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THE EVALUATION OF DUAL BEAM ECHO SOUNDERS
IN HYDROGRAPHIC SURVEYING

Dean Robert Seidel

NAVAL POSTGRADUATE SCHOOL

Monterey, California

THESIS

THE

EVALUATION OF DUAL BEAM ECHO SOUNDERS

IN HYDROGRAPHIC SURVEYING

Dean Robert Seidel

September 1979

Thesis Advisor: D. E.

Nortrup

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The Evaluation of Dual Beam Echo
Sounders in Hydrographic Surveying

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September 19 79

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Dean Robert Seidel

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1*. KEY WORDS Continue on rereree aid* II neceeeoty and Idmntltr or black number)

Dual Beam Echo Sounder

Echo Sounder

Hydrography

Dual Frequency Echo Sounder

20. ABSTRACT ( Continue an rovorao aide It n*c«*«arr and Idmntltr »F block number)

DO , ;S"ti 1473
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The dual beam echo sounder, equipped with a narrow and wide
beam, sounding concurrently, represents a relatively
inexpensive means to increase the detection capabilities,
while preserving the narrow beam operation.

The wide beams detected significant peaks that were
absent on the narrow beam trace. The wider hyperbolic
returns of the wide beams served to emphasize the narrow
beam returns over features with little horizontal extent.
The narrow versus wide beam depth differences over feature
peaks were found useful in isolating the peak's apex.

DD Form 1473

1 Jan 73 „ — - — „

S/N 0102-014-6601 2 iieumrv classification or this **cerw*»« »••• »-.»•«*)

ABSTRACT

A limited area hydrographic survey was conducted in
shallow water, using a launch equipped to sound concurrently
with three beam widths, in order to evaluate the benefits of
dual beam echo sounders. The narrow beam echo sounder has
become commonplace in hydrographic surveying. This has reduced
the bottom area insonified by the echo sounder's beam, which
decreases the probability of detecting navigational hazards.
The dual beam echo sounder, equipped with a narrow and wide
beam, sounding concurrently, represents a relatively inexpen-
sive means to increase the detection capabilities, while
preserving the narrow beam operation.

The wide beams detected significant peaks that were absent
on the narrow beam trace. The wider hyperbolic returns of the
wide beams served to emphasize the narrow beam returns over
features with little horizontal extent. The narrow versus
wide beam depth differences over feature peaks were found
useful in isolating the peak's apex.

Approved for public release; distribution unlimited,

The Evaluation of Dual Beam

Echo Sounders in Hydrographic

Surveying

Dean Robert Seidel

Lieutenant Commander, NOAA

B.S., University of Washington, 1969

Submitted in partial fulfillment of the
requirements for the degree of

MASTER OF SCIENCE IN OCEANOGRAPHY (HYDROGRAPHY)

from the

NAVAL POSTGRADUATE SCHOOL
September 1979

TABLE OF CONTENTS

I. INTRODUCTION- ----- - 11

A. HYDROGRAPHIC SURVEYING PROCEDURES ------ -H

B. SURVEYING WITH A NARROW BEAM ECHO SOUNDER - - -11

1. Horizontal Resolution ---------- -11

2. Bottom Coverage ------------- -14

3. Pitch and Roll Errors ----- 20

4. Frequency Factors - - - - - -20

C. DUAL BEAM ECHO SOUNDERS - -21

D. PRIOR STUDIES - - -------- -22

II. PROJECT - __--- -24

A. PROJECT DESIGN 24

B. EQUIPMENT ------ -26

1. Sounding Equipment- 26

2. Data Acquisition Equipment - - - -28

3. Artificial Targets- - -- -30

C. SURVEY AREA ------ -30

III. DATA COLLECTION AND PROCESSING- ----- 37

A. SOUNDING DATA -------- 37

B. ARTIFICIAL TARGET TEST 38

IV. DATA ANALYSIS ----------- - - -40

A. CHARACTERISTICS OF THE DATA --------- -40

1. Operating Characteristics of the Seven

and Twenty- two Degree System- -------40

2. Artificial Target Test- - - - - -43

B. NARROW AND WIDE BEAM SOUNDING OVER

INDIVIDUAL FEATURES - - - - - - - -43

1. Large-Scale Features- -----------43

2. Small-Scale Features- -----------50

C. PRIMARY FACTORS ------ - - 53

1. Peak Detection- --------------53

2. Peak Isolation ------57

D. SECONDARY FACTORS ----- - - - 60

1. Wide Beam Depth Error ------ 60

2. Pitch and Roll Error- --- 68

3. Bottom Type -- -------- 69

4. Back Scattering ------- 70

5. Minimum Depth -- - - - 70

V. CONCLUSIONS / 72

APPENDIX A: BEAM PATTERNS- --------- 74

BIBLIOGRAPHY -------- 81

INITIAL DISTRIBUTION LIST ---------------- 82

LIST OF TABLES

I. Sounding Equipment ---------------- -29

II. Result of Gain Mismatch on the Seven and

Twenty-two Degree System ------- - -42

LIST OF FIGURES

1. Idealized Trace for an Echo Sounder in Fairly

Shallow Water ------------- 13

2. Illuminated Bottom Area for a Simple Cone

Shaped Beam over Flat Bottom- ------------15

3. Position Plot of a Search for a Bottom Feature- - - - 16

4. Echo Duration due to Spherical Spreading and

Pulse Length- -- - 18

5. Functions of Beam Width ---------- 19

6. Seven and Twenty- two Degree Sounding System 2 7

7. Acoustic Targets- --- - 31

8. Project Area- ----- ______ 32

9. Position Plot Area 1 - 33

10. Position Plot Area 2- ------- 34

11. Bottom Topography ------- _____35

12. Position Plot of Target Test- ----- - - 44

13. Target Traces - - - - 45

14. Target Traces --------------------46

15. Target Traces ----------- 47

16. Position Plot, Top Portion of Eleven Fathom

Peak from Area Two- -------- ____48

17. Seven and Twenty-two Degree Beam Sounding Profiles- - 49

18. Top Portion of Three Fathom Peak from Area One- - - - 51

19. Three Fathom Peak ------------------ 52

20. Two Fathom Peak - - - - - - 54

21. Various Beam Width Returns from a North-South
Sounding Line Adjacent to a Three Fathom Peak - - - - 55

22. Bottom Slope and Depth to Obtain Minimum Depth
Difference of 1 Foot and 1/6 Foot between the

Narrow and Wide Beams ----------------59

23. Peak Isolation for Cone Shaped Features- ----- -61

24. Seven Degree versus Twenty- two Degree Beam

Width Depths for Area One- ---- 63

25. Seven Degree versus Twenty-five by Sixty Degree

Beam Width Depths for Area One ---------- -64

26. Seven Degree versus Twenty- two Degree Beam

Width Depths for Area Two- 65

27. Seven Degree versus Twenty-five by Sixty Degree

Beam Width Depths for Area Two 66

28. Seven Degree Beam Pattern- ---- -- -76

29. Twenty-two Degree Beam Pattern - 77

30. Sum of Seven and Twenty- two Degree Beam Patterms - -78

31. Twenty-five by Sixty Degree Beam Pattern

Sixty Degree Athwartship Pattern 79

32. Twenty-five by Sixty Degree Beam Pattern

Twenty- five Degree Fore and Aft Pattern- 80

ACKNOWLEDGEMENT

I would like to express my appreciation to CDR D. E.
Nortrup, NOAA, as thesis advisor for his assistance and
guidance .

I am indebted to CAPT Wayne Mobley, the officers and
crew of the NOAA Ship RAINIER, for their willing assistance
during the field work for this project.

I would also like to thank RADM E. Taylor, NOAA, for
his interest and support.

Finally, I would like to thank my wife, Lynda, for her
patience and support throughout this project.

I. INTRODUCTION

A. HYDROGRAPHIC SURVEYING PROCEDURES

The purpose of a hydrographic survey for nautical charting
is to delineate the bottom topography and to detect hazards.
A hydrographic survey is generally accomplished by running a
series of parallel sounding lines with ten to twenty per cent
crossing lines to provide a check. Typically, the initial
main sounding line scheme indicates areas where a further
reduction in sounding line spacing is required to define areas
of particularly rough bottom topography, or to find the least
depths of features. A substantial portion of the hydrographer 's
efforts is devoted to item investigations. An item investiga-
tion consists of proving or disproving existence of a
particular object or feature and obtaining a least depth, for
example, a submerged wreck. Detection of these features
commonly requires extremely small sounding line spacing to
achieve one hundred per cent bottom coverage. Coverage of
this extent is impractical with the echo sounders commonly in
use.

B. SURVEYING WITH NARROW BEAM ECHO SOUNDER
1. Horizontal Resolution

The echo sounder beam widths in use for hydrographic
surveying have generally decreased over the past twenty years,
and the narrow beam echo sounder is now common. This is pri-
marily due to an effort to obtain the true depth directly

below the survey vessel with the higher resolution of the
narrower beam width. The thirty to sixty degree beam widths,
common one or two decades ago, were ambiguous as to where
within the insonified bottom area the least depth of the
echo sounder trace had originated.

An echo sounder records a hyperbolic trace for each
point reflector as the survey vessel proceeds. The character
of the recorded hyperbola is affected by the following
factors :

a. Speed of Vessel

b . Beam Width

c. Water Depth

d. Recorder's Paper Advance Speed

e. Recorder's Vertical Scale and Calibrated Velocity
The trace may be considered a sum of hyperbolas for each point
on the bottom. These hyperbolic properties have been previous
ly well documented by Krause (1962) and Hoffman (1957). True
depths are recorded only while directly over the apex of a
peak, or over a flat bottom. These properties and the charac-
teristic hyperbolic equations are presented in Figure 1. The
figure illustrates the relative error in depth, and the
position of a sounding in shallow water, when only the beam
width has been altered. The maximum error in the horizontal
position of a sounding as a function of the beam width is
d(cos (8) ) (sin(9) ) , where d equals the true depth, and 9 equals
one half the beam width. Narrow beam, vertically stabilized
echo sounders of seven degrees or less have substantially

Parametric equations for hyperbola

x = d tan Q

y = (d/cos-0)-d

Peak illustrated without 9:1 vertical exaggeration

recorded trace
true depth

20 Beamwidth

Figure 1 Idealized trace for a Echo Sounder

in fairly Shallow Water

paper advance - 120 inch/hour
vessel speed - 8 knots
water depth - 30 fathoms

reduced the ambiguity by reducing the insonified area, and
placing the position of recorded depths within limits more
consistent with today's obtainable position accuracies.
2 . Bottom Coverage

The bottom coverage over a flat bottom, for a simple
cone shaped echo sounder beam, is a function of the beam width,
water depth, pulse repetition rate, and vessel speed. The
bottom area insonified by a single ping is illustrated in
Figure 2. Assuming the pulse repetition rate is high enough
to provide substantial overlap between insonified bottom
areas along the vessel's track, the bottom coverage may be
approximated by a swath of width equal to two times the tangent,
of one half the beam width, times the water depth.

As the hydrographer ' s echo sounder has evolved into
a higher frequency and narrower beam sounding instrument, an
increased problem with bottom coverage arises. The narrow
beam echo sounder has substantilly reduced the insonified
bottom area. The line spacing required to adequately detect
and delineate shoaling features is also reduced. The hydrog-
rapher's objective of detecting hazards, and the objective of
high resolution accuracy using narrow beam sounders, are con-
tradictive when using a single beam sounding system.

The problem of bottom coverage is well illustrated by
a recent National Ocean Survey hydrographic survey in Cook
Inlet, Alaska. Figure 5 is a position plot of a survey launch's
efforts to confirm reported shoals of about six fathoms in
sounding depths of 10 to 15 fathoms, using a seven and one

= 1/2 Bearawidth
= Vertical Depth
= Illuminated Area
= 7Tdxtan\Q)

Figure 2. Illuminated Bottom Area for a simple
Cone Shaped Beam over Flat Bottom

Scale
|^ . 1:10,000

Scale
1:2,500

Scale
1:1,000

Figure 3 Position Plot of a Search
for a Bottom Feature

half degree beam width transducer. The investigation
eventually led to a wire sweep. There is a natural tendency
to initiate an investigation of this type with the echo
sounder. When the echo sounder has a narrow beam width, the
investigation rapidly evolves into an attempt to cover fair-
ly large areas with sounding line spacing of only a few
meters. The result is a substantial investment of time by
the field hydrographer , and a disproportionate increase in
the time required to process and verify the data. Figure 3
was created from blow-ups originally requested by the survey
verifier, in order to manage the high density of soundings
in the investigation area.

The bottom area insonified by a simple cone shaped
beam is naturally not completely illustrated by the echo
sounder recorded trace. This is due to spherical spreading
and stretching of the outgoing pulse. The effect of spherical
spreading on the recorded trace is illustrated for a simple
flat bottom in Figure 4. The recorded trace starts with the
return from the shortest two-way travel time. For a flat
bottom this is the vertical path directly below the vessel.
The duration of the return develops as the curved wave front
continues to return out to the limits of the beam and over the
pulse length. In actuality, the wide beam echo sounder trace
becomes a complicated function of pulse length, bottom topog-
raphy, bottom penetration, and beam width.

transducer

resultant
e~c ho1 "Trace

echo trace begins

Figure 4. Echo Duration due to Spherical
Spreading and Pulse Length

Lgure

Functions of Beam Width

h i >

' i

100-'
90-
80-
70-
60-
50-
40-
30-
20-
10r

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10 20 30 40 50 60 70 80 90

Beam Width in degrees

6- Beam Width/2

Curve#1 - 2 *tan(#)

Diameter of the insonified bottom area

Curved - cos(#) sin(#)

Maximum horizontal error in the position of a
recorded depth

Curve#3 - 1 - cos (Q)

Height of a feature that may be hidden in the
echo smear by spher ical spreading at the
lateral limits of the beam.

3. Pitch and Roll Error

The heave, pitch, and roll of the survey vessel cause
sounding errors. The heave error is nearly the same for a
narrow or wide beam echo sounder. The pitch and roll cause
pointing errors for a non-stabilized, narrow beam echo sounder,
while a wide beam maintains a vertical return through a higher
degree of pitch and roll. The heave, pitch, and roll error
on an analog trace cannot be reliably differentiated from
the analog record of similar periodic topographic features.

4 . Frequency Factors

The development of narrower beam widths was accompanied
by increasing operating frequencies. The higher frequencies
facilitated narrow beam width echo sounder designs. The
advantages of higher frequencies are listed below (Watt, 1977).

a. Shorter Pulse Lengths

(1) Shallower Depth Capability

(2) Higher Resolution

b. Lower Level of Ambient Noise

(1) Better Signal- to-Noise Ratio

(2) Lower Acoustic Power Required

(3) Less Noise on Echogram

(4) More Definitive Bottom Traces

c. Smaller Transducers

(1) Narrower Beam Widths

(2) Easier Launch Installations

(3) Portable Sounders

The list indicates that, particularly for shallow water launch
hydrography, the higher frequency echo sounder is advantageous.

The attenuation of the sound intensity in the water
column and the bottom sediments is a function of the frequency.
The higher frequencies have greater attenuation, which reduces
the maximum ranges obtainable. A second factor, that may be
considered a disadvantage of the higher frequencies, is the
loss of information concerning the bottom's composition. The
high frequency allows little penetration or information below
the bottom's surface layer.

C. DUAL BEAM ECHO SOUNDERS

The acceptance of the narrow beam echo sounder has
resulted in a loss of the inherently beneficial factors of
the wide beam systems for hydrographic surveying. In particu-
lar, the wide beam's greater bottom coverage and peak detection
abilities were lost. Dual beam echo sounder systems, which
are readily available and relatively inexpensive, provide a
means of combining the desirable characteristics of both
narrow and wide beam echo sounders. The dual beam systems are
designed to operate with a narrow and wide beam concurrently.
Some systems offer selectable beam width operation only,
vice concurrent operation, which limits their potential
considerably. The concurrent operation of the narrow and
wide beams is made possible by using two frequencies suffi-
ciently different to prevent interference. The recorded

traces are typically displayed on the same recorder with
separate darkness controls.

Various means have been developed to deal with the
problems of spherical spreading in a simple wide beam (side
scan, outrigged transducers, multi-beam, and sector scanning).
These systems represent a higher technology, and typically a
higher price tag than dual beam echo sounders.

D. PRIOR STUDIES

A substantial amount of literature is available con-
cerning the properties of wide beam echo sounders, their
recorded effect on the shape of bottom features, and the
advantages of a narrow beam system. References that relate
directly to studies concerning the usage of dual beam echo
sounders systems in hydrographic surveying are fairly scarce.

Weeks (1971) discusses a survey conducted in the Marshall
Islands designed to find a route for underwater cables. The
survey was in an area of irregular bottom topography with
numerous coral outcrops. The echo sounder used was an ATLAS-
DESO AN 6014, which has a 50 kHz, twenty-eight degree beam
width transducer, and a 210 kHz, eight degree beam transducer.
Both frequencies were displayed simultaneously on the same
recorder, and differentiation was obtained by the use of separate
grayness controls. Weeks found that by setting the narrow
beam to a dark trace, and the wide beam to a lighter gray
trace, the high resolution narrow beam bottom trace was
continually discernible as a dark line, while maintaining
the side echo information from the wide beam. Weeks found

the dual beam system a vaulable aid for detecting the coral
outcrops as opposed to operating with a single narrow beam.
Cohen (1959) discusses the simultaneous operation of a
34 kHz, six and one half degree stabilized beam, and a 12
kHz, sixty degree beam in hydrographic operations. The paper
is generally oriented toward deep water ship hydrography, and
the advantages of stabilized narrow beam sounding. In this
study the two beams were recorded on separate recorders. A
deep water area was contoured using narrow and wide beam
sounding for comparison. The contour plot illustrated the
substantial depth errors in deep water generated by the wide
beam. The features were broadened and smoothed by the wide
beam echo sounder, and small scale features were lost. Cohen
discussed the possibility of using the narrow versus wide beam
depth differences as an aid in ship positioning.

II. PROJECT

A. PROJECT DESIGN

This project was designed to assist in evaluating any
possible benefits or problems encountered while using various
beam width and frequency echo sounders concurrently during
hydrographic surveying. The design was oriented toward
launch hydrography in shallow water (less than 100 fathoms) .
The project was directed toward launch hydrography, because
a dual beam system, which is considered a relatively
inexpensive and partial solution, applies better to launch
work. The multi-beam, swath systems require space for the
processors, peripherals and mounting the transducer array.
The installation and operation of a dual beam system is
relatively much simpler. The higher technology systems to
increase bottom coverage will be adopted first by ship
hydrography. Most of the prior study work has been done in
deep water, where the problems with spherical spreading of
the wide beam are not as severe as in shallow water.

The project was designed primarily to evaluate the dual
beam system abilities relative to two factors:

1. Peak Detection - The wide beam of the dual system
provides increased bottom coverage and increases the proba-
bility of detecting shoals of small horizontal extent.

2. Peak Isolation - The narrow versus wide beam depth
difference is zero on the apex of a peak. The wide beam
always records shoaler depths than the narrow beam on a

sloping bottom. This characteristic of a dual system assists
in locating the feature's apex.

To evaluate these factors, a limited area survey was
undertaken at a reduced line spacing, relative to National
Ocean Survey standards, to delineate small scale features.
The launch was equipped to sound simultaneously with three
beam widths and two frequencies. The peak detection capa-
bilities would be measured by the small scale features
detected by the wide versus narrow beams. The peak isolation
abilities would be measured by the depth differences, wide
versus narrow, as the sounding lines crossed adjacent to,
or over feature peaks.

In addition to the major interest factors cited above,
the following factors were subject to consideration:

1. Wide Beam Depth Error - The narrow beam provides
nearly true depths , while the wide beam is affected by bottom
slopes .

2. Pitch and Roll Error - The wide beam maintains a
recorded depth originating from the perpendicular to the bottom
over a higher degree of pitch and roll of the survey vessel
than does the narrow beam.

3. Bottom Type - The low frequency wide beam penetrates
the bottom sediment more than the high frequency narrow beam.
This indicates bottom acoustic impedence and correlates to
bottom composition.

4. Minimum Range - The high frequency narrow beam system
typically has shorter pulse lengths than a low frequency system,

allowing operation in very shallow water without losing the
trace in the reverberation.

B. EQUIPMENT

1 . Sounding Equipment

The National Ocean Survey hydrographic launches are
generally equipped with automated surveying systems that
include a seven degree echo sounder. All beam widths are
referred to the six db down, or half power level. A twenty-
eight foot launch from the NOAA ship RAINIER had been equipped
with an additional twenty-two degree transducer to assist in
locating reported shoals. The NOAA ship RAINIER subse-
quently requested the seven and twenty-two degree beam
transducers be designed to allow concurrent sounding to
evaluate the benefits during various hydrographic projects.
The launch's regular seven degree narrow beam system was
equipped by the Electronics Division of the Pacific Marine
Center to display the seven and twenty- two degree traces on
the same recorder. The two transducers operate at the same
frequency (100kHz) . The twenty-two degree beam width
transducer triggering was delayed by about six milliseconds,
or two and one half fathoms of recorded depth. The delay for
the twenty-two degree beam was generated at its transceiver.
The design of the seven degree and twenty-two degree system
is illustrated in the block diagram of Figure 6. The digitizer
received only from the seven degree beam. The launch pro-
cessing system recorded only narrow beam depths. The outgoing

2-1/2 fathom delay
in start pulse

"" — start pulse

digitizer

22° beam

7-1/2° beam

Figure 6. Sounding System.

"start" pulse from the recorder and the returning signals
from the two transceivers were simply connected together at
a junction box. The gain and mark intensity of the recorder
controlled signals from both transceivers.

For the study, an additional wider beam and lower
frequency system was requested and temporarily added to the
launch. This twenty-five by sixty degree beam system operated
independently. The transducer was mounted on a portable
strut on the starboard side of the launch with the sixty
degree beam athwart-ship and the twenty- five degree beam
fore and aft. The operating frequencies of 21 kHz and
100 kHz differed enough to prevent any interference problems.
This system added a second frequency and extended the beam
width to a degree that was envisioned as closer to the useful
limits in shallow water hydrography.

The sounding equipment is listed in Table 1. The
project was designed using the existing inventory of sounding
equipment from the National Ocean Survey, Pacific Marine
Center, with the underlying desire that a useful and readily
available permanent system might exist.
2 . Data Acquisition Equipment

The launch's "Hydroplot" automated data acquisition
system was used to collect and initially plot the hydrographic
data. The system collected narrow beam depths, time, position
and correctors. The corrections for tide, draft and control
calibrations were performed, and the narrow beam soundings
were plotted on-line. The data were stored on paper tape

TABLE I

* A. SEVEN DEGREE SYSTEM

1. Recorder

a. Range - 400 feet/200 fathoms

b. Phasing - 100 feet/50 fathoms per 6.5" Scale

2. Transducer

a. Frequenty 100 kHz

b. Beam width 7.5 degrees to 6 db level

3. Digitizer

* B. TWENTY-TWO DEGREE SYSTEM (consists of a transceiver

and transducer added to the seven degree system)
1. Transducer

a. Frequency 100 kHz

b. Beam width 22 degrees to 6 db level

**C. TWENTY-FIVE BY SIXTY DEGREE SYSTEM

1. Recorder

a. Range - 1 foot to 250 fathoms

b. Phasing - 50 feet or fathoms per 6-1/4" scale

c. Chart speed - 60 inches/hour, 120 inches/hour

2. Barium Titanate Transducer

a. Frequency - 21 kHz

b. Beam width - 25 degrees fore and aft to 6 db
level, 60 degrees athwart ship

* General Characteristics

1. Pulse repetition rate - feet (6/sec), fathoms (2/sec.)

2. Calibrated velocity - 4800 feet/sec.

**General Characteristics

1. Pulse repetition rate - feet (10/sec), fathoms (1-2/3/sec.)

2. Calibrated velocity - 4800 feet/sec.

with accompanying printouts. The system was also used for
the initial editing and plotting off-line. By using the
launch's "Hydroplot ," the soundings collected were received
only from the seven degree beam. The wide beam analogs were
hand scanned, and the printouts were annotated with the wide
beam depths.

3. Artificial Targets

A set of three portable acoustic targets were con-
structed from high density one-eighth inch masonite, with one-
eighth inch plastic foam packing material pasted to the
surfaces. The bubbles entrapped in the packing material
served as good reflectors. The targets were two feet wide by
three feet high. The targets were designed to be just slightly
buoyant, so that they could be placed at known depths by
hand from the launch. The acoustic targets were constructed
to serve crudely as sounding system calibrators. The objectives
were determine whether the three beam widths were performing
as expected and to measure the degree of side echo returns.

C. SURVEY AREA

The field work for this study was performed in conjunction
with a navigable area survey, conducted by the NOAA ship
RAINIER in the area of Auke Bay, Southeastern Alaska. The
survey areas are illustrated by the following position plots,
Figures 9 and 10 and the project area, Figure 8. Area One
is in the small bay at the southeast end of Auke Bay, and
Area Two is west of the southern end of Spuhn Island, and
north of Gibby Rock. These areas were pre-selected due to

Figure 7 . Acoustic Targets

0 ;

134 42

0 I

134 40

134 38

Indian Point

AUKE

Coghlan Island

Area Two

Gibby Rock

BAY

FRITZ COVE

o I
-58 23

o I
58 22

0 A

58 20

Figure 8. Project Area

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Area 1 Looking North

Figure 11. Bottom topgraphy

the roughness of the bottom topography. The area surrounding
Auke Bay has been heavily glaciated, and the bays have
received substantial sediment fill. The result is an area
with extensive flat sedimentary bottom, fairly steep slopes
approaching the shoreline, and generally large outcrops and
peaks extending above the sediment fill. Due to the limited
time available, and in order to avoid the relatively flat
bay basins, it was necessary to pre-select working areas in
which to operate the three beam width sounding system.

III. DATA COLLECTION AND PROCESSING

A. SOUNDING DATA

The data collecting and processing procedures generally-
followed National Ocean Survey hydrographic standards for a
one to five thousand scale survey. Using National Ocean
Survey standards, the sounding line spacing deemed appropri-
ate for the working areas was fifty meters. Area One was
developed with twenty meter sounding lines, and Area Two
with a forty meter grid pattern. The sounding data were
corrected for transducer draft, sound velocity, and predicted
tides. The velocity correctors were determined by S.T.D. and
C. T.D. casts in the survey area. The velocity was fairly
close to the calibrated 4800 ft/sec. velocity, and velocity
correctors' magnitudes were minimal. Bar checks, at one
fathom intervals to seven fathoms, were carefully observed
twice daily. The sixty degree beam transducer, which was
mounted on the starboard side, required a separate bar check
alongside the launch, in order to maintain the bar vertically
below the transducer.

Position control was obtained from a super high frequency
electronic ranging system. Area One contains a combination
of range-range and range-azimuth control. Area Two is total
range-azimuth. The azimuth was obtained from a theodolite
of known position ashore. The positioning system transponders
were calibrated morning and evening, using a known position
adjacent to the study area.

The narrow beam hydrographic data were transferred from
paper tape to magnetic tape. The magnetic tape contained the
position, time, sounding, and corrector information for an
eight second sounding interval. The original intent was to
edit this tape, with the sounding data from the twenty-two
and sixty degree beams, to create a data file for each beam.
Then contour plots of the area's bottom topography, and plots
of the depth differences between the various beams, could be
automated. The eight second sounding interval was found to
be too long, and would only create a generalized picture of
the effects over large features. The small scale features,
and significant depth differences between the beams at peak
apexes, would be lost. Therefore, the narrow beam sounding
data were plotted and contoured using automated means. These
plots served as a basis for plotting the depth differences
between the various beam widths. The depth differences were
obtained by manually scanning the three analog traces, with
particular attention to peak detection and peak apexes.

B. ARTIFICIAL TARGET TEST

The targets were fixed to a line, and set at known depths.
(See Figure 12, Data Analysis.) Sounding lines were run
adjacent to the targets at decreasing ranges to determine the
relative side-looking abilities of the various beam widths.
The initial plan was to anchor the targets in the working area
before surveying. This was attempted and proved to be
impractical. The size of anchor and buoy that could be
handled from a launch did not guarantee a vertical wire angle.

Therefore, the targets were suspended on a line off the stern
of the ship while at anchor. The wire angle remained vertical
during the tests. The launch was controlled by range-range
positioning, and the swing of the ship's stern, by visual
sextant fixes. This method appears to be awkward, but it was
the most expedient, and served the purpose.

IV. DATA ANALYSIS

A. CHARACTERISTICS OF THE DATA

1 . Operating Characteristics of the Seven and Twenty-
Two Degree Systems

A dual beam system usually operated at two different
frequencies to prevent interference between the beams. The
seven and twenty-two degree beam transducers operate at the
same frequency, which allows both transducers to receive from
the seven and/or twenty-two degree transmissions. The recorded
traces from the seven and twenty- two degree beams did not
perform quite as anticipated. The intended recorder trace,
with the system connected as in Figure 6, was a seven degree
bottom trace followed shortly by the delayed twenty-two
degree bottom trace. The actual characteristics recorded
were as follows. At low gain settings both traces reflected
narrow beam characteristics. At high gain settings both
traces converted to a wide beam character, and at intermediate
gains, the traces were narrow with fainter wide returns.
The system was operated at intermediate gains to retain a
narrow and wide trace. The first trace consisted of a dark
seven degree beam line, super-imposed with the lighter twenty-
two degree receiver trace which became visible on bottom
slopes. The delayed trace appeared essentially as a duplicate
of the first trace, but was generated by a twenty-two degree
transmission.

These traces may be explained, if the gain of the
narrow and wide transceivers were not very well matched. The
logic is illustrated in Table II. The gain of the narrow
beam system was higher than the wide beam system. At low gain
settings the narrow receiver dominates. At high gain settings
the wide receiver's bottom return overcomes the recording
thresholds, and the delayed wide trace becomes wide. But now,
the narrow transmit and wide receive combination were at a
high enough level for transmitted narrow beam side-lobes to
return through the wide receiver.

An examination of the signal excess at high gain
settings confirms the feasibility of this explanation. The
average depth in the operating area was thirty fathoms. The
manufacturer's maximum design depth is two hundred fathoms.
The difference in propagation losses due to spreading,
attenuation, and bottom backscatter for thirty fathoms versus
that for two hundred fathoms, results in an approximate signal
excess of plus thirty-seven db . This thirty-six db signal
excess level on the narrow transmit and wide receive beam
pattern generates a twenty-four to twenty-six degree beam.
The computations and beam patterns are included in Appendix A.

The gain and mark sensitivity of the seven and twenty-
two degree transceivers were both controlled at the recorder.
Unfortunately, while in the field, little attempt was made to
adjust the gain separately at the transceivers. Feasibly, a
darker wide beam trace could have been obtained, while still
maintaining the visibility of the narrow beam trace.

TABLE II

RESULT OF GAIN MISMATCH ON THE SEVEN
AND TWENTY -TWO DEGREE SYSTEM

I. ORIGINALLY EXPECTED RESULTS (Matched Gains)

NARROW BOTTOM RETURN WIDE BOTTOM RETURN

NARROW RECEIVER NARROW NARROW

WIDE RECEIVER NARROW WIDE

RESULT TO RECORDER NARROW WIDE

II. RESULTS LOW GAIN (Gain of Narrow Higher than Wide)

NARROW BOTTOM RETURN WIDE BOTTOM RETURN

NARROW RECEIVER NARROW NARROW

WIDE RECEIVER NO TRACE NO TRACE

RESULTS TO RECORDER NARROW NARROW

III. RESULTS HIGH GAIN (Gain of Narrow Higher than Wide)

NARROW BOTTOM RETURN WIDE BOTTOM RETURN

NARROW RECEIVER NARROW NARROW

WIDE RECEIVER WIDE WIDE

RESULT TO RECORDER WIDE * WIDE

* A plus thirty-seven db level on the narrow transmit and
wide receive beam pattern allows wide return to recorder.

2. Artificial Target Test

A few of the echo sounder traces are presented in
Figures 13, 14, and 15. Sounding line number eight was
obtained as the launch approached the targets head-on. The
hyperbolic return has a distorted and extended width in this
case because the launch slowed to maneuver directly over the
targets. The seven degree beam began to digitize on the
targets when the launch was stationed directly over the
targets. The hyperbolic return for line numbers four and
seven were obtained at a constant vessel speed, and the computed
hyperbola is included in Figure 14. The computed hyperbola
indicates horizontal extent for various beam widths, and
assists in indentifying main beam versus side-lobe returns.
The maximum lateral range for significant target returns for
the seven, twenty- two, and sixty degree beam widths were two
meters, ten meters, and twenty-two meters, respectively. These
ranges correspond to slant range returns at beam widths of
seven, thirty-three, and sixty-five degrees, respectively.
This indicates the relative lateral signal levels at inter-
mediate gain settings in relatively shallow water. Spherical
spreading is illustrated in line number four where a target
two and one half fathoms above the bottom has just become
lost in the bottom trace at a range of twenty- two meters.

B. NARROW AND WIDE BEAM SOUNDING OVER INDIVIDUAL FEATURES
1. Large-Scale Features

Figure 17 displays the forty meter sounding line

profiles over an eleven fathom peak from the northeast corner

134° 39' 05'

134* 39' Of

position of
target

4- 58° 22'

sounding
lines

scale 1:300

Figure 12. Position Plot of Target Test

ressel ? 1,° 2p ^ horizontal range in meters

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conical

Figure 13. Target Traces, Line 8

Approaching Targets Head-on

f- 18 degree main beam

- 60 degree side lobe

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2-5 by 60 degree beam

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-*— target 1

22 degree delayed trace

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Figure 14. Target Traces, Line 7
CPA of 7 meters

target 1

target 2

target 3

masked by spherical

spreading

25 by 60 degree beam

- 15

-*— no returns

-20

7 and ZZ degree beams

Figure 15. Target Traces, Line k .
CPA of 20 meters

26 8 3 2 684

6192

2680 2692

one fathom contours
7 degree beam
correctors applied

Scale 1:5000

Figure 16. Position Plot (if 5 meter line spacing)
Top Portion of Eleven Fathom Peak
from Northeast Corner of Area Two

22 return
7 return

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Figure 17. 7 and 22 Degree Beam Sounding Profiles

of Area Two. Each of the dual beam profiles has a difference,
narrow versus wide, indicating shoaler depths. Sounding line
2692-2694 gave an indication of where to look for the apex of
the peak. Line 6192-6194 is three fathoms shoaler. Unfortu-
nately, line 6192-6194 happened to be run by a narrow beam
only launch.

Figure 18 illustrates a broad three fathom deep peak
from Area One, with sounding lines at twenty meter spacings.
The dual sounding profiles are from the seven degree beam,
and the twenty-five by sixty degree beam. In this case, the
peak is not very well isolated by narrow versus wide depth
differences. Sounding lines numbered Two and Three contain
little indications of slope. The three to four fathoms water
has reduced the bottom coverage and the effectiveness of the
dual beam system.

2. Small-Scale Features

The usefulness of a narrow versus wide beam sounding
system is more apparent in the following figures of the
profiles over features with horizontal extent less than fifty
meters. The potential for large slope angles is naturally
greater with small features of any significance, and the area
of "zero difference" over the peak is small.

Figure 19 shows a small, three fathom peak approxi-
mately centered between two twenty meter line spacing sounding
lines. At this depth the sixty degree beam was supplying
nearly one hundred per cent coverage. The narrow beam did not
see the feature.

Line 1

Line 2

Line 3 Line k

7 Degree and 25 by 60 Degree Beam Sounding Profiles

Depths in Fatnoms

58 2125"-f
13439:

58 21

Figure i8. Top Portion of Three Fathom Peak
from Area One

mmmm

22 Return

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7° Return

r~*

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Figure 19. Snail three fathom Peak, between

tv/o twenty aeter line spacing Sounding Lines

Figure 20 illustrates that even in five to ten fathom
water, the narrow versus wide beam data may be useful in
locating and determining the least depth of small features.
The bottom coverage was very limited (22 beam = 4 meters) ,
but narrow versus wide depth differences are visible on line 2
and in line 3 on the steep slopes of the small peak.

Figure 21 indicates a wide beam return. The wider
hyperbolas of the wide beams strongly substantiate the narrow
beam returns, which might have missed the scanner's attention.
When surveying at speeds between eight and fifteen knots, the
narrow beam was transmitting at intervals of seven to thirteen
feet. Therefore, features of substantial height and hori-
zontal extent may be indicated by only a couple of narrow
beam marks on the analog trace. These isolated narrow beam marks
may easily be mistaken as "strays." The wide beam extends the
small scale feature returns to a point where they are more dif-
ficult to ignore. The difference in narrow versus wide depths
at the peak apex indicates this was not the peak's least depth.

C. PRIMARY FACTORS

1 . Peak Detection

Evaluating the benefits of increased bottom coverage
by using a wide beam is generally difficult to quantify be-
cause of the problem of spherical spreading and its dependence
on water depth. The detection of features between sounding
lines which were not indicated by the narrow beam would be
such a measure. Isolated small scale features similar to

Line 1

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Sounding Lines across two fathom Peak
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Figure 21 Various Beam Width Returns from a_
•North-South Sounding Line adjacent
to a three fathom Peak

those in Figures 19, 20, and 21 were disappointingly scarce.
The narrow beam analogs contained three isolated peaks of
less than fifty meter horizontal extent and of any significant
height. The wide beams reflected five isolated small scale
peaks that were not recorded by the narrow beam trace.
Figure 19 was one of a group of three small scale peaks that
were two to three fathoms high and less than twenty meters
in extent in a fairly flat area (58°21 ' 15"W, 134°38 ' 52MN,
Area One) of ten to fifteen fathoms deep. These peaks were
developed by a second set of north-south sounding lines at
twenty meter spacing and were still not picked up on the
narrow beam trace.

The sample size is too small to make any quantitative
estimate of the wide beam's potential to reflect features
totally absent on the narrow beam trace. The feature dis-
coveries that could be attributed solely to the wide beam's
side-looking abilities were a significant number because the
total dual beams' hydrography amounted to only sixty nautical
miles or one typical launch working day.

The average launch speed was eight knots, or about
four meters per second. The pulse repetition rate at this
speed was fast enough to supply overlapping insonified bottom
areas for the wide beams, up to a depth of two or three
fathoms. The seven degree beam began to lose overlapping
areas in depths less than nine fathoms due to its smaller
insonified bottom area. The wide beam of a dual beam system

decreases the problem of maintaining overlap between pulses
in very shallow water.
2. Peak Isolation

The narrow versus wide beam depth differences may
assist the hydrographer by indicating the sounding line has
passed within some limits of the peak's apex. The difference
between the narrow and wide beam depths goes to zero over the
peak apex. The least depth would have to come from the narrow
beam trace in order to maintain positional accuracy, unless
the water was very shallow. The previously presented profiles
over individual features demonstrated cases where the sounding
line obviously did not find a least depth as well as cases
where the sounding line displayed a "zero difference" and must
have crossed near the apex. Theoretically a "zero difference"
while developing a feature would be a point directly over the
least depth. The resolution of the echo sounder limits the
minimum depth difference that is discernible before it is
considered zero.

For the seven and twenty-two degree system used in
this project, a one foot difference in narrow versus wide
beam depths was visible while using the fathom scale. This
resolution is not considered overly optimistic when both
narrow and wide beam traces are on the same recorder. The
timing errors will affect both the traces equally when they
are on the same recorder. A small difference in the wide
and narrow beam traces is readily discernible if the wide

beam directly overlays the narrow beam trace, and if it is
recorded with a lighter mark intensity.

The minimum bottom slope required at a particular
depth to generate a one foot difference between the seven
degree beam and the twenty-two degree beam is plotted in
Figure 22. Features with slopes and depths that plot above
this curve will have some degree of peak isolation using
narrow and wide beam depth differences. Also plotted is the
dividing line for a forty degree wide beam with minimum
discernible depth differences of one foot. The minimum
discernible depth difference could have been decreased to
one-sixth foot by operating the system using the feet scale.

The one foot dividing line for features that will
develop narrow versus wide beam depth differences and allow
some degree of peak isolation agrees with the project data.
For example, Figure 17 has fifteen to twenty degree slopes
and depths of ten to fifteen fathoms near the peak. Sounding
lines adjacent to the peak indicate that the apex was not
found. Figure 18 has slopes averaging about ten degrees and
depths of two to five fathoms, which is below the useful peak
isolation line. The large scale features had average bottom
slopes across their apex in the five to fifteen degree range.
The significant small scale features typically had slopes
greater than twenty degrees, which requires depths of at least
four fathoms for peak isolation.

Assuming cone shaped features, the degree of peak
isolation has been plotted in Figure 28. This illustrates

Figure 22. Bottom Slope and Depth to obtain minimum
Depth Difference of 1 Foot and 1/6 Foot
between the Narrow and Wide Beams

Depth, d in fathoms

D =

D =
d =

minimum discernable difference
narrow versus v/ide depths

d cos/3/cos(^~Qn)
d.cos£7cos(/?-^)

wide beam depth
narrow beam depth
bottom slope
1/2 angle narrow beam s
1/2 angle wide beam

true de-Dth

the diameter of the circular area of "zero difference" over
the cone shaped feature for a seven degree narrow beam versus
a wide beam of at least forty degrees. The features on this
figure are plotted against peak depth. Figure 23 illustrates
the peak isolation limits for a minimum discernible difference
of one foot.

The narrow versus wide beam depth difference can
assist the hydrographer in isolating and determining a least
depth. The feature to be developed may be run, using a line
spacing based on a reasonable wide beam's bottom coverage.
For example, the spherical spreading of a thirty degree beam
is not excessive. A thirty degree beam will indicate shoals
within its insonified area that are greater than five percent
of the vertical depth. Also, the thirty degree beam has an
insonified area that is still five meters in diameter, in only
five fathoms of water (Figure 5) . The narrow versus wide
beam depth differences will isolate the features peak to a
degree that, if necessary, is more reasonable to further
develop using only the narrow beam.

D. SECONDARY FACTORS

1. Wide Beam Depth Error

Features substantially larger than the line spacing

were common in the work areas. The bottom slopes of these

large scale features averaged eighteen degrees, with a few

maximum slopes of about forty degrees. These slopes naturally

generated the most extensive differences in recorded depths

Figure 23. Peak Isolation for Cone Shaped Features

Peak Depth, dp in fathoms

D =

r =

d
dP

minimum discernable difference

narrow versus wide depths

1/6 fathom

radius of peak isolation area

in meters

true depth

peak depth

wide beam depth

narrow beam depth

bottom slope

1/2 angle narrow beam

1/2 angle wide beam

between the various beam widths. The depth differences are
illustrated in Figures 24, 25, 26 and 27. The fine lined
contours were generated from the narrow beam corrected depths.
The fine line contours may be considered very nearly the
true depths and actual feature shapes. The bold contours
are the differences obtained between the wide beams and the
seven degree narrow beam. The bold contours are the depth
errors created by the wide beam echo sounder, relative to the
narrow beam echo sounder. For example, in Area One, there
is a central north-south trending ridge. The western slope
at the northern end of the ridge had the maximum bottom
slopes (=45°) for the area. The twenty-two degree beam and
the sixty degree wide beam recorded depths two fathoms and
four fathoms shoaler than the narrow beam depths.

The depth differences obtained from the three beam
widths generally agreed within one half fathom to computed
values for respective bottom slopes. The computed difference
may be derived from the following relations:

a. Bottom Slopes Less than One Half the Wide Beam
Width D = dn(l - cos(B - 9n)) .

b. Bottom Slopes Greater than One Half the Wide Beam
Width D = d (1 - cos(S - 9n)/cos(3 - 6W)

D = Depth Difference for Wide versus Narrow Beams

9 = One Half Wide Beam Width
w

d = Recorded Narrow Beam Depth
n

3 = Bottom Slope Angle

0 = One Half Narrow Beam Width
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The twenty-five degree, fore and aft, by sixty degree
athwart-ship rectangular beam should show maximum differences
on slopes tangent to the vessel track, and very little dif-
ference on slopes normal to the vessel track, in comparison
to the twenty-two degree conical beam. This is apparent in
the east-west elongation of the features of Area Two.

The figures illustrate the substantial depth error
obtained by the wide beam echo sounder in relatively shallow
water, and is a consideration when using prior surveys in
comparison with current surveys. The positional errors of the
depths recorded by the wide beam system in areas of sloping
bottom averaged roughly five to seven meters. The maximum
shift in position of the contours is simply limited by the
echo sounder's beam width and depth (d sin 6 cos 9). The
measured shifts in contour position, due to the wide beams,
agree, to within a few meters, with the computed values for
the working area depths. The shifts were only a few meters
greater than the vessel's positional accuracies, but only
because of the water depth. In one hundred fathoms a twenty-
two degree echo sounder may cause thirty-five meter shifts in
contour position. The requirements for the resolution and
positional accuracy of a reasonably narrow beam echo sounder
is unquestionable.

Figures 24, 25, 26 and 27 indicate to some degree
the usefulness of the wide beam to the hydrographer on large
scale features. The extent to which the difference contours
surround the apex of individual peaks, and the size of the

"zero difference" area over the peaks, is indicative of the
ability of the concurrent sounding system to isolate the
apexes. In most cases the diameter of the area of "zero
difference" in recorded depth between narrow and wide beams
was larger than a 1:5,000 scale fifty meter sounding line
spacing, so the hydrographer has gained little. These large
features shoaled to around five fathoms. At five fathoms the
insonified area is limited (see Figure 5) , and a substantial
bottom slope is required to overcome the curvature in the
wide beam.

For example, the individual peaks on the north-south
trending ridge in the southwest corner of Area One were not
isolated by narrow versus wide depth differences of the east-
west sounding lines.

Figures 24, 25, 26 and 27 were generated from soundings
at the six to eight second sounding interval, and present a
generalized picture of the broad features.
2. Pitch and Roll Error

The dual beam system was considered during project
design as a means to preserve some indication of sea state
on the analog records, due to the difference in reaction to
pointing error of the narrow versus wide beams. The heave,
pitch, and roll error cannot be reliably identified from
bottom topography subsequent to the field work, unless the
records were annotated for sea condition.

The seas during the project were very calm, except
for the last day, which had a three foot chop. The difference

of the narrow versus wide beam depths, due to vessel pitch
and roll, was too similar to the result that would occur
due to the difference in horizontal beam resolution for this
characteristic to serve as an indicator of sounding in rough
water. In both cases the small scale periodic variations in
the narrow beam trace are smoother in the wide beam trace.
The top left corner profile of Figure 17 shows narrow beam
depth variations known to be caused by roll, while sounding
on a sloping bottom. The wide beam maintained a nearly flat
trace. Assuming these narrow beam variations were actual
bottom features, the narrow and wide profiles would be
expected to appear the same, due to the wide beam's poor
horizontal resolution.
3. Bottom Type

The predominance of high frequency narrow beam systems
has resulted in the loss of possible useful geological informa-
tion derived from the lower frequency's (Watt, 1977). A
recent concern is the possibility of an upper layer composed
of a "slurry," with sound velocity equal or less than that in
the water column. This may be detected with dual frequency
systems .

Bottom samples were obtained by the NOAA Ship RAINIER,
adjacent to the project areas, during the course of their
hydrographic survey. The bottom composition was fairly uniform,
and consisted primarily of silt and clay with rock outcrops.
The 21 kHz low frequency analogs were carefully compared with
100 kHz high frequency analogs in the flat bottom areas for

low frequency depths greater than high frequency depths, which
would indicate a "slurry." The relative depths remained equal.
Also, very little useful penetration was exhibited by the 21
kHz system, indicating a fairly consolidated bottom.

4. Back Scattering

The Auke Bay area has repeated plankton blooms in the
spring and early summer. A bloom was occurring during the last
few days of this project, which had a marked effect on the echo
sounder's ability to maintain a bottom trace. No attempt was
made to obtain a biological sample of the zooplankton responsi-
ble. But the problem with the traces occurred in patches that
correlated with the density of phytoplankton visible from
the surface. The plankton's scattering effects were greater
for the 21 kHz system, to such a degree that in some areas a
bottom trace could not be obtained. This problem illustrates
the frequency dependence on biological scattering and an
additional possible benefit of a dual frequency system. The
majority of the project's data was obtained before the plankton
became a problem.

5 . Minimum Depth

The minimum depth obtainable with an echo sounder is
related to the pulse length and the resulting initial rever-
beration. In very shallow water the bottom return becomes
lost in the initial reverberation. The 21 kHz and 100 kHz
systems have pulse lengths of .009 seconds and .001 seconds,
respectively. As expected, the 21 kHz trace was periodically
lost in the initial reverberation while maneuvering in shallow

water. But interestingly, the 21 kHz system was equally
useful in very shallow water with the transducer mounted on a
strut along the starboard side. The narrow beam trace, with
its transducer mounted near the keel, was repeatedly lost in
the propeller wash while maneuvering inshore to start an off-
shore line. The difference was probably due to transducer
location, rather than frequency, penetration and backscatter.
In a dual beam system for launch hydrography, it may be useful
to mount the wide beam transducer away from the keel on a
fairly flat-bottomed launch.

V. CONCLUSIONS

The negative effects of the wide beam's poor horizontal
resolution and the degree of wide beam depth error relative
to a seven degree narrow beam were plotted for the two pro-
ject areas. The plots illustrate the necessity of a narrow
beam echo sounder for accurate depth determinations. The
results confirm the usefulness of side-looking abilities of
the wide beam echo sounder, in spite of the problems with
spherical spreading. The sample size of the features detected
with the wide beams was too small to quantify the usefulness
of the sixty degree transverse beam relative to the twenty-
two degree beam. The wide beam trace was found to emphasize
the narrow beam profiles over small features that may have
missed detection when scanned.

A useful ability of peak isolation is exhibited by the
narrow versus wide beam depths over feature peaks. This
requires a visible narrow versus wide beam depth difference
in the recorded traces near the peak's apex, which is a
function of the bottom slope and peak depth. A model using
cone shaped features indicates the degree of peak isolation.

A number of desirable dual beam design concepts for use
with hydrographic surveying were obtained. The wide beam
and narrow beam trace should be displayed on the same recorder
This reduces the relative, narrow versus wide beam, time
error, and allows for easy visual comparison. The narrow and
wide beam trace should be set to directly overlap. This

allows small depth differences to be readily discernible. The
small depth differences between the narrow and wide beam were
significant over the peak apexes, and determined the degree of
peak isolation. Separate gain and mark sensitivity controls
are required to maintain a distinct difference in the narrow
and wide beam returns. The necessity of a difference in
operating frequencies, for the narrow and wide beams, was
confirmed for concurrent sounding with dual beams. The study's
seven and twenty-two degree beams both operated at 100 kHz.
This caused interactions between transducers and problems
in interpreting the results.

The dual beam echo sounder appears to be well-suited for
filling the void between narrow beam sounding and swath or
scanning sounding systems in shallow water launch hydrography.
The abilities and procedures with narrow beam echo sounding
are maintained, while the beneficial factors inherent in a
wide beam system are added. The wide beam trace becomes a
familiar and easy to operate descriptive tool for the
hydrographer

APPENDIX A

A. EXCESS SIGNAL LEVEL FOR SEVEN DEGREE BEAM TRANSMIT AND
TWENTY -TWO DEGREE RETURN

The recorder analog traces showed both seven and twenty-
two degree characters when the audio lines were combined to
the recorder. The following computation shows the beam pattern
and possible excess echo levels for a seven degree transmit
and twenty-two degree return. The result illustrates a
feasible origin for the wide return for the seven degree
transmitted analog trace.

Assuming a specular return from the bottom the sound

pressure at the receiver appears to arrive from a mirror

image source constructed across the bottom interface. The

sound pressure at the image source equals the pressure at

the original source times a factor for bottom losses, the

reflection coefficient. The excess echo level is equal to

the difference in propagation losses for the shallow water

case and the maximum operating range. Assuming the same

bottom reflection coefficient the propagation losses will be

due to spherical spreading and attenuation in the water

column over twice the range.

Excess Echo Level = Propagation loss 200 fathoms -

Propagation loss 30 fathoms

Excess Echo Level = 20 log 2R + a2R - 20 log 2r - a2r

= 20 log R/r + 2a(R-r)
16.5 + 20.4 = 36.9 dB

R = 200 fathoms (maximum operating range)
r = 30 fathoms (project operating range)
a = .06 dB/fathom

Figure 28. 7 degree Beam Pattern

Figure 29. 22 degree Beam Pattern

Figure 30. Sum of 7 and 22 degree Beam Patterns

Figure 31. 25 by 60 degree Beam Pattern
60 degree athwartship Pattern

Figure 32. 25 by 60 degree Beam Pattern
25 degree Fore and Aft Pattern

BIBLIOGRAPHY

1. Clay, C. S. and Medwin, H. , Acoustical Oceanography,
Wiley, 1977.

2. Cohen, P. M. , "Directional Echo Sounding on Hydrographic
Surveys," The International Hydrographic Review, v. 36,
No. 1, p. 29-42, July 1959.

3. Hoffman, J., "Hyperbolic Curves Applied to Echo Sounding,"
The International Hydrographic Review, v. 34, No. 2, p. 45-
bb, 1957.

4. Hurley, R. J., "Bathymetric Data from the Search for USS
THRESHER," The International Hydrographic Review, v. 41,
No. 2, p. 43-52, 1964.

5. Ingham, A. E., Sea Surveying, v.l, Wiley, 1975.

6. Krause, D. C, Menard, H. W. and Smith, S. M. , "Topography
and Lithology of the Mendocino Ridge," Journal of Marine
Research, v. 22, No. 3, p. 236-247, 1964^

7. MacPhee, S. B., "Developments in Narrow Beam Echo Sounders,"
The International Hydrographic Review, v. 53, No. 1, p. 43-
bl, January iy/6.

8. Raytheon Company, Bathymetric Systems Handbook, Revision 1,
July 1977. "

9. Umbach, M. J., Hydrographic Manual, U.S. Dept. of
Commerce, 4th Ed., Washington, D.C., 1976.

10. Urick, R. J., Principles of Underwater Sound, McGraw-Hill,
1967.

11. Watt, J. V., "Towards a Maximization of Information
Recorded on Hydrographic Echograms," Lighthouse, Journal
of the Canadian Hydrographers ' Association, Ed. 15 , p . 2~5-
TTt April 1977.

12. Weeks, C. G., "The Use of a Dual Frequency Echo Sounder in
Sounding an Irregular Bottom," The International Hydro-
graphic Review, v. 48, No. 2, p. 43-49, July 1971.

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DUDLEY KNOX LIBRARY

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