In general, amplitude is a typical variable used to describe the characteristics of a wave. It says something about the amount of energy transferred by the wave and can be seen as a measure of how strong or big the wave is. Regarding acoustics, a greater amplitude means a greater intensity and the wave sounds louder (in the audible frequency range).
The amplitude, or signal strength, for current measurements, can be defined as "a measure of the magnitude of the acoustic reflection from water". As the amplitude is a measure of the echoes, it is a function of the scattering conditions. The amplitude is given in the dimensionless units decibel (dB) or counts. The relation between the two units may vary slightly between instruments, but a conversion of 0.5 dB/counts is a good estimate.
ADCP instruments emit longitudinal sound waves into the water and measure the echoes. To receive an echo signal it is crucial to have enough scattering material in the water that can reflect the transmitted signal. When we talk about amplitude for current measurements, we usually refer to the amplitude of the echo signal. It is this amplitude that the instrument register and outputs. However, the transmitted sound pulse also has an amplitude, one of the factors that tells how much energy is emitted from the instrument. The echo signals reflected from scattering materials usually contain far less energy, i.e. lower amplitude, than the transmitted signal. As the transmitted signal propagates further along the profile, its amplitude will constantly decrease as the pulse loses energy for each reflection (and from absorption). This also implies that the strength of the echo signal decreases because there is less energy to reflect. This decrease follows the Sonar equation and will look similar to the left amplitude profile in Figure 1. After a gradual decrease in signal strength, the amplitude reaches a constant limit known as the noise floor. At this point, the instrument only measures noise, and the standard deviation becomes large. All instruments have an individual noise floor due to internal electronic noise inside the instrument. In this way, the amplitude can determine the profiling range for a current profiler because data with such a weak echo signal that the noise dominates and gives a high standard deviation should be discarded. A common way to determine whether the signal is strong enough is to assess the signal-to-noise ratio (SNR). This is a ratio of the received echo signal and the noise floor. We recommend using a 3 dB SNR threshold, meaning that the recorded signal should be at least 3 dB higher than the instrument's noise floor. How far the instrument can measure before this threshold is reached depends on several factors. The type and amount of scattering material play a decisive role. The upper layer of the water column normally contains more scattering materials, which implies that an upside-looking instrument might have a longer range than an instrument measuring from the surface and down. The range is also affected by the instrument frequency, where lower frequencies generally enable longer ranges. Cell size is also a factor when it comes to the maximum profiling range, as this determines the length of the transmit pulse. Bigger cell size means longer transmit pulses and more data points within each cell. A drawback is that increasing the cell size reduces the spatial resolution. Increasing the power level means more energy is emitted, which is useful when a longer range is desired. A consequence of this is higher power consumption and possible shorter deployment. The configured profiling range also depends on chosen blanking distance (start of profile) and the number of cells. A maximum profiling range is given in the technical specifications of every current profiling instrument. But keep in mind that this is a nominal value, with the possibility that the actual values obtained could be longer or shorter.
Figure 1: Typical amplitude behavior along a profiling range. Left: The amplitude gradually decreases according to the Sonar equation and reaches the noise floor when the values level off. This applies in situations when the instrument does not detect blockages or boundaries. Right: After a gradual decrease in amplitude the signal strength increases as the signal approaches the surface. This increase in amplitude can also be seen when measuring the seabed or other blockages. The area in which the amplitude increases is affected by sidelobe interference.
Amplitude data can show both spatial (profiler) and temporal (profiler and current meter) variations. Figure 2 shows an example of this from Ocean Contour. Figure 2-a presents the amplitude readings for Beam 2 in space and time. The amplitude along the horizontal dotted line is illustrated in Figure 2-b and shows how the amplitude changes with time within that specific cell. Figure 2-c shows the amplitude along the vertical dotted line in Figure 2-a and tells how the amplitude changes along the profiling range at that specific time. The amplitude values always refer to the along-beam signal strength and are independent of the chosen velocity coordinate system.
(a) Amplitude readings in space and time
(b) Time variation of amplitude
(c) Spatial variation of amplitude
Figure 2: Amplitude can show both temporal and spatial resolution for current profilers. (a) Show both in an example of overall amplitude, (b) shows the temporal variation in one cell, and (c) gives spatial at one time.
An amplitude quality test should be applied to each beam and to each cell. Whenever the amplitude profile deviates from the Sonar equation you should look closer at the data. If the amplitude increases with distance in one or more beams it may indicate a solid boundary such as the surface, bottom, or an obstruction. The right amplitude profile in Figure 1 is taken from an instrument measuring the sea surface. Here you can see that the amplitude at first decreases, but before it reaches the noise floor the amplitude then increases until the surface is detected at the peak value. A single, unusually high amplitude may indicate a passing thing that reflects stronger than water. In some areas, seasons and times of day can affect signal strength. More of this is discussed here: Why does my data have low SNR? The amplitude data can also give information about other kinds of occurrences, read more about this in QA/QC current measurement (comprehensive).
Correlation is a statistical measure of similar behavior between two observables, which in our case is how similar the transmitted signal is to itself at a delayed time (when the signal is received after reflection). Correlation is output in %, where 100% means perfect correlation and 0% means no similarity. The magnitude of the correlation is thus a quality measure of the velocity data, and as the correlation decreases so does the data accuracy. Correlation decreases with distance from the instrument and establishes the maximum usable profiling range. A commonly accepted threshold for range when considering correlation data is 50%. Figure 3 shows how a threshold of 50% can be justified, by how the standard deviation increases as the correlation drops.
Figure 3: Upper: Amplitude and correlation. Lower: Standard deviation. Note how the standard deviation increases as the correlation drops.
Amplitude vs. correlation
Both amplitude and correlation are used as quality parameters of velocity data, and both can be used to establish the maximum range. Since they are based on different things, they are also sensitive to different abnormalities. The correlation should drop to 50% approximately where the SNR reached the 3 dB threshold. When this is not the case, take it as a sign that the data need more careful analysis. Note that not all instruments measure correlation, for these the focus has to be on the amplitude.