Measuring the bottom elevation near a pier with a swath multibeam bathymetric system.
By tilting the echosounder, it may be possible to expand the survey footprint under the pier.
A pier deck that extends well below the water level creates a shadow for the multibeam swath that cannot be removed without dropping the depth of the transducer.
techniques were considered. Diver investigations of the entire hull were considered ineffective as a standalone solution because the data scarcity would be unacceptable to the project, and the horizontal positioning of divers under the hulls would be problematic due to the sheer size of each hull and the difficulty in providing accurate landmarks to the diver. Remote methods, such as surveying with a multibeam echosounder were also considered. Clearly, the technology could provide an outstanding data density and the survey quality would be highly accurate. The beam of each aircraft carrier was 130 ft., and the draft was approximately 30 ft. Surveying from the surface with a multibeam echosounder was predicted to produce a coverage of about 15 percent of the footprint under each aircraft carrier. The carriers' drafts of approximately 30 ft. also meant that rotating the transducer at the surface would not produce any appreciable effect on increasing the survey footprint. The only way to increase the coverage of the survey foot-
print would be to drop the transducer in elevation to near 30 feet so that the transducer could be rotated in order to transmit under each vessel. This approach, too, was considered unacceptable because it would be very difficult and potentially unsafe to survey with the multibeam echosounder on a pole mount extending nearly 30 ft. below the water surface. It was also a concern that, even if we could drop the transducer to 30 ft. of depth, the massive beam of the carrier would still prevent us from surveying the entire footprint. Taking into consideration the size of the carriers and the accuracy and density of bathymetric data that the project required, the survey required aspects of both diver and remote approaches. SeaVision needed the ability to gain access to the complete underside of each aircraft carrier so that it could collect bathymetric data within the footprint of each carrier. SeaVision also needed the positioning accuracy and the data density that a traditional vesselbased survey strategy would provide in accessible waters. Attention was turned towards using a remotely-operated
The raw data from the survey merged both the berth floor data and the hull data into a 3-D image model within CDL's Tunnel Viewer software. Notice the sloping wall of the hull of the aircraft carrier, and the slope approaching the vertical feature on the right, where the berth meets the slope under the pier. www.seadiscovery.com Marine Technology Reporter 25
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