A stable vertical reference for bathymetric surveying and tidal analysis in the high Arctic

Abstract

The EM300 multibeam sonar mounted on the icebreaker CCGS Amundsen is one of the prime tools used for marine geomorphologic and hydrographic investigations as part of the ArcticNet research program. The area of operations for the Amundsen extends from the Labrador Shelf to the Beaufort Sea. Depths of interest range from ~ 20m to 2500m but the prime focus is on the Arctic Island Archipelago where depths are generally less than 300m and include zones of critical navigational interest. Within that area, whilst tidal predictions are available, there are insufficient active tide gauges to provide proper vertical control. The WebTide hydrodynamic model is available for predictions away from the active coastal sites but remains untested in many areas. An alternate approach to a stable vertical datum is to adopt the ellipsoid as the reference. In doing so it completely bypasses the need for draft and squat measurements. As part of an experiment, almost all data acquired so far with the Amundsen has had synoptic measures of the vessel ellipsoid elevation derived using the CNav GcGPS service. To make use of the bathymetric data for hydrographic survey, however, an ellipsoid-geoid and ultimately geoid-chart datum model are required. At this time the EGM96 and the GPS-H models are being tested. EGM96 provides a separation model at 15 minute intervals from global spherical harmonics whereas the GPS-H is available at 2 minute intervals thereby providing shorter spatial wavelength undulations (higher spherical harmonics). Areas of particular concern, where the two might diverge, are those where the gradient in the ellipsoid-geoid separation is steep. This notably includes the Labrador Shelf and the east coast of Baffin Island. An additional byproduct of this technology is that, as long as appropriate draft and squat models are applied, the same data can be used to examine the variations in the phase and amplitude of the tide at locations remote from the existing tides network. This will allow calibration of existing hydrodynamic models and even perhaps the establishment of viable chart datum interpolations for these remote areas.