Factors affecting Sonar Image

[different_stroks]

Hello all,

Recently i operated side scan sonar system in a river. Water was somewhat muddy & channel depth was around 25 m max.

surprisingly,i did not get any image at all in long range ..ie . beyond 150m or so..( I was targeting huge under construction Jetty pillar in 400-500m Range)

I would like to know from u ,the probable reasons .... or any such documentation available on web.

Is ther any role played by operating conditons??

Thanks for ur time.
Hope to hear from u all...

[GaryF]

Hello,

Assuming that you are using a frequency that is adequate for covering the ranges that you describe then the other major factors affecting range would be transducer down-angle and towfish altitude.

Quote from the book "Sound Underwater Images" regarding proper towfish altitude and it's affect on range.

"FISH HEIGHT AND POSITION:

The quality of the sonar data is often a function of the height of the towfish above the bottom, or bottom targets, during a survey or target imaging. In general, with standard sonar configurations, surveys are performed with the towfish positioned a distance above the bottom approximately equivalent to between 10% and 20% of the RANGE SETTING of the sonar. This will depend upon the tilt of the transducers down from horizontal. Most side scan systems are designed with the transducers tilted down from horizontal between 10-20 degrees. More acoustic energy is then dispensed toward the seafloor when the fish is flown at normal working altitudes. The tilt to the transducers is often adjustable for working at higher and lower altitudes.

The optimum height of the fish is a function of range, because at the outer edges of the record, the angle of incidence of the outgoing acoustic pulse on the seafloor is one of the factors that determines the amount of backscatter and returning echoes to the towfish. If the transducer array is towed high off the seafloor, shadowing will be lessened and target recognition may be reduced. If towed too low, the reflectivity at the outer ranges will be reduced limiting the effective range of the system."

To get good imaging at 400m range you would need to position the towfish between 40m and 80m above the bottom. This obviously poses a problem scanning at long range settings in shallow water.

I hope this is helpful.

Gary