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Combined Current Profiling and Biological Echosounding Results from a Single ADCP

Authors:
  • Nortek Group

Abstract and Figures

The present work describes a newly-developed Acoustic Doppler Current Profiler (ADCP) that has a fully integrated single-beam wide-band biological echosounder, thus serving a dual purpose: current measurement and biomass assessment. The system comprises a traditional 4-beam Janus configuration head, which is responsible for profiling the currents, with a vertically oriented center beam for collecting high-resolution acoustic backscatter data for subsequent biomass analysis. The system belongs to the Signature Series family of ADCPs launched in 2013 by Norwegian scientific instrumentation company Nortek. Named Signature100, it is powered by the AD2CP electronics platform, described in United States Patent 7.911.880. The four slanted beams (current profiling beams) operate at a center frequency of 100 kHz and have a range of up to 400 m with 4 m spatial resolution and sampling rate up to 1 Hz. The center vertical beam (echosounding beam) has a wider frequency band of approximately 70-120 kHz with a high dynamic range (~130 dB), and presently operating in up to three discreet pulse characteristics from a single beam set: 1) 70 kHz monochromatic, 2) 120 kHz monochromatic, and 3) 91 kHz chirp with 50 percent bandwidth and pulse compression. Acoustic pulses from the echosounder beam are interweaved with pulses for the current profiling beam for synchronous data collection. In this work we describe the system’s configuration, capabilities and results from initial trials.
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Combined current profiling and biological
echosounding results from a single ADCP
David W. Velasco
Nortek Group
Boston, USA
david.velasco@nortekgroup.com
Sven Nylund
Terje Pettersen
Nortek Group
Oslo, Norway
Abstract— The present work describes a newly-developed
Acoustic Doppler Current Profiler (ADCP) that has a fully
integrated single-beam wide-band biological echosounder, thus
serving a dual purpose: current measurement and biomass
assessment. The system comprises a traditional 4-beam Janus
configuration head, which is responsible for profiling the currents,
with a vertically oriented center beam for collecting high-
resolution acoustic backscatter data for subsequent biomass
analysis. The system belongs to the Signature Series family of
ADCPs launched in 2013 by Norwegian scientific instrumentation
company Nortek. Named Signature100, it is powered by the
AD2CP electronics platform, described in United States Patent
7.911.880. The four slanted beams (current profiling beams)
operate at a center frequency of 100 kHz and have a range of up
to 400 m with 4 m spatial resolution and sampling rate up to 1 Hz.
The center vertical beam (echosounding beam) has a wider
frequency band of approximately 70-120 kHz with a high dynamic
range (~130 dB), and presently operating in up to three discreet
pulse characteristics from a single beam set: 1) 70 kHz
monochromatic, 2) 120 kHz monochromatic, and 3) 91 kHz chirp
with 50 percent bandwidth and pulse compression. Acoustic
pulses from the echosounder beam are interweaved with pulses for
the current profiling beam for synchronous data collection. In this
work we describe the system’s configuration, capabilities and
results from initial trials.
Keywords—echosounding, ADCP, currents, biomass
I. INTRODUCTION
The continual global increase in human population is
prompting governments to assess protein sources with greater
detail. Global demand for animal-derived protein is expected to
double between now and 2050 [1], driven by increasing
urbanization (especially in emerging economies), improved
recognition of protein’s role in a healthy diet, and increased need
for protein in the elderly community. Fish stocks are one source
of animal-derived protein which is receiving considerable
attention due to their potentially vast contribution to addressing
global protein requirements. In fact, global fish production far
surpass the production of all other animal protein in the world,
and fish also contain many essential micronutrients, minerals
and essential amino acids [2].
Fisheries scientists use a variety of tools in understanding the
structure, dynamics, function and quantity of fish stocks.
Acoustic technology (in the form of biological echosounders) is
widely used in quantifying fish stock biomass volumes and their
behavior. Acoustic technology (in the form of ADCPs) has also
been used to accurately measure currents in all of the world’s
major water bodies over the last 30 years.
As ADCP and echosounder data complement each other
well, they are often used in the same project and deployed
together. However, these two technologies have historically
been developed by separate companies, with different
objectives, leaving the end user to integrate the two solutions
together. Nortek’s approach has been to leverage its expertise
in underwater acoustic technology, transducer manufacturing,
electronics and firmware architecture design to combine these
two tools into a single instrument. This reduces the complexity
of the system, increases the ease of use to the operator (data are
precisely synchronized) and drives down cost as a single
instrument can do the job of two.
Fig. 1. Nortek’s Signature100 ADCP. The four yellow transducers are
responsible for measuring the currents. The center black transducer is a
biological echosounder. All dimensons in mm.
II. SYSTEM DESCRIPTION
A. Core AD2CP Functions
Powered by the AD2CP platform [3], the Signature100 (Fig.
1) belongs to Nortek’s Signature Series family of Doppler
profilers, which presently span operating frequencies from 1000
kHz to 55 kHz. Although some application-oriented features are
unique to just some of the Signature Series systems, there are
several elements common to all of them:
Current profiling using proper Broadband processing,
with frequency coding of the transmit pulse as opposed
to phase coding, thus providing improved
configuration flexibility.
Concurrent Mode Technology (described under US
Patent 7.911.880), allowing for different types of pings
to be collected at the same time (within measurement
interval).
Support for vertical beam operation independent of
velocity beams.
Ethernet communication, allowing for high bandwidth
communication and support of standard network
protocols (HTTP, FTP, UDP, PTP, and Telnet).
Automatic recording of raw magnetometer data,
enabling post-deployment compass calibration.
Large memory bank (presently up to 128 GB) for
extended deployment time as well as ability to record
high volume data such as multiple echograms.
Significantly reduced power consumption, owing to
modern electronics, high efficiency transducers, and
intelligent firmware architecture.
External LED indicator for visual confirmation of
instrument’s status.
Support for internal high-accuracy Attitude and
Heading Reference Sensor (AHRS) for real-time bin
mapping in dynamically moving installations, such as
surface buoys.
B. Dual Purpose Applications
The above characteristics come together in the two key
functions of the Signature100: current profiling and biological
echosounding. The current profiling (traditional ADCP) portion
of the Signature100 is comprised of four slanted beams in a
Janus configuration, operating at a center frequency of 100 kHz,
with 6.1° beam width. These can profile currents over a range
of up to 400 m with 4 m spatial resolution and sampling rate up
to 1 Hz. The biological echosounder portion of the instrument
is comprised of an independent center beam with a wider
frequency band (70-120 kHz), a high dynamic range of
approximately 130 dB, a beam angle of 15° at 70 kHz and 8.7°
at 120 kHz and maximum output power of 120 W. This center
beam can ping at the same rate as the velocity beams and also
reach the same full range (400 m). When both current profiling
and biological echosounding are used together, it is expected
that the Signature100 can last one full year with standard battery
power options in a typical configuration.
C. Novel Transducer Design
Traditional ADCP transducer stacks are generally comprised
of three main parts: a piezoelectric ceramic, a matching layer at
the front, and damping material on the back. These are encased
in a cup-like housing. This housing acts to support the
transducer stack and connect it to the ADCP body, but otherwise
plays no active part in the acoustic performance of the entire
assembly. The Signature100’s slanted (current profiling)
transducers are unique in the ADCP industry in that all
mechanical parts in the transducer design, including the cup,
actively contribute to its acoustic performance and efficiency.
This approach allows the entire assembly to have a depth rating
of at least 1500 m yet have a total thickness of only 43 mm,
never before achieved on an ADCP of this frequency (100 kHz).
In addition to the novel design, the piezoelectric element
used in the Signature100 slanted transducers are composite
broadband ceramics, rather than solid disks often used on
ADCPs. Although not unique to the Signature100 (Nortek’s
Signature55 also uses composite broadband ceramics), they are
made by dicing standard ceramics and filling with a special
epoxy resin. This process increases the final transducer’s
sensitivity while maintaining a wide usable bandwidth, two
critical parameters in determining the system’s ultimate
profiling range, flexibility and power efficiency.
D. Echogram Processing for Wideband Chirp
The main function of the center single-beam transducer in
the Signature100 is to record echograms that provide
information on the structure and dynamics of marine biota.
Echograms can be generated from up to three different pulse
types: 1) 70 kHz monochromatic, 2) 120 kHz monochromatic,
and 3) wide bandwidth (50%) linear chirp ranging from 68 kHz
to 113 kHz centered at 90.9 kHz. The monochromatic pulses
are processed internally using standard algorithms, but the chirp
can be processed in one of two ways: using pulse compression
or using a binned frequency response. The pulse compression
technique is widely used in echosounder applications and it
allows for increased range resolution as well as improved
Signal-to-Noise Ratio (SNR). The resolution after pulse
compression is 1.65 cm, and the internal processing can average
this into a minimum bin size of 37.5 cm. In the binned frequency
response method, the return is processed into five separate
echograms each containing approximately one fifth of the total
bandwidth.
In this work, the Signature100’s echosounder beam was not
calibrated for absolute backscatter. However, Nortek is
developing a process to allow operators to perform this
calibration such that accurate Volume Backscatter Strength (S
v
)
and Target Strength (TS) can be computed. As such, all
backscatter data presented here is relative to the instrument itself
and reported as SNR in decibels (dB).
E. Raw Return Signal Storage
In order to expand the system’s flexibility, the Signature100
is able to store the complex demodulated return signal. The
system allows storage of the in-phase (I) and quadrature-phase
(Q) components of the return signal at 45.45 kHz (50% of the
center frequency). Additionally, the system can also store the
transmit pulse for a complete data set. Despite the system
having a large memory bank, recording of raw demodulated
signal can surpass the memory’s limits, so a configurable
recording scheme is implemented. The operator can specify the
spacing between the pings or their interval (e.g. store every Nth
ping, or all pings for N minutes every hour). The ability to store
the raw return signal allows the operator to post-process the data
using whatever technique is most suitable for their particular
application, as well as perform post-processing calculations for
TS and S
v
.
III. FIELD VALIDATION
Field trials have been done as part of the Signature100’s
development, and here we highlight one such deployment
carried out in the Mediterranean Sea. The location was just
south of Toulon, France, and the deployment lasted from the
morning of 10/Nov/2017 until the afternoon of 15/Nov/2017.
Water depth at the site was about 470 m and the instrument was
mounted up-looking on a subsurface buoy at the top of a short
mooring. Raw heading data from the Signature100 (not shown)
indicates the buoy observed a strong spin moment on its 7
minute descent, rotating at about 3-4 revolutions per minute,
which is not unreasonable during such deployments. For fixed
installations (not this case), this spinning can be a source of raw
magnetometer data allowing for compass calibration in post-
processing. After about 3 hours on the bottom, the buoy
stabilized and subsequent data shows the mooring was very
stable throughout the rest of the deployment, with only a gentle
rotation (less than one revolution every few hours) and a very
minor variation in tilt (less than 1°).
A. Current Data
The ADCP portion of the instrument was configured to
transmit 60 pings at 0.25 Hz, repeating the sequence every 5
minutes, with single ping data being recorded for quality control
purposes. Current profiling was set for 60 depth cells of 10 m
each (15 ms pulse) with a blanking distance of 1 m. Variable
particle distribution in the water column caused the maximum
usable range to oscillate from about 230 m to 420 m (beyond the
instrument’s specifications), especially during the first half of
the deployment, as evidenced by the SNR data of the four
slanted beams shown in Fig. 2. As expected, times of reduced
SNR coincide with lower along-beam signal correlation values,
indicating limit of usable data which is taken at 50% correlation.
But despite the variations in particle distribution, over 68% of
the data is above this 50% correlation threshold.
Fig. 2. Signal correlation (top) and SNR (bottom) from the slanted beams for
the duration of the deployment.
A mid-depth current maxima is observed early in the
deployment, although most of the fastest currents appear to be
confined to the top of the water column (Fig. 3). Unfortunately
these are not well captured some parts of the deployment due to
limited quantity of scattering particles from about 300 m above
the instrument, but can be seen during other times when
increased particle distribution drives longer profiling ranges.
Fig. 3. Current speed for duration of deployment.
B. Echosounder Data
The echosounder portion of the Signature100 was
configured to transmit all three pulse types supported, although
only the 70 kHz monochromatic is presented here. Each pulse
had a 1 ms transmit duration and were transmitted at 1 Hz. The
echosounder pulses interweaved with the current profiling
pulses at a ratio of 3:1 (i.e. every three echosounder pings to one
current profiling ping). The echosounder pulses’ return was
recorded in 0.75 m depth cells.
From the echosounder data (Fig. 4) it is possible to identify
schools of fish, swarms of smaller organisms (krill and/or
zooplankton) and even individual fish. Although the present
lack of a calibrated return signal prevents calculation of Sv,
insights about the distribution, structure and presence/absence of
biota over time and depth can be drawn from this deployment.
Additionally, echograms coupled with interweaved current
profiles into a single system provide valuable information on the
behavior marine life.
Fig. 4. Echogram from 70 kHz monochromatic pulse for duration of
deployment. Scale is relative to instrument (not S
v
) in dB. Closer details are
provided in subsequent figures.
Four different features were selected from the echogram for
presentation: plankton/krill diel migration, internal wave
structures, passing surface vessels, and migration due to changes
in current regime. The first type of feature is shown on Fig. 5.
Although no trawling was conducted during the deployment to
ground-truth the nature of the scatterers, it is reasonable to
assume the features shown represent either plankton or krill (or
both) migration, as the same patterns are widely observed in
similar data [4]. The diel nature of the movement drives
measurable vertical currents of approximately 5 cm/s upward
during dusk hours, with the reverse pattern at dawn.
Fig. 5. 70 kHz Echogram (top), vertical velocity (middle), and horizontal current speed (bottom) for the first two days of the deployment.
Another interesting feature identifiable in the echograms are
internal oscillations at varying depths and lasting up to a few
hours. The echogram on Fig. 6 clearly shows their presence,
distribution and duration. Internal waves are a common feature
in the Mediterranean Sea, being described as earlier as the 1960s
[5]. They are generated by the interaction of the mostly
semidiurnal tidal flow with the variable bottom bathymetry and
especially through narrow passages such as the Straits of
Gibraltar and others. For the sample shown, it is estimated that
some of these reach 10-15 m in height with periods of
approximately 90 minutes. Although the spatial resolution of
the current data (10 m depth cells) does not allow for as clear
identification of these oscillations as the echogram does,
nevertheless we can approximate a downward and upward
velocity of about 1 cm/s with the passage of this particular event.
Fig. 6. Internal oscillations observed 70 kHz echogram on 12/NOV/2017 (top). Inset is a particular structure showing the echogram (middle) and the vertical
velocity (bottom).
Following the terminology of [5], undesirable echogram
features (noise) can be divided into three main categories:
Impulsive Noise (IN), Transient Noise (TN), and Background
Noise (BN). IN occurs over less than one acoustic ping and can
often be traced to interference from a nearby acoustic source of
same or similar frequency. TN can last several pings and have
variable sources, such as ships passing near the echosounder (for
fixed systems) or waves colliding with the vessel’s hull (for
vessel-mounted systems). BN lasts for hours or longer and may
be traced to any continuously generated signal of same or similar
frequency, such as an underwater turbine. Fig. 7 shows both IN
and TN from what we interpret as a passing surface vessel. The
TN reaches almost the entire profiling range of the instrument,
with increasing attenuation with depth, an inverted arch pattern
typical of a single strong reflector, followed by increase near-
surface noise in the vessel’s wake. The echogram also suggests
the vessel had an active acoustic source onboard, as IN features
are clearly visible in the leading and trailing edges of the main
signal.
Fig. 7. Echogram showing the passage of a surface ship.
The echogram data also captures a change in the near-bottom
activity, which is correlated with a change in current direction
near the bottom (Fig. 8). Starting around midday on 14
November, and lasting through the end of the deployment, the
echosounder returned signal strength first increased within the
bottom 100 m of the water column, and then it diminishes
noticeably. Overall currents speeds around this time indicate
negligible change (not shown). Although the data is
inconclusive as to the reason for this reduction, the correlation
with the relatively sudden change in current direction may be a
driver. For the 30 hours preceding this change, the mean current
direction at 50 m above the instrument was holding relatively
steady towards the SSE, but then changed (over the course of
less than 3 hours) to flow primarily to the NNW, following some
further variations over the subsequent 24 hours. This means that
currents that were flowing mainly from the continent switched
to flow mainly from the open Mediterranean Sea. It is therefore
speculated that this change impacted the local biota, shifting its
location towards the continent, away from the instrument.
Fig. 8. 70 kHz echogram for second half of deployment (top) and current
direction at 100 m above the instrument (bottom).
IV. CONCLUSIONS
A newly-developed Acoustic Doppler Current Profiler
(ADCP) with a fully integrated single-beam wide-band
biological echosounder has been developed and presented. The
system belongs to the Signature Series family of ADCPs
launched in 2013 by Norwegian scientific instrumentation
company Nortek and is powered by the AD2CP electronics
platform (US Patent 7.911.880). Named Signature100, it
performs two key functions simultaneously over a maximum
nominal range of 400 m: current profiling and biological
echosounding. Some of its key features include a novel
transducer design, three distinct echosounder pulse types, data
processing with pulse compression, and the capability of
recording the complex demodulated return signal. Details from
a field validation deployment in the Mediterranean Sea were
presented, with a focus on the 70 kHz echogram created by the
instrument. Four main features of the echograms were
discussed: plankton/krill diel migration, internal wave
structures, passing surface vessels, and migration due to
changes in current regime. The current profiling data
complemented the echosounder data, providing greater insights
into the distribution, structure and behavior of the marine life at
the test site during the deployment.
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[3] A. Lohrmann, S. Nylund, “A New Long Range Current Profiler,
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[4] C. Winant, T. Mettlach, S. Larson, “Comparison of buoy-mounted 75-
kHz acoustic Doppler current profilers with vector-measuring current
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[5] J. Ziegenbein, “Short internal waves in the Strait of Gibraltar,” Deep Sea
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Article
In December 1991, the National Data Buoy Center (NDBC) deployed two meteorological buoys in the Southern California Bight on a transect between San Diego and San Clemente Island. Each buoy consisted of a 10-m discus hull instrumented to measure a suite of meteorological parameters, and, for the first time in the NDBC buoy program, acoustic Doppler current profilers (ADCPs) were included to gather hourly current profiles beneath the two buoys. Moorings instrumented with seven vector-measuring current meters (VMCMs) were deployed adjacent to the NDBC buoys for several months and provided current observations for comparison with the ADCP measurements. When wave-induced buoy motion is not overly large, the observations of horizontal current made by the ADCP and the VMCM are highly correlated. Time series of differences between ADCP and VMCM measurements are characterized by a mean difference (bias error) of about 0.01 m/s and standard deviation of about 0.035 m/s for 1-h observations. Estimates of current spectra from ADCP and VMCM records suggest that the ADCP system can be characterized by a white noise level of 2 x 10(exp -3) sq m/sq s/cph. However, when the in situ environment is such that large surface waves are present (including breaking waves and whitecaps), erroneous current values are usually reported by the ADCP. Mean values of vertical velocities reported by the ADCP appear to be much larger than what could be physically expected and are therefore deemed unreliable.
Short large-amplitude internal waves have been investigated in the Strait of Gibraltar. A buoy system consisting basically of a moored thermistor chain has been developed to get a detailed picture of the wave trains and preceding internal fronts. The buoy system is briefly described. The recordings reveal that the internal (boundary) waves form definite wave trains consisting of 5–8 cycles, preceded by internal fronts. Amplitudes of the waves vary between a few metres and 40 m; the periods vary between 6 min and 20 min. Two categories of wave trains can roughly be distinguished. The first one shows a maximum amplitude at the onset of the oscillations being then followed by a relatively smooth decay of the amplitude, the frequency remaining almost constant. The second one is characterized by growing amplitudes and increasing frequency during the course of the oscillations, leading to an abrupt decay of the oscillations after the maximum amplitude has been reached.
A New Long Range Current Profiler, Development of the Signature75
  • A Lohrmann
  • S Nylund
A. Lohrmann, S. Nylund, "A New Long Range Current Profiler, Development of the Signature75," MTS Oceans Conference 2013, San Diego, CA.