Recommendation for fishery research investigations
Non-linear effects in hydro acoustics are described in the literature. A brief theoretical explanation of the phenomena and results of measurements and simulations in the frequency range 38 - 200 kHz are described below.
This article was first published as a news bulletin in March 2002.
Non-linear effects occur at high sound intensities, and when sufficiently great, limit the amount of energy that a sound source can radiate into the water. This is caused by harmonic distortion and cavitation. Only harmonic distortion is described in this article.
Normally, the sound level at a certain range from a transducer will increase proportional to the power at the source.
As the power at the source is increased to very high levels, one will find that the sound level at the 1st harmonic in the field will not result in the same increase, and finally, no further increase in the received signal will occur, no matter how much power is radiated by the source.
The reason for this effect is that at high intensities, the normally sinusoid signals, are distorted by the creation of higher order harmonics in the water, resulting in a sawtoothlike waveform or repeated shock wave. This is caused by non-linearities in the water.
As the higher harmonics will suffer higher absorption in the water, the signals will, at a certain range, return to sinusoid. However, as the higher harmonics have dissipated into heat, the sound energy is lower.
The non-linear effect will also have influence on the transducer beam pattern. The main lobe will become flatter and thus wider, and the side lobes will increase compared to the main lobe. The change in the beam pattern means that target strength (TS) measurements and integration values will not be correct.
Simrad has performed extensive measurements on this effect on the most commonly used echo sounder frequencies, 38, 70, 120, and 200 kHz, using standard echo sounder transducers and Simrad EK60 transceivers. The transducers were mounted 2.5 m below water surface during the measurements.
The result from these measurements are:
- The non-linear effect is negligible on 38 and 70 kHz within the standard maximum limits for the output power, 2000 W and 1000 W respectively.
- For 120 kHz, non-linear effects cause a reduction in signal level of approximately 1 dB at 10 m range at full power (1000 W).
- For 200 kHz, the reduction in signal level is 4.5 dB at 10 m range at full power (1000 W).
To avoid the influence from non-linear effects, we recommend that the output power is reduced to approximately:
- 500 W for 120 kHz and transducer beamwidth 7 degrees
- 100 W for 200 kHz and transducer beamwidth 7 degrees
when performing fishery research investigations.