Principles of Operation 17
© 2018 Nortek AS
SNR
The Signal-to-Noise ratio (SNR) is defined as
SNR is displayed for each receiving beam. It is a measure of the level of the signal with reference to
the background noise level. Strictly speaking, we are unable to measure the signal without the noise
present, so Amplitude_signal should read Amplitude_signal+noise. However, for SNR values in the
magnitude applicable to typical Velocimeter situations, the difference is negligible.
The particles that is necessary to generate reflections of the acoustic signal can be naturally
occurring suspended sediments (rivers, ocean, sand-filled or polluted model basins), bubbles
entrained from pump systems (often the case in flumes), or artificial ("seeding").
When measuring in basins or flumes, seeding is often needed. If practical, we recommend that
stagnant, clear basins or flumes are seeded with hollow spheres that have a density close to that of
water and a size around 10 µm. Particles that are close to neutrally buoyant will remain in
suspension without additional stirring and are ideal for low-flow experiments. The choice of size is
motivated by the relatively strong echo that these particles generate per unit of concentration.
Smaller particles (e.g. 1 µm particles used to seed Laser Doppler systems) are not recommended
because the required concentration is quite high.
When collecting raw data (e.g. at 25 Hz), we recommend an SNR that is consistently above 15 dB.
When collecting mean data (for example, 10 second averages), we recommend an SNR of at least 5
dB. If the SNR remains low even after seeding, try to clean the receive and transmit transducers by
gently rubbing the surface with a finger. This will clean off residues or bubbles that may have
collected and are blocking the sound. Take a look at the Troubleshooting Guide for more about how
to increase SNR.
The determination of the quality of the data is the primary use of the signal strength, for the Vectrino
Profiler it also determines the range over which the instrument can measure velocity accurately. It
may also be used to retrieve information about the suspended sediment or the concentration of the
scattering materials. Note that different types of scattering elements produce different echoes, and it
is difficult to distinguish between conditions with few, relatively big scattering elements or many
small elements. For users interested in sediment concentration estimates it is recommended to take
a look at this subject in the Frequently Asked Questions (FAQ) on the Nortek web.
Figure: Signal to Noise Ratio from one Vectrino Profiler beam. A fixed boundary can be seen at about 6cm
from the transmitter
1.8 Correlation
Correlation is a measure of the similarity of the two pulse echoes being measured. Zero correlation
means nothing at all is similar between the two echoes, and a correlation of 1 means the two echoes
are identical. The number reported by Nortek instruments is a normalized correlation value and will
always lay in the interval 0-100%. The aim is high correlation because it gives confidence the system
measured the two pulses it originally sent out and is determining a valid phase shift. In practice we
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