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.

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.

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.

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

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.

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.

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.

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.

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.

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.