Image Corrections; Causes Of Image Defects - C-Max CM2 User Manual

CM2 Sidescan Sonar User Manual 9. Interpreting the Sonar Image
If an object casts a shadow on a flat bottom the height of the object is given by simple geometry, using the
shadow length, the towfish altitude, and the true (slant) distance of the object from the centreline. Some
acquisition software, including MaxView, includes the ability to calculate the object height by running the
cursor along the length of the shadow.

Image Corrections

For safety when gathering sonar records use the uncorrected, normal geometry (not slant-range corrected)
image. When reviewing the records, or when taking measurements, corrected geometry may be useful.
Selecting slant-range corrected geometry (SRC) corrects for the 'slant range' distortion, by differentially
stretching the image, and removing the water column.
Over rocky or very uneven ground the closure at the centre of the SRC image may not be perfect. If the
ground is uneven it cannot be assumed that the whole bottom is varying in unison, so a smoothed altitude
value is used. The SRC process exaggerates any mis-closure.
Although SRC geometry corrects for slant range distortion, it does not correct the display 'aspect ratio', i.e.
ensure that a square area of the bottom is represented by a square on the display. To do this requires
replicating or suppressing ping lines and is not generally desirable.

Causes of Image Defects

It is important that the towfish is towed on a steady track and without excessive heave motion transmitted
down the tow cable. Any erratic motion of the towfish will transfer to the image. Roll oscillations may produce
light and dark banding, alternating between port and starboard sides. Turning stretches the image on one
side and compresses the other.
If the towfish is near the surface it may pick up reflections from the surface waves. This can reduce the
maximum range at which a good bottom image is obtained. Also in calm conditions, particularly in shallow
water, the echoes can reflect off the surface as well as returning directly. This 'multi-path' effect shows up in
the image as ghosting. The CM2 HF towfish has a sharper surface cut-off than most sidescan sonars and is
inherently more resistant to both the above effects.
The CM2 towfish transducers are adjustable in angle. In the normal position they are depressed nominally
10º from the horizontal. This angle gives maximum range. In the other position they are depressed
approximately 20º from the horizontal. This gives greater immunity from spurious echoes from the surface,
which may be useful in shallow water, but sometimes at the cost of reduced effective range.
The wakes of motor vessels contain a vast number of microscopic bubbles, which may take ten minutes or
more to dissolve. Wakes left by other vessels can show up prominently on the sonar image and should not
be confused with permanent features.
Note that a large object, such as a wreck or a piling structure for example, may appear to be distorted if it tilts
towards the direction from which it is being viewed or extends above the altitude of the towfish.
Version 3.6 (2008)
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