Interpreting The Sonar Image; Water Column And Bottom Image; Highlights And Shadows - C-Max CM2 User Manual

CM2 Sidescan Sonar User Manual

9. Interpreting the Sonar Image

9. INTERPRETING THE SONAR IMAGE
(This topic may be covered in additional detail in help text associated with MaxView or with third-party
acquisition software.)
Typical Sonar Image (780kHz)

Water Column and Bottom Image

At the very centre of the image is a line that corresponds to the track of the towfish.
On both sides of the central line, the (normal geometry) picture is occupied by what is called the 'water
column'. This represents the time before the echo first returns from the bottom directly below the towfish. The
water column normally shows up as a white area. Occasionally, if the towfish is close to the water surface,
faint echoes of the surface may show up in the water column. Wake bubbles, fish shoals, and other mid-
water objects, may appear in the water column on one or both sides, depending on whether they are to one
side of the towfish (but still nearer to the towfish than to the bottom) or are directly below the towfish.

Highlights and Shadows

The towfish sees most of the bottom in its view at quite a low angle, as little as 5º at the outer regions. This
means that the sonar picture normally includes both highlights and shadows.
Highlights, shown as darker points or smudges, indicate that the beam has struck a stronger echo reflector.
Hard surfaces reflect more strongly; so do surfaces that face the beam direction. The air-filled swim bladders
of fish can also act as good reflectors.
Shadows show up as lighter areas usually lying beyond a highlight or line of highlights. They can indicate the
presence of an object and can also provide clues about its height and shape. If there are no associated
highlights, the shadow may just indicate a depression in the bottom.
Version 3.6 (2008)
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