ICES Journal of Marine Science, 53: 487–491. 1996
Comparison of acoustic backscatter measurements from a
ship-mounted Acoustic Doppler Current Profiler and an EK500
scientific echo-sounder
Gwyn Griffiths and José I. Diaz
Griffiths, G. and Diaz, J. I. 1996. Comparison of acoustic backscatter measurements
from a ship-mounted Acoustic Doppler Current Profiler and an EK500 scientific
echo-sounder. – ICES Journal of Marine Science, 53: 487–491.
We compare the calibration of a 150 kHz RD Instruments Acoustic Doppler Current
Profiler against the 200 kHz channel of a SIMRAD EK500 scientific echo-sounder.
Over a range of mean volume backscattering strength of "88 to "68 dB (relative to
a scattering cross-section of 1 m "1), from 10 to 90 m depth, the data from the two
instruments were well correlated but with significant slope and offset errors (~10%
and &3 dB, respectively). After correction for these systematic errors the residual
differences between the ADCP and the EK500 were less than 1 dB. This accuracy is
sufficient for many qualitative studies of the coupling of ocean physics and biology.
? 1996 International Council for the Exploration of the Sea
Key words: acoustic backscatter calibration, Acoustic Doppler Current Profiler,
EK500, zooplankton.
G. Griffiths: Southampton Oceanography Centre, Empress Dock, Southampton SO14
3ZH, England. J. I. Diaz: Institut de Ciencias del Mar (CSIC), Pasco Juan de Borbon
S/N, 08039 Barcelona, Spain. Correspodence to Griffiths [tel: +44 1703 596004, fax:
+44 1703 596149].
Introduction
The instrumentation used to gather acoustic data on
biological distributions includes commercial scientific
echo-sounders, specialized research sounders, and,
recently, a tool used primarily by physicists studying
ocean currents, the Acoustic Doppler Current Profiler
(ADCP), e.g. Heywood et al. (1991) and Roe and
Griffiths (1993). These investigators used relative acoustic backscatter from the ADCP rather than calibrated
mean volume backscattering strength (MVBS) because
of the difficulties of calibration. More recently, papers
using calibrations based on manufacturer’s data have
appeared, e.g. Zhou et al. (1994). However, none of
these papers has addressed the calibration problem
through intercomparison.
The RD Instruments ADCP was not designed to
measure acoustic backscatter and, as a consequence,
even as a single frequency sounder, it has its drawbacks.
In this paper we deal with one drawback, the problem of
calibration. It is impractical for the user to calibrate the
instrument using the standard target method adopted
by many researchers (e.g. MacLennan and Simmonds,
1992), because the four beams of the ADCP are inclined
at 30) to the vertical and there is no aid to help position
the target within the narrow (2.5)) beams.
1054–3139/96/020487+05 $18.00/0
However, it is possible to compare the acoustic backscatter measurements from an ADCP with those from
a well-calibrated SIMRAD scientific echo-sounder. We
show that the backscatter strength measurements from
the ADCP have a linear relationship to those of the
200 kHz channel of a SIMRAD EK500.
Equipment and methods
The comparison between the ADCP and the EK500 was
carried out on the ship ‘‘Hesperides’’ in May 1994. Data
were gathered on a passage leg from Funchal, Madeira
to Cartagena, Spain. Both instruments have a large
number of user-selectable parameters, some of which are
vital for optimum use of both instruments in this role.
The main parameter settings are given in Table 1 and are
discussed in more detail below.
RD Instruments ADCP
The ADCP uses four transducers with acoustic axes
inclined at 30) to the vertical and driven by a common
power amplifier, but with four separate receiver channels. On ‘‘Hesperides’’, data from the ADCP deck unit
were acquired by a computer using the RD Instruments
? 1996 International Council for the Exploration of the Sea
488
G. Griffiths and J. I. Diaz
Table 1. Main EK500 and ADCP parameters.
Parameter
Power output
Transmit pulse length
Receiver cell length
Bandwidth
Averaging interval
MVBS resolution
MVBS threshold
EK500–200 kHz
1
0.45
0.02
20
23
0.01
"100
ADCP–150 kHz
Units
24
8
8
Proprietary
information
120
0.42
"112 at 200 m*
"86 at 400 m
kW
metres
metres
kHz
seconds
dB
dB
*The ADCP does not operate with an explicit threshold. The values given in the table are for a 0 dB
signal-to-noise ratio at the specified ranges.
Transect software. Acoustic backscatter data from the
four individual beams, in ensembles of four transmissions, were gathered at approximately 6 s intervals. Each
of the four beams was calibrated separately to account
for individual circuit and transducer parameters (Table
2). In addition, the user must estimate the noise floor for
each beam. However, this cannot be done accurately
when the vessel is moving, as flow noise is likely to be
present and will add to the wanted instrument noise
level. We adopted the procedure in RDI (1990) of using
a short (1 m) transmit pulse length, and a long (16 m)
receiver length interval, coupled with the maximum
number of intervals (128) to allow the receiver outputs
to reach the instrument noise level.
As neither the transmitter nor the receiver of the
ADCP are temperature-compensated, the temperatures
of both the transducer and deck unit affect the calibration and need to be considered in the calibration
equation.
The ADCP records the backscatter strength in the
form of Automatic Gain Control (AGC) counts; the
counts from each beam may be calibrated to give MVBS
expressed in decibels (dB) relative to an acoustic crosssection of 1 m "1. This form of MVBS is used in the
signal-processing firmware of the EK500, and we
adopted the same form for the ADCP.
The MVBS for each beam is obtained from the raw
AGC counts according to the following equation (RDI
1990):
where K2 is the instrument-dependent system noise
factor (dimensionless), Ks the frequency-dependent system constant, Kc the conversion factor for echo intensity
(dB count "1), E the AGC counts in the ensemble and
receiver interval considered, Er the instrument noise
level (counts), r the range from the transducer to the
Table 2. ADCP calibration parameters.
Parameter
Beam 1
Beam 2
Beam 3
Beam 4
Er noise counts
K1c watts
K2 noise factor
41
5.529
3.590
33
5.153
4.111
33
4.669
3.999
40
4.626
4.111
depth interval (m), c the speed of sound (1500 m s "1), P
the pulse length (m), K1 a power output term dependent
on system supply voltage, and á the absorption coefficient (dB m "1). The constants K2, Ks, Kc, K1 were
supplied by RD Instruments, we measured Tx (20)C)
and Er, and á was estimated at 0.049 dB m "1 by the
Transect software which uses the sound absorption
equations of Fisher and Simmons (1977). The electronics
temperature was 27)C.
SIMRAD EK500
The EK500 was fitted with three transceiver frequencies
– 38 kHz and 120 kHz using split beam transducers and
signal processing and a single beam 200 kHz transceiver
although, unfortunately, the 120 kHz channel was
faulty. During the comparison period, digital data from
the EK500 were acquired by a PC connected to the serial
interface.
In contrast to the ADCP, the EK500 has a wellcontrolled acoustic backscatter intensity calibration. In
this paper we assume that the absolute MVBS values
from the EK500 at 200 kHz may have an error of
&0.5 dB, based on the original calibration in 1990 by
SIMRAD and the study of Simmonds (1990), who
found the long-term stability of an earlier SIMRAD
instrument (the EK400) to be better than 13% or 0.5 dB
over 5 years.
We used the default value built into the sounder for á,
0.053 dB m "1.
Acoustic backscatter measurements
489
–60
–60
–61
ADCP 150 kHz MVBS (dB)
–65
ADCP 150 kHz MVBS (dB)
–62
–63
–64
–65
–66
–70
–75
–80
–67
–68
–85
–90
–69
–70
–75
–74
–73 –72 –71 –70
EK500 200 kHz MVBS (dB)
–69
–68
Figure 1. Scatterplot of mean volume backscattering strength
from the four beams of the ADCP at 150 kHz against the
EK500 200 kHz channel, for a depth layer of 10–50 m.
.=ADCP Beam 1; /=ADCP Beam 2; ,=ADCP Beam 3;
0=ADCP Beam 4.
–85
–80
–75
–70
EK500 200 kHz MVBS (dB)
–65
Figure 2. Scatterplot, and the linear regression, of the beam
average ADCP at 150 kHz against the EK500 at 200 kHz, for
depth layers of 10–50 m (upper cluster) and 50–90 m (lower
cluster). The outliers in the lower cluster were due to interference signals at 200 kHz.
The observations were taken on 21 May 1994 during
daylight, prior to the onset of upward diel migration of
zooplankton.
Data processing
The EK500 digital data were acquired for six 40 m layers
centred on 30, 70, 110, 150, 190, and 230 m. MVBS data
from individual pings were acquired at intervals varying
between 2 and 6 sec. These data were later averaged into
2 min periods to correspond exactly with the 2 min
periods of ADCP data, representing horizontal averages
of some 700 m at the transit speed of 6 m s "1. All
averages were arithmetically computed by converting
from dB to linear units, averaging, then converting back
to dB. The ADCP data, gathered in 8 m depth intervals,
were averaged over five bins to correspond with the 40 m
bins from the EK500. This procedure avoids tackling the
very difficult issues involved in making ping-by-ping
comparison of the two sounders because of their dissimilar spatial sampling characteristics. The footprints of the
main beams of the four ADCP transducers were circular, with a diameter at 70 m depth of 4 m at the "3 dB
points, spaced on a circle of 40 m radius. Samples from
each beam were taken every 9 m along the track (1.5 sec
ping interval at 6 m s "1). The footprint of the 200 kHz
channel of the EK500 was somewhat larger, at 6 m
diameter at 70 m depth, and adjacent samples were
between 12 and 36 m apart along the track. Beams 3 and
4 of the ADCP most closely matched the spatial track of
the EK500 footprint.
Results
Figure 1 shows the data from both instruments for the
30 m depth layer. Clearly, the calibrations applied to the
four beams of the ADCP have failed to converge.
Consistently, they are in increasing order: beam 1, beam
4, beam 2, beam 3. Slope and offset errors combined
such that the ADCP recorded MVBS of 4–8 dB higher
than the EK500.
The slopes of regression for each ADCP beam against
the EK500 were not statistically different at the 5% level
of significance but the offsets for each beam were
different at the 5% level of significance. Over the range
of MVBS from "74 dB to "68 dB the difference
between beam 1 and beam 3 was consistently 3.8 dB.
However, the scatter between the EK500 200 kHz
MVBS and that from an individual ADCP beam was
much lower, the standard error of the residual being
0.58 dB. One set of points, at the lowest MVBS indicated by the EK500, was anomalous, and has been
excluded from the analysis.
The MVBS data at 30 m spanned a range of barely
6 dB during the 90 min comparison period. By using the
beam average data for the 30 m and 70 m layers we
extended this comparison range to 20 dB (Fig. 2). The
gap between the two data clusters is due to the difference
490
G. Griffiths and J. I. Diaz
Table 3. Correlation analysis for the EK500 at 200 kHz and the ADCP at 150 kHz for depth layers centred at 30 m and 70 m, for
each beam and for the combined beams. The correlation statistics for the ADCP beam 1 and beam 4 data are shown
for comparison.
Correlation
statistic
Beam 1
and
EK500
Beam 2
and
EK500
Beam 3
and
EK500
Beam 4
and
EK500
Combined
beams
and EK500
ADCP
Beam 1
and
Beam 4
Slope
Intercept
Standard error of the slope
Standard error of the intercept
R2
F statistic
Regression sum of squares
Residuals sum of squares
Standard error of the y estimate
Degrees of freedom
0.895
"3.363
0.014
1.086
0.984
4069
3043
49.4
0.87
66
0.899
"0.680
0.014
1.095
0.984
4038
3072
50.2
0.87
66
0.928
2.815
0.013
1.015
0.987
5008
3271
43.1
0.81
66
0.901
"2.163
0.013
1.027
0.986
4618
3087
44.1
0.82
66
0.892
"1.963
0.012
0.940
0.989
5398
2842
30.5
0.73
58
0.994
"1.113
0.005
0.372
0.998
38 311
3935
8.4
0.32
82
in MVBS between the two layers. Some of the 70 m data
points were in error. These were found to be noise spikes
caused by other equipment on ‘‘Hesperides’’ interfering
with the EK500. As they are clearly identifiable they
have been excluded from the correlation analysis in
Table 3.
It is clear from the Table that the main cause of
variability in the ADCP calibration was the different
offsets of the four beams. Equation (1) shows that the
source of offset error is Er, the noise counts value. A
noise count error of one leads to an offset error of
0.42 dB. The implication from Figure 1 and Table 3 is
that the noise counts we measured when stationary were
not valid for the period of the comparison.
Beams 3 and 4 of the ADCP most closely tracked the
spatial sampling of the EK500 200 kHz beam, and in
Table 3 these two beams have the highest values of R2.
The significance of the higher R2 for beams 3 and 4
compared to beams 1 and 2 was examined after using a
Fisher-z transformation on the values of R. At the 5%
level of significance, all four of the correlation coefficients were shown to be samples of the same population,
and so the higher values are not statistically significant.
Conclusions
Our data show that the RD Instruments ADCP estimate
of calibrated MVBS at 150 kHz is highly correlated (R2
of 0.989) with the well-calibrated EK500’s 200 kHz
channel over a range of MVBS from "88 to "68 dB.
The slope of the correlation differs from unity, but this
may well be due to the difference in frequency. The
standard error of the difference between the beam average ADCP and 200 kHz EK500 for individual data
points of 0.7 dB is encouragingly low. Systematic error
in the individual beam ADCP absolute MVBS data was
due primarily to uncertainty or error in the measurement
of the noise levels of the four channels. This points to
a need for further study of the method for obtaining
estimates of noise levels and their application to the
calibration of the instrument. Some of the systematic
errors may be due to the difference in the angle of
incidence of sound on the targets, 0) for the EK500 and
30) for the ADCP (Inigo Everson, pers. comm.).
The implications of this comparison are that the
ADCP, given careful application of the manufacturer’s
calibration and particular attention to the measurement
of the beam-by-beam noise levels, can give estimates of
MVBS at the 95% confidence limits to better than
&1.5 dB. This accuracy is sufficient for qualitative
studies of the coupling of ocean physics and biology on
the scales of 10 m in the vertical and 1 km in the
horizontal.
Acknowledgements
Without the enthusiasm and help of M. Manriquez and
P. Rodriguez this work could not have been completed.
J. Rucabado kindly provided calibration data for the
EK500. The British Council provided a travel grant for
G.G. to work on the ‘‘Hesperides’’, and the analysis of
the data was partially supported by the U.K. Defence
Research Agency under contract PDN1A/201. Their
support is gratefully acknowledged.
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