Underwater ultrasonic measurements

Transcription

1 Underwater ultrasonic measurements S Licentiate course in measurement science and technology Jussi Kaasinen Content Sound as a measurement tool Underwater measurements Underwater imaging applications Demo Same physical principles applied as used in measurements on dry land NDT: pulse-echo imaging Flow meters: doppler 006 Jussi Kaasinen Basic setup

2 Underwater measurements, background Skin depth for EM radiation Only very low frequencies suitable High freq. required for resolution! Generally not good Water transparency to light questionable Good: few ten s of meters Bad: some centimeters skin depth/m δ 1 πfμσ 0 Acoustic approach is the only known form of imaging for which our murky waters are transparent to Pressure waves travel rather nicely over long distances, just like predicted by the theory Gee, whales and dolphins were clever! frequency/hz Jussi Kaasinen Some math Speed of sound d=depth, T=temperature, S=salinity in ppt V~1500m/s Empirical result 3 c = T T T + ( T)(S - 35) d Sound generation & sound pressure Water is non-compressible Relatively high pressures easily achievable p = ρcωξ = Zωξ Intensity in db relative to 1uPa TX level: dB not uncommon SPL ref P =10*log 1μ Pa Total SPL takes antenna gain into account SPL = SPLref + DI Jussi Kaasinen

3 Attenuation Spherical spreading I=P 0 /(4πR ) Transmission loss Attenuation rises dramatically in higher frequencies (>100kHz) Mainly due to salinity, MgSO 4 α = TLspherical = log = 10log r =.11 f 1+ f 4πr 10 0 log r 4π (1m ) 0.70 f f ( S /35) 0.03 f + + (1 65*10 ft + f ft 0 T 6 / d) db km Total attenuation TL = 0logr + αr alpha, db/km S=0ppt 80 S=6ppt 60 S=30ppt kHz 100kHz 00kHz 500kHz S=0ppt 0,3 4,4 17, 106,8 S=6ppt 0,9 10,0 4,7 115,0 S=35ppt 3,4 3,6 54,7 148,1 frequency, khz 1000kHz 46,7 435,1 468,7 Attenuation in db/km at various frequencies as a function of salinity Jussi Kaasinen Transducers, TX & RX Equation used for RF work here too Radio & radar antennas Directivity Index, Gain L DI 10log λ Main lobe, side lobes Jussi Kaasinen

4 Target strength Target strength Ratio of reflected to incident intensity TS of a round sphere Signal level increases with increasing frequency TS = 10log I I r i 4 6 π a 100 TS = 10log 9λ Selection is a compromise between attenuation and resolution (Active) Sonar equation DT=SL + DI T + TS - TL - (NL-DI) Detection threshold, source level, transmitter directivity, target strength, transmission loss, noise level, receiver directivity Jussi Kaasinen Materials Transducers generally piezoelectric (khz s Mhz s) Quartz Lead-zirconium-titanate, barium titanate Different geometries (bar, plate, ) various shapes, thickness at resonance ~λ/ Tonpilz generator structures for low frequencies (ultrasonic cleaners!) Also ferroelectric for low frequencies (few khz few ten s of khz) For really low frequencies speaker-type sensors (100 s of Hz few khz) PVDF V V Jussi Kaasinen

5 Acoustic impedance Acoustic impedance Z 1 Z Z=ρc Water: Z= 1.5*10 6 kg/m s ( rayl ) Al: 17Mrayl, W=105Mrayl 1 R 1-R Transducer matching to media Mismatch reflection just as in the case of RF & 50ohm matching Matching layers: thickness + acoustic impedance Quarter wave matching R Z Z = 1 T = 1 Z + Z R 1 Water Jussi Kaasinen Acoustic impedances of some materials density ρ, kg/m 3 speed of sound v, m/s (+4 C) Acoustic impedance Z, MRayl water ,5 ice , PZT ,3 tungsten steel ,7 air 1, , Jussi Kaasinen

6 Sound ray bending Acoustic rays bend towards lower speed of sound P, T, S Sofar channel Depth ~1000m Blind areas who s hiding there? Jussi Kaasinen Sonar Sound Navigation And Ranging History Colladon, 186 WW I WW II, ASWF Usage of underwater acoustics Main driving force: underwater warfare Geological surveys - boom in usage due to oil industry Scientific use Fisheries Recreational use Surface acoustics: medical, NDT, distance measurement, parking radars, bats, Jussi Kaasinen

7 Active vs. Passive Active - ping Tx+Rx (Reveals source location) Passive listen only RX hydrophones only (Hidden from enemy) See or be seen? Geological research: X kg of TNT hydrophone array Jussi Kaasinen Active sonar Principle: 1. Generate burst of acoustic energy. Record echoes 3. Process and display results 4. Goto 1. 1 > 1) Ch 1: 500 mvolt 100 us Jussi Kaasinen

8 Active sonar Source level determines how loud returns can be expected back Water incompressible relatively high pressure levels easily achievable E.g dB (re 1uPa) not uncommon Distance measurement d=c*t/ Echo after 1 second range=1500m/s*1s*0.5=750m Resolution Along track beamwidth Across track pulse duration =ping length Jussi Kaasinen Example of one active sonar ping Sonar ping signal, target at t=3s 8000 Sonar ping spectrogram, target at t=3s amplitude Frequency time, s time, s Jussi Kaasinen

9 Phased arrays Multiple sensors Beam steering by altering phase Several beams can be synthesized Calculation intensive A B A+B A B A+B time time Constructive interference, φ=0 Destructive interference, φ= Jussi Kaasinen Pulse-echo devices Depth finder Measures distance between sensor and reflecting surface (=bottom) + fish in mid-water Sector scanning sonar Maps around the sensor Phased array or rotating sensor Sub-bottom profiler Low frequency for good penetration into mud ADCP current profiler - doppler Jussi Kaasinen

10 LIMIS Acoustic camera Multibeam sonar Uses acoustic lenses to enhance quality Tx: 11dB, RX:18 beams, 4-9 frames/s University of Washington Jussi Kaasinen! Echo sounders Wide swath system? 1 h/km 1. h/km Jussi Kaasinen

11 Side scan sonar Theory of Operation Design Mapping of large areas, mosaicking Artefacts Inventors Doc Edgerton Also other activities Jussi Kaasinen Side scan sonar based on utilization of narrow measurement beam narrow strips of seabed are measured at a time, acoustic back scattering vs. time is plotted complex, expensive, military uses comparable to remote sensing: SLAR Towfish, cable, display 006 Jussi Kaasinen

12 Side scan sonar One line of data isn't very informative alone......but when several lines are stacked together, the big picture becomes very informative Jussi Kaasinen Image interpretation Image = sonogram Surface echo Tx pulse 1st bottom return Target Acoustic shadow Left ch Right ch Measurement controls Jussi Kaasinen

13 Interpretation difficulties Finnish Coast Guard Jussi Kaasinen Acoustic shadows Comparable to shadows caused by light sources Nature has some sense of humor Jussi Kaasinen

14 Sources of oddities Image generation assumes that the tow vehicle travels along a straight track Image artifacts caused by non-idealities Jussi Kaasinen Multibeam Example of multi-channel phased arrays (e.g. 18 channels) Many measurement beams measure distance + direction bottom topography Mill s cross Bottom topography mapping Measurement platform stability crucial Jussi Kaasinen

15 Mosaicking Scan narrow strips at a time, store data with accurate location Build larger picture out of narrow strips Jussi Kaasinen Sample data Jussi Kaasinen

16 Another example 500kHz Klein Jussi Kaasinen