Principles of Sonar & Sonar Signal Processing

Acoustics for Underwater applicationsDr. G. A. Ramadassramadass@niot.res.inNIOTChennai1Under water soundWhy so much noise about sound ?E M waves suffer heavy losses in the waterRange Hardly a few metresSound wave is a mechanical wave Travels 100s of kilo metersIt is the only radiation that can travel long ranges in water2Lead In: Go over the basic concept of Underwater sound and detection by another ship, sub, aircraft

Two ways to derive information from underwater sound:Active => send out sound wave and listen for it to return (analogous to radar); get a PING off the sub

Passive => listen for sounds of other vessels (analogous to passive EM detection); listen for the sounds of the subTechnological Applications of Underwater Sound

Geophysical measurements-Echo SoundersDepth Profiling-Multibeam SonarsAcoustic Imaging Systems-Sidescan Sonars-Acoustic camerasGeotechnical measurements-Sub bottom Profiler, Boomer, SparkerOceanographic measurements -Current meters/Profilers3

Technological Applications of Underwater Sound

Obstacle AvoidanceUnderwater Acoustics Positioning - Long Base Line (LBL)- Short Base Line (SBL) - Ultra Short Base Line (USBL)Underwater Communications- Under Water Acoustic ModemsUnderwater Surveillance 4Light vs. SoundLightSoundEM WaveMechanical WaveDoes not need mediumNeeds mediumVelocity 3 X 108 m/s330 m/s in air1500 m/s in waterTransverseLongitudinal FluidsCan be polarizedCan not be polarizedTravels millions of kM in airTravels only tens of kM

Travels only metres in waterTravels hundreds of kM in waterEasy to focusDifficult to focusBandwidth very highBandwidth very lowLight Vs. Sound

Under water soundHow does it travel?Sound is mechanical waveIt is longitudinal wave in water and in air

UnitsFrequency 1 Hertz (Hz) = 1 cycle per second - a measure of frequency1 kilo Hertz (kHz) = 1,000 Hertz

7Lead In: Go over the basic concept of Underwater sound and detection by another ship, sub, aircraft

Two ways to derive information from underwater sound:Active => send out sound wave and listen for it to return (analogous to radar); get a PING off the sub

Passive => listen for sounds of other vessels (analogous to passive EM detection); listen for the sounds of the subSound Air vs UnderwaterAirWaterVelocity330 m/s in airVelocity 1500 m/s in waterTravels tens of kM in airTravels hundreds of kM in waterLow Impedance High ImpedanceLess dispersiveMore dispersiveRef pressure 20 PaRef pressure 1 PaReference Level ConventionsLocationReference IntensityReference Pressure*Air1 x 10-12 W/m220 mPaWater6.67 x 10-19 W/m21 uPa

Difference is 20 log10(20) i.e 20*1.3 =26 dB9Underwater hearingCan human beings hear underwater? Do we use our ears? The eardrums that connect the outer to the inner ear are too soft to be able to pick up sound underwater. We hear underwater sounds through a phenomenon called 'bone conduction'. The neck and skull bones are hard enough to pick up the sound, which is then transmitted directly to the inner ear, which is filled with fluid. Underwater, the outer and the middle ear (filled with air) are bypassed.

Concepts of SoundThree (3) elements required for this to workSourceMediumDetector (Receiver)The source VIBRATES causing a series of compressions and rarefactions in the medium11 Source: Vibrates like a speaker in your stereoMedium: Need a transmission medium, particles excited to vibrate. The particles in the medium move! But not much! KINETIC ENERGY OF PARTICLES IN MOTION!Detector/Receiver: HEAR the return

Compression: Higher PressureRarefaction: Low PressureIf students are having a hard time with this concept, consider a series of balls, each connected by a spring. Move one end, and they all will through a series of compressions and rarefactions. See page 178.

Relationship between Frequency and Wavelength is the same as EM wavesUnderwater soundPressure1 Pascal (Pa) - a measure of pressure1 microPascal (Pa) = 10-6 Pa - commonly used when measuring sound pressures1 atmosphere (atm) = 10 332.275 548 kg-f/sq.m= 101,325 PaIntensity Watts/ sq m

I = (p2/c)a

12Lead In: Go over the basic concept of Underwater sound and detection by another ship, sub, aircraft

Two ways to derive information from underwater sound:Active => send out sound wave and listen for it to return (analogous to radar); get a PING off the sub

Passive => listen for sounds of other vessels (analogous to passive EM detection); listen for the sounds of the subUnderwater soundPressure is analogous to Voltage P ~ V Particle Velocity ~ Electric CurrentOhms law V = IRUnderwater sound P = z v where v is particle velocity z = c where is density & c is sound velocityMeasurements of SoundSound related quantities are measured on logarithmic scale (dB), because sound pressure and intensity vary over 12 orders of magnitude

Mean distance between the Sun and the Earthsize of a pocket rulerIntensity at threshold of painIntensity at threshold of hearing 14Multiplication to Addition Let a = 100000000b= 1000000000a*b= 100000000000000000 = 1017

log10 (a) = 8log10 (b) = 9log10 (a*b) = log10 (a) +log10 (b) = 8+9 =17

15Decibel scaleEars judge loudness on a logarithmic vice linear scale

Sound Pressure Level (dB) = 20 log (p/pref) where pref is the reference pressureSound Intensity Level (dB) = 10 log (I/Iref) where Iref is the reference intensityI p2thereforeSIL(dB) = 10 log (I/Ir) = 10 log (p2 water / p2 ref-water) = 20 log (pwater/1Pa)In other words, once we start using the decibel scale, SIL and SPL are pretty much the same thing.16Reference Level ConventionsLocationReference IntensityReference Pressure*Air1 x 10-12 W/m220 mPaWater6.67 x 10-19 W/m21 uPa

Difference is 20 log10(20) i.e 20*1.3 =26 dB17UnitsReference unit of intensity (I0) in underwater sound:The intensity of a plane wave with rms pressure equal to 1 PaIn physical units: .67x10-22 W/cm2Acoustic intensities usually referred to in terms of decibels (dB)XXX dB re 1 Pa = 10log10(I1/I0)Multiplications of intensities are additions of decibel levels18Sound Speed

AirWater SteelBulk Modulus1.01 x 105 Pa2.2 x 109 Pa ~2.5 x 1011 Pa Density1.21 kg/m31000 kg/m3 ~104 kg/m3 Speed343 m/s1500 m/s 5000 m/s19Transducers for underwater applicationsHydrophone- equivalent of microphone in airTransmitter- equivalent of speaker in airTransceiver- underwater transmitter cum receiverTransponder- replies to an underwater commandResponder- replies to command through cables20Conversion Between Electrical and Acoustic EnergyTransductionPiezoelectricity and magnetostrictionReceiving responseXXX dB re 1 V/PaTransmitting responseXXX dB re 1 Pa/V @ 1 mSource level (SL) specified in dB re 1 Pa @ 1 m Efficiency: E=Pa/Pe (usually .2 listen for sounds of other vessels (analogous to passive EM detection); listen for the sounds of the subDeterminants of Underwater DetectionTransmission Loss (TL)Noise Level (NL)Self Noise (SN)Ambient Noise (AN)Source level (SL)Directivity Index (DI)Target Strength (TS)

30SL: :- sound level of targets noise source.TL: Transmission Losses: (reflection, absorption, etc.)NL: Noise Level: (AN+SN)DI: Directivity IndexDT: Detection Threshold

What is Ambient noiseIt is the prevailing, sustained unwanted background noise in the ocean, typical of the location & depth against which a signal such as the sound of a submarine or the echo from a target must be detected Excludes

All forms of self noise such as the noise of the platform on which the measuring system is mounted eg. Ship, buoy etc and all forms of electrical noise

31Ambient Noise in OceansOceans are inherently noisy. In some places, the sound levels can be comparable to that of a busy traffic junction at peak hours !

32Need for characterization of underwater ambient noiseAmbient noise because of its masking the signals received in the underwater acoustic systems, such as the sonar, echo sounder, etc limits their useful range.

To enhance the signal to noise ratio of acoustic instruments the ambient noise field must be detected33Noise Levels in OceanNosie Source Maxiumum Source LevelUndersea Earthquake255 dBSeafloor Volcano Eruption 250 dB+Lightning Stike on Water Surface 250 dBSeismic Exploration Devices 212-230 dBContainer Ship198 dBBlue Whale 190 dB (avg. 145-172)Open Ocean Ambient Noise 74-100 dB (71-97 dB in deep sound channel)34Spectral Classification of ambient noise Ultra low band (< 1 Hz), Infrasonic band (1Hz to 20 Hz )Low sonic band (20Hz to 200Hz)High sonic band (200Hz to 50,000Hz)Ultrasonic band (> 50 kHz)High sonic band (200Hz to 50,000Hz)35Sources of noise in the oceanGeophysical Seismic waves, rain, hail and snow, hydrostatic and hydrodynamic sources such as bubbles, waves and wind turbulence.Human made Ship traffic, Coastal of off shore activity, aircraft flying over the sea.Biological sources Marine speciesThermal noise due to bombardment of molecules.

36Sources of NoiseSourceFreq. Band

Tides /Swell (internal waves) 100kHz

37Sources of Noise.Biological sourcesFreq. Band

Marine Mammals 10 300 HzFish 100 300 HzCrustaceans 0.1 10kHz

Industrial

Oil / Gas RIGS 10 100 HzOff-shore exploration10 100 Hz Distant ship traffic 10Hz 1kHz 38Blade-rate frequency of propeller driven vesselsShip noise Infra sonic band (1 to 20Hz)Low sonic band (20 to 200 Hz) Noise of distant shipping

Man made activities other than shippingHigh sonic band (200 to 50KHz) Noise due to wind dominates in this region

Noise due to heavy rain39Ambient Noise in Oceans

40Ambient noise at different depthsDeep water behavior more predictable than shallow water oneIn shallow water reverberation sets the limitIn noisy boats their self noise sets the limitAs the underwater warfare is shifting to shallow waters water studies have become more important Boom in the off shore operations is leading to noise pollution in the shallow watersIts effect on environmental aspects needs to be studied

41Noise Level (NL)NL = SN + AN42Source LevelPassive Generated by the targetBroadband Propeller, Flow Noise, Propulsion SystemNarrowband Pieces of Machinery on targetPumps, Electrical Generators, MotorsActiveNarrowband pulse generated by SONAR.ReverberationCaused by producing sound. Medium pushes back43Directivity IndexAbility of receiver to look in one direction and at individual frequencyIncreases the likelihood of separating the desired sound from the Ambient and Self Noise.44How do we see underwater?Detect the reflected SIGNAL (Active)Detect Submarine produced Signal (Passive)SONAR (Sound Navigation Ranging)SONAR equationsLook at losses compared to signalProbability of detection45Signal, Noise, Hmmm, Signal to Noise ratio?

Signal to Noise Ratio (SNR)Same as with RADAR. The ratio to the received echo from the target to the noise produced by everything else.Detection Threshold (DT)The level, of received signal, required for an experienced operator to detect a target signal 50% of the time.S - N > DT461. Signal-to-Noise ratio or SNR is essentially the same as with radar. - High SNR is good.

2. Detection Threshold (DT). - Go over definition. - What this means is that if a target is out there the detection threshold is the SNR level when an operator will detect the target 50% of the time. (see the targets return over the noise). - The DT is a function of equipment and the operator.

3. Bottom line: To achieve a detection with a specified degree of probability, the signal minus the noise must be equal to or greater than the detection threshold.

S - N > DT (all in dB)

This is the foundation for all sonar equations to predict performance.Passive Sonar Equation

SL - TL - NL + DI > DTSL: Source level:- sound level of targets noise source.TL: Transmission Losses: (reflection, absorption, etc.)NL: Noise Level: (AN+SN)DI: Directivity IndexDT: Detection Threshold47{Pg. 217, Fig 8-11 Old Book}1. The Passive Sonar Equation represents the ability to detect a target without using active sonar. Just listening for the noise generated by the Target itself.2. From the basic equation of: S - N > DT a. . The Source noise level (S) is the Source Noise Level (SL) minus the sound losses due to the water environment (reflection, absorption, scattering , etc.). These losses to the environment are called Transmission Losses (TL) b. The Noise is the self noise (noise from own ship) and the environmental noise. Together they are the noise level (NL) c. The Noise portion of the equation is further modified by what is call the Directivity Index (DI) . DI comes into play because the sonar can look in specific directions rather than just 360 degrees. If you are looking the direction of the target you have a better chance of seeing it so the DI increases the Detection Threshold. (it is positive) (1) Note that N = NL - DI and noise is always positive so DI can never be more than the Ambient Noise Level (NL).SLTLNLDIDTSonar EquipmentSL-TL-NL+DI=DT48Recall from previous slide:

From the basic equation of: S - N > DT a. . The Source noise level (S) is the Source Noise Level (SL) minus the sound losses due to the water environment (reflection, absorption, scattering , etc.). These losses to the environment are called Transmission Losses (TL) b. The Noise is the self noise (noise from own ship) and the environmental noise. Together they are the noise level (NL) c. The Noise portion of the equation is further modified by what is call the Directivity Index (DI) . DI comes into play because the sonar can look in specific directions rather than just 360 degrees. If you are looking the direction of the target you have a better chance of seeing it so the DI increases the Detection Threshold. (it is positive) (1) Note that N = NL - DI and noise is always positive so DI can never be more than the Ambient Noise Level (NL).

Active Sonar EquationsAmbient Noise Limited:Reverberation Noise Limited: (Reverb > ambient noise)RL > NL+DI SL - 2TL + TS - NL + DI > DT SL - 2TL + TS - RL > DTTS: Target Strength, A measure of the reflectivity of thetarget to an active sonar signal. 491. Just like the passive sonar, there are equations that describe the performance of active sonar.

2. Same basic equation applies: S - NL > DT except for:

a. Involves the signal traveling twice the distance. (to the target then back) So the Transmission losses are double (i.e. 2TL)

b. TS: Target Strength factor is added. Target strength is the amount of the active sonar signal that is reflected back towards own ship.

c. Involves two equations depending on what noise is the most limiting. (1) If the ambient noise the the most limiting than use equation 1. (2) If the reverberation noise becomes stronger than the ambient noise, then must use the second equation.- RL: difficult to quantify. A time-varying function resulting from the inhomogeneities in the medium If the noise level or reverberation level is too high, you wont detect the target

Lead in: So what good is all of this?SL2TLNLDIDTSonar EquipmentSL - 2TL + TS - NL + DI > DTTS50Figure of Merit (FOM)FOM = the maximum allowable one-way transmissionloss in passive sonar, and the maximum two-way trans-mission loss in active for a detection probability of 50%. PFOM = SL - NL + DI - DTAFOM = SL + TS - NL + DI - DT511. We cant rate acoustic gear by range so we use Figure of Merit (FOM).2. FOM is the ability of a SONAR to detect a level of sound energy.3. To get the equations just move DT to the other side of the equation.4. ASK : From the equations, how can we improve the FOM? a. FOM improves by increasing the source level (1) Increasing transmitted power for active sonar (2) Finding a noisier target for passive sonar b. Improves by decreasing the ambient noise level. c. Improves by increasing the Directivity Index (DI) d. Improves by decreasing the detection threshold (DT)

5. FOM is used for measuring sonar capabilities. It is a key tool for deciding detection probabilities and estimated range of detection (if propagation losses are known).6. Lead in: That is the key. If we can understand and measure the propagation losses of the sound received by the sonar then we can predict the range at which we will detect a target. We previously stated that many things affect the the loss of sound. Those included spreading, scattering, absorption. a. The speed of sound in the water and the path the sound takes also greatly affect the propagation of the sound energy.Factors that affect Sound WaterTemperaturePressureSalinitySound is lazy. Sound bends towards areas of slower speed

52Speed of sound in water-temperature, pressure, and salinity

53Sound Transmission in WaterDepthDepthDepthSalinityPressureTemperature Salinity Pressure TemperatureVariable Effects of:54Pg 183 in book1. The speed of sound in water is determined first by the water itself. a. The elasticity of the medium (for compression and expansion of the sound energys longitudinal wave) is the most important factor in determining the speed of sound b. The effect of medium density is also very important.2. In addition to the normal density of water, there are several factors which can cause the density of the water to change. They are salinity, pressure and temperature.3. Salinity (has the smallest affect on sound speed) (an incr in 1 pp thous = an incr in spped of 1.3m/sec) a. As salinity increases the sound speed increases. b. Salinity can be a big factor near rivers. c. Salinity increases with depth4. Pressure Biggest factor below ~1500ft (Every 3 feet of depth = 0.017 m/sec incr in speed) a. As pressure increases sound speed increases b. Pressure increase is constant and predictable5. Temperature (The major factor affecting sound speed above 1500 feet) (The warm spot in the POOL) a. Below 1500 feet temperature of the ocean is constant, roughly 34F. b. One degree Celsius increase in temperature will change water speed by 3 meters/sec.Typical Deep Ocean Sound Velocity ProfileDepth of Water (meters)Speed of Sound (meters/sec)150015201480100020003000Surface LayerSeasonal ThermoclinePermanent ThermoclineDeep Isothermal Layer55Pg 183, Fig 15-41. The graphic shows the typical deep ocean sound speed profile. It is the summation of the effects of destiny, salinity, pressure and normal temperatures.2. Surface layer: - Sound speed is susceptible to daily and local changes in heating cooling and wind mixing action.3. Seasonal Thermocline: - Thermocline is a layer where the temp. can change rapidly with depth. - Seasonal can change with the seasons. (wind and storms can cause mixing of water temperatures)4. Permanent Themocline - Affected only slightly by seasonal changes.5. Deep Isothermal Layer Sound speed is constant - Has nearly a constant temperature (about 4 degrees C)6. Ocean Currents can create an unexpected thermal layer - This layer can trap sound waves and let the sound travel further.7. Ocean fronts are boundaries of large masses of different temperature water (like weather fronts). These can cause large horizontal gradients of Temperature and pressure.

Ray Propagation Theory The path sound travels can be depicted as a RAY or VECTOR

RAYS will change direction when passing through two mediums of different density. REFRACTION!

Snells Law!!!!!

Sound bends TOWARDS the region of SLOWER sound speed.

Really valid only at high frequencies 56Why do we care about these layers? The answer lies in ray propagation theory and how sound behaves when its speed is changed.

1. Just like electromagnetic waves we can depict the travel of the sound wave as an arrow or ray. This will show how the wave front will travel.

2. Snells law say the when rays pass through different mediums (different densities) then the ray will bend when passing through the interface. a. In water the change of densities occurs gradually so the ray is bending slightly all the time. b. The effect is that the ray appears to bend. c. The more the difference the more the bend.

3. Sound rays will always bend towards the area where the speed of sound is slower.

4. Can explain bending by the following example: If have a curved surface (wave front) and the bottom of the surface moves slower than the top the surface will have a torque on it which will tend to turn the curved surface towards the slow speed area.ISOVELOCITYRangeMaximum Echo RangeDepthTransducerTemperature57Pg 186, Fig 15-8

1. Isothermal water is where the water temperature is essentially constant.

2. Rays will travel is a straight line in these waters (very little bending)Negative GradientDepthWater WarmShadow ZoneWaterCoolSound Bends Down When Water Grows Cooler With DepthDepthDirection of IncreasingTemperature and VelocityNegative Gradient Thermal StructureTC581. Negative gradient profile is when the temperature of the water (and sound speed) decreases with depth.

2. Rays will tend to bend DOWN.

3. This is common near the surface where the sun heats the shallower water.

4. Sound rays can bend so much that a shadow zone exists where the sound can not get to that area.

Shadow zone : Blind spot where you cant hear any return..Positive GradientWater CoolShadow ZoneWater WarmWhen Temperature Increases withDepth, Sound Bends Sharply UpDepthDirection of IncreasingTemperature and VelocityPositive Gradient Thermal StructureTC591. A positive gradient is when the water temperature (and sound speed) increases with depth. 2. Sound in a positive gradient will tend to bend up.3. Sound will reflect off the surface, good for longer ranges unless the sea is rough, then we get scattering.

Lead in:

4. Unfortunately the sound velocity or temperature curves are not straight lines but are combinations of isothermal and positive and negative gradients.

Layer DepthTemperatureCoolShadow ZoneIsothermalSound Beam Splits When Temperature IsUniform At Surface and Cool At BottomDepthDirection of IncreasingTemperature and VelocityIsothermal Gradient Thermal StructureTCDepth601. Layer depth phenomenon - When there is a layer of isothermal water over water with a negative gradient. - The speed of sound is maximum at the boundary.

Layer depth is the depth of the greatest sound speed above the seasonal Thermocline.Shadow Zone p.188 fig. 15-12

Sound ChannelWater CoolShadow ZoneWater WarmDepthDirection of IncreasingTemperature and VelocityNegative Gradient Over PositiveTCDepth61Sound Channels - Negative gradient over a positive gradient. - Tends to refract rays back and forth essentially trapping the ray in the channel - Rays in a sound channel can travel great distances

Surface Duct is a sound channel near the surface. - Just below surface and is susceptible to daily and local changes of heating, cooling and wind direction. - These are rare and not stable.Convergence Zone (CZ)

3-4 degCTDEAP BOTTOM DEPTH ESSENTIAL!62Convergence zone (CZ) p.190 fig. 15-14 - Negative gradient over a positive gradient in extremely deep water (so rays bend totally before striking the bottom. - Temperature decreases and the sound bends down when deep pressure causes the beam to go back up. - About 50 km the beam hits the surface and refracts back towards the bottom. - Can be multiple CZ zones. - Can result in very long range detection's.HistorySound Navigation And Ranging (SONAR) developed during WW IISound pulses emitted reflected off metal objects with characteristic pingLike radar and time of flight is measured to determine distance echo ranging Early sonar gave only distance and direction to targetModern sonar used for mapping63

Present Technological Applications of Underwater Sound

Underwater Survey Systems Single Beam Echo sounder

Multibeam Echosounder

Sidescan Sonar

Sub-bottom profiler

64PrinciplesIn environmental work there are three kinds of sonarSide-scanUsually in the range of 100 kHz - 400 kHzSingle beamUsually at 12 kHz - 200 kHzMulti-beam Usually at 12 kHz - 200 kHzLower frequency = longer range and reduced spatial resolution65Single Beam SonarUsed primarily for mapping channels and for engineering applicationsUses only a single sourceDoes not produce much data, because only a single point for each pulse under the transducer66EchosounderAn echosounder sends out a sound pulse and waits for its echo reflected from the seabed. The velocity of sound is a known quantity and hence the time elapsed is a measure of depth.

67More about Echosounding

68

More about Echosounding69

More about Echosounding70Multi-beam SonarSensor uses an array of sources and receiversEnergy focused on a narrow strip beneath the shipMultiple echoes give a profile of depths along a strip perpendicular to ship trackSwath width depends on depth Usually 2-4x water depth71Multibeam Echosounder

72

Multibeam EchosounderMBES can increase the coverage to as much as 240 beams (profiles) as against one profile of conventional echosounderLarge areas can be surveyed with full coverage in much shorter timeRequires much more survey hardware and softwareNot suitable for very shallow area as the swath is dependent on depth

73Comparison of SBES & MBES

74

75

Tide .CDEchosounding includes water column caused by tidal variationHence tide is observed during sounding operation and raw soundings are reduced to Chart DatumMSL76Side Scan SonarDF-1000 Towfish

Records tonal variation of reflected signal from objects / seafloor The tonal variation depends on material characteristicsIt is possible to qualitatively judge the sea bed composition sand/rock/clay or objects ship wreck, pipeline, structures

Normally towed behind the survey vessel to avoid vessel noiseUsed in seabed engineering investigations, geological mapping and search operations

5077Side Scan Sonar

Track of Tow Fish78

Side Scan Sonar79Side Scan Sonar Sample of Ship Wreck

80Sub-Bottom Profiler

Sound waves travel into the seabed and get reflected at different depths depending on their frequency and material nature. This property is used in SBP to investigate the sub-bottom layers and formation details81Sub Bottom ProfilerSB-216S

Towfish ModelSB-424

SB-216S

SB-0408

SB-0512

Towfish ModelSB-424

SB-216S

SB-0408

SB-0512

Boomer and Sparker are used for sub-bottom profiling. They use very low frequency and High Energy sound waves to accomplish the sub-bottom penetration.The later types known as chirp sonar produces a pulse sweeping over a band of low frequency (eg. 0.5 to 12kHz). The reflection of signal is a function of frequency used. Different layers of seabed respond to different frequency component of outgoing signal. The reflection of each component corresponds with a layer change. 82

Sub Bottom Profiler Sample record83Sample Sub-bottom profile

84Under Water Acoustic Positioning SystemsThe distance between acoustic baselines is generally used to define the type of system:Ultrashort / Super Short Baseline (USBL/SSBL):