SIDE SCAN SONAR

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SIDE SCAN

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TOPIC OUTLINES

• THEORY OF OPERATION

• SIDE SCAN SONAR GEOMETRY

• SSS RECORDS

• SSS OPERATING SYSTEMS

• INSTALLATION

• CALIBRATION

• SURVEY PREPARATIONS

INTRODUCTION

– Hydrography is not restricted to navigation but includes the description of the features of the seas.

– The introduction of sonar enables a more complete and detailed description for safer navigation and other uses.

– Major engineering advances in equipment used for underwater acoustic imaging were made during 1970’s and 1980’s

– Side scan sonar (SSS) has enable the geophysicists to quickly survey a large undersea area and to make a rapid qualitative investigation of the seafloor materials.

– The SS technique is similar to the aerial photography, except that acoustic beams are used instead of light beams. The system transmit and receive signal via transducer; transceiver.

– The active sonar system transmits sound and receives the returning echoes from objects.

– A key to the application of SSS is the proper control and manipulation of the instrument to gain the highest resolution and most accurate images.

– SSS although becoming easier to use, is still a fairly complex instrument, such as the integration of other equipment (DGPS) and subject to the erratic motion of the sea.

THEORY OF OPERATION

• SSS is used to produce images of the sea bottom, which in turn are used for geological investigations and the search for objects like wrecks, mines and pipelines.

• SSS is a method of underwater imaging, based on the principles of underwater acoustics.

• Typical uses of SSS include:-

– Object detection (mines, sunken ships, pipelines)

– Bottom classification (sediment type, rock, sand ripples)

– Inspection of underwater constructions (wellheads, oil pipes, bridges)

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• Side-scan sonar is similar to multibeam echosounding in that it covers a swath of the seafloor, except that a time-series of the strength of the return signal is measured, not the time of the first return.

• Hence, side-scan cannot measure depth but rather returns a grayscale image showing objects and shadows not unlike a photograph.

• In addition, the sound waves are usually transmitted and received from a "fish" towed close to the seafloor, like the one shown here, although they may be hull-mounted in shallow water.

Towing the fish closer to the seafloor allows a different perspective when surveying: one from near the seafloor that allows more of a side-ways profile.

SSS GEOMETRY

• Slant range

• Horizontal range – Can be calculated from

sonar height and slant range (pythagorean teorem)

• Maximum range – System setting, telling the

sss how far it should scan

• Insodified area – The total area insonified by

the sonar beam

The numbers on the diagram show...

1. Depth to inside of acoustic path.

2. Vertical beam angle.

3. Range setting in software (maximum acoustic range).

4. Swath width accross seafloor. 5. Tow depth of side scan sonar.

6. Port and starboard channel separation. 7. Horizontal beam width.

A Sidescan Ping • Water column is the blank

area in the sidescan record.

• Seafloor reflects at a typical amplitude.

• Normal incidence = strong return from target.

• Features on the seafloor block the signal and give no return – producing a “shadow” on the record

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Geometry of sidescan sonar and definitions of some basic parameters

Typical beam pattern of a sidescan sonar having a narrow beamwidth in the horizontal and a broad beamwidth in the vertical plane

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Side Scan Sonar Records Side Scan Sonar Interpretation

Sonar measurement

• A= Trigger pulse

• B= first surface return

• C= Sea clutter

• D= first bottom return

• E= Water column

• F= sunken fishing vessel

• G= shadow

• H= data channel

• I= System operational settings

• J= 25m scale marks

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Side Scan Sonar

Side Scan Sonar Components and working principles

SSS Components

• SSS components basically consists of three main parts, namely as:

Sonar tow fish

Electromechanical cable

Display/processing unit

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Side Scan Sonar Side Scan Sonar Components and working principles Side Scan Sonar Towfish • The SSS tow fish, a transducer, which consists of piezoelectric crystal has the property of

physically changing shape when a voltage is applied across it. • Separate port and starboard transducers are located on each side of the towfish

• The transducer converts the oscillating electrical field produced by the transmitter into

a mechanical vibration, and transferred into the water as an oscillating pressure, called the sound pulse.

• The sound travels away from the sonar transducer until it strikes something, such as the seafloor or a target in the water.

• Absorption reduces the strength of the outgoing pulse and the returning echoes due to

physical and chemical processes in the ocean.

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Side Scan Sonar Side Scan Sonar Components and working principles Side Scan Sonar Towfish • Absorption in the ocean is much more rapid than in fresh water.

• Other sound loss factors are beam spreading and scattering

• Some of the outgoing sound is reflected and returned back to the transducer.

• As the pulse travels out farther away from the towfish, it becomes more attenuated, scattered and

absorbed.

• The returning echoes from great distances are extremely low level and require very high amplification to normalize them by Time Varied Gain (TVG).

• The transducer converts the returned sound into electrical energy, and is further amplifier, before being recorded.

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Towfish Across-track swathe

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Side Scan Sonar Towfish

• The two-way-travel (twt) time is recorded that is the time taken for the sound to travel from the transducer to the target of interest and reflected back to the same transducer.

• The distance or depth could be determined using the known velocity of propagation of sound in the medium, which is in this case is the seawater.

• In other words, SSS system converts twt directly into the distance.

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Side Scan Sonar Cables and Connectors

• The towfish is connected to the tow cable by an underwater (wet) connector for

electrical connections.

• An armoured cable is used for deep operation or a reinforced multiconductor cable for shallow area.

• The cables are winch onto a winch drum and mechanically connected to the recorder.

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Side Scan Sonar Components and working principles Side Scan Sonar Recorder

• The control/display unit, which produced the graphic records, contains tuning controls for the system.

• Provides the operator with controls required such as gain levels, frequency, chart speed, and slant range correction for survey operations.

• Equipped with either electro-sensitive printing head or thermal printers • Has a variety of interfaces for keyboard, navigation inputs, video

displays, data storage and computer processing. • Receive data via the tow cable from the towfish. • The SSS recorder is the command component of the sonar system.

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Side Scan Sonar Components and working principles The main characteristics of the SSS are: • Sideways look: Sonar tansducers are located on both sides of the tow fish, which look at a series of

echoes across the sea bottom.

• Two channels:

As there are two sonar transducers, one looks at each sides of the survey vessel, thereby doubling the coverage.

• Narrow beam: Side scan uses a pulse, which is narrow in the horizontal plane and wide vertically.

• Towed body: Tow fish can be either in towing mechanism or fixed side mounted.

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Towfish and recorder

SSS Transducers

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EG & G towfish

A complete SSS system

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An “A” frame use during deployment

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Side Scan Sonar Side Scan Sonar Offshore Survey Operations

Deployment

• Cable out from the stern to the tow fish must be known for latter calculation and interpretation.

• The determination of the cable length out can be easily notified by marking the cable at 10m intervals.

• An “A” frame of the vessel with pulley can be used so that the cable will go through the pulley in order not to damage and kink the SSS cable.

• The SS cable is coiled into a drum with winch and normally welded at the centre alignment of the vessel deck.

• The surveyor needs to winch in and winch out the cable to the specific length of the cable paid out.

• This cable drum is linked to the processing unit in a controlled room.


Deployment: • Prior to deployment, check all shackles and fitting that attach the towfish to the

ship.

• For deployment and recovery of the towfish, the speed of the vessel should be reduced to 2-3 knots. Care should be taken to keep the towfish away from the propellers.

• A constant watch must be kept on the sonar trace to ensure adequate clearance between the towfish and the seafloor.

• Care must be taken during turns as the reduced speed will result in lowering of the towfish.

• If the towfish bottom clearance is inadequate, accelerate rapidly to raise the fish or to heave in on the towcable.

• The ship can neither stop nor turn rapidly without risk to the towfish and towcable.


Deployment:

Installation and deployment

System on deck


Installation

Winch, cable drum and “A” frame

Recorder unit in control room


Towfish Height and speed of survey vessel

• After the deployment of the tow fish, need maintain the fish height above the sea floor at the safe allowance height, normally at 10% of the range scale or 10 meters and above depending on the survey requirements.

• In general, for most work the optimum height of the towfish above the seafloor is 10% of the range-scale in use, i.e. on the 150 m scale the towfish should be 15 m above the seafloor.

• SSS transducers are directed slightly downwards so flying the towfish too close to the seafloor may reduce the range from which returns can be received.



• If the towfish is too high, acoustic shadows may not be formed behind obstructions making them more difficult to detect.

• This is especially true in deep water when a compromise has to be made between the need for getting the towfish down to a useful depth and maintaining a reasonable speed of advance.

• The higher the fish, the less resolution but the more scan range coverage (small range scale). The lower the fish height, the higher resolution but bigger range scale and less scan range.

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• The fish height also subject to the speed of the survey vessel and cable paid-out.

• The more the speed of the survey vessel applied on, the higher the fish.

• The longer cable paid-out the lower the fish level and vice-versa.

• Speed over ground of 4 to 7 knots is normal use. In fact for practical reasons the towfish should not be towed at speeds over the ground in excess of 8.0 knots, or small features will be missed, or 10 knots through the water since above this speed the towfish is liable to yaw.



• In areas of very high seafloor relief it may be prudent to tow the sonar higher than normal; in this event the reduction in acoustic shadow on features standing proud of the seafloor must be borne in mind. This effect is worst close in to the towfish where detection of small contacts is already at its most difficult.

• In shallow water it may not be possible to get the towfish as high off the seafloor as desirable. Although the recorder will be giving a background trace across the entire width of the paper, the sonar beam may not be ensonifying the entire range. Under these conditions the only solution is to reduce both the range scale and the line spacing.



• As a further limitation in shallow water the transducers may be very close to the surface with little tow-cable streamed. This will introduce the problem of surface noise (such as waves and ships wake) degrading performance and may also lead to the towfish being adversely affected by the motion of the ship.

• When investigating contacts with sonar, the towfish should always be sufficiently high above the seafloor to allow it to pass over the obstruction in the event of an accidental "on top". The least depth over a feature can usually be estimated initially from the shadow length obtained during the area search.



• If it becomes necessary to tow the towfish at a height other than the optimum, the towfish height can easily be controlled by a combination of wire out and ship's speed.

• Quickly heaving in a length of cable will "snatch" the towfish upwards rapidly, after which it will settle back down more slowly.

• This technique can be very useful in lifting the towfish over unexpected dangers.

• As the length of wire streamed increases this method becomes less effective.



• The 150 m scale is usually best (use of the 75 m scale may result in the shadow from a large contact extending off the trace). Speed should be kept to about 3 kt, to reduce distortions in the record, with the towfish about 15 m clear of the seafloor.

• The zone where small contacts may not be detected can be calculated for a given range scale in use and speed over the ground.

• Line spacing can then be adjusted so that sweeps from adjacent lines at least cover the gap. Alternatively, line spacing can be fixed and speed adjusted to ensure that full coverage is achieved.


Pre-Determine scan range and scale

• Across-track Scan range can be pre-determined depending on the object size to be search. The shorter the scan range the better resolution of the image.

• Say the analogue paper record width is 20cm, speed of sound 1500m/s and the twt is set to 1/10 sec.

• The range per channel can be calculated as follows:

D = (1/2) v t

D = (1/2) x 1500 x (1/10)

= 75

• Across-track scale = 10 : ( 75 x 100)

= 1 : 750

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Pre-Determine along-track scale

Fix lines with coordinates on side scan image



• Effective Sonar Range. The presence of marks on the sonar trace does not necessarily indicate that returning echoes are being received. Transmission losses, interference from other sources of noise, water conditions and recorder limitations all restrict the useful range of SSS.

• A maximum range of 270 m is about all that can be expected for even large wrecks, with small contacts (1-2 m) unlikely to be detected beyond about 120-150 m.



• Detection range varies between different SSS models and frequencies - the higher the frequency the less the detection range, although the resulting picture may be better.

• The best results will usually be achieved by restricting the range scale to 150 m to take advantage of the higher pulse rates and greater definition.


Single or dual Channel scanning

• SSS system can be operated in two ways: single channel and dual channels.

• Single channel is operated when only one side of transducers is set to transmit and receive signal. In this way either port or starboard transducer is set working.

• The advantage of using single channel is that the resolution of the image can be set higher as shorter range scale is used. This is a normal case in pipeline as-built survey.

• Dual channel SSS system is where both transducers are set to transmit and receive signal. In normal SSS survey, this system is fully utilized.


Tow Fish positioning and error

• The towfish (sub-towed transducers) is towed at some depth below the sea surface and some altitude above the sea floor.

• The transducers emit the sonar pulses towards the sea floor, reflect from a target and returns to the fish.

• Therefore the distance of the target on the seafloor is measured in relation to the position of the fish, which is positioned in relation to the reference datum point of the survey vessel.



• Position of the Sidescan Towfish: Towing the sonar transducers astern of the vessel has several advantages including removing the sensor from the effects of vessel motion and operating it at a height above the seafloor which will enable the optimum shadow.

• However, there is a disadvantage in that it also introduces uncertainty as to the position of the towfish.

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• This error has three components:

– an along-track component, caused by uncertainty in how far the towfish is astern of the vessel; this depends on the length of cable out, depth of towfish and vertical catenary of the cable (the last two also vary with the ship's speed);

– an across-track component, caused by deflection of the towfish by tidal stream or current, and by ship manoeuvres;

– errors in the position of the ship or boat, which will be transferred to the towfish.



• Towfish position can be determined using an Ultra Short Baseline (USBL) positioning system which requires transducers/receivers to be fitted in the vessel and towfish; however the accuracy of this system deteriorates rapidly depending on the length of tow.

• In addition, the attitude of the towfish may vary both longitudinally and about its axis and thus the direction of the transducer beams may fluctuate. This is especially true if the ship's course or speed are frequently changing.



• Hull Mounting: • SSS can be mounted in the hull of a surface vessel. • Advantages:

– its position and orientation are accurately known and therefore the positioning of detected features is relatively easy.

– Enables freedom of manoeuvre for the vessel which is no longer required to tow the sensor.

• Disadvantages: – the effect of vessel motion on SSS ensonification and performance, – interference with other hull mounted sensors, e.g. MBES, and – it is unlikely that the SSS will be operated at the optimum height above the

sea floor.


Tow Fish positioning and error • Hull mounting is often the best method when operating in shallow water or in

areas where the seafloor topography is potential hazardous, e.g. reef. • Under most conditions the towfish is largely decoupled from the effects of

ship's motion by the flexibility of the tow-cable. • The assumption is usually made that the towfish is completely stable in

roll, pitch and yaw, although some motion in all these planes undoubtedly occurs.

• Roll probably has relatively little effect on the sonar picture, being

compensated for by the wide beam angle in the vertical plane.

Side Scan Sonar Side Scan Sonar Offshore Survey Operations Tow Fish positioning and error • The problem of towfish stability is believed to be less important than that of

towfish position.

• In rough weather the effects of towfish oscillation can usually be clearly seen on the trace. Under these conditions the reduction in the probability of detecting small features must be considered.

Towfish Operating Height

• Typical operations call for the side scan to have a altitude of 8% to 20% of range scale off the seafloor.

• Ensures the best angle of incident (reflection of an object) for a sonar return.

Range Scale Example Optimum Fish Height

75 m 6-15 m

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Side Scan Sonar

Side Scan Sonar Interpretation

Measurements from Sonar Records

Layback: Layback is the distance astern of the navaid position that the towfish is assumed to

be.

Side Scan Sonar



In the normal course it can be computed as follows: Layback = DT + [WO2 - DS 2 ]

Where

DT = horizontal distance from fix point to tow point,

WO = amount of wire out from tow point, and

DS = depth of towfish below surface.

As previously mentioned, side-scan sonar can not accurately measure depth. The correct position and orientation of the fish is not well known due to several variables, including the length of cable out, bends in the cable, and pitch and roll of the towfish. However, the position can be approximated as shown in this diagram.

Side Scan Sonar



Geometry of Heighting from SSS.

One of the most important capabilities of SSS is its ability to enable the height of a feature to be measured from the length of its shadow on the sonar trace.

However, this capability depends on the SSS being operated at the correct height above the seafloor and selection of the optimum range scale.

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Side Scan Sonar



Geometry of Heighting from SSS. The geometry of heighting from SSS is shown at figure below:

Therefore, by similar triangles - H = S x h

R + S

H = height of the feature

S = length of feature shadow

R = slope range

H = height of towfish above seafloor

Side Scan Sonar



Geometry of Heighting from SSS.

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Side Scan Sonar


Interpretation from Sonar Records The horizontal dimension along the direction of the survey track:

The corresponding fix interval could be determined from the navigation data, and used to calculate the dimension of the sonar feature along the survey track.

What to look for inside a Sidescan Record

Water Column – information about towfish height. Target – features on the bottom with shadow can easily define a target and

height of object Bottom sediment and “classification”. Signal will differ based upon return

angle on incident. Differing bottom types will show up as light or dark returns. Easily group sections of the bottom for similarities

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While we tend to look at side-scan sonar images as pictures of the seafloor, what are we really looking at? As this schematic diagram shows , ridges and projections on the seafloor reflect more sound energy resulting in a brighter color. These features also leave a shadow zone which no sound energy reaches, so none can return. Knowing the height of the towfish above the seafloor and the length of the shadow, one can approximate the height of the object above the seafloor.

SSS INTEPRETATION Measurement – Height Above Bottom

• A = Towfish Altitude

• R = Range to far side of shadow

• L = Length of Shadow

• H = Target Height

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The side scan will provide the target height off the seafloor (not depth of object).

Simple Formula: H = L * A / R

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Typical display of recorded sonar data. Side Scan Sonar images

Boulders

Side Scan Sonar

11.0 Side Scan Sonar: Sonar Images

Connectors failure

Side Scan Sonar


Exposed cable

Side Scan Sonar


Exposed pipeline

Side Scan Sonar


Pockmarks

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Side Scan Sonar


Shallow gas

Side Scan Sonar


Structure

Side Scan Sonar


Pipelines and anchor scars

Side Scan Sonar


Pilings

Side Scan Sonar


Pipeline

Side Scan Sonar


Rock

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Side Scan Sonar


Wreck and shadow

Application of SBP

Seismic

Seismic sources

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Hydrophone

SIDE SCAN SONAR

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