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Echosounders Types
Before we Illustrate any data or classifications about
Echosounders we have to know what is Echo sounders do?
Its making or help to draw Seabed Map or mapping of seabed
How it works?
An echo-sounder sends an acoustic pulse directly downwards to
the seabed and
records the returned echo. The sound pulse is generated by a
transducer that emits an
acoustic pulse and then listens for the return signal. The time
for the signal to return is recorded and converted to a depth
measurement by calculating the speed of sound
in water. As the speed of sound in water is around 1,500 metres
per second, the time
interval, measured in milliseconds, between the pulse being
transmitted and the echo
being received, allows bottom depth and targets to be
measured.
The value of underwater acoustics to the fishing industry has
led to the development
of other acoustic instruments that operate in a similar fashion
to echo-sounders but,
because their function is slightly different from the initial
model of the echo-sounder,
have been given different terms.
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The methods to obtain the seabed:
Multibeam Echosounder
Multibeam echosounder systems (MBES) first appeared in the
1970s
and primarily consisted in an extension of a singlebeam
echosounder. Echosounders are capable to transmit a pulse of
controlled length, repetition rate and frequency to an
underwater acoustic transducer
and to accurately time the returning echoes (propagation
velocity of sound waves in
water: ~1,500 m/s). Instead of transmitting and receiving a
single
vertical beam, the MBES transmits several tens of beams
(typically
100 to 200) with small individual widths (1 to 3) in the form of
a
fan perpendicular to the navigation line. This configuration
provides
depth informations out to several hundred meters each side of
the
vessel which allows to survey large areas of the seabed with a
higher
density and a better accuracy than singlebeam echosounders.
During recent years MBES have greatly evolved and nowadays they
are a broadly
accepted tool for seabed mapping. Acoustic transducers were
developed with
frequencies ranging between a few hertz and several megahertz
depending on the
region and the water depth surveyed. A low frequency of 12 kHz
is used for deep sea
research, whereas recently developed MBES with high frequencies
between 200 and
300 kHz are applied for the investigation of shallow-water
areas.
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Seabed Classification System
Conventional methods to classify the seabed are in situ sampling
of
bottom sediments or optical methods like analyzing of photos
and
videos. However, remote acoustic classification techniques
are
becoming of increasing importance because they are less
expensive
and time-consuming and provide a higher spatial resolution
in
determining the seabed characteristics. Especially,
echosounder-based
techniques, first developed for fishery purposes , are a useful
tool to
classify a big area of the seabed in a relative short time
period with a
high spatial resolution. In general, echosounders are used for
bathymetric surveys.
However, acoustic signals reflected by the seabed contain more
information than just
the water depth. The intensity and shape of a returning acoustic
signal is affected by a
number of factors, primarily sediment grain size and sorting,
seabed roughness,
bedforms, and presence, concentration and type of benthic fauna
and flora. For
instance, the harder or rougher the seabed, the more energy is
scattered back to the
transducer and vice versa. Therefore, echosounder-based
classification systems are
utilized to reveal geological structures of the seabed composed
of various types of
sediments and rocks.
QTC VIEW
The QTC VIEW system uses the first returning echo from the
seabed
only and analyzes the shape of each echo with a series of
five
algorithms. These algorithms characterize the waveform by using
energy
and spectral components, yielding 166 descriptors of each
echo.
Principal-Component-Analysis (PCA) reduces the large quantity
of
information to three most useful descriptors (Q1, Q2, Q3), which
prove
enough to recognize the different types of seabed. When plotting
the points defined by
Q1, Q2, and Q3 on a three-axis plot, echoes of similar character
form clusters that
stand for distinct acoustic classes. To correlate each acoustic
class with seabed
characteristics groundtruthing by sampling bottom sediments has
to be carried out.
The result is a georeferenced trackplot classified by sediment
types.
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Sidescan Sonar
A basic Sidescan Sonar System consists of a topside processing
unit,
a cable for electronic transmission and towing, and a subsurface
unit
(a towfish) that transmits and receives acoustic energy for
imaging.
Sidescan Sonar is a method of underwater imaging using
narrow
beams of acoustic energy (sound) transmitted out to the side of
the
towfish and across the bottom. Sound is reflected back from
the
bottom and from objects to the towfish. Certain frequencies
work
better than others, high frequencies (>500 kHz ) give
excellent
resolutions but the acoustic energy only travels a short
distance.
Lower frequencies (50-100 kHz ) give lower resolution but
the
distance that the energy travels is greatly improved.
The towfish generates one pulse of energy at a time and waits
for the sound to be
reflected back. The imaging range is determined by how long the
towfish waits before
transmitting the next pulse of acoustic energy. The image is
thus built up one line of
data at a time. Hard objects reflect more energy causing a
lighter signal on the image,
soft objects that do not reflect energy as well show up as
darker signals. The absence
of sound such as shadows behind objects show up as very dark
areas on a sonar
image.
Multiparameter Probe with inductive Current Meter
The Multiparameter Probe Series 2001compact consists of 1+4
sensors, e.g. an inductive current sensors, as well as of an
internal
compass and memory, cable interface and internal batteries.
for more information contact
Dr.-Ing. Helmut Schlter VDI, HS Engineer
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Types of Echo sounders:
SideScan sonar
The Geological Lineaments are single or plural linear pattern
that appear in satellite images. The Analysis of natural linear
structures provides important information to us. Lineament is
considered that they reflect geological structure such as faults or
fracture zones as well as artificial structure such as railroads
and power lines. To classification lineament according to
artificiality requires geologists, well-trained specialists, to
find such as hidden seismic faults in the urban area at this
moment. OHTI has been developing a mapping system for lineament
classification to propel satellite image analyses. Those are shown
below.
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Single Beam Echosounder
A single beam echosounder or 'depth sounder' is a system capable
of accurately
measuring the water depth below a vessel (Figure 1). This is
achieved by measuring
the two-way travel time (e.g. from the ship to the seabed, and
back again) of an
acoustic pulse (or a burst of sound) emitted by the sonar
transducer. The acoustic
pulse typically ranges in frequency from 12 - 200 kHz, with
lower frequencies
required in deeper water. The reliability of the depth
calculation is dependant on
accurately knowing the sound velocity in sea water, which is
usually around 1500 m/s
depending on water temperature, salinity, and other factors.
Single beam
echosounders are routinely mounted on most sea-going vessels,
and when attached to
a GPS and recording device, provide an inexpensive seabed
mapping tool.
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Multibeam Sonars
OVERVIEW
A multibeam sonar operates much like a single-beam echo sounder,
ensonifying the
bottom and detecting echo arrival times.
Unlike the single-beam sonar, which illuminates a single point
beneath the sonar, the
multibeam sonar illuminates a narrow swath elongated across the
bottom and
perpendicular to the path of the boat. This illuminated swath is
then sampled with
multiple, discrete "receive beams" formed by the sonar at known
angles. For each
beam, the sonar attempts to determine the "best" echo arrival
time. With the known
beam angle, the determined "best" echo travel-time, and the
water-column sound
velocity, the cross-track distance and depth can be
determined.
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Sub-bottom profilers
A sub-bottom profiler is effectively an echosounder that
transmits a relatively low-
frequency acoustic pulse that can penetrate the seabed. This
signal is reflected off sub-
surface boundaries between sediment or rock layers that have
different acoustic
impedance, which is related to density and sound speed within
each layer (Figure 1).
The strength of the reflected signal depends on the degree of
impedance contrast. The
returning sound waves are recorded by an array of hydrophones
(towed behind the
vessel), or by a transducer/transceiver, depending on the type
of system. The first
useful signal received represents the seabed-water interface,
and shows the
morphology of the seabed in a manner similar to a single beam
echosounder. The time
of arrival and intensity of subsequent impulses provides
information about layers that
exist below the seabed.
Several physical parameters of the acoustic signal emitted, such
as output power,
signal frequency, and pulse length affect the performance of the
instrument and
influence its usefulness in various marine environments.
Increased output power
allows greater penetration into the substrate. However, in the
case of harder seabeds
(e.g. gravels or highly compacted sands) or very shallow water,
higher power will
result in multiple reflections and more noise in the data.
Higher frequency systems (up
to 20 kHz) produce high definition data of sediment layers
immediately below the
seabed, and are able to discriminate between layers that are
close together (e.g. 10's of
cms). Lower frequency systems give greater substrate
penetration, but at a lower
vertical resolution. Longer pulse length transmissions (or
'pings') yield more energy
and result in greater penetration of substrate. However, they
decrease the system
resolution. The depth of penetration also depends on the
hardness of the upper layers
and is significantly limited by the presence of gas
deposits.
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The commercial names There is avarios commercial names for thes
products
according to its company
And the following types is acommercial names for one
company:
Atlas Fansweep
The FANSWEEP multibeam system is manufactured by Atlas
Hydrographic, Sydney, Australia.
The ATLAS FANSWEEP has 100-200kHz
broadband coverage that includes Simultaneous
Multi Ping technology allowing the system to
transmit up to eight pings into the water column
at any one time. This enables the system to
operate in smaller vessels at higher speeds and in
higher sea states than conventional multibeam
systems. The FANSWEEP uses proprietary "
High Order Beamforming", a technique that
provides superior resolution across the entire
swath of 8x water depth. The FANSWEEP
transmits survey data including bathymetry,
automatic feature detection, bottom type
classification and in-water column analysis.
Atlas Hydrosweep
The name ATLAS HYDROSWEEP
comes from "HYDROgraphic Multi-
Beam SWEEPing Survey
Echosounder". The HYDROSWEEP
MD (Medium Depth) is a beam-forming
multi-beam system suitable for surveys
from 10m to 2,000m, 4,000m, 5000m or
6000m depending on the transducer and
frequency configuration. Operating at
30kHz or 50kHz the ATLAS
HYDROSWEEP MD is optimized for
surveys on the continental shelf and on
the continental slope. The
HYDROSWEEP has a bathymetric
swath width of up to 10 times water
depth, and a side scan imagery swath of
12 times water depth.
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Odom Echoscan
The Odom Echoscan is manufactured
by Odom Hydrographic Systems, Inc.,
of Baton Rouge, Louisiana.
The Echoscan has a 200 kHz operating
frequency, with 2.5cm resolution, high
survey speed capability (17 knots max),
sidescan and heave/pitch/roll sensors
co-located in the transducer, a built-in
single-beam transducer and independent
bottom-tracking channels.