overall system of this invention;
FIGURE 4 is a typical example of an electrical signal derived from an acoustical signal received by the hydro~hones shown in FIGURE 3;
FIGURE 5 is a typical example of electrical signals derived frorn the acollsti.c signals received at the two hydrop}lones, as shown in F'IGI.IRE; 3, to i.llustral~ e arlgular difference<: there'bel~weerl; an(l FIGIJRE 6 i].lustrates, typlcally, the calculated cross-covariance of the signals received at the two hydrophones shown in FIGURE 3.

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As indicated in E~IGURES 1 and 2, this invention is particular useful. in providi.rig acoustic flow measl3relnellts rnade perpendicular to tlle direction of flow across a liquid flow area, and particularly across an expanse of water 9, such as a channel (the term "channel", as used herein, is meant to include any like expanse, or area, of water having a measurable flow rate). In addition, and for simplicity of discussion, the term "water" will be utilized, but it is meant to be realized that the invention could be utilized with other 1.iquids, and the invention is not meant to be l.imited to use in collnectioll witll water. To accomplish neasurement according to this invention, it is necessary to acoustically deploy a projector array 11 and a receiver array 13 at different sides 15 and 16, respectively, of the channel with the projectors and receivers of the arrays facing one another. Projector array 11 incl.udes at least two high frequency acoustic transducers, or projectors, 18 and 19 (as indicated in E'IGURES 2 and 3), and receiver array 13 includes at least two high ~requency receivers, such as a hydrophones 21 and 22 (as also indicated in FIGURES 2 and 3).

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B~tll projector arrdy 1l ancl receiver arr~y 13 are rigidly rno~nted, by tripods 24 an~ 25, respectively, as indicated in FIGURES 1 and 2, or by any other assembly fixed to the yround which could include, for example, pilings, bridge piers, and the like. The transducers are maintained at a fixed position horizontally suaced a short distance with respect to each other, which spacing includes a component parallel to the direction of flow of the water to be measured so that parallel transmission paths can be established through the water. Likewise, the receivers are maintained at a fixed position horizontally spaced a short distance with respect to each other, which spacing also includes a cornponent parallel to the direction to the flow of water to be measured so that the receivers are aligned with the projectors. When the projectors and the receivers of the arrays are mounted with the transducers and receivers aligned and facing one and another across the channel, a pair of parallel acoustic signal paths 27 and 28, as indicated in FIGURE 2, are established through the water perpendicular to water flow in the channel. In this manner, forward scattered ~ound waves in the water (as opposed to back scatter) are utilized to derive information about flow speed perpelldlcu]ar to the acoustic beams.

~ s in(3icated in IIGi~RIS 1 arld 2, projector array ~1 is connected with radio transi,litter 3~ tllrough cable 31, and receiver array 13 is connected with radio receiver 33 throuyh cable 34 to th~reby establish a radio transmission link between the arrays utilizing antennas 36 and 37 at radio transmitter 30 and radio receiver 33, respectively.
This radio transmission iink is used for transferring t:ime, or synchrollization informa-tion between arrays so that the phase of the receive~ signal can be measllred precisely with respect to that of the transmitted signal. To accomplish this end, the sync}lronizing signal co~nonly has the same format as, and is in phase with, the acoustic signal (which also may be col~trolled by the same synchroni~ing siynal).
Each of the acoustic signal paths 27 and 28, extending across the channel from the transducers to the receivers, have three principal components, as indicated in FIGURE 1, including a direct path 39 and two reflected paths, with one path 40 being reflected from surface 41 of the water and the other path 43 being reflected from the bottom 44 of the channel. Where needed or desired, additional transducers and receivers could be utilized to form additional acoustic signal pat}ls, and/or cross paths could likewise be utilized (i.e., between transducer 18 and receiver 22 and between transducer 19 and receiver 21). When utilizing cross paths, ~5~$~9 spatial aperture filteril~g tec~ ues, sucl~ as have heretofore been developed for remote sensing oE
electromagnetic radiation ~see, for example, Lee, "Remote Probing Using Spatially Filtered Apertures, J. OPT. SOC.
AM., Volume 64(10), pages 1295-1303, 1974) can be utilized to derive flow speeds at different points across a channel.
For this purpose, more than two projectors and two hydrophones may usefully be employed for improved spatial resolution.
A blocX diagram of the overall device of this invention is shown in FIGURE 3. As shown, projector side 46 includes projectors, or transducers, 18 and 19 separately connected with signal yenerator 48 through power amplifiers 50 and 51, respectively. Signal generator 48 provides a pulse output to each projector with the pulse output from transducer 18 occurring timewise before the pulse output from transducer 19 (which can be accornplished by providing the pulses from the generator in succession to each transducer). Pulse outputs are repeatedly provided to the transducers with a predetermined pause between each pulsing cycle.
In the simplest form, signal generator 48 can produce simple rectangular pulses at a desired frequency. More sophisticated processes can be utilized involving coding or spread spectrum techrliques to improve signal-to-noise ratios and tlle st~l~ility o t1~e resu1ting signal. Wl1i~e such so~ isti~:ate(l sigll~ls mig~1t 1~ desira~le f~r i,arti~ml~r applicatior1s, tl1ey are not considered essential to thiS
lnvention.
Clock 52, shown in FIGURE 3 connected with radio transmitter 30 and siynal generator 48, provides a synchronizing, or timing, signal to both the projector side 46 and to the receiver side 53, with the receiver side 53 receiving the timing signal through the radio transmission link.
As indicated in FIG~RE 3, receiver side 53 includes hydrophones 21 and 22. Hydrophone 21 is connected through preamplifier 55 to complex demodulator 56, while hydrophone 22 is connected througl1 preamplifier 58 to complex demodulator 59. Complex demodulators 56 and 59 receive the timing input (clock) input signal from radio receiver 33.
The output signal from complex demodulator 56 is coupled through low pass filter 61 to analog-to-digital converter 62, with the digital output signal from converter 62 being coupled to microcomputer 64. In like manner, the output signal from complex demodulator 59 is coupled through low pass filter 66 to analog-to-digital converter 67, with the digital output signal from converter 67 being also coupled to microcomputer 64.

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lrl thi.s i.n~ nti.o~., tlle use oI pllaSe l.le~3SUrt~ llt, toge~her with alllplitude lileasurelilellt, as oppose~ to t~le use of amplitude meclsurelnent alone, is important. With each of the projectors driven by the same clock, the sound transmitted by each is in phase with that of the other. At the receiving end, the useful portion of the signal is encoded in the flucuations in phase and amplitude, rather than in the actual value of the phase relative to the source. It is merely sufficient that the clock be stable enouyh such that over the period taken for inhomogeneities in the flow to pass through the successive acoustic paths, no aupreciable uncertainty in phase drif~ exists between the projector and the receiver clocks. Clocks having this stability are presently readily available.
Utilizing a pair of projectors and a pair of receivers, deployed as shown in FIGURES 1 and 2, small temperature or salinity (and hence sound speed) fluctuations can be sensed since they are always present in a water mass moving through a channel (at speed v). These fluctuations resu].t in phase perturbations and amplitude scintillations (analogous to the effect of a twinkling star) at the receivers 21 and 22.
Such fluctuations occur at many scales in the water, from a few millimeters up to the full channel width.

As the c~rrent passes tl-rouyh the acoustic beams, it carries these sound speed variations with it, resulting in a corresponding horizontal translation of tlle phase of the amplitude fluctuations across the receiving array. When using two parallel paths, as indicated in the example of FIG~RE 2, time of travel through the paths is x/v, where x is the path spacing and v is the current speed. Correlation analysis (as brought out hereinafter) can then be carried out by the microcomputer in real time to provide a direct measurement of the translation of the fluctuations, and hence of the flow speed, perpendicular to the acoustic paths.
Separation between acoustic signal paths is accomplished on the receiver side using an appropriate bit width in the signal coding (or pulse width if signal coding is not employed), which bit width (or pulse width) must be appreciably less than the separation of arrival times of the direct path and reflected signals.
Complex demodulation of the signal in each receiver channel is standard ~nd ~imply requires prod~lct detection by multiplication of the signal with the in-phase and quadrature of the carrier wave referenced to the clocX
signal. After demodulation in each receiver channel, two outputs are produced representing the in-phase and ~s~

-quadrature cor,lpollents of the signal. The high frequency portion of each of these components is removed by low pass filtering, and the signals are then digitized by the analog-to-digital converter, in conventional manner.
For each receiver (hydrophone), the digitized in-phase and quadrature components are fed to microcomputer 64 for processing. The task of the microcomputer is to determine the time taken for the inhomogeneties in refractive index to pass between one path and the other.
Following transmission of each pulse by a transducer at the projector side of the channel, the signal travels throuyh the water to a hydrophone at the receiver side of the channel via several possible paths (as indicated in FIGURE 1). The direct route 39 is of primary interest and must be separated from surface and bottom reflections 40 and 43.
An example of the received signal strengths (derived from the modulus of the complex signal) is shown in FIGURE
4. The direct path is identified by peak (i), the surface and bottom paths are identified by the peaks tii) and (iii), and Inore cornplex paths are also indicated that decay afterward. The same process i9 repeated with the next transmission, for which (i)', (ii)', and (iii)' are identified in FIGURE 4 as the corresponding peaks.

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Tlle microcolnL)uter first identifies the I)eak corresponding to the direct pulse. This task is simplified by searching only within an appropriately located narrow window. Having identified the peak, the corresponding phase of the angle is determined (proper combining of the in-phase and quadrature signals in each channel yields phase information, while processing in either signal yields amplitude information). This is done repeatedly for each hydrophone to generate phase time series as shown typically in FIGURE 5 (wherein Rl refers to hydrophone 21 and R2 refers to hydrophone 22). Similar times series may likewise be generated for amplitude.
While many techniques can be used for determining the time taken for passage of inhomogeneties between the two paths, the now preferred technique is to identify two appropriate algorithms for application to the phase and amplitude information that is available from received acoustic signals (numerous algorithms have been described for application to electromagnetic radiation experiments in the atmosphere - see, for example, Wang, Ochs, and I.awrence, App. Optics, Dec. 1981). Phase information is not normally available, however, in studies using electromagnetic radiation, because of the much higher frequencies involved.

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In one techllique t~ t has been fou~ suc~:essf,ll in experimental veLification of certain components of this invention, the cross-covariance of the two phase signals is calculated (shown as a solid curve in FIGURE 6). The displacement of the pea~ is determined, and the physical separation of the acoustic paths divided by the time displacement thus calculated, gives the mean flow speed.
In a second approach, known as the Briggs approach, the auto-covariance function (shown as a dashed curve in FIGURE
6) is also calculated, and the intersection point of the two curves (auto- and cross-covariance) identified. The mean flow speed is then determined.
An output from rnicrocornputer 64 is thus provided in the form of mean flow speed. If several transmitters and receivers are used with spatial aperture filtering, the output from the microcomputer can then be in the form of flow profile information. This output information can be transmitted as needed for real time displa~, and/or, if desired, can be coupled to a voice synthesizer for real time navigation information via radio.
The system, as indicated in FIGURE 2, shows the acoustic paths following straight lines. Over sufficiently long paths, refraction will cause the paths to become curved and under certain circumstances may preclude a direct and ~?c5~ g ur~reflecte~ ath bet~een projectors and hydropllones. In this situation, iL will, under some circumstances, still be possible to derive flow speed estimates from the reflected paths, but the depth of the measurernent will be spread over the depths traversed by the reflected sound paths.
Moreover, even if direct paths are available, the reflected paths (as indicated in FIGURE 2) will provide additional information on flow speed in different parts of the water column. For measurements of flows in channels where the desired result is an estimate of volume flow per unit time, it is necessary that the acoustic system be installed at more than one depth so that an integral volume flux may be inferred.
Alternately, a sound channel may exist, in which case the range of measurement over which wholly refracted (rather than reflected) paths are available will be greatly increased. The techniques described herein will be applicable in this case also.
Choice of acoustic frequencies and repetition rates are governed by the range over which arrangements are to be made in the closeness o multipath arrival time.
As an ancillary measurement, the transverse, or cross channel component, of the current (as opposed to cross-channel measurements) can also be recovered by using ~5~

reciprocal tral-lslilissiorl, in wllich acoustic signals are transmitted and received from b~th ends of t~le path.
Processing of tlle resultant travel times in each direction yields both a mean sound speed value and also a mean current along the axis of the acoustic path. Since only a single reciprocal path is necessary for these measurements, the results are unambiguous. Moreover, the measurements exploit the phase of the received signal, rather than the amplitude alone, thus greatly enhancing the overall sensitivity and accuracy.
It is meant to be realized that changes and modifications can be made with respect to the exact embodiment of the invention, as disclosed, without departiny from the intended scope of the invention. For example, the synchronizing, or timing, signal transmission link could be established utilizing cables, microwaves, and/or optics, the clock could be at the receiver side with the transmission link being then from the receiver side to the transmitter side, separate clocks could be utilized to eliminate the need for a timing signal transmission link (or to at least greatly simplify the link) where such clocks have the necessary sufficient accuracy and stability for measure~nent purposes as contemplated by this invention, the signal generator could be incorporated into the receiver side and ~ ~54~

transl~litted therefrolll to the projectors at the projector side, and/or components ShOWIl in specific signal paths could be combined through use of appropriate switching circuitry.
From the foregoing, it is to be appreciated that this invention provides a novel system and method for measuring liquid flow through a predetermined area utilizing acoustic beams positioned perpendicular to the direction of the cornponent ofliquid flow to be measured..

Claims (14)

1. A system using acoustic signals for measuring the flow characteristics of water flowing through a predetermined area, said predetermined area having at least first and second opposite sides and a bottom with the distance between the sides being such that an acoustic signal transmitted through said water therebetween would include a direct acoustic signal path and at least one reflected acoustic signal path formed by said acoustic signal reflecting from at least one of the surface of said water and said bottom of said predetermined area, said system comprising:
- a receiver array located at said first side of said predetermined area, said receiver array including at least two receivers spaced with respect to one another in a direction having at least one component parallel to the direction of flow of said water through said predeter-mined area;
- a projector array located at said second side of said predetermined area, said projector array including at least two transducers spaced with respect to one another in a direction having at least one component parallel to the direction of flow of said water through said predeter-mined area, said projector array being mounted with respect to said receiver array so that at least a portion of the acoustic signals transmitted from said transducers pass through said water in a direction substantially perpendi-cular to the direction of flow of said water so as to be CLAIMS (cont.)

1. (cont.) modified relative to flow direction before being received by said receivers;
- timing signal generating means for generating signals providing both phase and amplitude references;
- acoustic signal generating means connected with said timing signal generating means and said projector array to cause each of said transducers to transmit pulsed acoustic signals through said water; and - signal processing means including signal selec-ting means, said signal processing means being connected with said timing signal generating means and said receiver array to receive signals from said receivers indicative of received acoustic signals travelling through said direct and reflected acoustic signal paths between said projectors and said receivers, said signal selecting means being capable of separately selecting received acoustic signals from said direct and reflected paths to thereby enable utilization by said signal processing means of at least received acoustic signals travelling through said direct acoustic signal paths for derivation of phase and amplitude information therefrom relative to said phase and amplitude references of said timing signal generating means so that, responsive thereto, said signal processing means provides an output indicative of the speed of said water flowing through said predeter-mined area.

CLAIMS (cont.)

2. The system of claim 1 wherein said signal selecting means enables utilization of received acoustic signals travel-ling through at least one of said reflected acoustic signal paths in addition to enabling utilization of received acoustic signals travelling through said direct acoustic signal paths for derivation of phase and amplitude information therefrom so that, responsive thereto, said signal processing means provides an output indicative of the speed of said water flowing through different parts of said predetermined area.

3. The system of claim 2 wherein said at least two receivers and said at least two transducers are each installed at more than one depth at their respective said sides of said predetermined area, and wherein said signal selecting means separately identifies and selects said signals indica-tive of said received acoustic signals travelling through said direct and said reflected acoustic signal paths between each of said projectors and said receivers for derivation of phase and amplitude information therefrom so that, responsive thereto, said processing means provides an output indicative of the volume flow per unit of time of said water through said predetermined area.

4. The system of claim 1 wherein said projector array and receiver array are positioned at opposite sides of an open channel, and wherein different ones of said transducers and receivers are substantially aligned with respect to one another so that separate paths are established through said water between said transducers and receivers.

CLAIMS (cont.)

5. The system of claim 1 wherein said timing signal generating means includes a clock directly connected with one of said signal generating means and said signal processing means, and connected to the other of said signal generating means and said signal processing means through a transmission link.

6. The system of claim 5 wherein said transmission link is a radio transmission link.

7. The system of claim 1 wherein said signal gene-rating means includes a signal generator for generating a pulsed output signal and a pair of power amplifiers for separately receiving said pulsed output signal and coupling the same to said transducers at predetermined different times.

8. The system of claim 1 wherein said signal process-ing means includes first and second electrical channel means for separately processing the signals received from each said receiver, with each of the first and second electrical channel means including signal multiplying means for pro-viding in-phase and quadrature output components of said signals indicative of phase and amplitude information.

9. The system of claim 8 wherein each of said elec-trical channel means also includes low pass filter means and analog-to-digital converter means.

10. The system of claim 9 wherein said signal process-ing means includes a microcomputer for providing said out-put indicative of the flow speed of said liquid.

CLAIMS (cont.) 11. A method for using acoustic signals for measuring the flow characteristics of water flowing through a predeter-mined area, said predetermined area having at least first and second opposed sides and a bottom, with the distance between the sides being such that an acoustic signal trans-mitted through said water therebetween would include a direct acoustic signal path and at least one reflected acoustic signal path formed by said acoustic signal reflecting from at least one of the surface of said water and said bottom of said predetermined area, said method comprising:
- generating timing signals providing both phase and amplitude references;
- generating pulsed acoustic signals based upon said timing signals and transmitting said pulsed acoustic signals from at least two locations at said first side of said predetermined area through said liquid substantially perpendicular to the direction of flow of said liquid with said pulsed acoustic signals being modified relative to said flow direction during passage through said liquid, said locations being spaced with respect to one another in a direction having at least one component parallel to the direction of flow of said water through said predetermined area;
- separately receiving said pulsed acoustic sig-nals at said second side of said predetermined area after passage of said signals through said liquid and forming electrical output signals indicative of said received pulsed CLAIMS (cont.)

11. (cont.) acoustic signals travelling through said direct and reflected acoustic signal paths;
- separately processing said electrical output signals, including separating said electrical output signals indicative of said received acoustic signals travelling through said direct paths from said electrical output sig-nals indicative of said received acoustic signals travelling through said reflected paths, for derivation of amplitude and phase indications therefrom relative to said phase and amplitude references provided by said timing signals; and - utilizing said derived amplitude and phase in-dications to provide an output indicative of the speed of said water flowing through said predetermined area.

12. The method of claim 11 wherein said step of sepa-rately processing said electrical signals includes multiply-ing and filtering of said signals and wherein said indications are the in-phase and quadrature components of said signals.

13. The method of claim 12 wherein said step of sepa-rately processing said electrical signals includes digitally converting said signals after multiplying and filtering has occurred.

14. The method of claim 12 wherein said step of sepa-rately processing said electrical signals includes providing a microcomputer for receiving said indications and process-ing the same to thereby provide said indication of the speed of said liquid in said predetermined area.