Use of Side Scan Sonar in identification of submerged objects in the shallow sea area
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Date
2013
Journal Title
Journal ISSN
Volume Title
Publisher
Uva Wellassa University of Sri Lanka
Abstract
North east coast is playing a significant role in economy of the country via fishery. Beach seine
fishery has been initiated in this area after 30 years of civil war. Under water war remnants
buried in the bottom of the north east shallow sea acts as a major impediment in improving of
beach seine fishery in the area. Generally, unidentified buried objects are discovered with the
help of divers and underwater cameras. However, this technique is ineffective, due to absurd
searching in the sea bottom, time consuming and high cost. So survey becomes unsuccessful,
most of the time. In this study, Side Scan Sonar (SSS) survey technique which has been
developed using medical ultrasound technology was used to detect the specific objects that
affect the beach seine fishery, their exact location and distribution on the seafloor. North east
coast of Mullaithivu area in Northern Province was selected as the study area for four waypoints
/
80°49'15.06"E, Z-9°17'30.00"N/80°48'30.00"E) using the Integrated Global Positioning System
(inbuilt GPS or DGPS Garmin Colorado 300 handheld GPS). Survey was carried out by towing
vessel along predetermined survey lines just above the bottom of the seafloor depending upon
the water depth. Data was collected using Imaginex Model 872 “YellowFin” side scan sonar
combination with data acquisition using “YellowFin version 2.0.1.4” software. Image
processing techniques of “sonarWiz” 5 and “ArcGIS” software was used to detect and classify
buried objects in side-scan sonar images. According to the results, three objects were identified
in three different locations. Object 01was at 09˚ 17.97244’ N/080˚ 48.66892’E with 85 m length
and 21 m width, while object 02 was at 09˚ 18.71599’ N/080˚ 48.02634’E with 55 m length and
15 m width at the widest point. Location of object 03 was at 09˚ 18.87650’ N/080˚ 47.48114’E
with 120 m length and 25 m width at its widest point. Highest coverage (with greatest length
and width), was recorded for object three (3000 m ) which was distributed over a larger area of
sea bed. Distance from coastal area to object one, two and three are reported as 400 m, 575 m
and 200 m respectively. Third object is in close proximity to beach environment compared to
other two. Average depth of the studied area was recorded as 10 -12 m. Sharp edges and curvy
features of first and second objects indicated that these two objects are ship wrecks. Images
show vast amounts of ship debris around these identified ship wrecks. Since these two artificial
objects were identified within boundary of coastal area, there is a high potential to damage
coastal fishing gears, especially beach seines by entangling and will waste time and money in
repairing the damaged nets. Third object exhibits blunt edges with smooth curvatures and can be
a natural structure such as a bed rock or a sand bar. The identified objects can be marked as the
objects that directly influence on commercial beach seine industry. Side Scan Sonar can be
recommended as a modern technique useful in detection of underwater objects with their precise
location.
Description
Keywords
Technology, Aquaculture and Fisheries, Aquatic Resources, Aquatic Products, Fisheries, fish Industry, Animal Sciences