The amplitude or signal strength of the received signal, can be used to describe the scattering conditions in the environment and its variation throughout the water column can be used to judge the quality of data received from the recorded signal. For a detailed description of the parameter, please see Amplitude - theoretical background.
Amplitude data can show both spatial (profiler) and temporal (profiler and current meter) variations. Figure 1 shows an example of this from Ocean Contour. Figure 1a presents the amplitude readings for Beam 2 in space and time. The amplitude along the horizontal dotted line is illustrated in Figure 1b and shows how the amplitude changes with time within that specific cell. Figure 1c shows the amplitude along the vertical dotted line in Figure 1a 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.
Data analysis
An amplitude quality check should be performed for each beam and each cell. If the observed amplitude profile deviates from what is expected based on the sonar equation, the data should be examined more closely.
An increase in amplitude with distance in one or more beams may indicate the presence of a solid boundary, such as the sea surface, seabed, or another obstruction. For example, the amplitude profile shown in Figure 1c comes from an instrument measuring toward the sea surface. In this case, the amplitude initially decreases with range, but before reaching the noise floor it begins to increase again, eventually reaching a maximum value at the surface. For further details on how reflections from the sea surface influence the measurable range, see How to determine the range of a dataset from amplitude.
Amplitude data can also be used to identify other conditions affecting data quality. Several of these are discussed in the section Common sources of data loss, including Beam blockage, Bio-fouling, Diurnal migration of plankton and Acoustic interference.
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