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SIDE SCAN SONAR
The main features that distinguish side scan sonars from other forms of sonar are: sideways looking, narrow
beam, two channels, and towed transducers.
Sideways look:
Originally, sonars 'looked' straight down and were used to measure bottom depth or locate large objects
resting on the bottom. During the 1950's researchers turned the transducers on their sides and begin
looking at the series of echoes that returned from the bottom rather than a single echo from a discrete
target. The problem was that the beam was so wide that resolution was very poor.
Narrow Beam:
A narrow horizontal beam angle is required to obtain a high resolution picture of the sea floor.
Two Channels:
Side scan sonars look to both sides of the survey vessel. Not only does this double the effective coverage
area, but there are economic gains to be had as both channels can share a common controller, tow fish
etc.
Towed Transducer:
The narrow beam angle makes control of the beam direction very important which would be difficult in
rough sea if the transducer were mounted to the survey vessel. Thus the transducer is towed behind the
boat where the effects of boat pitch, roll, etc. are minimized. Further, the towed body allows the sonar
to be operated in any water depth so it can be close to the target, which produces the most detail.
PC Monitor:
A side scan sonar display builds up its image by laying down successive scans of the sonar image
producing a composite image.
To achieve high-resolution, side scan reduces the beam width to a narrow 1 to 2 deg, and the transducer is
mounted on a towed fish designed for stability. Waves and a rocking boat have very little impact on the resultant
printout. The result of the narrow beam, a rock stable transducer, and a display, is a high-resolution image with
surprising details of the bottom.
Operating frequencies of SSS are generally 50 Khz, 100 Kkz, or 600 Khz (higher frequencies are available, but
they have very limited range). The 50 Khz units have excellent long range (2000 ft+), but not very good resolution.
The 100 Khz units have good long range (1800 ft), and good resolution. The 600 Khz units have limited range
(400 ft), but excellent resolution. The 100 Khz side scan sonar is the best overall general purpose unit, and the
most popular in use today. The SSS-100K is a 100 Khz system. The SSS-600K is a 600Khz system that offers
significant increased resolution and is ideal for Police and Rescue Units (ideal for body recovery). The SSS-
100K/600K is a dual frequency system that allows the operator to choose the best frequency for each application.
SIDE SCAN SONAR BASICS
Top View of Acoustic Wave Path
9
Acoustic Wave
(1-2 deg thick)
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