introduction to sonar - lesson 5, inf-geo4310 · introduction to sonar lesson 5, inf-geo4310 roy...

Introduction to SonarLesson 5, INF-GEO4310

Roy Edgar Hansen

Department of Informatics, University of Oslo

October 2009

RH, INF-GEO4310 (Ifi/UiO) Sonar Oct. 2009 1 / 70

Outline

Outline1 Basics

IntroductionBasic PhysicsUnderwater sound

2 Sonar TheorySonar typesPosition EstimationSignal processing

3 Sonar ApplicationsFish findingHUGIN AUVImagingMapping

4 Summary


Sonar Basics

Outline1 Basics




4 Summary


Sonar Basics Introduction

History

Sound Navigation And RangingIf you cause your ship to stop and placethe head of a long tube in the water andplace the outer extremity to your ear, youwill hear ships at a great distance fromyou.

Leonardo da Vinci, 14901914 Fessenden: first active sonar system(detect iceberg 2 miles)

Images from wikipedia.org.



The masters in sonar

From wikipedia.org.



Similar technologies

SONAR = Sound Navigation And RangingRADAR = Radio Detection And RangingMedical ultrasound, higher frequencies, shorter range and morecomplex mediumSeismic exploration, lower frequencies, more complex medium


Sonar Basics Physics

Basic Physics

Sound is pressure perturbationsOr: compressional wave, longitudal wave, mechanical waveThe acoustic vibrations can be characterized by

I Wave period T [s]I Frequency f = 1/T [Hz]I Wavelength λ = c/f [m]I Sound speed c [m/s]


Sonar Basics Underwater sound

Underwater sound

Acoustics is the only long range information carrier under waterThe pressure perturbations are very smallObtainable range is determined by

I free space loss and absorptionI the sensitivity to the receiver

The ocean environment affects sound propagation:I sea surfaceI seafloorI temperature and salinityI currents and turbulence

Underwater sound propagation is frequency dependent



Geometrical spreading loss - one way

The acoustic wave expands asa spherical waveThe acoustic intensitydecreases with range ininverse proportion to thesurface of the sphereThe acoustic wave amplitudeA decreases with range RThe intensity I = A2

In homogeneous media

I ∼ 1R2



Geometrical spreading loss - two way

The acoustic wave expands asa spherical wave to thereflectorThe reflected field expands asa spherical wave back to thereceiverIn homogeneous media, thetwo way loss becomes

I ∼ 1R2

1R2 =

1R4



Absorption

Seawater is a dissipative mediumthrough viscosity and chemicalprocessesAcoustic absorption in seawater isfrequency dependentLower frequencies will reach longerthan higher frequencies

f [kHz] R [km] λ [m]0.1 1000 151 100 1.510 10 0.15100 1 0.0151000 0.1 0.0015



Transmission lossTransmission loss is geometrical spread + absorptionLogarithmic (dB) scale: IdB = 10 log10(I)A certain frequency will have a certain maximum range - criticaldesign parameter



The Ocean as Acoustic Medium

The sound velocity - environmental dependencyLayering and refraction - waveguidesThe sea floor and the sea surface - scatteringNoise sources



Reflection and Refraction in Acoustics

Recall from first lecture on opticalimagingThe reflection angle is equal to theincident angleThe angle of refraction is given bySnell’s law

sin θ1

c1=

sin θ2

c2

The index of refraction n = c2/c1

Snells law can be derived fromFermats principle or from thegeneral boundary conditions From Wikipedia



Refraction and the sound velocity

Medvins formula:

c = 1449.2 + 4.6T − 0.055T 2 + 0.00029T 3

+ (1.34− 0.010T )(S − 35) + 0.016D

The sound velocity depends on 3 major parts:Temperature T in degrees CelsiusSalinity S in parts per thousandDepth D in meters

The sound velocity contains information about the ocean environment.Example: T = 12.5 ◦C,S = 35 ppt,D = 100 m gives c = 1500 m/s



Deep sound velocity variation

The surface layerThe seasonal thermoclineThe permanent thermoclineThe deep isothermal layer



Sound refraction

The sound will refract towards areas of slower speedSOUND IS LAZY



Underwater sound channelwaves are trapped in a guideThe energy spreads in one dimension instead of two I ∼ 1/RMuch longer rangeThe effect of the medium on underwater acoustics is used inmapping the medium: acoustical oceanography



Coastal variability

Factors that affect soundpropagation:

The sound velocity profileThe sea surfaceinternal wavesturbulenceocean current



Coastal variability

The sound is trapped in awaveguideThe boundaries of thewaveguide changes theproperties of the soundwave



Reflection: basic physics

Characteristic impedance Z0 = ρcI ρ is the density [kg/m3]I c is the sound speed [m/s]

Reflection coefficient (normalincidence)

R(f ) =Z − Z0

Z + Z0

Transmission coefficient (normalincidence)

T (f ) =2Z0

Z + Z0

The characteristic impedance isa material propertyMaterial ImpedanceAir 415Seawater 1.54× 106

Clay 5.3× 106

Sand 5.5× 106

Sandstone 7.7× 106

Granite 16× 106

Steel 47× 106



Reflection: the sea surface

The sea surface (sea-air interface)Air: Z = 415Seawater: Z0 = 1.54× 106

Reflection coefficient

R =Z − Z0

Z + Z0≈ −1

The sea surface is a perfect reflector



Reflection: the sea floor (or bottom)

The sea floor (sea-bottom interface)Sand: Z = 5.5× 106

Seawater: Z0 = 1.54× 106

Reflection coefficient

R =Z − Z0

Z + Z0≈ 0.56

Sandy seafloors partially reflects,partially transmitsEstimated reflection coefficient canbe used in classification of bottomtype



Scattering - smooth surfaces

Scattering from rough surfacesThe sea surfaceThe seafloor

Other scattering sourcesVolume scattering fromfluctuationsScattering from marine life

A smooth surface gives mainly specular reflection



Scattering - rough surfaces

Scattering from rough surfacesThe sea surfaceThe seafloor

Other scattering sourcesVolume scattering fromfluctuationsScattering from marine life

A rough surface gives specular reflection and diffuse scattering



Ambient Noise

The ocean is a noisy environmentHydrodynamic

I Tides, ocean current, storms,wind, surface waves, rain

SeismicI Movement of the earth

(earthquakes)Biological

I Produced by marine lifeMan made

I Shipping, industry



Marine Life and Acoustics

Dolphins and whales use acousticsfor echolocation and communication.Whale songs are in the frequencybetween 12 Hz and a few kHz.Dolphins use a series of highfrequency clicks in the range from50 to 200 kHz for echolocation.

From wikipedia.org. Courtesy of NASA.

From wikipedia.org. Author Zorankovacevic.


Theory

Outline1 Basics




4 Summary


Theory Sonar types

Active sonar

Transmits a signalThe signal propagates towards theobject of interestThe signal is reflected by the targetThe signal is recorded by a receiver


Theory Sonar types

Active sonar

Transmits a signalThe signal propagates towards theobject of interestThe signal is reflected by the targetThe signal is recorded by a receiver


Theory Sonar types

Passive sonar

Passive sonar only records signals


Theory Positioning

Range Estimation

Estimation of time delay (or two way travel time) τRelate time delay to range

R =cτ2

Sound velocity must be known


Theory Positioning

Range ResolutionThe minimum distance two echoes can be seperatedRelated to the pulse length Tp for non-coded pulses

δR =cTp

2Related to bandwidth B for coded pulses

δR =c

2B


Theory Positioning

Pulse forms 1 - active sonarDifferent pulse forms for different applications

Gated Continuous Wave (CW)Simple and good Doppler sensitivity but does not have high BTLinear Frequency Modulated (LFM) (or chirp)Long range and high resolution but cannot handle Doppler


Theory Positioning

Pulse forms 2 - active sonarDifferent pulse forms for different applications

Hyperbolic Frequency Modulated (HFM) pulsesLong range and high resolution and Doppler resistivePseudo Random Noise (PRN) BPSK Coded CWHigh resolution and good Doppler sensitivity but low efficiency


Theory Positioning

Directivity

Transducers (or antennas or loudspeakers) are directiveThe beamwidth (or field of view) of a disc of size D is

β ≈ λ

D

The beamwidth is frequency dependent. Higher frequency givesnarrower beam.


Theory Positioning

Bearing estimation - arbitrary Rx positions

Direction of arrival can be calculated from the time difference ofarrival

θ = sin−1{

cδtL

}


Theory Positioning

Bearing estimation - array sensor

By delaying the data from each element in an array, the array canbe steered (electronically)


Theory Positioning

Bearing estimation - array sensor

Direction of arrival from several reflectors can be estimated byusing several receivers.


Theory Positioning

Imaging sonar

Echo location is estimation of range and bearing of an echo (ortarget)Imaging sonar is to produce an image by estimating the echostrength (target strength) in every direction and range

Algorithmfor all directions

for all rangesestimate echo strenght in each pixel

endend


Theory Positioning

Imaging sonar resolution

Range resolution given bypulse length (actuallybandwidth)Azimuth resolution given byarray length measured inwavelengthsField of view is given byelement length measuredin wavelengths

Array signal processing in imaging is the primary topic in INF 5410


Theory Signal processing

Sonar signal model



Active sonar processing

The basic active sonar processingconsists of

PreprocessingPulse compression (range)Beamforming (azimuth)DetectionParameter estimation - positionClassification



The sonar equationThe sonar equation is an equation for energy conservation forevaluation of the sonar system performace.

In its simplest form: Signal − Noise + Gain > ThresholdMore detailed (for active sonar):

SL− 2TL + TS − NL + DI + PG > RT

SL is source levelTL is transmission lossTS is target strenghtNL is noise levelDI is directivity indexPG is processing gainRT is reception threshold


Applications

Outline1 Basics




4 Summary


Applications Fish finding

Echosounders

The echosounder is orientedverticallyThe target strength is estimated inevery range (depth)The ship moves forward to make a2D map of fish densityThe target strength is related to fishsize (biomass)Different frequencies can be usedfor species characterisation



Stock abundance and species characterisation

From www.simrad.com. Courtesy of Kongsberg Maritime



Fish Detection Range

Modern echosounderscan detect a single fishat 1000 m range.Some fish have aswimbladder (air filled)which gives extra largetarget strength

From www.simrad.com. Courtesy of Kongsberg

Maritime


Applications HUGIN AUV

The HUGIN autonomous underwater vehicle



The HUGIN autonomous underwater vehicle

Free swimming underwater vehiclePreprogrammed (semi-autonomous)Used primarily to map and image the seafloorRuns up to 60 hours, typically in 4 knots (2 m/s)Maximum depth: 1000, 3000, 4500 m



Acoustic sensors on HUGIN

Multibeam echosounderImaging sonarAltimeterAnti collision sonarDoppler velocity loggerSubbottom profilerAcousticcommunications


Applications Imaging

Sidescan sonar

Sidescan sonar: sidelooking sonar to image the seafloorTypical platform: towfish, hull mounted, AUVAn image is created by moving and stacking range linesTypically frequency 100 kHz - 500 kHzTypical range 100 - 500 m



Sidescan Area coverageRange resolution is given by the pulse length (or bandwidth)Along-track resolution is range dependent



Sidescan Example - traditional technology

Image collected by HUGIN AUV. Courtesy of Kongsberg Maritime / FFI



Synthetic aperture sonar principle

Collect succesive pulses in a large synthetic array (aperture)Increase the azimuth (or along-track) resolutionRequires accurate navigation - within a fraction of a wavelength



Synthetic aperture sonar principleThe length of the synthetic aperture increases with rangeAlong-track resolution becomes independent of rangeAlong-track resolution becomes independent of frequency



Sidelooking Example - very high resolution (SAS)




Resolution matters




Example: Fishing boat




Properties in a sonar image

Geometry: Rangeand elevationResolutionRandom variability -speckleobject and shadow


Applications Mapping

Multibeam echosounders

Multibeam echosounders maps the seafloor by estimating therange in different directionThe map resolution is determined by the 2D beamwidth and therange resolution



MBE Example 1

Data collected by HUGIN AUVMaps from the Ormen Lange fieldThe peaks are 50 m high

Courtesy of Geoconsult / Norsk Hydro.



MBE Example 2Data collected by HUGIN AUVMaps from the Ormen Lange fieldThe ridge is 900 m long and 50 m high.

Courtesy of Geoconsult / Norsk Hydro.



MBE Example 3Data collected by HUGIN AUVExample area with large sand ripples

Courtesy of Kongsberg Maritime / FFI.



MBE Example 3

Hull mounted MBE70 - 100 kHzMagic T (Mills cross) layout




MBE Example 4Hull mounted MBE70 - 100 kHzColour coded seafloor height



Summary

Outline1 Basics




4 Summary


Summary

Summary

Acoustics is the only long ranging information carrier under waterSound velocity variations cause refraction of acoustic wavesThe ocean is lossy: higher frequencies have shorter rangeSONAR is used for

I positioningI velocity estimationI characterisation

Applications:I Fish findingI Imaging of the seafloorI Mapping of the seafloorI Military


Summary

Pa Norsk

Engelsk Norskbeam stralebeamwidth stralebredderange avstandbearing retningechosounder ekkoloddsidescan sonar sidesøkende sonarmultibeam echosounder multistrale ekkolodd


Summary

More information

Course text: sonar_introduction_2009.pdfThis presentation: inf-geo4310-2009-lecture-5.pdfwww.wikipedia.org

I sonarI underwater acousticsI side-scan sonarI biosonar, animal echolocationI beamforming

Xavier Lurton, An introduction to underwater acousticsSpringer Praxis, 2002Ocean Acoustics Library http://oalib.hlsresearch.com/


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