Multi-Beam Depth Sounding
CRA has been using multi?beam fathometer systems from Reson Inc. (“Sea?Bat” models 9001, 8101, 8124 and 8125) since the summer of 1994. CRA has used the Sea Bat in Milwaukee, Two Harbors Minnesota, and Ashland Harbor Wisconsin to examine the breakwaters. In the case of Milwaukee, the data was used to compute the volume of new rip?rap necessary to build up to a new design template. We have used multi-beam systems for naval vessel mooring in the Houston Ship Channel, to produce a navigation chart for Lake Charles, Louisiana for the casino vessels based there, and to look at the debris pile for the accident site at the Queen Isabella Causeway in southern Texas. Lately, surveys were carried out over the entire Pascagoula entrance channel for the tow out of an oil platform, for the Gulf Marine Fabricators dock area at Ingleside, Texas, for the Central Arizona Project irrigation canal, for the Neches River and Freeport Entrance Channel, and for the naval small vessel basin on the island of Diego Garcia.
In December 2007, CRA purchased a new Odom ES-3 multibeam system. Two of our vessels have been specially modified to use this system. The equipment package consists of: the ES-3, a Trimble SPS461 GPS unit with heading capability, a T.S.S. DMS05 motion sensor, and Odom Digibar Velocimeter. This system can give coverage of the sea bed up to 3.5 times the water depth with a measurement accuracy of around half an inch. Conversely, a traditional single-beam unit operated on a 50 foot transect spacing would only provide 28 percent of bottom coverage.
Multi-beam data is acquired and edited using “Hypack” and “Hysweep” software. Once a clean x,y elevation data set is made, it is transferred to our Bentley Microstation CAD system for the generation of terrain models in 2D and 3D modes. The system can then compute volumes based either on the separation between 3D surfaces, or sections cut through the model and the average end area method. Final drawings are usually plan view charts superimposed on base maps provided by clients or the U.S. Army Corps of Engineers, as well as aerial photographs. These charts show bottom elevations plotted along the vessel’s track and may also include contour lines at 0.5, 1, and 2 foot intervals. Cross-sections and volume computations referenced to design templates are also sometimes produced. Edited data can be submitted to the client in standard x,y, and elevation ASCII format on CD ROM.
The image to the right shows a post-dredge multibeam survey of a berthing area containing a “deep hole” for ballasting down heavy lift offshore vessels. Note the detail in the bottom, which shows not only cut lines but the marks from each swing of the cutterhead.