ICES Journal of Marine Science, 53: 407–413. 1996
Improved precision of acoustic surveys of benthopelagic fish by
means of a deep-towed transducer
Rudy J. Kloser
Kloser, R. J. 1996. Improved precision of acoustic surveys of benthopelagic fish by
means of a deep-towed transducer. – ICES Journal of Marine Science, 53: 407–413.
Acoustic measurements of deepwater fish from a deep-towed transducer deployed at
depths from 500 to 650 m over a seamount (rising from 1000 to 600 m depth) are
compared with those from a vessel-mounted transducer. Algorithms were developed to
correct for noise and to analyse signals near the bottom. The analysis showed that the
vessel-mounted transducer can significantly underestimate the volume reverberation of
deepwater fish by an average factor of 1.86, equating to 2.7 dB. This bias was largely
attributed to beam thresholding and calibration, acoustic attenuation from nearsurface bubbles, ship motion, and uncertainties in the sound absorption constant. On
the steepest slope of the seamount the near-bottom shadow zone was reduced by 50%
and the extrapolated biomass in this zone reduced by 47% using the deep-towed
transducer. These results confirm that a deep-towed transducer will improve absolute
abundance measurements of deepwater resources.
? 1996 International Council for the Exploration of the Sea
Key words: acoustic surveys, orange roughy, towed body.
R. J. Kloser: CSIRO Marine Laboratories, PO Box 1538, Hobart, Tasmania 7001,
Australia [tel: +61 02 325 222, fax: +61 02 325 000].
Introduction
The commercial exploitation of deepwater species in
southern Australian and New Zealand waters has
increased substantially over the past 10 years. One such
species, orange roughy (Hoplostethus atlanticus Collett)
is vulnerable to over-exploitation owing to its aggregating behaviour, slow growth, and longevity. Approximately half the Australian orange roughy catch has
been obtained from a single spawning aggregation that
forms from early July to early August around a small
seamount (area: 210 km2) off the north-east coast of
Tasmania. The seamount, known as St Helens Hill, rises
from approximately 1000 m to 600 m depth. The orange
roughy typically occur in a large aggregation around the
hill and echo traces suggest that they occur from the
bottom to as high as 150 m into the water column.
Echo-integration theory and methods are well established (MacLennan and Simmonds, 1992) and recently
these techniques were applied to orange roughy in New
Zealand waters using shallow-towed bodies (Do and
Coombs, 1989) and in Australian waters using hullmounted acoustics (Elliott and Kloser, 1993).
Several factors limit the usefulness of vessel-mounted
or near-surface deployed acoustic systems for the assessment of deepwater fish. The major factor, which arises
1054–3139/96/020407+07 $18.00/0
from the association of orange roughy with steep
bottom topography, is the large acoustic shadow zone
caused when the acoustic beam reverberates off the side
of the hill, such that fish at greater depths are difficult to
distinguish from the bottom echo. Reducing this zone
for vertical echosounding techniques can be achieved by
using very narrow beam-width vessel-mounted transducers or by deploying deepwater transducers and greatly
reducing the range between transducer and target (or
bottom). Other factors which are weather- and vesseldependent include acoustic attenuation from nearsurface bubbles (Dalen and Lovik, 1981) and the effects
of ship motion (Stanton, 1982), which vary with sea
conditions. The effects of ship motion greatly increase
with range and a decrease in beam width of the transducer (Stanton, 1982). Uncertainty in the sound absorption constant (Fisher and Simmons, 1977; Francois and
Garrison, 1982) and beam thresholding (Foote, 1991)
would also affect deepwater surveys. These factors are
also range-dependent and their effect is increased as the
range to the target increases. All the problems listed
above may be reduced or entirely alleviated by deploying the acoustic transducer on a deep-towed body.
This paper compares acoustic measurements of deepwater targets obtained from a calibrated (with depth)
deep-towed transducer and a vessel-mounted transducer.
? 1996 International Council for the Exploration of the Sea
408
R. J. Kloser
Material and methods
Acoustic equipment and calibration
The acoustic equipment on the 65 m research vessel
FRV ‘‘Southern Surveyor’’ consisted of a scientific
SIMRAD EK500 echosounder connected to both a
standard SIMRAD 38 kHz (7) full angle) split-beam
hull-mounted transducer and an EDO Western 38 kHz
(6.5) full angle) split-beam transducer rated to 1000 m
mounted in a towed body. The towed body was built to
perform echo-integration surveys at depths down to
600 m at 5–6 knots and in situ target strength (TS)
measurements to be made at depths down to 1000 m at
2–3 knots with 2000 m of non-faired cable in the water.
The towed body (750 kg) was a passive type (relying on
its weight to achieve its depth) housing the transducer,
transmitter, preamplifiers, and a ‘‘monitoring’’ pressure
case. The ‘‘monitoring’’ pressure case housed equipment
to measure physical parameters such as towed-body
pitch, roll, depth, and operating voltage, all displayed
continuously aboard the research vessel. The towed
body was connected to the vessel by 2700 m of electromechanical cable that consisted of five single conductors
required for power, trigger, and monitoring signals and
four twisted-shielded-pairs that carried the preamplified
split-beam acoustic signals. The acoustic ping and summary data were logged on a personal computer (PC) via
ethernet and the serial port.
Connecting the EDO transducer to the 38 kHz transceiver in the SIMRAD EK500 was complicated by
the cable characteristics. Cable attenuation (20 dB at
38 kHz) and noise were overcome by placing the transmitter and preamplifiers in the towed body. The 2.5 kW
transmitter, triggered by the SIMRAD EK500, was
transformer-matched (60 ohms) to all 112 elements of
the transducer and diode-isolated for reception. On
reception, the transducer was divided into a split-beam
configuration, and four high-impedance preamplifiers
with line drivers amplified the acoustic signals up the
cable. Twisted-shielded-paired cabling with isolating
and matching transformers at the cable ends reduce
far-end crosstalk to less than "80 dB. The transformer
isolated split-beam signals were connected to the 38 kHz
transceiver in the SIMRAD EK500, which had its
transmitter disconnected and placed in passive mode to
avoid transmitting down the cable.
Calibration of acoustic equipment was performed
with a standard "33.6 dB, 60 mm copper sphere
(Foote, 1982 and SIMRAD, 1992) to obtain the target
strength and on-axis echo-integration constants. The
on-axis echo-integration calibration combined the electrical and acoustic constants of the system such as
transmitter power, transducer transmit and receiving
efficiency, and receiver gain. The only additional parameter required to specify the system, the equivalent beam
angle (EBA), was calculated by the transducer manufac-
turers. No independent measurements were made to
verify the EBA measurements. The coefficients in the
EK500 were adjusted to meet the theoretical TS of the
standard sphere ("33.6 dB) and the area backscattering
coefficient (sA) dependent on target depth. The towed
transducer was calibrated from 100 to 1000 m to correct
for the depth-dependent changes in transducer sensitivity and to examine any change in beam pattern. This was
performed by suspending a sphere fore and aft 10 m
under the towed body and lowering and raising the
towed body through the water column with calibration
performed at each 100 m depth interval for 15 min. At
each depth typically 400 TS values were recorded for the
calibration sphere. The seawater parameters of absorption (Francois and Garrison, 1982) and sound velocity
(MacKenzie, 1981) were calculated from temperature
and salinity profiles obtained from a Neil Brown
conductivity–temperature–depth recorder.
Program (ECHOX) for manipulating data
A program (ECHOX) was developed that enabled the
acoustic data to be displayed and edited on a PC. The
acoustic data were displayed as an echogram with 16
colour levels, each level separated by approximately
3 dB; 3 dB represents an approximate doubling of intensity. The program permitted setting of background and
spike noise thresholds, corrections for calibration and
absorption changes, and editing of bottom and shadow
zone positions on the echogram. The sA above the
bottom was calculated by integrating up from the bottom 150 m. The data were stratified by the 100 m depth
contour intervals.
Near-bottom analysis
Editing of the bottom signals was required where the
automatic depth tracking in the SIMRAD EK500 could
not predict the bottom signal due to the steepness of the
bottom. These resultant high bottom-signal values could
potentially corrupt the acoustic data. Using ECHOX,
the acoustic bottom line could be redrawn with the PC
mouse. This proved to be a better method than an
algorithmic approach due to the merging of fish echoes
with the start of the bottom signal. The true bottom
depth was set on the highest acoustic bottom signal for
each ping; the shadow zone height was then estimated as
the difference between the two depths, as shown in
Figure 1. Area backscattering within the shadow zone
was extrapolated from the volume backscattering
strength data in the 10 m above the acoustic bottom
signal into the shadow zone height, d in m:
sA =1010/sv*d*18522*4ð m2 nm "2.
Noise
Noise spikes recorded at very high values (commonly
Sv > "25 dB) were due mainly to unsynchronized
Improved precision of acoustic surveys of benthopelagic fish
409
650
700
Depth (m)
Acoustic bottom
750
Shadow-zone height 'd'
800
True bottom
850
–90
–70
–50
–30
Volume reverberation (dB re 1 m)
–10
Figure 1. Details of the analysis of the shadow zone. One acoustic ping is presented with volume reverberation (dB re 1 m) shown
for every 2 m from a depth of 670 m.
soundings for the hull-mounted transducer or occasional
high vibration and electrical noise associated with the
towed-body system. Noise spikes greater than the
applied threshold of "30 dB in any 2 m depth interval
was replaced by the Sv value recorded in the 2 m depth
interval immediately above in the water column. The Sv
data were also corrected for non-reverberation background noise. To assess background noise, a variant on
Nunnallee (1990) was performed. Instead of turning the
echosounder to passive mode at each alternate ping, as
suggested by Nunnallee, acoustic data were collected
well below the bottom signal where no reverberation
signal was evident. The bottom line was then redrawn
well below the original bottom (usually 200 m) and a
layer similar in height and length to the above-bottom
data was integrated. The mean volume reverberation
noise (Svn) values obtained were adjusted using the
appropriate time varied gain and subtracted from the
above-bottom backscattering strength data for each
depth interval.
interval over a range of 1000 m and logged on a PC.
Volume reverberation (Sv in dB re m "1) values were
corrected for towed body depth and the system calibration constants. Typically, 3 Mbytes of data were
collected on a transect of 2–3 nm. Five transects were
spaced every 0.5* of longitude in north–south direction
and five transects at 0.5* of latitude in an east–west
direction.
Echo integration of the diffuse deep-scattering layer
away from St Helens Hill was performed to compare the
vessel-mounted and towed transducers. The vesselmounted transducer sampled a much greater volume of
water than the towed transducer, and comparing data in
the uniformly distributed scattering layers reduced the
effects of schools seen close to the hill. The acoustic data
were subsequently stratified by 50 m depth bins from 500
to 950 m and for 0.2 nm length intervals and averaged
for each depth interval. No vessel-mounted data were
obtained if there was evidence of aeration in the 0.2 nm
length interval.
Hull and towed transducer comparison
Results
The spawning aggregation of orange roughy on St
Helens Hill (eastern Tasmania 41)14.0*S 148)45.4*E) was
acoustically surveyed in July 1992. Echo integration
data were recorded from each ping simultaneously from
the vessel-mounted and towed transducers. The towed
transducer was at between 500 and 650 m depth at 5 to
7 knots. Acoustic data were recorded for each 2 m depth
Table 1 gives the calibration parameters for both the
hull-mounted and towed transducers used for the survey. The towed transducer proved to be depth-sensitive
and shows the effects of hysteresis for up and down casts
(Fig. 2). A time series of TS measurements at a given
depth did not show any gradual change due to temperature equalization. Nor was change in beam width
410
R. J. Kloser
Table 1. Calibration parameters for both hull-mounted and
towed transducer. The towed transducer is specified at 600 m
operating depth and absorption and sound velocity at a mean
depth of 800 m.
Parameter
Apparent target strength (dB)
Pulse length
Bandwidth
Equivalent bean angle
Sv gain
Absorption 8 0 0 m
Sound velocity 8 0 0 m
Hull
Towed
body
Units
3
0.38
"20.7
27.7
9.6
1498
1
3.8
"21.1
31.4
9.3
1490
ms
kHz
dB
@600 m
dB km "1
m s "1
Consistent bias was recorded when comparing the
area backscatter of the deep-scattering layer for the
vessel and towed transducers for depths from 500 to
950 m in the deep water away from the hill (Table 2).
When combining all depth strata, omitting the values
where the signal-to-noise of the vessel-mounted transducer was less than 10 dB, the towed body data were on
average consistently higher than the surface-mounted
transducer by a factor of 1.86, equating to 2.7 dB. An
increase in the ratio with depth may be evident but,
given the low signal-to-noise ratios and higher variance
with depth, the result is not conclusive. Noise values
have been subtracted from the data and the Svn from
900–950 m are "76 dB for the towed body and "78 dB
for the vessel-mounted transducer.
–31
Discussion
–32
–33
–34
–35
0
200
400
600
Depth (m)
800
1000
Figure 2. Plot of calibration profile for down (+) and up (#)
casts for the towed deep water transducer. Mean apparent TS
for a "33.6 dB copper sphere at each 100 m interval is shown.
observed from contouring (using LOBE, program supplied by SIMRAD) of the TS data to the original beam
pattern. The beam pattern, varying from 7.1) to 6.3),
was not depth-dependent and tended to reflect the
amount of coverage of the calibration sphere within the
beam. However, the coverage of the beam was limited
by the amount of movement of the towed body as the
vessel pitched and rolled.
The acoustic data were collected in relatively calm
weather. A typical echogram (Fig. 3) displaying the raw
data from both transducers shows the difference in
resolution and the reduction in width of the shadow
zone between the two systems.
The steepest slope was encountered on the north–
south transect directly over the hill. For this transect, the
shadow-zone height, and the ratio of the area backscattering coefficient, sA, the projected shadow zone to
that of the 150 m above the bottom for each 100 m
contour interval was calculated. The mean shadow zone
height was reduced from 32 m to 16 m by using the
towed transducer. Likewise, the estimated sA (or biomass) in this zone has been reduced from 47% to 25% of
the value recorded in the 150 m above it.
The calibration of the towed transducer is made difficult
because of the hysteresis observed in the calibration
profile (Fig. 2). The transducer is air-backed with 112
pre-stressed elements and a rubber matching face. It is
possible that the rubber face at pressure is deforming
into and around the elements producing the observed
hysteresis (G. Snow, acoustic engineer EDO Corporation, pers. comm.). To reduce the effect of hysteresis on
a survey, the transducer should be maintained within a
narrow depth range. This has the effect of greatly
reducing the hysteresis loop and improving the precision
of the survey. The calibration parameters used for the
survey can then be obtained from the downward profile.
The reduction in the shadow zone by 50% was
expected from basic geometry as the mean range to the
bottom decreased by 50% in deploying a towed transducer. This reduced the estimated proportion of sA
represented by the shadow zone for the steepest slope
from 47% to 25%, which substantially reduced a major
source of uncertainty associated with the survey. Area
backscatter trends within the shadow zone supported
the use of a linear method of extrapolation into that
zone for orange roughy during the survey. This was
consistent with qualitative analysis of echograms from
hull and towed transducers which did not show any
change of school structure close to the bottom. It is
possible that the method here may overestimate the
shadow zone height for the towed transducer by 2–4 m,
which was 1–2 screen pixels. The 2 m depth intervals
were coarse compared to the 1 ms pulse length used
and an operator always erred on the side of caution
when marking the acoustic bottom to avoid including
high-value bottom echoes.
Values for sA for the deep water away from the hill,
recorded by the deep-towed transducer, were consistently higher than those recorded by the near-surface
transducer. Because the two transducers were recording
data over the same ground, the consistency of this
Improved precision of acoustic surveys of benthopelagic fish
411
Figure 3. Echogram from hull-mounted (top) and towed transducers (bottom). They are of the same ground. Note the greater
definition of the acoustic marks from the deep-towed transducer and the diminished shadow zone shown by the reduced thickness
of the bottom echo in the towed transducer echogram.
412
R. J. Kloser
Table 2. Comparison of the area backscatter (sA) and variance in parentheses for the 50 m depth
layers, relative to the surface, and 0.2 nm log intervals in the deep scattering layer away from the
seamount. The mean Log ratio has been calculated excluding data with SN ratio of <10 dB (*).
Area backscatter
Mean depth
m
Vessel
m2 nm "2
Towed body
m2 nm "2
525
575
625
675
725
775
825
875
925
Mean (500 to 950 m)
181
(437)
227
(945)
123
(109)
75
(125)
39
(248)
21
(841)
"6
(294)
85 (19 453)
99 (10 535)
331
367
169
128
126
199
227
216
179
difference indicates a constant source of bias in one of
the data sets. The difference may be due to a constant
bias in the calibration of the hull-mounted and/or towed
transducer. This could occur if the equivalent beam
angle calculated by the manufacturers changed significantly when mounting the transducers. This has been
investigated by Simmonds (1990) who found that the
EBA could vary by 0.5 dB for various transducer
mountings. Variation in the EBA between the two
transducers due to transducer mounting may give a
constant bias of up to &1 dB. This constant calibration
bias does not fully explain the signal attenuation
observed. Several other sources of signal attenuation
may combine to cause the large difference observed.
Vessel movement and bubble attenuation may contribute to the reduction in the hull-mounted transducer
values. Weather conditions were good at the time of the
survey, with 10–15 knot NW wind and a 1 m sea. For
the sea conditions experienced, typical vessel movement
is in the order of 1–2) s "1 and the expected signal
attenuation for the 7) beam-width transducer at 475 m
would be less than 5% (MacLennan and Simmonds
1992). Attenuation by surface bubbles is difficult to
quantify, as this depends on past and present wind
conditions and the vessel characteristics (Dalen and
Lovik, 1981; Bruno and Novarini, 1983; MacLennan
and Simmonds, 1992). According to Dalen and Lovik
(1981), the attenuation expected for a windspeed of
10–15 knots is in the range of 13–29%. Both sources of
signal attenuation outlined above would account for
between 0.8 and 1.7 dB of the 2.7 dB mean difference
observed between the two systems.
The remaining potential sources of attenuation could
be attributed to the acoustic sampling volume changing
with depth (Foote, 1991) and an error in the absorption
coefficient. Owing to threshold effects, the effective
sampling volume of the near-surface transducer may be
(974)
(1207)
(156)
(114)
(3943)
(60 555)
(79 605)
(33 718)
(32 658)
Log ratio
sA
dB
2.6
2.1
1.4
2.3
5.2*
9.7*
NA*
4.1
2.6
2.7 dB
less than would be estimated from simple extrapolation
of the beam geometry through the water column. This
would explain the very high difference observed when
the signal-to-noise ratio of the vessel-mounted transducer was below 10 dB. The absorption coefficient
formula of Francois and Garrison (1982) results in
vessel-mounted backscattering values approximately
1.5–2 dB higher than the formula of Fisher and
Simmons (1977) as used by Do and Coombs (1989) for
deepwater surveys. The difference observed between the
vessel and towed transducers would be approximately
doubled if the Fisher and Simmons’ formulation had
been applied. The results suggest that the Francois and
Garrison formula is more appropriate and may even
underestimate the sound absorption at these ranges and
depths.
It has been demonstrated that the use of a deeply
towed transducer for acoustic surveys of deepwater fish
can improve data quality and greatly reduce the shadow
zone (Fig. 3). Quantitative improvement has been
obtained in the areas of shadow zone reduction and
improved signal levels.
Acknowledgements
This project was supported by a grant from the
Australian Fisheries Research and Development
Council. Thanks go to Dr T. Koslow and Dr N. Elliott
and an anonymous reviewer who provided valuable
advice on earlier drafts of the manuscript.
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