Underwater Acoustics including Signal and Array Processing

Transcription

1 Underwater Acoustics including Signal and Array Processing William A. Kuperman Scripps Institution of Oceanography of the University of California, San Diego OUTLINE

2 Underwater Acoustics including Signal and Array Processing

3 Helmholtz Equation Let p(r,t) = p(r)e iωt Rectangular Coordinate: Plane Waves p(x, y, z) ~ e ±ik r Spherical Symmetry: Spherical Waves With p(r) ~ e±ikr r k And note that is the equation of a plane and therefore a surface of constant phase

4 Energy, Energy Density,Power and Intensity (plane wave) Instantaneous Energy Density: E(t)=P.E. +K.E (Average) Energy Density: E Intensity, I, is average rate of flow of energy through a unit area normal to the direction of propagation Power is energy rate (e.g., watt) = ρ 0 v 2 rms = p 2 rms ρ 0 c 2 = p rms v rms c = ρ 0 v 2 = p2 ρ 0 c = pv 2 c I ~ c X E = ρ 0 cv rms 2 = p rms 2 ρ 0 c = p rms v rms

5 GEOMETRIC SPREADING

6 UNITS DECIBEL (db re_ ): 10 LOG (Intensity/Intensityref) = 20 LOG (Pressure/Pressureref) Air: Pressureref = 20 µ Pa ( 1 Atmosphere = 194 db = 10 5 Pa) Water: Pressureref = µ Pa 20 LOG 20 = 26 db Same db in air is higher pressure!!!!

7 Units cont d Where do the intensity numbers come from? Intensity is flow of energy through a unit area = energy/ (time x area) Energy/time = power (e.g. watts) Intensity units => Watts/m 2 INTENSITY (plane wave) = P 2 rms /(ρc)

8 Units cont d ρc water = 1.5 x 10 6 kg/m 2 s ρc air = ρc water /3500 and 0 db re µpa in water: Intensity of µpa plane wave in water is =.67 x watts/m 2 Int of 20 µpa in air = /.67 x = 1.5 x 10 6 Intensity of µpa in water In db: 61.7 db which is (about) the same as Log 3500

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10 More Numbers Sound Speeds SPL s in air: db re 20µPa

11 ACOUSTICS IN THE OCEAN

12 GENERIC SOUND SPEED STRUCTURE

13 SOUND SPEED, SNELL S LAW AND ATTENUATION

14 GLOBAL SOUND SPEED STRUCTURE

15 ATTENUATION OF SOUND IN SEAWATER (URICK)

16 Deep Scattering Layer Fish/Scatterers deeper in the day than at night Day:deeper swim bladders-- smaller--hf scatterers Night: strong scattering within 100 m of surface Sunset/Sunrise: biggest change

17 BUBBLES Pop and make noise Have Resonances a. b. Bubbly media attenuate an incoming field by Absorbtion Scattering Bubbly media have lower sound speeds Void fraction K m = µk b + (1 µ)k w 1 B m = µ 1 B b + (1 µ) 1 B w Example: µ =.0001 and.001 c = 930m / s and 370m / s

18 SNELL S LAW: SOUND LIKES LOW SPEEDS

19 GLOBAL SOUND SPEED STRUCTURE

20 SCHEMATIC OF SOUND PROPAGATION PATHS

21 DEEP SOUND -CHANNEL PROPAGATION (NORWEGIAN SEA)

22 Historical Underwater Acoustics Mid latitudes polar latitudes array Layers of constant sound speed Radiated noise Ray trapped in the Deep Sound Channel (DSC) Depth ~10000 ft Sea mountain or continental shelf Typical midlatitude sound speed profile Typical northern sound speed profile C (m/s)! Range ~1000 miles Box 1!

23 LLOYD MIRROR EFFECT

24 CONVERGENCE ZONE PROPAGATION (NORTH ATLANTIC)

25 SURFACE-DUCT PROPAGATION (NORWEGIAN SEA)

26 ARCTIC PROPAGATION

27 Sound propagation over a seamount

28 PROPAGATION OVER A SEAMOUNT (NORTH PACIFIC) data: Chapman and Ebbeson

29 PROPAGATION IN A RANGE DEPENDENT ENVIRONMENT

30 Propagation Modeling! Sound Speed Profile!! Propagation Model! Normal Mode Methods" ORCA : Evan Westwood (1996)!! Pulse Shape!

31 Pressure (R = 60km, SD = 100m) TL db

32 Pressure (R = 300km, SD = 100m) TL db

33 SHALLOW WATER SOUND SPEED PROFILES

34 REFLECTIVITY AND SHALLOW WATER PROPAGATION

35 CONSTRUCTIVE INTERFERENCE: MODAL PROPAGATION

36 SHALLOW WATER PROPAGATION

37 a) θ c θ 2 θ 1 θ n b) z r c ω% k r1 k r2 k rn ρ b,c b Box 2!

38 Mode Cutoff

39 SHALLOW-WATER PROPAGATION (SUMMER, MEDITERRANEAN)

40 OPTIMUM FREQUENCY CURVES

41 CONTOURED PROPAGATION LOSS: OPTIMUM FREQUENCY CURVES BARENTS SEA ENGLISH CHANNEL

42 HIERARCHY OF UNDERWATER ACOUSTIC MODELS OCEAN ACOUSTICS LIBRARY:

43 AMBIENT NOISE SPECTRA (WENZ)

44 Some Applications

45 SONAR

46 Underwater acoustic bottom mapping

47 Fish Finding Display Includes Bottom

48 Problems Associated with Undersea Acoustic Communication Tranfer Function and Symbol Spread in Shallow Water Amplitude ( a.u.) 1 0 Fo=3500, 1 ms,r=10 km Depth 120m MUST deal with Intersymbol Interference ( ISI ) (a) (b) Amplitude ( a.u.) Amplitude ( a.u.) Fo=6500,.5 ms,r=4km Depth 50m Fo=15000,.1 ms,r=.16km Depth 12m (c) Time (s)

49 Ocean Tomography Different rays have Different group speeds Therefore have different Arrival times Which ray corresponds To which arrival time?

50 Ships Underway Broadband Source Level (db re 1 Pa at 1 m) Tug and Barge (18 km/hour) 171 Supply Ship (example: Kigoriak) 181 Large Tanker 186 Icebreaking 193 Seismic Survey Broadband Source Level (db re 1 Pa at 1 m) Air gun array (32 guns) 259 (peak) Military Sonars Broadband Source Level AN/SQS-53C (U. S. Navy tactical mid-frequency sonar, center frequencies 2.6 and 3.3 khz) AN/SQS-56 (U. S. Navy tactical mid-frequency sonar, center (db re 1 Pa at 1 m) 235 frequencies 6.8 to 8.2 khz) SURTASS-LFA ( Hz) 215 db per projector, with up to 18 projectors in a vertical array operating simultaneously Ocean Acoustic Studies Heard Island Feasibility Test (HIFT) (Center frequency 57 Hz Acoustic Thermometry of Ocean Climate (ATOC)/North Pacific Acoustic Laboratory (NPAL) (Center frequency 75 Hz) Animal Sounds 223 Broadband Source Level (db re 1 Pa at 1 m) 206 db for a single projector, with up to 5 projectors in a vertical array operating simultaneously 195 Man Made Sounds Source Broadband Source Level (db re 1 Pa at 1 m) Sperm Whale Clicks Beluga Whale Echolocation Click (peak-to-peak) White-beaked Dolphin Echolocation Clicks (peak-to-peak) Spinner Dolphin Pulse Bursts Bottlenose Dolphin Whistles Fin Whale Moans Blue Whate Moans Gray Whale Moans Bowhead Whale Tonals, Moans and Song Humpback Whale Song Humpback Whale Fluke and Flipper Slap Southern Right Whale Pulsive Call Snapping Shrimp (peak-to peak)

51 QUESTIONS?

52 Underwater Acoustics including Signal and Array Processing

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55 HIERARCHY OF UNDERWATER ACOUSTIC MODELS

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59 GEOMETRIC SPREADING

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64 SPECTRAL METHOD (FFP) X ONE LAYER ALL LAYERS or Axisymmetric Geometry

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66 Lloyd Mirror Example

67 Fluid-Fluid Interface

68 θc

69 REFLECTIVITY AND SHALLOW WATER PROPAGATION

70 CONSTRUCTIVE INTERFERENCE: MODAL PROPAGATION

71 a) θ c θ 2 θ 1 θ n b) z r c ω% k r1 k r2 k rn ρ b,c b Box 2!

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73 Transmission loss k m -k n

74 Bottom Attenuation

75 COMING ATTRACTION NON CONSTANT PROFILE Munk sound speed profile

76 Ray equivalent of a mode GROUP SLOWNESS (1/SPEED) ~ MODE WEIGHTED SSP SLOWNESS

77 Deep Ocean Propagation MODAL/RAY GROUP SPEED L/T

78 Non Model Based Processing Or DATA BASED PROCESSING

79 Utilizing First Order Baseline Properties of Waveguide Propagation Group vs Phase Speed and Waveguide Invariant

80 Reminder: Phase and Group Speeds Our waveguide speeds are horizontal velocities Phase speed is related to ray or mode angle Horizontal ray has phase speed of medium Vertical Ray has infinite phase speed Refracting ray has phase speed of SSP at turning point Group Speed of horizontal path is ~ speed of medium Group Speed of Vertical path is zero Group Speeds of rays are horizontal range/time Group Speeds of modes are related to mode weighted SSP slowness

81 Remember triangles for Range Independent Env.: Modal wavenumbers Modal phase speeds: v mp Modal group speeds: v mg k=ω/c(z) c k zm (z) θ m k rm = ω/v mp Prop down the waveguide v mg =c cos θ m

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83 SHALLOW WATER ARRIVALS (Bottom Reflected Paths) Lowest Mode-most direct Arrival comes in first. DEEP WATER ARRIVALS (Refracted Paths) Deep Refracted Arrivals come In before Deep Sounds Channel

84 Waveguide Invariant: β Lines of Constant Intensity I(ω,r) = c ΔI = 0 I r Δω Δr Δr + I ω Δω = 0 = I / r I / ω = βω r

85 Spectrogram for shallow water data β 1

86 Simple Ranging in shallow water Δω Δr = β ω r Δω Δr Range along track or array r ω Note: a single receiver spectrogram has (t,f) not (r,f) axes One solution: use long, horizontal array that provides a measu

87 Waveguide Invariant: β β changes sign depending on the environment β = +1 β = 3 Burenkov, Sov. Phys. Ac., 1989

88 MACRO Properties of the Sound Field Waveguide Invariant: (Chuprov ) β Lines of Constant Intensity I(ω, r) = const ΔI = 0 I r Δr + I ω Δω = 0 Δω I / r = Δr I / ω = β ω r REFLECTION Dominated β 1 = S & g = v p S ( p ' v g 2 ) vg + * v p

89 Waveguide Invariant β ~Slope of GP-GV curve Reflection β > 0 Refraction β < 0

90 Phase and Group Speeds vs Path Types: SIMPLE RANGING Δt Δu X X ~3430km (=3514 km)

91 GENERALIZED Waveguide Invariant Theory Pressure / Intensity Field P( r) = Am exp( ik mr), m I( r) = Am An cos( Δk m, n Stationary Phase Condition (Constant Intensity Lines : striation) Waveguide Invariant Theory: Chuprov (1982), Grachev (1993), Weston (1971,1979),D Spain & Kuperman (1999) mn r), δr δδk Φ Δk mn r, mn δφ = + = 0 r Δk δr r Δk mn mn 1 δω γ δh + = 0 β ω β h ( ω,h,c ) = k m k n β = 1, γ = 2

92 Group Speed vs Phase Speed 20 m 40 m Group Speed (m/s) m m/s 1600 m/s Phase Speed (m/s) Idealized Summer Profile

93 Shallow Water Group Speed vs Phase Speed WHERE THE ACTION IS - BUT FIRST: MODE # "

94 QUESTIONS?

95 CAN WE OVERCOME SINGLE SENSOR INVARIANT RESTRICTION THAT WE MEASURE A TIME DIFFERENCE BUT NEED A RANGE DIFFERENCE

96 HOW DO WE DO INVARIANT-BASED RANGE- LOCALIZATION FROM A FREQUENCY-TIME SPECTROGRAM Rakotonarivo, Kuperman,JASA, 2012.