Geoscience Reference
In-Depth Information
2 Heave Compensation Method
The essence of the mechanical side scan sonar stabilization is to physically remove
intrusive sonar and towing vessel movements and imaging noises that cause poor
quality sonar image [
13
]. These movements are mostly caused by the sea waves,
sudden manoeuvring of the towing vessel sonar and underwater currents. The
biggest and the most common issue addressed on side scan sonar systems is the
poor quality of the seafloor images, obtained by the side scan sonars, due to heaving
of the ship caused by the rough sea waves. There are several methods to cope with
this problem. The easiest way to avoid the swinging of the ship is to plan the
sea
oor-scanning expeditions at the time when sea waves at their minimum. Even
though this method is effective, it limits the number of days per year for expedi-
tions. Another method is to choose the preferable swimming route. When the ship
swims lengthwise but not perpendicular to the waves to reduce the impact of
rebounding waves.
The depth of the Baltic Sea in Lithuania
fl
is territorial waters is up to 100 m.
However there are shallow areas, which also need to be researched. The optimum
operating height of the sonar is about 12 m above the seabed. But when scanning
shallow areas near coast, the sonar has to be towed just 3
'
4 m above the seabed. It
can be done by using a short cable (from 2 to 5 m) which connects the ship and the
sonar. Because of the turbulence caused by the ship, the sonar towed with a short
cable is more disturbed. It happens because a short cable does not absorb the
imbalances of tension. Exploring seabed in the deeper waters long cable is used and
the sonar is towed far from the ship. This reduces the impact of the turbulence
caused by the ship. Also Long cables absorb shock caused by the ship. Scanning the
seabed in deeper waters it is faced with a problem of maintaining the sonar in a
constant depth. Towed sonar tends to rise to the surface. It moves away from the
optimal 12 m height above the seabed. As a result, the quality of images is worse
than needed.
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3 Current System Parameters
The standard Current system uses Side Scan Sonar. Its main parameters are pre-
sented in Table
1
. For our tests we used standard a typical side scan sonar system
(see Figs.
3
and
4
). It consists of dual frequency (100/500 kHz) sonar
“
Edgetech
272
controller and a portable computer with sonar and GIS
software, for real time map producing.
This system is mounted on Klaipeda University ship. Sonar cable is mounted on
the cable drum, which helps to adjust sonar immersion depth. At end of ship sonar
cable goes through pulley, it is the last point in ship where sonar and ship are
connected. All further measurements of ship movement will be captured at this
point.
TD
”
,
“
Edgetech 560P
”
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