NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Techniques
Noise
Acoustical
A common type of acoustic noise is a discreet spike caused by another echo sounder or sonar
operating within the frequency bandwidth or a harmonic of the scientific echo sounder. The
solution is to identify the source of the interference and shut it down. A list of all acoustic
systems with associated operating frequencies can aid in identifying the interfering system.
Interference can be eliminated if acoustical instrumentation essential for safe ship operation is
synchronized with the survey echo sounder. Removal of acoustic noise during post-processing is
sometimes possible, but difficult, so eliminating it during the survey is always preferable.
Electrical
Electrical noise can be of many types. Electrical interference caused by improper grounding
or other electrical systems can cause low-level voltage interference, spikes, or cyclical
interference. A low level voltage introduced to the echo sounder can be amplified with range by
the TVG function, and may pose a problem only in the deeper parts of the survey area. Problems
can be reduced or eliminated by ensuring proper grounding of the scientific echo sounder, by
using an uninterruptible power supply (UPS) for the scientific echo sounder, and by eliminating
electrical interference during data collection. Electrical interference not eliminated during data
collection should be removed during post-processing, either manually or with signal processing
techniques. If signal processing techniques are used, care should be taken to ensure that target
data is not modified, or correction factors may be required.
Bubble Attenuation
Bubbles can have a strong effect on propagation and transmission of sound. Due to the high
acoustic impedance between air and water, bubbles are efficient scatterers of sound. Bubbles can
increase attenuation (loss of signal strength) and potentially increase the probability of
misclassification of gas-bearing organisms. Bubbles near the sea surface are generally
associated with increased sea state and/or the position of the transducer relative to the vessel’s
hull. The transducer location on the hull must be chosen to minimize potential problems caused
by bubbles. To prevent degradation of survey data, it is necessary to slow vessel speed or
suspend acoustic survey operations when sea state causes unacceptable bubble attenuation.
Currently, this decision is based on the judgment of the scientific field party chief, but explicit
criteria need to be developed. In some cases, bubble backscattering can be removed from S v data
during post-processing, but this will not correct signal loss from targets of interest.
Transducer Motion
Excessive transducer motion is associated with increased sea state. Transducer motion
affects bottom tracking, target strength and volume backscattering measurements. ‘Dropouts’
(i.e., reduction or elimination of S v values over one or more pings) observed on the echogram are
a clear indication of excessive transducer motion. If the vessel is outfitted with a motion sensor,
these data should be recorded. Motion sensor data may be used for objective decisions on
acoustic data quality or for making corrections to the acoustic data. Such corrections may be as
high as 30% (MacLennan and Simmonds 1992). When sea state or vessel motion is excessive,
as judged by the field party chief, survey speed must be slowed or operations must be suspended.
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