Seaforth conducts cable route survey off Newfoundland

Seaforth Engineering Group Inc. was contracted by International Telecom to conduct a hydrographic and geophysical survey of a proposed cable route from Nova Scotia to Newfoundland and along the southern coast of Newfoundland. The survey was several weeks in duration and involved the mobilization and execution of a complex integrated survey operation.

Acquired survey data allowed for an accurate interpretation of water depths and seabed profiles; the establishment of seabed and shallow sub-geological and geotechnical parameters; potential geological, biological features, archaeological features, and/or geotechnical phenomena that may have the potential to influence the construction or operation of the proposed site. Any significant features were Identified and mapped. Throughout the campaign, special attention was given to identification and charting of possible boulders, hard-grounds, reefs, shallows, areas of archaeological or cultural significance, shipwrecks, and manmade structures, facilities and debris. All man-made structures, significant debris or cultural resources within the immediate vicinity of the site were identified and mapped. Fishing gear and fishing gear scar positions were also mapped to provide an assessment of their density and orientation.

A challenge for the project was the stabilization of the RESON 8101 multibeam sytem on board the survey vessel. Both the survey vessel and mulitbeam were outsourced and thus the installation was not permanent. The figures below illustrate the problems encountered with an unstable/vibrating multibeam pole mount. Seaforth contracted the services of SolutionSmith Engineering Services  to do a design and analysis and subsequent manufacturing of a innovative multibeam pole mount that would reduce, if not eliminate, the bathymetric artifact associated with such a configuration.

Another challenge for the survey was based on the fact that while towing a sidescan sonar, the drag of the tow cable in the water column imparts a significant upward force on the towfish. In normal operations, water mass movement relative to the towcable is balanced against the amount of cable out and the weight of the towfish which maintains fish height above the bottom. The position of the Sidescan is determined by range and bearing observations from a Ultra Short BaseLine (USB) device attached to the survey vessel, normally protruding out of the hull or installed on the side of the vessel. In instances of very long cables, the signal attenuation no longer makes it possible to position the sidescan sonar. Consequently, the user is constrained by the amount of cable the can be deployed and thus this confines the user to actual water depth.

“Depressors” are not commonly used to place a counter weight on the sidescan sonar, to sink the sidescan closer to the ocean bottom. This also brings the sidescan closer to the Survey Vessel and thus permits extended use of the USBL technology mobilized on the survey vessel. This particular survey tested the boundaries of the standard Depressor approach. Seaforth designed and deployed a new design to the Depressor. The approach allowed for weight adjustment, optimizing its use for varying depth.

The depressor weight decouples the sonar sled from the ship heave and creates a very stable platform which is essential for the acquisition of high resolution sonar imagery. Running the sonar system close to the seafloor removes the problems associated with variations in the water column. Towfish depth is monitored with a precision pressure sensor which can be combined with towfish altitude (detected from sidescan data) to yield highly accurate bathymetric profiles along track. Swath interferometry yields up to 38 precise soundings across track in water depths to 6000m.