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UNDERWATER ACOUSTICSUNDERWATER ACOUSTICS

• Basic Sonar PrinciplesBasic Sonar Principles• Sound Sound

Transmit/ReceiveTransmit/Receive• Speed of SoundSpeed of Sound• Spherical SpreadingSpherical Spreading• Absorption of SoundAbsorption of Sound• Refraction of SoundRefraction of Sound• Reflection of SoundReflection of Sound• Backscatter Backscatter

(Reverberation)(Reverberation)• Time-Varied GainTime-Varied Gain

• Noise (Ambient, Self Noise (Ambient, Self Made)Made)

• Sound in SedimentsSound in Sediments• Sonar EquationSonar Equation

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Basic Sonar PrinciplesBasic Sonar Principles

• SonarSonar ( (SOSOund und NNavigation avigation AAnd nd RRanging)anging)

• Active SonarActive Sonar

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Sound TransmitSound Transmit

• VibrationVibration

• FrequencyFrequency

• Source LevelSource Level

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Sound ReceiveSound Receive

• Type of MaterialType of Material

• Conversion to Conversion to electrical signalselectrical signals

• Pre-amplificationPre-amplification

• AmplitudeAmplitude

• FrequencyFrequency

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Speed of SoundSpeed of Sound

• 1500 meters per 1500 meters per secondsecond

• Affected by Affected by temperature, temperature, salinity & pressuresalinity & pressure

• Different for sea Different for sea water and sub-water and sub-bottombottom

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Spherical Spreading of Spherical Spreading of SoundSound

• Spreading lossSpreading loss

• When it hits the When it hits the sea bottom or sea bottom or surface, surface, spreading spreading becomes becomes cylindricalcylindrical

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Absorption of SoundAbsorption of Sound

• Vibrating Vibrating moleculesmolecules

• ViscosityViscosity

• ChemicalChemical

• Depends on Depends on Frequency of Frequency of SoundSound

High Frequency Sound

Low Frequency Sound

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Refraction of SoundRefraction of Sound

• Bending of a sound Bending of a sound wave towards a wave towards a region of slower region of slower sound speedsound speed

• Effects on imaging Effects on imaging the bottomthe bottom

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Reflection of SoundReflection of Sound

• Deflection of the Deflection of the path of a sound path of a sound wave by an object wave by an object or by the boundary or by the boundary between two mediabetween two media

• Acoustic properties Acoustic properties of the boundaries…of the boundaries…• Similar = less Similar = less

reflectionreflection

• Dissimilar = more Dissimilar = more reflectionreflection

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Scattering of SoundScattering of Sound

• Scattering affects Scattering affects the distance the distance sound can travelsound can travel

• Amount of Amount of scattering…scattering…• Size of scatterSize of scatter• Wave length of Wave length of

soundsound

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Noise (Ambient, Man Made)Noise (Ambient, Man Made)

• Traveling through the sea, Traveling through the sea, an underwater sound signal an underwater sound signal becomes delayed, distorted becomes delayed, distorted and weakened, reflecting and weakened, reflecting on boundaries of underside on boundaries of underside surface of waves, bottom surface of waves, bottom and shores, bubbles, and shores, bubbles, suspended particles and suspended particles and marine life.marine life.

• Tide, current, temperature Tide, current, temperature variances and wind also variances and wind also play on a sound's final play on a sound's final quality.quality.

• Man made noise can also Man made noise can also affect the resultsaffect the results

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Time-Varied GainTime-Varied Gain

• (TVG) accurately (TVG) accurately controlled controlled amplification amplification (gain) relative to (gain) relative to time after time after transmission, transmission, used to correct for used to correct for transmission loss transmission loss

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Sonar EquationSonar Equation

• EL = SL + TS - 2TLEL = SL + TS - 2TL

• Where EL (echo level) is the level of Where EL (echo level) is the level of the reflected sound, SL (source level) the reflected sound, SL (source level) is the level of the incident sound, TS is the level of the incident sound, TS is target strength, and 2 TL is two-is target strength, and 2 TL is two-way transmission loss due to way transmission loss due to spreading and absorption.spreading and absorption.

SUB-BOTTOM PROFILER SUB-BOTTOM PROFILER DESCRIPTIONDESCRIPTION

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What is a Sub-Bottom What is a Sub-Bottom ProfilerProfiler

• Seismic Seismic ReflectionReflection

• Sound SourceSound Source

• PenetrationPenetration

• HydrophonesHydrophones

• Time of TravelTime of Travel

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Typical Sub-Bottom Typical Sub-Bottom ConfigurationConfiguration• Tow VehicleTow Vehicle

– TransmitterTransmitter– HydrophoneHydrophone– Pre-AmpPre-Amp

• Tow CableTow Cable• AmplifierAmplifier• Display ProcessorDisplay Processor

– Video DisplayVideo Display– Mass StorageMass Storage– PrinterPrinter

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Uses of Sub-Bottom ProfilerUses of Sub-Bottom Profiler

• Geological SurveysGeological Surveys

• Geotechnical SurveysGeotechnical Surveys

• Hazard SurveysHazard Surveys

• Hydrographic Hydrographic SurveysSurveys

• Environmental Site Environmental Site InvestigationsInvestigations

• Sediment Sediment ClassificationClassification

• Cable And Pipeline Cable And Pipeline SurveysSurveys

• Archeological surveysArcheological surveys

• Mining & Dredging Mining & Dredging SurveysSurveys

• Scour & Erosion Scour & Erosion SurveysSurveys

• Deep Water Profiling Deep Water Profiling (Hull Mounted System)(Hull Mounted System)

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Sub-Bottom Profiler Record Sub-Bottom Profiler Record InterpretationInterpretation• Transmit (T0)Transmit (T0)

• Bottom EchoBottom Echo

• Sub-BottomSub-Bottom

• LayersLayers

• MultiplesMultiples

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Record Interpretation - Record Interpretation - LayersLayers

• DetectionDetection

• Grey Scale IntensityGrey Scale Intensity

• SeparationSeparation

• HiddenHidden

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Record Interpretation - Record Interpretation - Acoustic ShadowsAcoustic Shadows

• Hard ObjectHard Object

• Trapped GasTrapped Gas

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Record Interpretation - Record Interpretation - TargetsTargets• PipelinesPipelines

• Known ObjectsKnown Objects

• Unknown ObjectsUnknown Objects

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Record Interpretation - Record Interpretation - DistortionsDistortions

• Multi-PathMulti-Path

• Direct PathDirect Path

• Side EchosSide Echos

• Point Source Point Source ReflectionsReflections

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Distortions - Multi-PathDistortions - Multi-Path

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Distortions - Direct Path & Distortions - Direct Path & Water Surface ReflectionsWater Surface Reflections

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Distortions - Point Source Distortions - Point Source Reflections (Side Echo)Reflections (Side Echo)

Side Scan SonarSide Scan Sonar

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• SIDEWAYS LOOKINGSIDEWAYS LOOKING

• NARROW BEAM HORIZONTALNARROW BEAM HORIZONTAL

• WIDE VERTICAL BEAM WIDE VERTICAL BEAM

• TWO SIDESTWO SIDES

• TOWED BODY DECOUPLES SHIP TOWED BODY DECOUPLES SHIP MOVEMENT MOVEMENT

• TOW FISH IS BELOW SURFACE NOISETOW FISH IS BELOW SURFACE NOISE

SSS Features

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• AMBIENT NOISEAMBIENT NOISE

• SELF MADE NOISESELF MADE NOISE

• SEA SURFACESEA SURFACE

• BIOLOGICALBIOLOGICAL

• SURFSURF

• FLOWFLOW

• TERRESTRIALTERRESTRIAL

• MAN MADEMAN MADE

Ambient Noise Sources

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• OTHER INSTRUMENTSOTHER INSTRUMENTS

• SHIPS MACHINERYSHIPS MACHINERY

• FLOWFLOW

Self Made Noise Sources

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Sonar Performance

Thus: The resolution of a sonar is its ability to Thus: The resolution of a sonar is its ability to determine the presence of two small objects determine the presence of two small objects separated by a distance X. If the resolution of separated by a distance X. If the resolution of the sonar is greater than X, the 2 targets will the sonar is greater than X, the 2 targets will appear as one in the sonar image. If the appear as one in the sonar image. If the resolution is X or less the target should be resolution is X or less the target should be resolved into 2 distinct targets in the image. resolved into 2 distinct targets in the image.

For this purpose we use the following rule to For this purpose we use the following rule to determine if the targets will be resolved. determine if the targets will be resolved.

"The resolution of the sonar is twice the two "The resolution of the sonar is twice the two way 3 dB beamwidth.“ with conventional side way 3 dB beamwidth.“ with conventional side scan sonar systems.scan sonar systems.

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Data acquisition

Beam spreading

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Data acquisition

Target recognitionIn general, the side scan sonar system shall be operated so that it can detect an object on the sea floor that measures 1 m x 1 m x 1 m.

If the object in question is smaller than 1m x 1 m x 1 m the system should be operated in such a manner that at least three hits (pings) on a target is obtained. This is achieved by varying the speed and or repetition rate of the side scan system.

Speed The Surveyor should tow the side scan sonar at a speed such that an object 1 m on a side on the sea floor would be independently insonified a minimum of three times per pass. The 4200 Multi ping sonar allows for faster tow speeds due to the increased ping rates.

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Sonar Performance

The number of pings per second, or repetition rate, determines the speed at which the boat or tow fish can move along the track and still maintain the required coverage of the bottom. Longer sonar ranges have slower ping rates, which requires the vessel speed to be slower if the entire bottom is to be covered.

A minimum of 3 "hits" on a target at 100m range is the general standard used in target recognition and classification. Using this criteria we can find the maximum tow speed at which the conditions are met for 100m range for the 2 frequencies.

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Data acquisition

Bottom Coverage

Bottom coverage is the term used to describe the extent to which the bottom has been insonified by side scan sonar swaths, that is, the section of sea bed which has been insonified and logged along a track line. One hundred percent coverage is defined as an area insonified once, and two hundred percent coverage results in an area insonified twice.

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Data acquisition

There are three methods used to achieve the required There are three methods used to achieve the required coverage of an areacoverage of an area..

• Method 1. Method 1. Conduct a survey where the vessel track lines are Conduct a survey where the vessel track lines are separated by one-half the distance required for 100-percent separated by one-half the distance required for 100-percent coverage. coverage.

• Method 2. Method 2. Conduct two separate 100-percent coverages wherein Conduct two separate 100-percent coverages wherein the vessel track lines during the second coverage split the the vessel track lines during the second coverage split the difference between the track lines of the first coverage. Final track difference between the track lines of the first coverage. Final track spacing is essentially the same as technique 1. spacing is essentially the same as technique 1.

• Method 3. Method 3. Conduct two separate 100-percent coverages in Conduct two separate 100-percent coverages in orthogonal directions. This technique may be advantageous when orthogonal directions. This technique may be advantageous when searching for small man-made objects on the bottom as the searching for small man-made objects on the bottom as the bottom is insonified in different aspects. bottom is insonified in different aspects.

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Data acquisition

Horizontal Range Side scan sonar ranges refer to the range of the display on each side. For example, a range setting of 100 meters provides a 200 meter swath width.

What the sonar range should be set to is a function of several factors . such as sea state, thermoclines, sea bed composition, system characteristics,towfish altitude, tow fish (vessel) speed, size of a specific target being searched for.

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Side Scan range

Environmental Conditions:The acoustic environment can have a severe affect on

the operating range of a side scan sonar. There are 4 primary factors which may affect range performance:

1. ambient noise2. water temperature and salinity3. water depth4. sonar/towfish height above bottom (altitude)

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Side Scan range

Noise: The ambient noise level for sidescan sonar frequencies (100 to 500Khz typically) is mostly dominated by acoustic and waterborne electrical noise or interference from the vehicle platform itself.

The use of chirp technology with very well controlled receiver filter characteristics such as is used in EdgeTech's Full Spectrum systems can ensure that out of band system noise does not limit range.

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Side Scan range

Water Conditions: The water temperature and salinity can have a dramatic affect on range performance.Statements of outrageous or excessive range performance by a manufacturer may well be true under one of the extreme conditions that can be found in nature. The 2 way transmission loss (the total drop in intensity a signal undergoes traveling to and from the target) for a 410Khz signal is shown graphically in Figure 1for two extreme, but realistic conditions, of a warm tropical sea (Florida east coast), and a cold fresh waterlake.

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Data acquisition

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Side Scan range

The vertical scale is logarithmic, in dB (decibels) where every 20dB loss represents a ten fold weakening of the signal. A 120dB loss is therefore a million fold reduction and is a typical loss that can be tolerated bygood sonar systems.

A given sonar system with good range performance out to 120m in Florida waters, will achieve the sameimaging performance out to beyond 200m in the conditions of a cold fresh water lake !.

Water Depth: The effects of water depth are illustrated in Figure 2.

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Data acquisition

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Side Scan range

The beam shape shown in the previous slide is highly idealized but illustrates the essential concepts.The vertical beam shape of the sonar inevitably has some response to echoes returning from the surface. (surface reflection)As the ratio of depth (D) to Range (R) gets increasingly smaller either by way of shallower water or longer range, the masking effects of the surface backscattered echoes become more pronounced resulting in adecrease in range performance. In shallow waters this effect is pronounced in the lower frequency ( hence longer range) systems.It is worth noting that the frequency dependant loss experienced by the sound is weakly dependant on pressure (depth), and at extreme depths (>2000-3000m) longer ranges can be obtained.

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Side Scan range

Altitude: The backscattering strength of the sea floor (intensity of initial echoes) is a strong function of the grazing angle, or the acute angle between the line of sight to the sea bottom and the sea floor itself. Atlonger ranges/lower altitudes, this angle can become extremely small and all incident energy is reflected away from the source, and not back to the sonar. Thus maximum range is also limited by the altitude torange ratio. A typical and accepted value for Altitude/MaxRange for this purpose is 1/10. ( D/R in Figure2)Detection rangeThe meaning of "maximum range" itself is somewhat arbitrary and may vary from manufacturer to manufacturer, Maximum Range may be given to mean the ability of the operator/observer to see the echo of a large target(ship wreck or similar) above the obscuring noise, or, at the other extreme, the ability of the user to useshadows in the image to aid in target identification.The difference in the "maximum" range as defined by these two conditions for the same sonar may be as great as 30 to 50 % of stated range.

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Side Scan range

EdgeTech's statement of maximum range for a system is given for the following general and typical conditions:

Full Spectrum Sonar Systems.Expected Operational Ranges, depending on Water temperature and salinity. (See Figures 3 and 4).The absorption factor is estimated based on a model from Francois and Garrison, JASA 1982, and a depthof 50m.

Freq : 75kHz, Range: 700 to 800m. 1000m is possible at extreme depths and with special pulsesFreq : 120Khz, Range: 250 to 500m.Freq : 270kHz, Range : 150 to 300mFreq: 410kHz, Range: 130 to > 200mFreq : 540kHz, Range: 100 to 150m

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Side Scan range

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Side Scan range

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4200-FS system

Measuring targets

The height of an object off the sea floor can be determined using the formula, Ht=(Ls×Hf) ∕ R

DSCOVER Target logger software allows for computer aided mensuration of targets.

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4200-FS system

Measuring targets

Towfish height(Hf)

Range to end of shadow (R)

Height ofTarget (ht)

Length of shadow (Ls)

Ht=(Ls×Hf) ∕ R

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Discover Screen

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Measuring targets

Using TARGET loggerDiscover software

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Target logger Screen

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Target logger Screen

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Discover Screen

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