Jan Buermans

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.

Real-Time Pack Ice Monitoring Systems - Identification of Hazardous Sea Ice Using Upward Looking Sonars for Tactical Support of Offshore Oil and Gas Projects

There is an increasing requirement for real-time detection of sea ice hazards. These hazards include the identification of thick ice keels, large hummocky ice, fast moving ice, rapidly changing ice direction and multi-year ice. Such information is needed in real-time to support tactical applications for safe routing of ships in heavy sea ice concentrations. More recently, a need has emerged for tactical support of offshore oil and gas activities in ice infested waters of the Arctic Ocean and in marginal ice areas such as the Barents Sea, the Sea of Okhotsk, the Caspian Sea, Baffin Bay, the Labrador Sea and East Greenland waters. Reliable upward looking sonar (ULS) instruments, including the ASL Ice Profiler for ice keel measurements and the Acoustic Doppler Current Profiler for Ice Velocity measurements have been widely used in these areas for many years. These instruments, which record data internally, are operated from subsurface moorings that are deployed and recovered by ship during times of minimal sea ice coverage. Providing real-time measurements from the upward looking sonar measurements operating under heavy ice cover pose new technological challenges. The use of surface buoys to relay data from subsurface instruments to shore facilities or satellites is not possible due to the ice cover itself. A more feasible approach is to transmit the data from each instrument using underwater cables on the sea floor and which link the instruments on the subsurface moorings to a bottom mounted or floating structure. For a floating structure, the use of high performance acoustic modems may be required. Previous experience with real-time ULS ice measurement systems dates back to operational projects undertaken from 2002 to the present. More challenging requirements for real-time ULS ice measurement systems are being addressed in much deeper and more remote areas of the Arctic Ocean such as the Barents and Beaufort Seas. Conditions in these areas can vary from short episodes of hazardous ice to more prolonged and severe ice conditions. ULS ice systems may be deployed on a yearround basis or used episodically strategically just before hazardous ice episodes begin. The requirements for timely and accurate ice information demand high reliability in support of ship navigation, offshore oil and gas drilling and development applications. The real-time ULS ice measurement system must be capable of operating for multiple years without servicing in conjunction with other metocean sensors packages (e.g. ice radar, satellite, winds). Multiple ULS measurement arrays will be needed over operational areas spanning distances of many kilometers. For these Arctic Ocean applications, cabled ocean observatory technology and advanced underwater acoustic modems become key enabling technologies. Recent developments of automated detection techniques for deep keels, large hummocky (rubbled) ice, high ice speeds and rapid changes in ice direction derived from data collected from autonomous ULS systems will be described. Robust realtime versions of these algorithms, along with development of automated identification techniques of old-ice using acoustic backscatter data from the Ice Profiling Sonar, will be essential in providing the required tactical ice information.

Testing of Ice Profiler Sonar (IPS) targets using 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, along with long-term scientific studies of the underside of the sea-ice canopy in the Arctic and other ice-infested areas. ASLs Ice Profiler Sonar (IPS) has been widely used to provide continuous accurate measurements for ice draft at a horizontal resolution of 1 m, which enables the measurement of ice thickness values over periods from 1-2 seconds up to several years, as well as detailed characterization of many hundreds to thousands of keel shapes and other ice features. In 2014, an important redesign of the IPS instrument was initiated to provide improved performance of the original instrument developed in the 1990s and last upgraded by ASL Environmental Sciences Inc. in 2007-2008.

Near real-time transmission of reduced data from a moored multi-frequency sonar by low bandwidth telemetry

Moored, internally-recording acoustic instruments can acquire continuous profiles of echoes thro column, thus providing a low-cost method ughout the water of studying the behavior and abundance of fish and zooplankton in oceans and lakes. Calibrated sonars with several frequencies allow some information about species composition and abundance to be deduced from acoustic backscatter data. The same instrument can be configured to look up from the bottom, down from the surface, or horizontally from a CTD cage. In this presentation we describe additional capabilities of this low-power, batteryoperated, multi-frequency sonar capable of autonomously collecting data at high temporal and spatial resolution for periods of up to a year. The AZFP instrument (Acoustic Zooplankton and Fish Profiler) supports up to four frequencies in a single housing. The available operating frequencies are 38, 67.5, 125, 200, 455, 769 and 2000 kHz. The transducers are colocated, with the same nominal beam widths of 7° or 8°, except at 38 and 67.5 kHz, where the beam width is 12°. The standard AZFP can be moored at depths up to 300m, and with modified transducers as deep as 600m. The AZFP can store up to 32 GB of this data internally. The user can specify averaging in time and/or range to choose an optimum sampling scheme for the available storage. Because the AZFPs are deployed on moorings for long peri ods of time, there can be a requirement to monitor the data in near real-time. The volume of data collected is usually too great to allow monitoring of the results unless the instrument is connected by cable to shore. A capability to retrieve a subset of this data over low bandwidth satellite or other network links to provide this monitoring was developed. In this presentation we show how the stored data is further reduced to accommodate transmission over a low bandwidth network. The AZFP data is acquired in logarithmic form so compiling data averages requires conversion to linear values if a true arithmetic average is desired. Because the instrument is designed for low power consumption to allow long deployments, the onboard microprocessor has limited processing capabilities. In particular, floating point calculations are not possible and all processing is done using integer arithmetic. Conversion to and from logarithmic form is therefor done using lookup tables, but limited internal memory limits the size of the lookup tables. We will describe how the tables are constructed to provide adequate resolution. This capability is applicable to AZFPs deployed on AUVs and gliders as well as moorings where a surface buoy allows satellite or radio telemetry. If two-way serial communication is possible in a particular application, then the operation of the AZFP can be controlled by the platform or remotely by a communication link through the platform. Here we will give an example of a moored system where inductive modems are used to transmit data from the AZFP to the surface buoy for retransmission over a satellite link. In this case, because other types of data were to be transmitted over the satellite link, the fraction of the bandwidth available for the AZFP data was severely limited, so additional range averaging was required.

A low-cost calibration facility for high-frequency acoustic backscatter instruments

Full-system calibration of acoustic instruments used to measure high-frequency backscatter from zooplankton and small fish in the ocean is required if the data are to be used to estimate biomass and populations from signals at different frequencies. Here we describe a test tank facility for calibrating high-frequency, narrow-band sonars operating at one or more frequencies from 125 kHz to 775 kHz and with beam-widths between 1.8 and 11 degrees. The all steel tank is cylindrical with a diameter of 2.43 metres and a length of 6.10 metres and holds 28.5 cubic metres of water. The absorbing material on the end wall is a coarse artificial turf which is easily available at minimal cost and reduces high-frequency reverberations in the tank to negligible levels after 6 reflections. Pulsed operation at pinging rates up to 10 Hz is therefore possible without interference from reverberations. Echoes from the target are recorded over a series of pings; the difference between the target strength computed from the mean of those echoes and the known target strength is a measure of the accuracy of the nominal instrument response calculated from the manufacturers transducer characteristics and bench measurements of the system electronics, and therefore allows a calibration correction to be made. An acoustic propagation model of the tank has also been developed to characterize the response of instruments placed in it and to identify optimum target placements.