Real-time measurement of sea ice thickness, keel sizes and distributions and ice velocities using upward looking sonar instruments

Abstract

There is an increasing requirement for real-time underwater measurements of sea ice keel properties, including thickness and sizes of individual keels and of ice velocities. Such information is needed in real-time to support tactical applications for safe routing of ships in heavy sea ice concentrations and, more recently, a need 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 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 is presented based on operational projects undertaken from 2002 to the present. The projects are based on experience in the St. Lawrence Seaway (since 2002), and more recent work at the Confederation Bridge in Canada (2005–2008), and the Caspian Sea (2008). The approaches taken to addressing the real-time measurement of the subsea ice keels are summarized for each application. More challenging requirements for real-time ULS ice measurement systems are being addressed in much deeper and more remote areas of the Arctic Ocean. In these areas of more prolonged and severe ice conditions, the deployment of the system is limited to late summer periods when ice coverage is reduced. The requirements for timely and accurate ice information demand high reliability in support of ship navigation and offshore oil and gas drilling applications. The real-time ULS ice measurement system must be capable of operating for two to three years without servicing. 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.