YSI SonTek ADVField, Technical Documentation Manual

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SonTek/YSI
ADVField Software Manual (September 1, 2001) 22
Several important features can be identified in Figure 5. Each of these is described below, start-
ing at the left side of the plot.
•
The first ≈ 20 counts of the horizontal axis shows “ringing” following the transmit pulse. The
transmit pulse is generated by a strong electrical signal and requires time for the electronics
and transducers to recover.
•
When there is sufficient scattering material in the water, there will be an increase in signal
strength in a bell shaped curve that corresponds to the transmit pulse passing through the fo-
cal point of the receivers. The peak of this curve corresponds to the center of the sampling
volume. The exact location of the center of the sampling volume depends on the exact probe
geometry. For the 10-MHz ADV, the peak is typically located at 90-105 counts for 5-cm
probes, and at 175-195 counts for 10-cm probes. For 5-MHz ADVOcean probes, the peak is
typically located at 140-160 counts. All three receivers should see the peak in the same loca-
tion, although there will be variation in the height and shape of the curve. It is possible to cal-
culate the signal to noise ratio (SNR) from the height of this curve and the noise level (see
below). For the example in Figure 5, the mean height of the curve is about 130 counts and the
noise level is about 58 counts. SNR is then calculated by (130-58)*0.43, or about 31 dB.
•
If the ADV is reasonably close to a boundary (bottom, surface, or side), a sharp reflection
should be seen in
ADFCHECK
. The size and shape of this reflection will vary depending on the
nature of the boundary, its distance from the ADV, and the acoustic conditions of the water.
We can estimate the distance from the probe to the boundary by the location of the front edge
of the boundary reflection. In Figure 5, this occurs at about 300 counts or 18 cm from the
Figure 5 – ADFCHECK Display