mbes_ taicheng report

2014

SINGLE AND MULTI BEAM ECHO SOUNDER

Submitted By:

Taichengmong Rajkumar Indian School of Mines Dhanbad

Submitted To:

Ratan Srivastava Chief Scientist SK-318 NCAOR

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INDEX

Sl No. Contents Page No

2.1 INTRODUCTION 3

2.2 THEORY 4

2.3 TYPES OF ECHOSOUNDER 5

2.4 SINGLE BEAM ECHOSOUNDER 5

2.4.1 BASIC PRINCIPLES OF SBES 5

2.4.2 BASIC WORKING OF SBES 5

2.4.3 SBES SYSTEM 6

2.4.4 SBES SPECIFICATION 7

2.4.5 SBES CORRECTION 8

2.4.6 APPLICATIONS 10

2.4.7 DEMERITS 10

2.5 MULTI BEAM ECHO SOUNDER 11

2.5.1 INTRODUCTION TO MBES 11

2.5.2 BASIC PRINCIPLES OF MBES 11

2.5.3 MBES OPERATION 12

2.5.4 WORKING OF MBES 13

2.5.5 TYPES OF MBES 14

2.5.6 MBES INSTRUMENTS 15

2.5.7 MBES SYSTEM 17

2.5.8 MBES SURVEY 22

2.5.9 MBES SPECIFICATION 27

2.5.10 APPLICATIONS 28

2.5.11 DEMERITS 28

2.6 REFERENCES 29

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2.1 INTRODUCTION:

Echo sounding is a technique for measuring water depths by transmitting acoustic

pulses from the ocean surface and listening for their reflection (or echo) from the sea floor.

This technique has been used since the early twentieth century to carry out traditional

hydrographic surveys using single beam echo-sounders, which relied on a grid of tracks,

between which data was interpolated to produce a map of certain parts of the seafloor in

geographic reference frame. The echo-sounders used were of broad and narrow beam width

types. These types of echo-sounders could provide only a profile along a track and hence to

map certain area requires tremendous amount of time. In coastal areas it is very much

required to have a 100% information about the topography of the seabed and it is difficult

to achieve this by using single beam echo-sounders as they are time consuming and proved

to be very expensive. To overcome this problem the advanced method of multi-beam

mapping (swath mapping) been introduced, which has been one of the most significant

technological advance for marine geologists. Multi-beam echo-sounder systems

replaced the single beam transducers with 60 – 120 beams, simultaneously collecting data

from a broad swath, allowing digital terrain models of the seafloor to be produced,

equivalent to photographic and satellite imagery on land. Multi-beam echo-sounder systems

has provided the vital depth input to charts that now map most of the world’s water-covered

areas. These charts have permitted ships to navigate safely through the world’s oceans. In

addition, information derived, has aided in laying trans-oceanic telephone cables, exploring

and drilling for off-shore oil, locating important underwater mineral deposits, and improving

our understanding of the Earth’s geological processes.

Just after December 2005 trial, the ship sailed out for its first multi-beam mapping

operation in exclusive economic zone of INDIA, which failed due to malfunctioning of the

system. In May 2005, on visit of OEM engineer, the problem was identified and reported to

change the underwater multi-beam transducer. The system was made operational after re-

fitment of new underwater unit. And later from October 2006, the hydrographic surveys had

been started off Goa. The hydrographic chart produced by system will be of good help to

the port / river navigation authorities, geologists exploring the seas and oceans etc.

This report is stressed on the operational aspects, problems encountered in the acquired data,

exercise conducted to solve the sound velocity problems and the debugging/ maintenance

of the multi-beam echo-sounder system.

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2.2 THEORY:

Sound moves in water in a moving series of pressure fronts known as Compression

Wave carrying a certain amount of energy called Acoustic Energy. When these waves

encounter another medium Reflection, Scattering, Transmission take place. Amount of

energy that come back to medium is called ECHO. The angle of reflection αr is equal to the

angle of incidence αi ,part of the energy will be transmitted into the second medium at an

angle αt according to Snell’s law.

Fig 2.2(a): Reflection and transmission following Snell’s law.

----Eq: 2.1

Fig 2.2(b): Sound Wave as Compression Wave

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2.3 TYPES OF ECHOSOUNDER:

There are two types of Echo sounders

a) Single Beam Echo Sounder (SBES)

b) Multi Beam Echo Sounder (MBES)

2.3(a) Single Beam Echo Sounder (SBES):

It is also called as narrow beam echo sounder, contains one transmitter and

receiver and it covers a particular area.

2.3(b) Multi Beam Echo Sounders (MBES):

Multibeam Echo-sounder (MBES) systems are used to increase bottom coverage of

the water covered areas. It is the most advanced acoustic tool for remote observations and

characterisation of the seafloor. Multibeam echo-sounders consisted of an extension of

single-beam echo-sounders.

2.4 SINGLE BEAM ECHOSOUNDER:

2.4.1 Basic Principles of SBES:

An acoustic wave sent out by an Echo-sounder in the water will propagate through

the water column until it collides with the boundaries of the seabed or sea surface. These

boundaries will send back echoes of the transmitted signal.

2.4.2 Basic working of SBES:

The Single Beam Echo-sounder sends an acoustic pulse from the transducer down

into the water column towards the sea bottom. The travel time before the signal is received

back will give, together with the correct sound speed, the water depth under the transducer.

H = tc/2 -------- Eq: 2.2

Where

H is the water depth

t is the two way travel time and

c is the sound speed in the water column

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Fig 2.4(a): Working of Single beam Echosounder

2.4.3 SBES System:

In Sagar Kanya SBES model BATHY 2010 is used.

Fig 2.4(b): SBES Instrument

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In the above figure (A)–Display, (B)-CPU, (C)-Receiver, (D)-LPT, (E)-Depth measured

system.

SBES system is a ground isolated interface and provides the output signal for the high

frequency output channel. The BATHY-2010 system is configured as a flexible acoustic

measurement sensor device, capable of both shallow and deep water hydrographic and sub-

bottom profiling applications. The Bathy 2010 accepts ship's position and heading interface

via serial data from several external devices including DGPS Positioning Systems and

Heave Compensators.

2.4.4 SBES Specifications:

Table 2.1: Technical Specification of SBES BATHY-2010

2.4.4(a) LPT (Linear Power Transmitter):

The LPT utilizes pulse width modulation switching technology. The basic building

block for the LPT is a 5-kilowatt linear power amplifier module. The maximum operating

duty cycle is 20% at full power with a 1-50 KHz bandwidth. The LPT offers greater than 70

dB operating dynamic range. It receives its input waveform and control signals from the

BATHY 2010 via a ground-isolated interface.

Units Ft or m

Depth (ft.) 30000

Pulse Length 200usec – 1 sec at Max Power

Depth (m) 12000

Resolution 0.1 Ft, 0.1 m

Speed of Sound 1400-1600 m/S, 4590-5250 Ft/S

Environmental 0 to 50 degC , 0 to 95% Humidity

Format SEG-Y

Transducer SYQWEST, INC

Input Power: The BATHY-2010 requires AC power connection to the

LPT,

The Server, and the LCD Display.

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2.4.4(b) Frequency:

The frequency is directly related to the absorption in the medium. Typical SBES

frequencies range from 12 kHz to 300 kHz. Single beam bathymetry surveys carried on both

high and low frequencies. High frequencies used in shallow depths where as low frequencies

are used in deeper depths.

Fig 2.4(c) A,B & C: Beam patterns for 12, 38 and 200 kHz _ _ _

2.4.5 Corrections:

2.4.5(a) Roll-pitch effects:

. On larger vessel i.e. greater than 26 ft. roll and pitch are usually not excessive under

normal working conditions typically less than 5 deg. However, on smaller vessels (e.g., less

than 26 ft.) roll or pitch can easily approach or exceed 10 deg in rough seas. The correction

for roll and pitch varies with the angle of rotation and depth. However, the beam width of

the transducer may be greater than the overall roll or pitch, resulting in the first return still

being near vertical.

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Table2.2 & 2.3: Roll, Pitch and depth variations

Fig 2.4(c): Roll and Pitch effects on a Single beam depth

2.4.5(b) Heave compensation:

The major depth error component is heaved the long period up and down motion of

the vessel due to wave motion, other vessel wakes, etc. Heave is basically a function of

wave swell and period. Heave errors are normally excessive at coastal entrances and on

offshore approach channels large 65 ft. survey boats can typically work in swells up to 3 or

4 feet. Modern heave compensators can effectively record heave movement and smooth out

these effects.

2.4.5(c) Pulse duration:

Pulse duration is the length of time the sounder transmits power to the transducer.

It is related to the amount of energy propagated into the water. Short pulse duration does

not deliver as much energy to the seafloor as a long pulse and will likely contain less

information than with long pulse duration.

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The pulse length and the transducer diameter are inputs for the SBES.The single beam echo

sounder used for the data gathering is made by Simrad.

Frequency [KHz] Pulse length [s]

12 0.001

38 0.0003

200 0.0003

2.4.6 Applications:

• To obtain depths directly under the vessel.

• To improve the quality of the data in terms of both resolution and accuracy.

• To find any leakage underwater pipeline.

2.4.7 Demerits:

• To produce a narrow beam, larger size transducers are needed than for a wide

beam.

• This technique is time consuming.

• The equipment becomes bulky and expensive.

• Narrow beam echo sounders do not provide information off the sides of the ship.

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2.5 MULTI BEAM ECHO SOUNDER:

2.5.1 Introduction:

The Echo sounder is the earliest, most basic and the most widely used echo sounding device.

Echo sounders have been used for a long time. Historically they were used to determine the

water depth under a ship. Further researches showed that the signals could be used for many

more purposes, one of which is classification of the seabed.

Nowadays acoustic systems are used for depth measurements. In the beginning of

the 20th century the first acoustic systems were developed for measuring ocean depths and

for obstacle detection. It is an active system, emitting and recording sound, and these are

called SONAR (Sound Navigation and Ranging) systems.

Echo sounders are in widespread use for bathymetric mapping. These devices used

one source and one receiving device. Such systems are now known as single beam and multi

beam echo sounders.

2.5.2 Multi Beam Echo Sounder Basic Principle:

Sound travels in water in a moving series of pressure fronts known as a

compressional wave. These pressure fronts move (or propagate) at a specific speed in water,

the local speed of sound. The local speed of sound can change depending on the conditions

of the water such as its salinity, pressure, and temperature, but it is independent of the

characteristics of the sound itself all sound waves travel at the local speed of sound. In a

typical ocean environment, the speed of sound is in the neighbourhood of 1500 meters per

second (m/s).

In bathymetry, the object to be positioned is frequently the seabed. Acoustic pulse

transmitted by a transducer travels through the column of water and is then reflected by the

target (sea floor) back to the source and we can calculate depth from travel time.

Depth is calculated from the measured travel time

Depth = c 𝛥𝑇

2 ----- Eq: 2.3

Where c is the speed of sound in water

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2.5.3 Multi Beam Echo sounder operation:

Fig 2.5(a): Basic echo sounder operation

A T/R (transmitter/receiver) switches which passes the power to the transducer.

A transducer, mounted on the ship's hull, which converts the electrical power into

Acoustic power, sends the acoustic signal into the water, receives the echo and

converts it into an electrical signal.

A receiver which amplifies the echo signal and sends it to the recording system.

A recorder which controls the signal emission, measures the travel time of the

acoustic signal, stores the data, and converts time intervals into ranges.

The transducer is mounted on the ship's hull and is in contact with water. Its functions are

To convert electrical power into acoustic power.

To send the acoustic signal into the water.

To receive the echo of the acoustic signal.

Modern echo sounders usually offer a choice of two to three transmitting frequencies,

namely:

Low frequency - effective for deep water because the attenuation is lower, but it

requires a large transducer.

High frequency - the transducer can be compact but the range is more limited due

to a higher attenuation

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2.5.4 Working of Multi Beam Echo Sounders (MBES):

Multibeam echo-sounders consisted of an extension of single-beam echo-

sounders. Arrays of sonar projectors produce soundings not only along the track, but also

for significant distance across to the ship track. Instead of lines of single soundings, new

multibeam systems produce a swath of soundings (Fig.2.5.b). In modern deep-water

systems, the swath covered on the seafloor can be up to 7 times the water depth. This means

that if we are working in an area of 3000m water depth, the maximum width swept is of 21

km. To obtain a complete cartography of the seafloor, the vessel scans adjacent swaths at a

speed of 8 to 12knots, drawing up a mosaic of seafloor topography. Therefore, in deep water

(> 3000 m), a zone of 400km by 20km (8000 km2) could be surveyed in less than a day.

Multibeam Echosounder (MBES) systems are used to increase bottom coverage of

the water covered areas. It is the most advanced acoustic tool for remote observations and

characterisation of the seafloor. Multibeam echo-sounders are based on the principle of

acoustic wave transmission and reception in the water.

Multibeam echo-sounders consisted of an extension of single-beam echo-sounders.

Fig 2.5(b): Sketch of how multibeam echo sounder surveys the seafloor

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2.5.4(a) Swath:

A narrow single-beam echo sounder is performed at several different locations on

the bottom at once. These bottom locations are arranged such that they map a contiguous

area of the Bottom usually a strip of points in a direction perpendicular to the path of the

survey vessel, called as swath showing in Fig 2.5(c) The dimension of the swath in the

across track or athwart ship direction (perpendicular to the path of the ship) is called the

swath width, and it can be measured either as a fixed angle or as a physical size that changes

with depth

Fig 2.5(c): Multi beam sonar swath

2.5.5 Types of MBES:

A) Swath systems

B) Sweep systems

A swath system produces multiple acoustic beams from a single transducer system.

A sweep system simply consists of an array of single beam echo sounders mounted on

booms deployed on each side and perpendicular to the surface vessel.

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2.5.6 MBES Instrument:

Multibeam echo-sounders are consists of the following devices:

Transmission and reception arrays, transmission electronics, reception unit, user interface

(with system control options and real-time processing results) and ancillary systems, such

as a positioning system, attitude sensor unit (giving roll, pitch, heave and the heading

values), and sound velocity profiles (SVP).

The main characteristics of multibeam echo-sounders are acoustic frequency,

maximum angular aperture, number of beams, beam spacing, length of emission and

cadence of the emission. The resolution of systems increases with frequency, but so does

the attenuation in the water, so higher frequency systems will have shallower depth

limitations than lower frequency systems.

Therefore, acoustic frequency determines several types of systems:

Deepwater systems (50–12000 m) that work at 12 kHz for the deep ocean and

30 kHz for continental shelves; shallow-water systems (5–1000 m), work at 100-

200 kHz and are designed for mapping continental shelves, and

High resolution systems (few meters) work at 300-500 kHz and are used for local

studies (e.g. ports, bays, etc.).

The more the pulse lasts the higher the resolution is (typically between 1 ms in Shallow

waters and 15 ms in deep waters). The length between two successive emissions of the

sounder is referred to as the cadence of the emission and, at least, it is longer than the

duration of the return trajectory of the more external beams.

2.5.6(a) Transmitter and Receiver control units:

The transmitter control unit supplies the drive signals to the entire projector array.

Each output is separately controlled for power level, phase and frequency. This facilitates

programmable shading and steering, as well as transmits beam stabilization using Swept

Beam.

The receiver control unit controls the overall ping cycle. It contains the receiver

circuits for the hydrophones as well as the signal processor for beam forming, bottom

detection and data reduction. The control units are interfaced to the operator station via

Ethernet. The transmitter power amplifier for amplify the power signals. Then the transducer

converts these electrical signals to sound pulses. For this transmitter and receiver control

units there is an operating main switch box in the multi beam system. By switching these

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buttons we can get the signals from the control units. These pulses are recorded by Hydro

star acquisition software.

Fig 2.5(d): Transmitter and receiver control units

2.5.6(b) Transducer array:

The transducer array is made up of poly-urethane material and is sealed for under

water use. The array is used both for transmit and receive purpose. It is semi-circular with

a radius of 45 cm and 160° angular extent. Its weight in water is 90Kgs approximately. This

transducer is fixed to the hull of the ship which is protected from strong longitudinal

underwater currents by the dome called blister. The blister helps to eliminate bubble

formation and improves acoustic measurements from the ship speed of 2 to 7 knots. The

transducer consists of 8 Arrays and each array consists of 16 elements comprising of total

128 elements. Eight 12.5m long underwater cables connect the transducer to the junction

box. The cables are fitted with connectors on the dry end. The cables from the transducer

junction box to the transceiver unit are 5 m long. The function of transducer is to execute

the command given by TRB-32, which is to transmit and receive the beams.

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2.5.6(c) The Mills Cross Technique:

Here the projector and hydrophone arrays are perpendicular to each other. The strip

of the ocean floor ensonified by the projectors will intersect with the strip of the ocean floor

observed by the hydrophones. This occurs in only a small area with dimensions that

correspond approximately to the projector and hydrophone array beam widths. While

echoes occur along the entire ensonified area, and sound may be received from the entire

observed area, the only part of the bottom both ensonified by the projector array and

observed by the hydrophone array beam is the area where the two strips overlap. The

amplitude trace from the hydrophone array will contain only those echoes from the

transmitted ping that occur in this area. The perpendicular arrangement of the projector and

hydrophone line arrays is called a Mills Cross, named after a pioneering radio astronomy

instrument built in New South Wales, Australia.

Fig 2.5(e): Mills Cross Technique

2.5.7 Multi beam Echo Sounder system:

The EM1002 multi-beam echo-sounder system have different parts, Viz. operator station,

transceiver unit (which hosts maximum circuitry for signal processing), transducer and other

external sensors. The operational diagram is given below. Initially all the instrument offsets

pertaining to alignment or external sensor locations etc. were fed in the acquisition software

and the system is calibrated for roll, pitch, heave etc. During operation, the system is kept

in the service mode (available on TRU), which allows the automatic adjustment of the power

supplies in accordance with the power required by the system boards for transmission and

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reception. As given in the functional block diagram above the sensors are started prior to

EM1002 TRU and allowed for stabilization for at least 5 min, however the sound velocity

probe is made to stabilize for at least 30 min. (sequence for start-up is Sound Velocity probe

DGPS Motion sensor TRU ). Meanwhile the operator station and Ethernet switch

is started. After about 5 minutes stabilization time, the EM1002 TRU is started and the

Ethernet connection with HWS10 is established. Now first the Seafloor information

Systems (SIS) software is started. The planning and survey administration modules in SIS

allows survey planning of the cruise, which in-turn gives flexibility for entering survey

locations and marking areas having geological importance while on run. The module also

allows for real-time coverages and grids to be transferred to the Helsmann display, which

helps the sailor to manoeuvre the ship in proper direction. Once the SIS software is stable,

the status of inbuilt modules of the TRU Viz, Processing unit (PU), Beam-forming and

signal processing (BSP), shell status (SH) turn green which enables the operator to ping the

transducer array. Once pinging is started, the various parameter settings made on the

operator station are sent to the control processor of the Transceiver unit. The information is

interpreted and passed on to the signal processor RX (SPRX) in the receiver rack, which in

turn enables the TRB 32 (transmitter/ Receiver board – 32 Bits) to send the acoustic signal

towards the seabed. The angular coverage sector and beam pointing angles are variable with

depth, according to achievable coverage to always maximize the number of usable beams.

The beam spacing is normally equidistant, corresponding to 1.5% of depth at 90°, 2.5% at

120° and 4% at 140° angular coverages. The transmit fan is split in several individual sectors

with independent active steering according to the vessel roll, pitch and yaw to get a best fit

to a line perpendicular to the survey line and thus a uniform sampling of the bottom. Once

the signal gets reflected back from the seabed, the signal processor RX (SPRX) receive the

echoes and pass them to the two beam-former and signal processing (BSPs) boards. The

heart of the EM1002 Multi-beam echo-sounder system is the beam-forming and signal

processing circuit boards, where multiple digital signals are processed, hence also called as

Digital signal processor (DSP) boards. It is designed to perform the beam-forming and

signal processing in sonars and multi-beam echo-sounders. It is controlled from a host

processor. The task of beam-forming and beam steering is done considering the sound

velocity probe values and the beam transmission through the entire water column is taken

care by considering the sound velocity profile values. The beam forming is done using time

delay technique. The beam-forming and signal processing boards are multi digital signal

processor boards, which have fixed point digital signal processors working in parallel while

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beam forming the data. In addition there are four floating point digital signal processors

interconnected to use both parallel and sequential processing of the beam-formed data. Once

the beam forming is done, the data is sampled and geo-referenced in the control processor

unit. The Beam-steering is fully taken into account when the position and depth of each

sounding is calculated, as is the sound speed profile effect on ray-bending. All the data

logged gets transferred to the main operator station through an Ethernet cable 10/100 base

Tx. Hence all the parameters are also get logged with the data pings on the hard-disk in the

operator station. The data stored is in the binary simrad90 (used to save the space and CPU

time required to store data) format. The helmsman display is used on wheel house, which

helps the operator to give necessary instructions to the sailor for the proper navigation. On-

board data quality check been done online in SIS software panel and also by post processing

the data through Neptune (cross-line, backscatter and sound velocity checks modules). In

SIS software the quality checks were carried out by visualizing the cross-track, beam-

intensity and quality values of the data and also the seabed image compared with the grids

developed on screen.

Simultaneously the attitude and heading data is also been checked for any errors in post-

processing software. As all the systems are connected to each other by means of an Ethernet

cat5 cable. Hence data can be accessed at any time for post- processing purpose. The

acquired raw data from each research cruise are archived on HDD and later on compact

discs (DVD - ROM). Further analysis and bathymetry chart preparations are done at shore.

Fig 2.5(e): Block Diagram of MBES System

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Multibeam system consists of the following parts:

2.5.7(a) Main switch box:

This switch box consists of the transmitter, receiver switches for getting the

signals transmitter and receivers. There are the two hard disks for helmsman display for

both the systems in the laboratory and in the bridge which is shown in Fig 2.5(f) - A

2.5.7(b) DGPS:

Differential Global Positioning System (DGPS) allows users to obtain maximum

accuracy from the GPS system. DGPS requires the use of two GPS receivers. One receiver,

known as the Reference Station, is placed at a surveyed location, the coordinates of which

are precisely known. The purpose of the differential GPS system is to use the reference

station to measure the errors in the GPS signals and to compute corrections to remove the

errors. The corrections are then communicated in real-time to the navigators, where they are

combined with the satellite signals received by the navigators, thereby improving their

navigation or positioning. Fig 2.5(f) - F shows DGPS.

Fig 2.5(f): Multibeam system

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2.5.7(c) Water Column Imaging Workstation:

Sea Beam 3012 is WCI ready, no extra installation is needed. The Water Column

Imaging (WCI) functionality is utilized via an additional PC workstation that logs WCI data

and displays real-time images of backscatter from the water column and sea floor, both

below and to the sides of the vessel. WCI shows in Fig 2.5(f) - I. The WCI workstation

connects to the SeaBeam 3012 multibeam system via Ethernet, and receives data for each

ping from the multibeam.

2.5.7(d) HD’S-1, 2:

Fig 2.5(f) - E and D shows the hard disks for helmsman’s display. It contains two

HD’s for helmsman’s display 1 and 2.one HD is for system in laboratory and another for

system in bridge.

2.5.7(e) Navigation workstation:

In multibeam system there are two workstations one is for navigation and another is

for post processing of data. The Fig 2.5(f) - G shows the navigation work station for

multibeam system. It acts as the CPU for the navigational data. It can be stored all the

information of navigational data.

2.5.7(f) Post processing Workstation:

After recording the data there is a system which is used to store entire required data.

For this it needs a CPU for this system. Fig 2.5(f) - H shows the CPU for the post processing

workstation.

2.5.7(g) Track plotter:

Track plotter is used to plot the data. Fig 2.5(f) - J shows the track plotter.

2.5.7(h) Operator station:

The operator station, a PC of latest technology, provides a graphical user interface on high

resolution TFT monitors for controlling the system using L-3 ELAC Natick’s HydroStar

ONLINE software. It communicates with the sonar electronics via Ethernet both for control

and reception of sonar data and performs the sound velocity correction, heave compensation,

navigation merging and data record construction. A variety of real-time data displays are

available for quality control.

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Fig 2.5(g): Display for multi beam system

Fig 2.5(h): Navigational display

2.5.8 MBES Survey:

In finding of the bathymetry by using MBES, we have three major steps, are

a) Data acquisition

b) Processing

c) Interpretation

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2.5.8(a) Data acquisition:

The sonar transducer emits acoustic pulses propagated inside a wide across-track

and narrow along-track angular sector. The receiver array directed perpendicularly to the

transmit array forms a large number of receive beams that are narrow across track and

steered simultaneously at different across-track directions by a beam forming process. Thus

the system performs spatial filtration of acoustic signals backscattered from different

portions of the seafloor along the swath.

Fig 2.5(i): Typical geometry of the transmit and receive beams of MBES

The reflected pulses are recorded by the receiver, and we can observe these reflections are

recorded by the Hydrostar software. We are recording these reflections until our survey

area to be completed.

Calibration before Survey:

For any system to perform within the accuracy, requires it to be well calibrated with

proper offsets. The correct calibration of the vessel attitude sensors and the time delay of

the positioning system is vital to the quality of multi-beam echo-sounder data. Reliable data

can only be acquired after proper calibration has been performed on the system as a whole.

This calibration begins with the alignment and static offsets of the sensors referenced to the

centre line of the vessel and the transducer. The alignment will reduce the static corrections

of each sensor and can be performed with either GPS receivers or a total station geodetic

instrument. After the static offsets are determined, a patch test is performed. This test is

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designed to reveal the following residual biases: pitch offset, roll offset, positioning time

delay, and azimuthal offset. The test consists of a small survey of several lines that are

evaluated for inconsistencies and then corrected using software designed for multi-beam

surveys. There are several mathematical equations developed for analysing these biases that

are incorporated into the processing software for the patch test. The performance test is the

final check of the offsets and biases to verify whether the data meets accuracy requirements

for the survey. This test is a series of parallel and cross lines with significant overlap to give

redundant data. With the improved resolution and coverage comes the need for much greater

control and calibration to ensure that the sounding is recorded from the correct position on

the seafloor (geo-positioning). This geo-positioning is accomplished by using a high

accuracy differential global positioning system (DGPS), heave-pitch-roll (HPR) sensor and

a gyrocompass. In addition, the time synchronization for all these components is critical.

For this reason, the system accuracy is comprised not only of the multi-beam sonar accuracy,

but also of the various components that make up the total system. This overall quality control

assessment must be performed in the field before the actual survey been conducted. The

field procedures necessary for proper calibration are the alignment of each sensor, the patch

test, and the performance test. These are discussed below:-

Roll Calibration: The roll calibration is normally done in the area having no features or

smooth area. So the area was selected and lines were run in opposite direction with 100%

overlap. Once done, by activating the calibration module, the swaths of both the lines were

imported and by using the numerical control panel, the values were determined for roll

offset.

Fig 2.5(j): Roll Calibration

Pitch calibration: The Pitch calibration is normally requires a distinct object (preferred)

or feature or a slope of maximum 20° for determining the offsets. The ship is made to pass

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over the slope in both the directions. Once finished with the line survey, the swaths of both

the lines are adjusted in the calibration module and the offsets are stored.

Fig 2.5(k): Pitch Calibration

Heading calibration: In heading calibration preferably an object is considered by the side

beams. Due to oblique angle of the transducer it is necessary to run the lines in opposite

directions to get an overlap. This makes the calibration dependent on time and pitch. And

these parameters must be solved first.

Fig 2.5(l): Heading Calibration

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Outer-beam calibration: For outer-beam calibration it’s been required that the two lines

are surveyed perpendicular to each other and the outer beam are then calibrated taking

over a reference of center beams on each lines. The test is normally done in smooth

surface or flat areas, where there is no feature. Before performing this test the roll, pitch

and heading factors are well calibrated.

Fig 2.5(m): Outer-beam Calibration

Survey Lines:

Before starting the survey we map the area with parallel lines with fixed space. The space

between the lines is decided by observing the measured swath range. If the swath range is

more then we increase the line spacing. If the swath range is less we decrease the line

spacing.

Patch Lines:

After surveying the area we process the raw data. If we find any data is missing of a

particular area we again do the survey of that particular area along some cross lines called

Patch Lines.

2.5.8(b) Data processing:

After getting the data it needs to process the data. In this data processing we are

using “Caris Hips and Sips 7.1” software. In processing initially we have to create vessel

file and then data must be imported. We can remove noise by applying corrections, this

processing increases the quality of the data because we are applying corrections to the data,

then we can get the bathymetry of our interested survey area.

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SVP Correction:

The speed of sound in water increases with rising temperature, salinity, and pressure

(depth), causing it to vary slightly from less than 1,500 meters per second to more than 1,600

meters per second at depths greater than 2,500 meters. Therefore sound velocity is applied

to the data as part of the processing routine. The accuracy of sounding data depends on the

measurement of sound travelling through water at the time of acquisition.

2.5.8(c) Interpretation:

Swath-bathymetry and acoustic backscatter data allow us to identify the main

morphologies and structures of the seafloor and determine its nature based on the acoustic

faces. We can identify more structures and morphologies in addition with gravity coring.

2.5.9 Specifications of MBES:

• In Sagarkanya “ELAC nautik Sea Beam 3012” multibeam system is used.

• It is a Full ocean depth multi beam system. It is using Frequency of 12 kHz which is

for deep ocean system.

• It performs Up to 11,000 m full ocean depth and 31,000 m swath coverage.

Table 2.4: Specifications of MBES

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2.5.10 Applications:

• It covers wide area of interest.

• To identify the main morphologies and structures of the seafloor.

• Geography of the bottom can be established.

• These charts are used to navigation of ships safely.

• In laying Trans-oceanic cables.

• They represent the most significant advance in mapping large areas rapidly and

accurately, and are essential for the study of geomorphology and seafloor faces.

• Combined with detailed positioning information (acquired through modern GPS

navigation systems) and advanced computer graphics, multibeam systems provide

us with a whole new view of the seafloor.

2.5.11 Demerits:

More expansive

If we want to find the depth of an exact point on the sea floor we can’t get it with

MBES. But we can find this using Single Beam Echo Sounder.

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2.6 REFERENCES :

Seafloor classification using a Single beam Echosounder, P.A.I. Brouwer

Manual of Single Beam Acoustic Depth Measurement Techniques

Acoustic Techniques for Seabed Classification, -J D Penrose, P J W Siwabessy, A

Gavrilov, I Parnum, L J Hamilton, A Bickers, B Brooke, D A Ryan and P Kenned

Multibeam Sonar TheSory of Operation,-L-3 Communications Sea Beam

Instruments

mbes_ taicheng report

Documents

single beam echosounders

multi beam echo sounder

mbes system

types of mbes

single beam echosounder

single beam transducers

mbes operation

multibeam mapping operation