SonTek/YSI
Acoustic Doppler Velocimeter Principles of Operation (September 1, 2001)
7
in post processing (see §7.2). Probe geometry is calibrated at the factory for each ADV; no re-
calibration is required unless the probe has been physically damaged.
The accuracy of the probe geometry, with factory calibration, is specified to ±1.0% of the meas-
ured velocity (i.e., an accuracy of ±1.0 cm/s on a measured velocity of 100 cm/s). This represents
the limits of the calibration procedure to determine the angular alignment of the acoustic trans-
mitter and receivers.
The ADV specifications state a maximum zero offset velocity of ±0.25 cm/s. With Doppler proc-
essing, there is no potential for zero offset in velocity measurements; this specification is in-
cluded because of the difficulty in generating calibrated velocities at low flows. See §7.5 for ad-
ditional information about using the ADV to measure low flow velocities.
6.2. Signal Strength
Signal strength, recorded for each ADV receiver, is a measure of the intensity of the reflected
acoustic signal. This is recorded as raw signal amplitude in internal logarithmic units of counts;
one count is equal to 0.43 dB. With the ADV software, signal strength can be accessed either as
signal amplitude in counts or as signal-to-noise ratio (SNR) in dB. SNR is derived from signal
amplitude by subtracting the ambient noise level and converting to units of dB.
The main function of signal strength data is to verify there is sufficient particulate matter in the
water. If the water is too clear, the return signal may not be stronger than the ambient electronics
noise level. Without sufficient signal strength, the ADV is unable to make accurate velocity
measurements. As SNR decreases, the noise in ADV velocity measurements will increase. For
high-resolution measurements (e.g., sampling at 25 Hz), we recommend maintaining SNR of at
least 15 dB. For mean current measurements (e.g., sampling at 0.5 Hz, or once every 2 seconds),
the ADV can operate reliably with the SNR as low as 5 dB.
The strength of the return signal is a function of the amount and type of particulate matter in the
water. Signal strength values can be used as a measure of sediment concentration when the type
of sediment is known. While ADV signal strength data cannot be immediately converted to
sediment concentration, it does provide an excellent qualitative picture of sediment fluctuations
and, with proper calibration, can be used for reasonably accurate estimates of sediment concen-
tration. For more information about sediment concentration, contact SonTek.
6.3. Correlation Coefficient
The ADV correlation coefficient is a data quality parameter that is a direct output of the Doppler
velocity calculations. The ADV computes three correlation values, one for each acoustic receiver,
with each velocity sample. Correlation is expressed as a percentage: perfect correlation of 100%
indicates reliable, low noise velocity measurements; 0% correlation indicates the output velocity
value is dominated by noise (no coherent signal). Correlation can be used to monitor data quality
during collection and to edit data in postprocessing. For example, if the ADV is being used in
waves where it is periodically out of the water, correlation values can be used to determine what
portions of the data can be used.
Ideally, correlation values should be between 70 and 100%. Values below 70% indicate that the
ADV is operating in a difficult measurement regime, the probe is out of the water, the SNR is too
low, or that something may be wrong with the ADV. In some environments (highly turbulent
flow, highly aerated water), it may not be possible to achieve high correlation values. Low corre-
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