NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Performance Degradation
Definition & Importance
“Performance degradation is the reduction in echo sounder performance due to
mechanical, biological, or electrical processes.
Degradation in echo sounder performance can be caused by acoustical, vessel, and
electrical noise, bio-fouling of the transducer face, excessive transducer motion, and bubble
attenuation. Performance degradation differs from system performance in that the causes of
performance degradation are external to the echo sounder, where as ‘system performance’
concerns the echo sounder electronics.
Routine monitoring of data by scientific personnel during data collection is necessary to
ensure a high standard of data quality.” (NOAA Protocols for Fisheries Acoustics Surveys
and Related Sampling)
Techniques
Noise –Video displays of echograms are constantly monitored for the appearance of acoustical
noise. Examining the display while the sounder is in passive mode may also be useful in
identifying external sources of acoustical noise. A common source of acoustical noise is a
result of the bridge sounder or ADCP being out of sync with the EK500. If the source of the
noise can be identified as another piece of shipboard gear, the offending gear should be either
shut down (preferably) or synchronized with the EK500.
Small amounts of noise are edited during post processing. In the event of serious noise, the
position is determined where the noise began to affect the data. The chief scientist will decide
either to continue or lose those data, or to re-start the survey prior to the position of the noise.
The choice will depend on whether the data loss appeared to be significant. If data loss is
determined not to be significant and the survey is continued, the area of noise will be designated
as “bad data,” and will yield a zero data point at the position.
Electrical noise can result from grounding problems or other pieces of electrical equipment.
As with acoustical noise, electrical noise is often manifested in the data display or in unusual
system diagnostic values (see Protocol 1, Calibration and System Performance). To resolve,
ensure proper grounding of the sounder, use an uninterruptible power supply and/or “clean”
ship’s power, and shut off offending equipment if it can be identified. Additional remediation
methods during the cruise and in post-processing are the same as those given above for
acoustical noise.
Bubble Attenuation – Bubbles are strong sources of scattering. Bubbles can both lead to
increased signal attenuation and also be a source of misclassified backscattered energy on an
echogram (scattering from bubbles could be confused with scattering from fish). Bubbles near
the sea surface are often associated both with vessel speed and sea state. Transducers should
be located so as to minimize the effects of ‘bubble sweep down’. In rough seas, vessel speed
may have to be reduced or operations suspended to preserve data quality (see Protocol 4,
Sampling). If scattering from bubbles can be reliably identified on the echogram, it can be
identified and disregarded in post-processing. This will not correct for attenuation of the
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