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Old 09-25-2014, 04:06 PM   #5
JDCAVE
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City: Lions Bay, BC
Country: Canada
Vessel Name: Phoenix Hunter
Vessel Model: Kadey Krogen 42 (1985)
Join Date: Apr 2011
Posts: 2,271
I'll offer my two bits worth: Simrad probably has the best technology and research of any of the manufacturers. They build high-end systems for fisheries research and commercial fishing applications. However....

You should be aware that the devil is in the details and I suspect that most of the vendors oversell the capabilities of their units. Most units only show "targets". Whether these targets are fish, debris or entrained bubbles is uncertain for the most part and requires "biological sampling" for confirmation. In other words, you need independent knowledge of fish in the area and their likely abundance and target strength. Even in fisheries applications where users should be better informed, there is considerable uncertainty and misinterpretation of results. Where conclusions about target information are important, most researchers are moving towards imaging sonars. The prices of these units have declined to a more modest $80,000 USD. See for example DIDSON. Note however that these units are only capable in 2D, typically distance and width of the horizontal plane: In a side-scan application, you cannot determine the depth of a target. They are also very susceptible to pitch and roll of a boat, the effects of which increase dramatically with range.

"Bottom" or depth is more certain, however a pitching or rolling boat often results in overestimation of depth. In addition, the user should be VERY careful in interpreting side-scan sonar information for identifying near-field obstacles. Note that when one edge of the beam strikes a strong target, such as bottom, objects further away from the source of the beam are uncertain and not interpretable without bias. Beam-width becomes an important consideration for longer-range side-scan sonar applications, particularly in shallow water.

I am VERY suspicious of the claims of the "Simrad StructureScan". As the range of an imaging sonar is increased, the width of the individual beams increase and detail is declines exponentially with range. You WILL NOT get the same resolution at 100 meters as you would at 10 meters. In addition, you should be aware that these sonars ARE NOT 3D! Only very high end units such as CODA OCTOPUS achieve such capabilities and require very sophisticated echo integration technology. Also, in the 3D applications, the results are not real time: i.e., you don't get instant gratification--you achieve the 3rd dimension by moving the transducer platform, something that requires absolute platform stability. My team used DIDSON technology. I have since retired from fisheries work.

Other situations that are problematic for hydroacoustics technology occur when a vessel passes between or over water with different densities such as haloclines, thermoclines, or convergent tidal fronts. Large aggregations of plankton, jellyfish or small particulates can also blank out a sounder.

Jim
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