SIST EN ISO 6416:2018

SLOVENSKI STANDARD
01-februar-2018
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SIST EN ISO 6416:2005
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Hydrometry - Measurement of discharge by the ultrasonic transit time (time of flight)
method (ISO 6416:2017)
Hydrometrie - Messung des Durchflusses mit dem Ultraschall-Laufzeitverfahren (Transit-
time-/Time-of-flight-Verfahren) (ISO 6416:2017)
Hydrométrie - Mesure du débit par le temps de transit ultrasonique (temps de vol)
méthode (ISO 6416:2017)
Ta slovenski standard je istoveten z: EN ISO 6416:2017
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 6416
EUROPEAN STANDARD
NORME EUROPÉENNE
November 2017
EUROPÄISCHE NORM
ICS 17.120.20 Supersedes EN ISO 6416:2005
English Version
Hydrometry - Measurement of discharge by the ultrasonic
transit time (time of flight) method (ISO 6416:2017)
Hydrométrie - Mesure du débit par la méthode du Hydrometrie - Messung des Durchflusses mit dem
temps de transit ultrasonique (temps de vol) (ISO Ultraschall-Laufzeitverfahren (Transit-time-/Time-of-
6416:2017) flight-Verfahren) (ISO 6416:2017)
This European Standard was approved by CEN on 22 August 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6416:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
European foreword
This document (EN ISO 6416:2017) has been prepared by Technical Committee ISO/TC 113
“Hydrometry” in collaboration with Technical Committee CEN/TC 318 “Hydrometry” the secretariat of
which is held by BSI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by May 2018, and conflicting national standards shall be
withdrawn at the latest by May 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 6416:2005.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 6416:2017 has been approved by CEN as EN ISO 6416:2017 without any modification.

INTERNATIONAL ISO
STANDARD 6416
Fourth edition
2017-10
Hydrometry — Measurement of
discharge by the ultrasonic transit
time (time of flight) method
Hydrométrie — Mesure du débit par la méthode du temps de transit
ultrasonique (temps de vol)
Reference number
ISO 6416:2017(E)
©
ISO 2017
ISO 6416:2017(E)
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
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Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved

ISO 6416:2017(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Applications . 1
4.1 Types of applications . 1
4.2 Attributes and limitations . 2
5 Method of measurement . 2
5.1 Discharge . 2
5.2 Calculation of discharge from the transit-time measurement . 3
6 Flow velocity determination by the ultrasonic (transit time) method .3
6.1 Principle . 3
6.2 Sound propagation in water . 5
6.2.1 General. 5
6.2.2 Speed of sound in water . 6
6.2.3 Propagation losses . 6
6.2.4 Signal path bending . 8
6.2.5 Reflection . . 9
7 Gauge configuration .10
7.1 General .10
7.2 Single-path systems .10
7.3 Multi-path systems .11
7.4 Crossed-path systems .11
7.5 Reflected-path systems .13
7.6 Systems using transponders .14
7.7 Wireless systems (if a cable crossing is not possible) .15
7.8 Systems using divided cross-sections .16
7.9 Sloping paths .16
8 Determination of discharge .17
8.1 Single-path systems .17
8.2 Multi-path systems .18
8.2.1 General.18
8.2.2 Mid-section method .19
8.2.3 Mean-section method .20
8.3 Systems with transducers in the channel .21
9 System verification and calibration .22
10 Site selection .23
11 Site survey — Before design and construction .23
11.1 General .23
11.2 Visual survey .23
11.3 Survey of the cross-section .24
11.4 Survey of velocity distribution .24
11.5 Survey of signal propagation .24
12 Operational measurement requirements .24
12.1 General .24
12.2 Basic components of flow determination .25
12.3 Water velocity determination .25
12.4 Determination of water stage or depth .25
12.5 Determination of mean bed level .26
12.6 Channel width .27
ISO 6416:2017(E)
13 Gauging station equipment .27
13.1 General .27
13.2 Design and construction of equipment .28
13.2.1 Transducers.28
13.2.2 Transducer cables .29
13.3 Reflectors .29
13.4 Civil engineering works .32
13.5 Signal timing and processing .32
13.5.1 General.32
13.5.2 Signal-to-noise ratio .32
13.5.3 Signal maintenance (gain control) .33
13.5.4 Signal detection . .33
13.5.5 Post-detection filtering .34
13.6 System self-checking .34
13.7 Site-specific data (or site parameters) .35
13.8 Clock and calendar .35
13.9 System performance criteria .35
13.9.1 General.35
13.9.2 Operating environment .36
13.9.3 Water environment.36
13.9.4 Mechanical environment .36
13.9.5 Extreme environmental conditions .36
13.9.6 Power source .36
13.9.7 Measurement uncertainty .36
13.10 System output .37
13.10.1 Local display .37
13.10.2 Local record .37
13.10.3 Remote record .37
13.10.4 Diagnostic information .37
13.11 Installation .37
13.12 Commissioning .38
13.13 Operating manual .38
13.14 Maintenance .39
14 Measurement uncertainties .40
14.1 General .40
14.2 Definition of uncertainty .40
14.3 Uncertainty in discharge .41
14.3.1 Uncertainty equation .41
14.3.2 Effective number of paths .42
14.3.3 Uncertainty in the line velocity, U .
lv 42
14.3.4 Uncertainty in the channel width estimation, U .42
w
14.3.5 Examples of uncertainty estimation .43
14.3.6 Uncertainty estimate at low flow .44
14.3.7 Uncertainty estimate at high flow .45
Annex A (informative) Principle of measurement uncertainty .47
Annex B (informative) Performance guide for hydrometric equipment for use in technical
standard examples .54
Bibliography .58
iv © ISO 2017 – All rights reserved

ISO 6416:2017(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1,
Velocity area methods.
This fourth edition cancels and replaces the third edition (ISO 6416:2004), which has been technically
revised. The main changes from the previous edition are:
— the title has been changed;
— a new subclause (7.7) on wireless systems has been added;
— former subclauses 9.2 and 11.6 have been removed;
— Clause 10 on site selection has been revised;
— Annex A (Principle of measurement uncertainty) and Annex B (Performance guide for hydrometric
equipment for use in technical standards) have been added.
INTERNATIONAL STANDARD ISO 6416:2017(E)
Hydrometry — Measurement of discharge by the ultrasonic
transit time (time of flight) method
1 Scope
This document describes the establishment and operation of an ultrasonic (transit-time) gauging
station for the continuous measurement of discharge in a river, an open channel or a closed conduit.
It also describes the basic principles on which the method is based, the operation and performance of
associated instrumentation and procedures for commissioning.
It is limited to the “transit time of ultrasonic pulses” technique, and is not applicable to systems that
make use of the “Doppler shift” or “correlation” or “level-to-flow” techniques.
This document is not applicable to measurement in rivers with ice.
NOTE This document focuses on open channel flow measurement. IEC 60041 covers the use of the technique
for full pipe flow measurement.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 772, Hydrometry — Vocabulary and symbols
ISO 4373, Hydrometry — Water level measuring devices
ISO/TS 25377, Hydrometric uncertainty guidance (HUG)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
4 Applications
4.1 Types of applications
a) Open channels
b) Multiple channels
c) Closed conduits
This method does not need a man-made or natural control, as it does not rely upon the establishment of
a unique relationship between water level and discharge.
ISO 6416:2017(E)
4.2 Attributes and limitations
The following attributes and limitations shall be considered when deploying this measuring system.
Attributes
1. Potential for high accuracy
2. Tolerant of back water effects
3. Able to measure multiple channels and combine results to give total flow
4. Capable of determining individual velocities at distinct heights within the water column
5. Visually unobtrusive
6. Fish friendly
7. Mains power supply not essential
8. Intrinsically safe systems available for use in explosive atmospheres
9. No obstruction or head loss
10. Suitable for large range of channel widths and depths
11. Potential for built in redundancy
12. Potential for relatively low operating costs

Limitations
1. A site with an unstable cross section needs to be avoided if possible
2. Requires minimum depth of water to operate
3. May require cables to both sides of channel
4. Ragging of sensors by trash
5. Potential attenuation of acoustic signal by
suspended solids
weeds
entrained gasses
temperature gradients
salinity gradients
Detailed explanations of these attributes and limitations can be found in clauses throughout this
document.
5 Method of measurement
5.1 Discharge
5.1.1 Discharge, as defined in ISO 772, is the volume of liquid flowing through a cross-section in a
3 −1
unit time. It is usually denoted by the symbol Q and expressed in cubic metres per second (m ·s ). The
definition of discharge is the product of the wetted cross-sectional area and the mean velocity vector
perpendicular to it.
2 © ISO 2017 – All rights reserved

ISO 6416:2017(E)
Thus:
Qv=×A (1)
where
3 −1
Q is the discharge, expressed in cubic metres per second (m ·s );
−1
is the mean velocity, expressed in metres per second (m·s );
v
A is the cross-sectional area, expressed in square metres (m ).
The transit-time method is a velocity-area method using flow velocities which have been determined by
the equipment, and which are averaged along one or more lines which are usually, but not necessarily,
horizontal.
5.2 Calculation of discharge from the transit-time measurement
5.2.1 Discharge can be computed using the velocity-area method (see 5.1), provided that a relation
can be established between the velocities determined by the transit time ultrasonic system and the mean
cross-sectional velocity. If there are sufficient operational paths distributed sufficiently throughout the
vertical to define the velocity profile, the resulting samples of flow velocity can be vertically integrated to
provide an estimate of the mean cross-sectional velocity. Alternatively, if there are insufficient operational
paths, a relationship between measured velocity (index velocity) and mean velocity can be established
using a spot flow gauging technique, e.g. rotating element current meter or acoustic Doppler current
profiler (ADCP).
5.2.2 The discharge calculation also requires the cross-sectional area of the water to be known.
An ultrasonic transit-time system will, therefore, normally be capable not only of making sample
measurements of velocity, but also of determining (or accepting a signal from some other device
determining) water depth, and of storing details of the relation between water depth and cross-sectional
area. It will also normally be capable of executing the mathematical functions necessary to compute flow
from the relevant stored and directly determined data.
6 Flow velocity determination by the ultrasonic (transit time) method
6.1 Principle
6.1.1 An ultrasonic pulse travels in a downstream direction faster than a similar pulse travels upstream.
The speed of a pulse of sound travelling diagonally across the flow in a downstream direction will be
increased by the velocity component of the water. Conversely, the speed of a sound pulse moving in the
opposite direction will be decreased. The difference in the transit time in the two directions can be used
ISO 6416:2017(E)
to resolve both the velocity of sound in water as well as the component of the velocity along the path
taken by the ultrasonic pulses.
Key
1 v component of water velocity along the path
path
2 v component of water velocity in the direction of the flow
line
3 direction of flow
4 channel width
5 ultrasonic path length (L)
A, B transducers
θ angle between the path and the direction of flow
y downstream distance between transducers
Figure 1 — Schematic illustrating the general principle
6.1.2 For the path between transducers A and B in Figure 1, the transit-times for the ultrasonic
pulses are:
t = L/(c − v cosθ )  and  t = L/(c + v cosθ ) (2)
AB BA
where
t   is the transit time from transducer A to B, in seconds;
AB
t is the transit time from transducer B to A, in seconds;
BA
L is the path length (distance between transducer A and transducer B), in metres (m);
−1
c is the speed of sound in water, in metres per second (m·s );
θ is the angle between the path and direction of flow.
Resolving for line velocity:
v = L × (t − t ) / (t × t × 2 cosθ ) (3)
line AB BA AB BA
where v is the line velocity or the average velocity of the water across the channel in the direction of
line
−1
flow, in m·s .
4 © ISO 2017 – All rights reserved

ISO 6416:2017(E)
6.1.3 The calculation of water velocity is
— independent of the speed of sound in water,
— proportional to the difference in transit times,
— inversely proportional to the product of the transit times,
— critically dependent on the angle between the path and the direction of flow (see Table 1).
Table 1 — Systematic errors incurred if the assumed direction of flow is not parallel to the
channel axis
Path angle, θ Velocity error for 1° difference between
actual and assumed flow direction
degrees %
30 1,0
45 1,7
60 3,0
6.1.4 In open-channel flow measurement, practical considerations will normally dictate that
a) the transducers at either end of an “ultrasonic path” are located on opposite banks of the
watercourse;
b) the line joining them should be at an angle between 30° and 65° to the mean direction of flow to
minimize uncertainties.
6.1.5 The following limitations are encountered in open-channel flow measurement.
a) At intersection angles greater than 65°, the time difference between sound pulses in opposite
directions may become small and therefore subject to a relatively large uncertainty, especially at
low velocities.
b) At an angle of 90°, there will be no time difference between forward and reverse pulses, and thus
velocity cannot be determined.
c) With large angles, there is also an increase in the error in velocity computation that results from
assumptions made in the assessment of the angle. Table 1 demonstrates this effect.
d) At intersection angles less than 30°, the following problems can arise.
1) The length of the channel occupied by the gauge can become excessive, and cease to be quasi-
uniform.
2) The direction of flow relative to the path may not be constant.
3) There can be practical problems with site selection, due to the length of the channel which is
required to be set aside for the flow gauge, and maintained free of debris and weeds.
4) The excessive length of the paths can cause problems of signal strength and/or signal reflection
from the channel bed or water surface, especially if vertical temperature gradients are present.
6.2 Sound propagation in water
6.2.1 General
Sound is a mechanical disturbance of the medium in which it propagates. It encompasses a wide range
of frequencies. The audible range is from approximately 20 Hz to 20 000 Hz, and is generally referred
ISO 6416:2017(E)
to as “sonic”. Frequencies less than 50 Hz are usually termed “subsonic”, and those above 15 000 Hz
“ultrasonic”. Transit-time systems operate in the ultrasonic range at frequencies typically between
100 kHz and 1,5 MHz.
The performance of transit-time systems depends heavily on the characteristics of sound propagation
in water. These characteristics are briefly described here.
6.2.2 Speed of sound in water
The speed of sound in water is independent of frequency, but depends on the temperature, salinity and
pressure of the water. In open channels, the effect of pressure is negligible. Over the normal ambient
−1
temperature range, the speed of sound in fresh water varies from about 1 400 m·s to a little over
−1
1 500 m·s (see Table 2). This will vary dependent on the characteristics of the water. However, these
figures are offered as a guide based on a review of the available literature.
Table 2 — Speed of sound in non-saline water at different temperatures
Temperature Speed of sound (approximate)
−1
°C m·s
0 1 402
10 1 447
20 1 482
30 1 509
40 1 529
NOTE 1 The above figures apply to the water in most natural fresh-water rivers and foul sewers.
−1
NOTE 2 In seawater, the corresponding speeds are approximately 50 m·s higher.
The speed of sound c in water is given by:
cT=+1402,,45 01 −+0,,05510TT00022 +
(4)
2 2
13,,30SS+−00013 0,0013TS+0,,00010TS+ 016d
where
−1
c is the speed of sound in water, in metres per second (m·s );
T is the water temperature, in degrees Celsius;
S is the salinity of the water, in grams salt per litre water;
d is the depth of water, in metres (m).
6.2.3 Propagation losses
6.2.3.1 Transmission of sound in water
6.2.3.1.1 Only a portion of the acoustic energy transmitted reaches the target. The remainder is lost for
a variety of reasons. The loss in signal strength is called “propagation loss”, which consists of spreading
loss (6.2.3.1.2) and attenuation loss (6.2.3.1.3).
6.2.3.1.2 Spreading loss is the reduction in acoustic intensity due to the increase in area over which the
given acoustic energy is distributed. Losses due to this effect depend on the following factors:
— path length;
— diameter of ultrasonic transducer;
6 © ISO 2017 – All rights reserved

ISO 6416:2017(E)
— frequency characteristics.
6.2.3.1.3 Attenuation loss is the reduction in the acoustic intensity caused by the resistance of the
medium to the transmission of acoustic energy. It is analogous to the loss of electrical energy in a wire
where there is no spreading loss.
Attenuation loss is attributable to scattering and absorption.
— Scattering is the redirection in all directions of the incident acoustic wave energy by suspended
matter in the water, e.g. air bubbles and suspended solids. The effect is greater at higher transducer
frequencies.
— Absorption is the process by which acoustic energy is converted into thermal energy by the friction
in the water, when it is subjected to repeated compressions and expansions by a passing sound
wave. This effect is also frequency dependent.
Losses due to absorption and scattering increase exponentially with increasing path length. This
means that if the suspended solids loading in sewer water were such as to cause a loss of half the signal
energy when the signal propagates through a metre of water, then that signal would be halved again
after passing through another metre of water. For a path length of 20 m, the signal would be reduced to
one millionth of the value expected for clean water.
For a 5 m path length in a foul sewer, a signal reduction of a factor of 30 (a factor of about 5,5 in voltage)
would be tolerable. For a 20 m path length, it is unlikely that any signal would be observable.
For these reasons, transducers of lower frequency are used for the longer paths. The range of values of
transducer frequency, f, for a given path length, L, is illustrated in Figure 2.
6.2.3.2 Reverberation
Reverberation is the energy returned by reflectors other than the transducers. This is analogous to the
effect which reduces the effectiveness of car headlights on a foggy night.
6.2.3.3 Refraction
This is the bending of the acoustic pulse path if the water varies significantly in temperature or density.
For example in slow moving rivers, with poor vertical mixing, the effect of the sun on the surface may
produce a vertically distributed temperature gradient.
6.2.3.4 Reflection
Sound can be reflected from the water surface and/or the bed of the river which can cause errors in the
signal timing.
ISO 6416:2017(E)
Figure 2 — Commonly used transducer frequencies for various path lengths
6.2.4 Signal path bending
6.2.4.1 The path taken by an acoustic pulse is bent (refraction), if the water through which it is
propagating varies significantly in either temperature or salinity. In slow-moving rivers, with poor
vertical mixing, the effect of the sun upon the surface produces an upward directed temperature gradient
vector. This causes the speed of sound to be higher near the surface and, consequently, the acoustic path
to bend towards the river bed.
The acoustic wave propagates across the channel as a cone. If a vertical temperature gradient,
as described above, exists only that ray which starts in a certain upward direction will arrive at the
other end of the path. With a temperature gradient of 0,5 °C per metre of depth, over a horizontal path
length of 50 m the vertical deflection D (as defined in Figure 3) will be about 0,5 m. In contrast, the
r
effect of vertical density gradients (such as may be associated with salt water intrusion into the gauged
reach) is to create a higher speed of sound near the bottom and thus to bend the path towards the
surface.
Similar effects can be produced by horizontally distributed temperature or density gradients, as is the
case with partial shading of the water surface from insolation such as found at the confluence where a
tributary with waters of contrasting characteristics joins.
6.2.4.2 The approximate degree to which the signal path is bent is given by:
R = c (d − d ) / (c − c) (5)
1 2 1 1 2
where
R is the radius of curvature of the ultrasonic path, in metres (m) (see Figure 3);
−1
c , c are the speeds of sound at depths d and d respectively, in metres per second (m·s ). [These
1 2 1 2
speeds can be calculated using Formula (4).]

8 © ISO 2017 – All rights reserved

ISO 6416:2017(E)
Key
1 transducer
2 transducer
D deflection of the ultrasonic path
r
L path length
R radius of curvature of the ultrasonic path
Figure 3 — Signal bending as a result of a vertical temperature gradient
The deflection, D , of the ultrasonic path from a straight line is given by
r
DR=− RL−0,25 (6)
r ()
where L is the path length, in metres.
6.2.5 Reflection
6.2.5.1 Sound is scattered from the water surface and, to a lesser extent, from the channel bed. This
is due to the fact that the contrast in acoustic impedance is much higher between water and air than
between water and the bottom (sand, rock, mud).
Errors in signal timing will occur if the secondary signal interferes with the first cycle of the direct
signal. To avoid this effect, the difference in the two paths shall exceed one acoustic wavelength (speed
of sound/frequency). Th
...