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ContinuousWave: Small Boat Electrical
SONAR Survey Vessel
|Author||Topic: SONAR Survey Vessel|
posted 02-25-2012 12:34 PM ET (US)
In June of 2011 we spent an enjoyable day or two in the port of Leland on Lake Michigan. While there we shared the dock with the 56-foot twin-engine aluminum-hulled RV NORTHWESTERN, a research vessel operated by Great Lakes Maritime Academy in nearby Traverse City, Michigan. She was laying at the dock in Leland and conducting daily missions to perform a detailed SONAR survey of the lake bed in the Sleeping Bear Dunes National Lakeshore region, just to the south.
The crew were a bunch of friendly young fellows and were very nice to invite me aboard for a closer look at their sophisticated SONAR gear. Their SONAR sensor was mounted on a long aluminum mast or tube which traversed the width of the 14-foot deck and could be lowered over the Starboard gunwale in the aft cockpit. The working end of the tube contained a very large SONAR sensor and another device, whose function I do not recall.
The SONAR sensor was massive, about a foot in diameter, and it contained a number of individual transducers in an array.
The SONAR sensor is lowered over the side and the boat steered at slow speeds over the lake bed to be surveyed. The signals gathered by the sensor are sent to a computer and recorded, producing gigabytes of data from each day's survey runs. The system is made by KONGSBERG, a firm specializing in sophisticated SONAR equipment.
The survey system gathers the bathymetric data and correlates it with the vessel position. The vessel position is determined by a sophisticated satellite navigation system using two GPS receivers mounted on the superstructure about 12-feet apart. Using sophisticated signal processing, the ship position and heading can be precisely determined, as well as any roll on the vessel. The roll information is important as it affects the depth of immersion of the SONAR sensor and also its beam orientation toward the lake bed.
The signal processing system integrates the SONAR data and the position data, and produces a detailed mapping of the lake bed. Structure on the lake bed, such as the remains of old ships sunk to the bottom, can be clearly seen. The survey had revealed a number of wrecks, such as the one shown below.
(This is a camera image of the screen, not an electronic image of the screen.)
In the image above pay attention to the plow furrows that appear in the lake bed. The crew explained that these plow furrows were made by divers who were dragging an anchor across the bottom, trying to hook onto the wreck--that is the sort of detail and resolution this SONAR can capture.
I was very impressed with the sophistication of the gear being used. This sort of survey of the lake bed is producing incredible detail. The survey was being performed, as I recall, as a paid project of the federal government, and the crew of the boat were students or staff of the college.
I have also found a good article on the RV NORTHWESTERN and its survey activities at
posted 02-25-2012 12:45 PM ET (US)
I do not recall the precise function of that other sensor, but it might be a speed sensor, giving speed through the water data to the survey system. The signal processing of the SONAR is so sophisticated that it might need to know about any current present in the water to adjust the sensor signals to compensate. That is just my guess, as I can't seem to find any notes from my interview with the crew.
posted 02-25-2012 09:19 PM ET (US)
The Seafloor Mapping Lab at California State University Monterey Bay has a couple of similar vessels. They have a 30' catamaran (trailerable) and a 35' catamaran (not trailerable), and I think, about a 40' sportfisher (it's not on their website but turned up recently sporting their logo). Because of this, we are fortunate to have 1M and 2M resolution bathymetry of the entire coast from Big Sur up past San Francisco. Eventually, the whole state. The local dive community has been finding dive sites nobody knew were there.
posted 02-26-2012 01:43 PM ET (US)
I was prepared for the electronics package of the sonar to be sophisticated, but I was not prepared for the physical size of the transducer. In looking at some other modern SONAR devices used in surveying or collecting bathymetric data, it appears that large size is of no objection--some of these transducer devices are huge, even larger than the one I show above.
(Note: I added a comment above about the sea bottom image showing plow furrows; it just came back to me in memory.)
posted 02-26-2012 06:54 PM ET (US)
Here is another good article on the research vessel NORTHWESTERN and its SONAR mapping in Northern Lake Michigan:
posted 02-26-2012 09:06 PM ET (US)
The transducer size is mostly determined by the frequency (wavelength) being used, the lower the frequency, the bigger the transducer. Fishfinder sonars transducers are in the multi-hundred kilohertz range. They are high resolution but very short range. Humminbird's new system reportedly only goes out to 150 feet. That's not much good for a survey vessel working in depths down to 1000 feet. It's interesting that the Northwestern's system is very much like Humminbird's, the cylindrical transducer is lowered into the water on a mast and only works at low speeds.
posted 03-05-2012 05:14 AM ET (US)
Very interesting to me - a lot of engineering/mathematics/physics involved here - especially in the corrections to data enabling good images. For example the 2 gps receivers on the vessel + removal of the effect of vessel roll to produce good hi-res bottom images - without undulations from the boat's movement.
I studied surveying once - I would be the last to say I was a good surveyor - but it did give me an appreciation of some of the complexities involved.
Moving on from this - and considering deepwater oceanographic mapping - wow. In waters sometimes a couple of miles deep with thermoclines, differing densities, contending with signal loss etc. Together with crappy weather up top with the survey vessel/s pitching, rolling and heaving.
All I can say is wow.
posted 03-11-2012 02:20 PM ET (US)
You can see the two GNSS receivers mounted on the cabin top in the last picture above. (They're above the "S" and the second "T" in NORTHWESTERN.)
There are now a couple of high-resolution GNSS receivers with differential receivers designed for marine use. They are still too expensive for me, at about $3,000, but they do produce roll data.
posted 03-11-2012 02:23 PM ET (US)
I think the transducer head size in this case is also a function of the number of individual elements in the device. My impression from the conversation--as best I can recall it--was that there were multiple beams emitted from the transducer and focused in an array covering from side to side of the vessel.
Yes, there must be a lot of computation going on, as well as a lot of data recording. The great thing about a digital survey is the ability to replay the data and apply further processing to it after you have collected it.
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