5-MHz ADVOcean Principles of Operation
The SonTek ADVOcean (Figure 1) is a single-point,
high-resolution, 3D Doppler current meter. Doppler processing techniques provide several
important advantages: 3D velocity measurements in a remote sampling volume; invariant
factory calibration (no periodic re-calibration required); simple operation; direct
calculation of turbulent parameters such as Reynolds stress; and excellent low-flow
performance. This document presents the basic operating principles of the SonTek ADVOcean.
To learn more about specific configurations and applications, please contact SonTek.
The ADVOcean measures the velocity of water using a physical principle called the
Doppler effect. If a source of sound is moving relative to the receiver, the frequency of
the sound at the receiver is shifted from the transmit frequency.
Fdoppler = -2Fsource (V / C)
In this equation, V is the relative velocity between source and
receiver, C is the speed of sound, Fdoppler
is the change in frequency at the receiver, and Fsource
is the transmitted frequency.
Figure 2. Bistatic Doppler Current Meter
Figure 2 illustrates the operation of a bistatic Doppler
current meter such as the ADVOcean (bistatic systems use separate acoustic transducers for
transmitter and receiver). Both transmitter and receiver are constructed to generate very
narrow beam patterns. The transmitter generates sound with the majority of the energy
concentrated in a narrow cone, and the receiver is sensitive to sound coming from a narrow
angular range. The transducers are mounted such that their beams intersect at a volume of
water located some distance away. The beam intersection determines the location of the
sampling volume (the volume of water in which measurements are made).
The transmitter generates a short pulse of sound at a known frequency, which propagates
through the water along the axis of its beam. As the pulse passes through the sampling
volume, the acoustic energy is reflected in all directions by particulate matter
(e.g., sediment, small organisms, bubbles). Some portion of the reflected energy travels
back along the receiver axis, where it is sampled by the ADVOcean and processed by the
electronics to measure the change in frequency. The Doppler shift measured by one receiver
is proportional to the velocity of the particles along the bistatic axis of the receiver
and transmitter. The bistatic axis is located halfway between the center axes of the
transmit and receive beams.
Each transmitter/receiver pair measures the projection of the water velocity onto its
bistatic axis. The ADVOcean uses one transmitter and three acoustic receivers. The
receivers are aligned to intersect with the transmit beam pattern at a common sampling
volume. The velocity measured by each receiver is referred to as the bistatic velocity,
and is the projection of the 3D velocity vector onto the bistatic axis of the acoustic
receiver. Bistatic velocities are converted by the ADVOcean to XYZ (Cartesian) velocities
using the probe geometry. XYZ velocities give the 3D velocity field relative to the
orientation of the probe. As it is not always possible to control instrument orientation,
the ADVOcean can be equipped with an internal compass and tilt sensor. The compass/tilt
sensor allows the ADVOcean to report velocity data in an Earth (East-North-Up or ENU)
coordinate system, independent of probe orientation.
The location of the sampling volume is determined by the physical construction of the
probe, and is nominally 18 cm from the tip of the probe. The size of the ADVOcean sampling
volume is determined by the sampling configuration used. The standard sampling volume is a
cylinder of water with a diameter of 12 mm and a height of 18 mm. For specialized
high-resolution applications, the height of the sampling volume can be reduced to as
little as 2.4 mm with only software modifications.
The ADVOcean records nine values with each sample: three velocity values (one for each
component), three signal strength values (one for each receiver), and three correlation
values (one for each receiver). Naturally, the velocity data are of foremost interest;
signal strength and correlation are used primarily to determine the quality and accuracy
of ADVOcean velocity data.
5.1. Velocity
ADVOcean velocity data can be reported in XYZ (Cartesian) coordinates relative to probe
orientation or Earth (East-North-Up or ENU) coordinates for systems using the optional
compass/tilt sensor. The ADVOcean's velocity output data can be used directly without post
processing. The calibration will not change unless the probe has been physically damaged.
One of the most important ADVOcean operating parameters is the velocity range setting.
This determines the maximum velocity that can be measured by the instrument; standard
settings are ±5, ±20, ±50, ±200, and ±500 cm/s. The user should select the lowest
velocity range setting that will cover the maximum velocity expected in a given
experiment. Instrument noise in velocity data is proportional to the velocity range
setting; higher velocity ranges have higher noise levels. The typical noise level in good
operating conditions is 1% of the velocity range when transmitting data at 25 Hz (i.e.,
each sample is ±2 cm/s when using the ±200 cm/s velocity range and a sampling rate of 25 Hz).
The ADVOcean is designed to measure velocity as rapidly as possible. A single estimate
of the 3D velocity field is referred to as a ping. The ADVOcean pings 150-300 times per
second. The noise in a single ping is too high for practical use, so the ADVOcean averages
a number of pings before outputting a velocity sample. The number of pings averaged is set
to meet the user specified sampling rate within the range of 0.1 to 25 Hz. For example,
when sampling at 25 Hz the ADVOcean will collect as many pings as possible over a 40-ms
period and output the average as one sample. An important result of the ADVOcean sampling
scheme is that reducing the sampling rate decreases the noise in each sample (by
increasing the number of pings averaged per sample).
5.2. Signal Strength
Signal strength, recorded for each ADVOcean receiver, is a measure of the intensity of
the reflected acoustic signal. The primary function of signal strength data is to verify
that there is sufficient particulate matter in the water. If the water is too clear, the
return signal may not be stronger than the ambient noise level of the electronics. Without
sufficient signal strength, the ADVOcean is unable to make accurate velocity measurements.
In general, the ADVOcean requires a minimal amount of scattering material (typically 10 mg/L) for excellent operation.
Since the return signal is a function of the amount and type of particulate matter in
the water, signal strength values can be used as an indicator of sediment concentration.
While ADVOcean signal strength data cannot be directly converted to sediment
concentration, it does provide an excellent qualitative picture of sediment fluctuations.
With proper calibration, signal strength data can be used for reasonably accurate
estimates of sediment concentration.
5.3. Correlation Coefficient
The ADVOcean correlation coefficient is a data quality parameter that is a direct
output of the Doppler velocity calculations. The ADVOcean computes three correlation
values (one for each acoustic receiver). Correlation is expressed as a percentage: perfect
correlation of 100% indicates reliable, low-noise velocity measurements; 0% correlation
indicates that the output velocity value is dominated by noise (no coherent signal).
Ideally, correlation should be between 70 and 100%. Values below 70% indicate that the
ADVOcean is operating in a difficult measurement regime, the probe is out of the water,
the signal-to-noise ratio (SNR) is too low, or that something may be wrong with the
ADVOcean. In some environments (highly turbulent flow, highly aerated water), it may not
be possible to achieve high correlation values. Low correlation values will affect the
short-term variability in velocity data (i.e., increase the noise), but will not bias the
mean velocity measurements. For mean velocity measurements, correlation values as low as
40% can be used.
Because of the remote 3D velocity measurements, the ADVOcean is extremely well suited
to flow studies in boundary layers. The ADVOcean can be used for detailed boundary layer
studies and direct measurement of turbulent parameters such as Reynolds stress.
Additionally, the ADVOcean automatically measures and records the distance to the boundary
at the start of each data collection cycle (the boundary measurement can be made when the
sampling volume is between 2 and 100 cm from the boundary).
Under good operating conditions, the leading edge of the sampling volume can be placed
within about 1 mm of a boundary. The vertical extent of the sampling volume is precisely
defined; thus, this leading edge can be placed very close to a boundary without
interference. Using the reduced sampling volume size mentioned earlier, with a minimum
height of 2.4 mm, allows the user to make detailed flow measurements within a few
millimeters of a boundary.
One significant advantage of the ADVOcean is that there is no minimum measurable
velocity, with no potential for a zero offset or zero drift. The lowest ADVOcean velocity
range, ±5 cm/s, will yield good results for flows down to about 0.1 cm/s. If working in
an environment with extremely low flows, the ADVOcean software can be modified to use
lower velocity ranges to further improve performance.
More details about the ADVOcean can be found at:
