David Lemon

Inverted Echo Sounder on a Cabled Observatory

High-frequency acoustic backscatter measurements have long been used as a method to detect zooplankton populations in the ocean. Ship-borne echo-sounders can map distributions over relatively large areas, but are not practical for following developments over long periods of time. Self-contained echo-sounders, either moored at depth looking upward, or mounted on surface buoys looking downward produce time series of acoustic backscatter which are a means for monitoring long-term behaviour of zooplankton populations. Combining these instruments with a cabled observatory allows this information to be acquired and monitored in near real-time, and thus to contribute to understanding and monitoring the state of one of the key components of coastal marine ecosystems. In this paper, we will discuss what we believe is the first operational example of an inverted echo sounder as part of a cabled seafloor observatory. The first component of the VENUS seafloor observatory, in Saanich Inlet, British Columbia, began operation in February, 2006. The instrument platform is located 3 km from the Institute of Ocean Sciences dock, at 96 m depth. A 200 kHz upward looking echo sounder (Acoustic Water Column ProfilerTM, also known as the ZAP, for Zooplankton Acoustic Profiler) is located on the platform. The instrument collects a profile of acoustic backscatter strength throughout the water column once per second and transmits the data over the cable to the VENUS shore station. Power and control communications are also supplied over the cable. The techniques for command and control of the instrument over the VENUS VPN are described, as are the means for real-time display of the data using TCP/IP protocols. Continuous data retrieval and image posting to the VENUS website are performed automatically using the observatorys DMAS (Data Management and Archiving System). Echograms of acoustic backscattering strength as a function of depth and time are available on the web at hourly intervals, and as daily summaries. These plots, and the data files from which they are constructed, may be used for the detection if zooplankton, fish, near-surface bubbles and internal waves. Their evolution and development can be monitored in real time, and the archived data now provide a year-long record at the Saanich Inlet site. At least two more of these instruments will be deployed at sites in Georgia Strait, when the next phase of the Venus observatory system is installed in October 2007. It is anticipated that the data from these locations will provide a valuable tool to further the understanding of the state of this coastal sea. A new generation of the AWCP is currently under development, which will provide greater A/D resolution and greater data storage for self-contained operation.

Real-time measurements of ice draft and velocity in the St. Lawrence River

The Canadian Coast Guard monitors winter ice conditions in the St. Lawrence River as part of its responsibilities to prevent and break ice jams in order to minimize the risks of flooding and maintain safe navigation conditions on the St. Lawrence River throughout the winter months. Near real-time information about the coverage, thickness and motion of the ice cover in the navigation channel are required to coordinate icebreaking for maintaining the shipping route, and to prevent and identify ice jams as they develop. Aerial and satellite surveillance provides ice coverage data, but not thickness. This work describes a test installation in the St. Lawrence that provides real-time ice thickness, ice motion, current velocity and meteorological data from a remote site. The IPS (Ice Profiling Sonar) and ADCP Data Display System (IADDS) consists of two submerged instruments (IPS and ADCP), connected by cable to a nearby lighthouse that is equipped with a computer, weather station, appropriate display software and data transmission capability to shore and the Fisheries and Oceans Departments network. The principles and operation of the IPS and the use of an ADCP to measure ice velocity are described. The IPS and ADCP are installed at 13 m depth in the navigation channel in Lac St. Pierre in the St. Lawrence River. Real-time data from the instruments and the weather station are collected at the lighthouse site, and then formatted and transmitted to the Coast Guard headquarters in Quebec City, approximately 200 km away. Web-compatible graphs of the data are then produced for display on the Coast Guard Intranet. The structure of the control, data transmission and storage software is described, and examples are given of the data and its use for managing navigation and detecting ice jams. The results of on-site validation measurements made in the winter of 2002-2003 are also described.

Modeling cooling water discharges from the Burrard Generating Station, BC Canada

A three-dimensional numerical model was applied to examine the impact of the Burrard Generating Station cooling water on the circulation patterns and thermal regime in the receiving water of Port Moody Arm. A key aspect of this study involved properly incorporating the submerged cooling water buoyant jet into the 3D model. To overcome the scale and interface barriers between the near-field and far-field zones of the buoyant jet, a sub-grid scheme was applied, and the coupled system of equations of motion, heat conservation and state are solved with a single modeling procedure over the complete field. Special care was taken with the diffusion and jet entrainment by using a second order turbulence closure model for vertical diffusion and the Smagorinsky formula for horizontal diffusion as well as jet entrainment. The model was calibrated and validated in terms of buoyant jet trajectory, centerline dilution, and temperature and velocity profiles. Extensive modeling experiments without and with the Burrard Generating Station in operation were then carried out to investigate the receiving water circulations and thermal processes under the influence of the cooling water discharge. The model results reveal that under the influence of the cooling water discharge, peak ebb currents are stronger than peak flood currents in the near-surface layer, and the reverse is true in the near-bottom layer. Meanwhile, the model revealed a well-developed eddy at the southeast side of the buoyant jet in the near-surface layer. It is also found that the warmer water released from the cooling water discharge is mainly confined to the upper layer of the Arm, which is largely flushed out of the Arm through tidal mixing processes, and a corresponding inflow of colder water into the Arm occurs within the lower layer.

Comparison of acoustic measurements of zooplankton populations using an Acoustic Water Column Profiler and an ADCP

Self-contained, moored echo sounders are a means of monitoring the behavior of populations of zooplankton and small fish over extended periods of time. Such instruments, either moored at or near the seafloor looking upward, or mounted on a surface buoy looking downward, record profiles of acoustic backscatter as a time series, and thus can provide insights into the long-term behavior and distribution of these populations. Single-frequency instruments are not capable of identifying the source of acoustic backscatter as species, but nevertheless can provide valuable information with low-cost, easily-deployed instrumentation over extended periods of time. This type of data can be collected either with an echo sounder designed for the task, or as an auxiliary output of an ADCP, using the RSSI (Received Signal Strength Indicator) output. In each case, without precise instrument calibration, an estimation of volume backscatter strength can be made from the data recorded by the two types of instrument. In this paper, we will compare the capabilities of an example of each type of instrument in terms of their spatial and temporal resolution and deployment endurance for extended monitoring. Calibration issues will also be discussed. In June 2004, a 200 kHz Acoustic Water Column Profiler (Version 4) and a 300 kHz RDI ADCP were co-located in Saanich Inlet, BC. The instruments were mounted on a surface buoy looking downward in 150 metres water depth for a period of 10 days. The two instruments were configured to operate with similar range and time resolution. The ADCP recorded a 30-ping ensemble every minute in 1 metre range bins. The AWCP4 recorded a 3-ping average every 12 seconds with 0.5 m range resolution. The sampling regions of the two instruments were not exactly co-located, but were close enough to show the same larger scale features. Echograms from the two clearly show the two primary zooplankton populations in the Inlet, one migrating diurnally, and the other remaining at depth. Data from both instruments, when converted to volume backscatter strength are in agreement within the limitations of their approximate calibrations. Examples of these will be shown. The difference in performance between the two instruments appears when longer or more-frequently sampled deployments are considered. With the spatial and temporal resolution used in the Saanich Inlet deployment, the ADCP is limited to a 20-day deployment on a standard battery pack (80 days would be possible if 2 more batteries were added in an external case). The AWCP4, in contrast, would last over 8 months operating with those parameters. Operating the ADCP to achieve high-resolution backscatter data also degrades its performance in measuring current velocity in most cases. The AWCP4 therefore allows greater temporal and spatial resolution over the extended monitoring periods of many months that are one of the primary motivations for using a single-frequency instrument. The AWCP4 has recently been replaced by a new model, the AWCP5, which has increased data storage (up to 16 GBytes vs. 138 Mbytes), greater flexibility in choice of sampling strategies and 16-bit as opposed to 8-bit digitization for greater dynamic range. The AWCP5 offers even greater advantages in time and space resolution and length of operation for acoustically monitoring zooplankton populations.

Assessing the site potential for underwater turbines in tidal channels using numerical modeling and advanced ocean current measurements

A combination of advanced ocean current profiling measurements and high resolution 3D numerical models was used to assess site potential for underwater turbines in tidal channels of the inland waters off the coast of British Columbia, Canada. The measurements involved the use of ADCP transects through potential sites. Due to the very strong tidal currents of up to 10 knots or more, special procedures are required to generate accurate and reliable maps of the very strong ocean currents. The three-dimensional, coastal circulation model COCIRM was used to map these detailed flows under different scenarios and assess the potential at various sites for operation of underwater turbines after validated using available water elevation and ocean current data.

The acoustic water column profiler: a tool for long-term monitoring of zooplankton populations

Measuring acoustic backscatter in the water column is a low-cost, reliable method for examining the long-term behaviour and distribution of zooplankton populations. Backscatter at acoustic frequencies above 20 kHz is useful for profiling those quantities, which, when tracked over long periods of time, can provide a valuable contribution to understanding and monitoring the state of marine ecosystems. The Water Column Profiler/sup TM/ is a self-contained echo-sounder, designed for long-term, autonomous operation. The instrument can be used in either downward-looking mode, from a moored surface buoy, or in upward-looking mode from a submerged mooring. The instrument has selectable parameters for pulse length and sampling interval. The data are recorded in digital form, and averaging in both time and range is available. On-board storage of up to 64 Mbytes of non-volatile Flash RAM allows operation for periods up to 6 months in length for 150 m water depth sampled at 1m intervals every minute. Interfacing to a real-time data link is possible for buoy-mounted installations. A 200 kHz Water Column Profiler/sup TM/ has been operating on a moored buoy in Saanich Inlet, British Columbia, since September 1999. In June, 2000, a 50 kHz unit was added. Time series of acoustic backscatter at both frequencies many months long have been collected, showing the evolution of the behaviour and abundance of the dominant species of zooplankton (Euphausiids and Amphipods) in the inlet over time scales from diurnal to seasonal. Examples of these data are presented and discussed, comparing the results at the two frequencies and the implications for acoustically discriminating size with a multi-frequency instrument.

An Acoustic System for Measuring Ocean Flows by Space-Time Scintillation Analysis

We describe an acoustic measurement system built to test several techniques for analyzing a forward-scattered sound field in the ocean. The fluctuations of the sound field contain information about the field of flow along the acoustic path. The instrument consists of a microprocessor-controlled, radio-linked transmitter with a 4-element array, a 4-element receiving array and a receiver incorporating signal processing, data logging and control electronics on a VME bus. Pseudo-random noise coding is used to enhance the signal, with the decoding being carried out in real time by a high-speed multichannel hardware correlator.The transmit and receive arrays may be configured to test spatial aperture filtering for measuring the mean flew field or to measure the structure function of the refractive-index turbulence. Dual-frequency operation is also available for differential absorption measurements. Preliminary results of a field test in a turbulent tidal channel are presented.

Bubbles near the surface of the ocean and their influence on wind‐generated ambient noise

Bubbles of radius 40–400 μm are formed by breaking wind waves and are known to influence significantly air‐sea gas exchange. Do they also modify wind generated ambient noise? For frequencies greater than 500 Hz, wind‐generated noise typically has a constant spectral slope, but a layer of bubbles will scatter and absorb the sound, changing its spectral and directional properties at high wind speed. Observations in Queen Charlotte Sound, B. C., in bands centered at 4.3, 8.0, 14.5, and 25.0 kHz illustrate these effects; at sufficiently high wind speeds noise at 14.5 and 25.0 kHz actually decreases with increasing speed. The changes in spectral slope as a function of wind speed and frequency allow bubble populations and size distributions to be inferred. These were found consistent with previous photographic and bubble trap measurements, but the range of wind speeds encountered permits determination of a more complete relationship. Scattering and absorption by the bubble layer has implications for the use of ...

Development of ice profiler sonar (IPS) target sonar with a logarithmic detector

Upward-looking sonar (ULS) instruments have become the primary source of data for high resolution and long duration measurements of sea ice drafts to support engineering requirements for oil and gas exploration projects in Arctic and other ice-infested areas. ULS instruments, in the form of ASLs Ice Profiler Sonar (IPS), provide accurate measurements for ice draft on a continuous year-long basis and allow detailed characterization of keel shapes and other ice features. The IPS instrument was originally developed in the 1990s and it was last upgraded by ASL Environmental Sciences Inc. in 2007-2008 through improved instrument design based on more capable microprocessors and more advanced on-board firmware. Another upgrade of the IPS instrument platform is presently underway with the design, testing and implementation of a logarithmic detector module in place of the previously used linear detector module which has been used for the past decade in the instrument. The linear detector module involves the use of an echo sounder detector which generates an analog voltage output from the raw transducer input supplied which is constant, i.e. independent of the time elapsed since the acoustic pulse was originally emitted. While this approach has proven reasonably serviceable, it has the disadvantage that the dynamic range of the instrument is curtailed from the alternative approach of using a logarithmic detector module which has previously been implemented in other ASL upward looking sonar instruments. The larger dynamic range of the log detector avoids using approximate TVG compensation. With the logarithmic sonar detector, the use of discrete threshold values for target detection is avoided and the resulting target detection capability is more robust. The project involved three principal components: (a) construction of a prototype 420 kHz log sonar card; (b) simulations of the response of the IPS log sonar instrument from previous IPS data sets which guided the development of operating firmware; and (c) assembly and field testing of a prototype IPS log sonar unit operated simultaneously with a standard IPS5. The simulations of the IPS5 log sonar outputs derived from previous standard IPS5 data indicate that there are occasional differences in the target detection for borderline cases, but they will not be significant. After iterations to improve the robustness of the target detection algorithm, development of the remaining functions of the IPS5 operating firmware was then carried out and further tested. Finally the prototype IPS5 log sonar instrument unit, along with a standard IPS5 instrument, was field tested in a deep open water environment (to 200 m water depth) in order to test the accuracy of the acoustic range of the sonar targets.

Real-time monitoring of currents and water level at second narrows to improve port efficiency in Vancouver harbour

A real-time monitoring system for currents and water levels was installed at Second Narrows in the Port of Vancouver in March 2001. Second Narrows restricts the movement of marine traffic into the upper harbour because of its strong tidal streams, limited depth and the presence of both a vehicular bridge and a rail bridge. The Port of Vancouver undertook a joint project with the Canadian Hydrographic Service (CHS), CN Rail and terminal operators in the upper harbour to replace the present system of predicted water levels and currents with an on-site measurement system at the rail bridge. Water levels and currents are measured at the Narrows; bridge and draft clearances are calculated and the results telemetered to the Port management and traffic control centre for real-time display. Current data are provided by an Acoustic Scintillation Flow Meter (ASFM), manufactured by ASL Environmental Sciences Inc. of Sidney, BC. Water level data are provided by an Aquatrak sensor manufactured by Bartex of Annapolis, MD. Currents are measured at two depths, and water level at one point under the bridge, every 5 minutes. The display shows a 24-hour history of currents at both levels, and the measured water level (the full data history is also stored). The current data show significant departures from the predicted tidal flows as, to a lesser extent, do the water level data. The system has operated in a trial mode for the past two years to assess its operational reliability. During this period two significant outages occurred. Modifications to the installation and to maintenance procedures to prevent their recurrence are described, and future plans for the operational use of the system are discussed.