David B. Fissel

Improvements in Upward Looking Sonar-Based Sea-Ice Measurements: A Case Study for 2007 Ice Features in Northumberland Strait, Canada

Scientific and engineering studies of polar and marginal ice zones require detailed information on sea ice thickness and topography. Accurate information on sea ice thickness and topography data is required for basic ice-covered ocean studies and, increasingly, for addressing important navigation-, offshore structure design/safety- and climate change-issues. Since the early 1990s, upward-looking sonar (ULS) instrumentation have been developed and applied to provide under-ice topography data with high horizontal and vertical spatial resolution. Such internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. The ASL Model IPS4 Ice Profiler, which has been widely used in studies of the Arctic Ocean, as well as in numerous seasonal ice zones and in the Southern Ocean, is being upgraded to allow much expanded data storage capacity (from 69 Mbytes to 1-8 Gigabytes) and 16 bit A/D resolution for ice ranges and other parameters. With typical ping rates of 0.5 or 1 Hz, the enhanced capability of the Ice Profiler provides very high resolution measurements of ice keel drafts and the under-ice topography of sea-ice keel features. The Ice Profiler is often used in conjunction with an Acoustic Doppler Current Profiler which provides direct measurements of ice velocity. The combination of high resolution ice draft time series with ice velocities allows for computation of quasi-spatial ice drafts as a function of horizontal distance. The results from the first deployment of an upgraded Ice Profiler, operated just off the Confederation Bridge in Northumberland Strait, from November 2006 to April 2007, are presented and compared with the results of previous sea ice studies at the same location. The much larger onboard data capacity allows for realization of multiple targets for each ping and, on a subsampled basis, offers data on acoustic backscatter returns over the complete water column. This additional information is being analyzed to examine the nature and cause of occasional false target returns. In past measurement campaigns, there have been episodic occurrences of deep targets detected which are not consistent with sea ice features that can be reasonably expected to occur in Northumberland Strait. These features are often associated with occurrences of the largest (spring) tidal currents, leading to the hypothesis that these anomalous targets may be associated with velocity shears in the water column resulting from strong tidal flows past the bridge support structures. Based on these analyses, improvements to the target detection algorithm are being developed and tested.

Upward looking ice profiler sonar instruments for ice thickness and topography measurements

Scientific and engineering studies in polar and marginal ice zones require detailed information on sea ice thickness and topography. Until recently, vertical ice dimension data have been largely inferred from aerial and satellite remote-sensing sensors. The capabilities of these sensors are still very limited for establishing accurate ice thicknesses and do not address details of ice topography. Alternative under-ice measurement methodologies continue to be major sources of accurate sea ice thickness and topography data for basic ice-covered ocean studies and, increasingly, for addressing important navigation, offshore structure design/safety, and climate change issues. Upward-looking sonar (ULS) methods characteristically provide under-ice topography data with high horizontal and vertical spatial resolution. Originally, the great bulk of data of this type was acquired from ULS sensors mounted on polar-traversing submarines during the cold war era. Unfortunately, much of the collected information was, and remains, hard to access. Consequently, the development of sea-floor based moored upward looking sonar (ULS) instrumentation, or ice profilers, over the past decade has begun to yield large, high quality, databases on ice undersurface topography and ice draft/thickness for scientific, engineering and operational users. Recent applications of such data include regional oceanographic studies, force-on-structure analyses, real-time ice jam detection, and tactical AUV operations. Over 100 deployments of moored and AUV-mounted ice profiler sonars, associated with an overall data recovery rate of 95%, are briefly reviewed. Prospective new applications of the technology will be presented and related to likely directions of future developments in profiler hardware and software.

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.

The capabilities of Doppler current profilers for directional wave measurements in coastal and nearshore waters

The adaptation of Doppler current profilers to measure directional wave spectra has provided a new instrumentation approach to coastal and nearshore oceanographic studies. Past studies have shown favorable comparisons between Doppler current profiler wave instruments with bottom mounted PUV (pressure-velocity) sensors sampling at wave frequencies and wave buoys. In this paper, we examine the capabilities and limitations of two different Doppler current profilers for directional wave measurements in shallow coastal waters of 0-25 m water depth. Data collection programs using Doppler current profilers for wave measurements have been conducted for one month long periods in the early spring of 2002, 2003 and 2004 on Roberts Bank in the Fraser River foreslope region of the Strait of Georgia, British Columbia, Canada. In 2004, an RD Instrument ADCP along with the newly-released 1000 kHz Nortek AWAC current profiler and wave instrument were co-located in 7 m water depth at a different site on the edge of Roberts Bank. Inter-comparisons between these bottom mounted instruments are used to examine the capabilities of the directional wave spectral parameters, in terms of: resolvable frequencies for directional and nondirectional wave spectra; wave directional resolution and reliability, and limitations arising from the use of linear wave theory. For a preliminary assessment of the capability of Doppler wave spectra in deeper waters of 20-25 m depths, in particular for very long wave periods, some experiences derived from a long-term measurement program being conducted off the west coast of Africa are presented

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.

Three-dimensional numerical modeling of sediment transport for coastal engineering projects in British Columbia, Canada

Quantitative understandings of sediment transport for coastal engineering projects, such as removing and installing underwater cables, installing and operating underwater turbines and disposal of dredged marine sediment (or terrestrial overburden) are one of the key requirements in planning these projects, assessing potential environmental impact and obtaining regulatory approvals from government agencies. In support of the environmental assessment and approval, the highly-integrated, three-dimensional finite difference COastal CIRculation Model COCIRM-SED was recently adapted and optimized to predict the sediment transport processes associated with a number of coastal engineering projects in Roberts Bank, Canoe Pass and Brown Passage, British Columbia, Canada. In these applications, the circulation module was validated using historical ocean current data located in the study areas.

Assessing the feasibility of a cabled marine observatory in the Canadian Arctic

Our understanding of physical and biogeochemical processes in the Arctic, especially related to marine ecosystems, is rudimentary yet it is precisely here where we are witnessing the most rapid and profound impacts of global environmental change. Many national and international organizations have stressed the need for long term monitoring of Arctic ecosystems to understand better how they function and how they will respond to global climate and oceanographic change.

Upward looking sonar-based measurements of sea ice and waves

With the recent reduction in summertime ice cover in the Arctic Ocean, year-long moored measurement programs require detailed information on sea ice thickness and topography data throughout most of the year, as well as ocean wave measurements during summer periods of major sea-ice retreat. This information is required for basic ice covered ocean studies and, increasingly, for addressing important navigation-, offshore structure design/safety- and climate change-issues. Since the early 1990s, upward looking sonar (ULS) instrumentation have been developed and applied to providing under-ice topography data with high horizontal and vertical spatial resolution. The internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. In the winter of 2007-2008, a new generation of ULS instrumentation was field tested, initially in Northumberland Strait near the Confederation Bridge separating the Canadian provinces of New Brunswick and Prince Edward Island. With typical ping rates of 1 Hz, the enhanced capability of the Ice Profiler provides very high resolution measurements of ice keel drafts and the under-ice topography of sea-ice keel features. The upgrades intrinsic to the ULS instrument feature much expanded data storage capacity (from 69 Mbytes to 1-8 Gigabytes) and 16 bit A/D resolution for ice ranges and other parameters. The offered combination of much increased dynamic range (via the 16 bit A/D converter) combined with the greatly expanded data storage capacity enables the instrument to operate at much lower gain levels. This facility allows extraction of information on the strength of the backscattering associated with sea-ice in contrast to the larger amplitude acoustic returns from open water, as well as detection of multiple targets from each regular 1 Hz ping. The instruments firmware also provides an ocean wave sampling mode in which a 2 Hz ping rate is used, typically over 20 minutes once each hour, from which non-directional wave spectra and wave parameters can be derived in post processing of the raw data. The new firmware allows the user to program the instrument to operate in up to 12 different sampling schemes over the course of the full deployment. For a typical Arctic Ocean deployment, this enables the instrument to be programmed to measure ocean waves in late summer and early autumn, then both waves and sea ice in autumn, sea ice in the winter and spring, sea ice and waves in the late spring and early summer. These features were utilized in the Northumberland Strait deployment, operated from Nov. 2007 to April 2008, to optimally detect the floating ice cover targets of interest, avoiding alternative false or null targets. Results are also presented on the measurement of ocean waves with wave heights of up to 3 m, and the early winter measurement of scattered ice keels in the presence of ocean waves.

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.