Comprehensive Manual18
© 2018 Nortek AS
will never see correlations of zero because of noise due to electronics, temperature fluctuations, and
other factors that will always correlate above zero.
Discarding measurements with correlations lower than a threshold significantly reduce the variance
of the data set, which is generally the goal of most data screening operations. Removing low
correlation measurements is still a good idea because correlation is a strong indicator of data quality
in the sense of a valid Doppler phase shift determination. However, high correlation is not always
indicative of a valid measurement of the flow. It is also important to note correlation is tied to a
specific beam velocity measurement, not to an orthogonal coordinate direction such as X, and any
screening should take this into account.
In the early days of the acoustic Doppler velocimeter, if a user did some tests and determined
correlations were above ~70% then the instrument was considered to be generating good quality
data. Many users still use this number for screening out bad data, although a generalization to some
universal value is unwarranted and a close examination of the dataset is the best way to set a
correlation threshold (if any) for discarding bad data points.
Figure: Correlation from one Vectrino Profiler beam. This plot corresponds to the SNR figure in the
previous section. Note how the boundary show s up in this plot
1.9 Distance Measurement / Bottom Check
The distance check (also referred to as Bottom Check) is used to measure the distance to the bed
from the central transducer. The return from this pulse is listened to over some time to obtain an
amplitude profile. The amplitude profile is checked for peaks, and an interpolation scheme is used to
locate the peak location based on the largest amplitudes recorded.
The Distance Check function is available in the Vectrino Plus firmware, the Vector and the Vectrino
Profiler, but not in the Vectrino standard firmware.
Vector: The Vector reports the distance to the boundary at the start and end of each burst, or at the
start when running the instrument in Continuous mode. That means that it is possible to do very
short bursts and get almost simultaneous measurements (requires Firmware V1.20 or higher).
Vectrino Plus: The Vectrino+ firmware comes with a separate boundary distance mode (10 Hz),
where the distance to the boundary is measured. The distance measurement is not done at the
same time as the velocity measurements. The Boundary Distance functionality is typically used in
experiments where bottom changes as a function of time are needed, or when the Vectrino+ is
mounted on traverse systems for bottom topography measurements.
Vectrino Profiler: The Vectrino Profiler has an interleaved boundary distance functionality. In other
words, you get both velocity profiles and distance to the bottom at the same time. The reasons for
this are both that it can be done technically, and also that the problem of weak spots is much more
challenging with the Vectrino Profiler than with a single point instrument. Hence, in order to operate
the Vectrino Profiler properly we depended on interleaving velocity and distance measurements, such
that the time lag between the two pulses in every pulse pair can be adjusted adaptively.
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