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 Introduction
Understanding the ambient noise environment in shallow waters is
essential for marine scientists and navies of today. The
characteristics of shallow water ambient noise often influences the
performance of underwater acoustic communications and remote
detection systems. Additionally, as the trend towards numerous
autonomous systems grows, underwater communication issues become
vitally important.
The objective of the ambient noise research at ARL is to provide
a better understanding and complete knowledge of ambient noise
characteristics in shallow water environments. The resulting
research would enhance and benefit research and development on
marine underwater systems.
Ambient Noise Database
The ARL has created an ambient noise database application with a
graphical interface. The database is constantly updated with
ambient noise data collected around local waters. The database
supports for marine researchers, allowing them ready access to
information needed for planning experiments and in the development
new underwater systems. The database may also have future uses in
Rapid Environmental Assessment (REA).
The ambient noise database is designed to be incrementally
improved and extended over the years as a continuing effort
collecting the ambient noise data profiles around Singapore waters.
The collected data can also be integrated with Marine Data
Information System (MDIS), a Geographic Information System
based database engine, undertaken by Marine
Information Technology Laboratory (MITL) at the Tropical Marine Science
Institute (TMSI).
As a part of this project, different data collection locations
in local waters are carefully selected to cover a large ensemble of
ambient noise environments, such as deep channel, shipping channel,
shipping anchorages, shallow reef, shallow mudflat and reclamation
sites. These data is collected using equipment that has been
successfully developed by ARL, such as the Pop-up Ambient Noise
Data Acquisition system (PANDA) and the High Frequency Data
Acquisition System (HiDAQ).
PANDA
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Ambient noise data are often collected by deploying a hydrophone
or towed array from a vessel. The data are usually contaminated to
some degree by the self-noise generated by the vessel, though towed
arrays are less prone to this than single hydrophones.
Contamination can be largely reduced if it is possible to silence
the ship by shutting down all noise generating machinery, but this
is rarely a practical option. When deploying a hydrophone at any
distance from a stationary vessel, we also run the risk of another
vessel running over the hydrophone. Collecting ambient noise data
from busy shipping lanes and fishing grounds poses a double problem
in that any system with a surface expression may be run down or
removed by other vessels. With the PANDA, we eliminate self-noise
contamination by the support vessel and also prevent vessels from
running over our equipment.
The PANDA system is designed as a self contained data
acquisition system equipped with one or more hydrophones. The whole
system sinks to the sea bed when deployed and thus does not have
any surface expression. The PANDA system can be scheduled to
collect the ambient noise, normally after the support vessel
transits away. The system is designed to allow deployment periods
from several hours to several days (depends on usage). To retrieve
the system, an acoustic release is activated on return of the
deploying vessel. The entire system can then be recovered by
hauling in the line.
PANDA leaves no anchor or deadweight behind and is
environmentally-friendly. Since PANDA remains anchored to the
seabed even after recall, recovery is simpler in that the surface
buoy marking the recovery cable is tethered to the seabed and
cannot drift away. Since PANDA utilizes microcontroller,
programming is made easy and data acquisition can be made via COTS
components and thus scheduled as best suits the data needs.
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Preparing PANDAs for deployment
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One PANDA surfaces for recovery
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Selected pictures taken during data collection field trials
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HiDAQ
HiDAQ is a data acquisition system with four simultaneously
acquiring acoustic sensor. The system was designed and built based
on COTS components. It permits four analog input channels to be
sampled simultaneously at up to 5MSa/s total sampling rate. It is
powered by rechargeable Lithium Ion batteries, and records via an
Ultra-Wide SCSI bus (maximum through put rate at 160Mbps) into a
conventional hard drive of 80 Gigabyte. It runs off an embedded
Pentium processor that acquires data The four acoustic sensors are
miniature hydrophones that connected to the analogue signal
conditioning board through underwater plugs using 4 to 5 meters of
flexible thin cables. Modifications have been done to these
hydrophones to increase its robustness and flexibility for this
experiment. Such specification might be easily found in full size
commercial PC systems but the distinction of HiDAQ is that it is a
self-contained and occupies a small cylindrical space of 230 mm
diameter and 600 mm length including its underwater housing,
weighting 16 kg.
The hydrophone array can be flexibly arranged into different
configurations, allowing it to form array of different
realizations. The system deployment is extremely flexible and can
be deployed surface or bottom mounted. When operating as a surface
mount system, it is lowered through a pole or a rope from a barge,
directly hanged from a buoy, a boat, a pier etc. When it is bottom
mounted, the system is attached on a tripod or simply rest on sea
bed. Regardless of how it is mounted, the system can either operate
as stand-alone system or it can be cable attached from surface to
provide power and real time communication.
The complete electronics package of HiDAQ
The system has been utilized intensively for various experiments
to study local high frequency ambient noise thus helping in Ambient Noise Imaging experiments. The high
frequency local ambient noise studies also include the High Frequency ambient noise directivity
study and spatial and temporal
distribution of snapping shrimp noise.
HiDAQ system on field waiting to be deployed, the hydrophones
are keep in a box, avoid them from being damaged
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A sample arrangement of hydrophones
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