Echograms
An echogram is perhaps the most commonly encountered way of displaying the pattern of echoes received during a hydroacoustic sounding or survey. Especially in the field of fisheries acoustics these will be seen on the displays of echosounder hardware and they will typically be one of the first things you see when using Echoview.
Structurally most echograms will present a series of vertical lines, one for each ping an echosounder transmits and listens for echoes from. The echosounder will keep listening until it is next ready to ping and repeats this cycle. While listening it will sample the incoming sound (at the frequency of the transmitted ping) and record the echo trace. Such an echo trace may appear as follows:
In which a very large echo is visible around 50m depth typically indicative of the sea bed, and a moderately large and long echo is visible between roughly 25m and 35m which is typically indicative of a school of fish (Sv is an interpretation of the volume of the echo).
When each successive ping in plotted side by side on a chart which encodes the magnitude (loudness) of the echo with colours, what results is an echogram.
The following echogram is a good example which illustrates a clear sea bed with some very clear schools of fish in the water column above it.
And the following echogram displays a clear trough in the seabed in which single fish are identifiable by the traces they leave on the echogram.
A major challenge in fisheries acoustics is the interpretation of such echograms to produce quantitative estimates of fish abundance. As such data are cost effective to collect and store, so the conversion of these images into fish abundance estimates must be cost effective ... typically implying automatic assessment using software tools like Echoview.
