Underwater Acoustics OCEN 201

Transcription

1 Underwater Acoustics OCEN 01

2 TYPES OF UNDERWATER ACOUSTIC SYSTEMS Active Sonar Systems Active echo ranging sonar is used by ships to locate submarine targets. Depth sounders send short pulses downward and time the bottom return. Side-scan sonars are used for finding objects on sea floor and mapping. Fish finding sonars are forward looking sonars for spotting fish schools. Diver sonars are hand held sonars used for locating of underwater objects. Position marking beacons transmit sound signal continuously. Position marking transponders transmit sound only when interrogated. Acoustic flow meters and wave height sensors are used. Multiple beam echo sounders used to map the seafloor in great detail. Seismic Systems Subbottom profilers are used to explore the rocks and sediments making up the ocean floor. The acoustic pulses used are basically unidirectional pressure pulses that are generated by air guns. Results show the geological features below the ocean floor.

3 SUBMARINE SONAR 3

4 TYPES OF UNDERWATER ACOUSTIC SYSTEMS 3. Underwater Communications and Telemetry Systems and Navigation a. Underwater telephone is a device used to communicate between a surface ship and a submarine or between two submarines (UQC). b. Diver communications - diver has a full face mask which allows the diver to speak normally underwater and a throat microphone is used to obtain speech signals. A transducer is used to transmit the signal. The same transducer is used to receive, and the signal is passed to the diver via an ear piece. c. Telemetry systems - data from a submerged instrument is transmitted to the surface. d. Doppler navigation - pairs of transducers pointing obliquely downward to obtain speed over the bottom from the Doppler shift of the bottom returns. 4

5 TYPES OF UNDERWATER ACOUSTIC SYSTEMS 4. Passive Systems a. Passive ship sonar is a hydrophone array that detects acoustic radiation from another vessel or object; i.e. JP or JT hydrophone used by WWII submarines. b. Acoustic mines - mines explode when acoustic radiation reaches a certain value. Torpedoes - home on acoustic radiation of submarine or ship. 5

6 ACOUSTIC TRANSDUCERS 6

7 ACOUSTIC DOPPLER CURRENT METER 7

8 SIDESCAN SONAR 8

9 Decibel Scales Sound intensities and sound pressures are expressed as logarithmic scales known as sound levels. Reasons: 1. A very wide range of sound pressures and intensities are encountered in the ocean..the human ear subjectively judges the relative loudness of two sounds by the ratio of their intensities. The most generally used logarithmic scale for describing sound levels is the decibel scale. The intensity level (N) of a sound of intensity I1 and reference intensity I is defined by: Intensity Level ( IL)N = 10log Sound Pressure Level ( ) I I SPL N = 0 log 1 p 1 / p 9

10 FUNDAMENTALS OF UNDERWATER SOUND (CONTINUED) For the case of a plane wave of sound, the acoustic pressure (p) is related to the particle velocity (u) by p = ρ c u Where p - pressure ρ - density c - propagation velocity of the plane wave ρc - is called the specific acoustic resistance u - particle velocity ρc seawater = 1.5 x 105 g/cms ρc air = 4 g/cms The energy involved in propagating acoustic waves through a fluid medium is of two forms: 1. Kinetic Energy - particle motion. Potential Energy - stresses set up in elastic medium 10

11 FUNDAMENTALS OF UNDERWATER SOUND (CONTINUED) For a plane wave, the acoustic intensity (I) of a sound wave is the average rate of flow of energy through a unit area normal to the direction of wave propagation. The instantaneous intensity is I = p / ρ c The average intensity is I = p ave / ρ c Where p ave is the time average of the instantaneous acoustic pressure squared. Units: p = dynes/cm ρ = gm/cm 3 c = cm/s I = ergs/cm s Since Intensity is also power/unit area and the units are often watts/cm. One watt is equal to 10 7 ergs/s then I = power/area = p ave / ρ c x 10-7 watts/cm 11

12 Decibel Scales (continued) In general, if we have a quantity x such that I = ( x / ) a 1 / I 1 x then the ratio of the values on the decibel (db) scale is I = ( ) 10 log 1 10 log 1/ I a x x db For a =, then 10 log (I 1 /I ) = 0 log (x 1 /x ) = 0 log (p 1 /p ) The reference level must be known to insure proper interpretation of the db value. (Note that 1 psi x 6895 = number of Pascal). Also 1 Pascal = 1 N/m The old reference levels are: 1) 1 dyne/cm ) dyne/cm The current reference level is: 1 micropascal (1 μpa). Note: 1 μpa = 10-5 dyne/cm 1

13 Decibel Scales (continued) N p3 p3 To convert from one reference (p ) to another (p 3 ). N p = 0 log (p 1 /p ) N p3 = 0 log (p 1 /p 3 ) Subtract N p3 from N p, N p p = 0 [ log( p / p ) log( p / p )] 1 3 [ log p log p log p log p ] N N = 0 + N p3 N p = 0 N = N + p3 1 [ log p log p ] p 0log 3 3 ( p / p ) Example: Express 15 db relative to dyne/cm in db relative to 1 dyne/cm. Let p = dyne/cm N p3 = 15 + N 3 p 3 = 1 dyne / cm 0 log( /1) p = = 51dB 13

14 Decibel Scales (continued) The level of a sound wave is the number of decibels by which its intensity, or energy flux density, differs from the intensity of the reference sound wave. In the case of a sound wave with an intensity of I 1 and a reference intensity of I, the level of the sound wave is equal to: N db = 10 log I For clarity the level should be written: N db re { / 1 I the intensity of a plane wave of pressure equal to 1μPa If a sound wave has an intensity 500 times that of a plane wave of rms pressure 1 μpa, then the level N is: N = 10 log 500/1 = 7 db re 1 μpa 14

15 Sonar Equations Active SL-TL+TS=NL-DI+DT Active (Reverberation) SL-TL+TS=RL+DT Passive SL-TL=NL-DI+DT 15

16 Active Sonar Equation Detection Threshold (DT) Directivity Index (DI) or Array Gain (AG) Receive Electronics Electronics Headphones Source Level (SL) Noise Level (NL) One-way Transmission Loss (TL) Target Strength (TS) 16

17 Example: A passive sonar system is being used to detect an object that has a source level of 80 db re dynes/cm and a directivity index of 1 db. If the detection threshold is 15 db and the transmission loss is 50 db, determine the noise level which will permit detection of the target. N p3 1μPa = N Given: Find: NL Solution: p 0 log N = N dyne / cm SL = 80 db re dynes/cm DI = 1 db DT = 15 db TL = 50 db p 3 p 1μPa 0 log dynes / cm N 1μPa N Pa 5 10 = 80 0 log μ = 80 0 N Pa ( 1.3) 1 μ = = 106 db re 1μPa SL = 106 db re 1μ Pa Passive Sonar Equation Sl TL = NL DI + DT = NL = NL + 3 NL = 53 db re 1μ Pa 17

18 Beam Patterns Line Array Circular Plane Array 18

19 19 Line Array with Equally Spaced Elements b( ) n d n d θ π λ θ π λ θ = sin sin sin sin beam width at -3 db acoustic axis db 9 x x x x x x x elements θ

20 Beam Pattern Spreadsheet 0

21 Spherical Spreading and Absorption Propagation measurements made in the ocean indicate that spherical spreading together with absorption yields a reasonable approximation to measured data for a wide variety of conditions. Therefore, transmission loss may be expressed by TL = 0log r + α r 10 3 where r is range in yards, α is absorption coefficient in db/kyd, and TL is transmission loss in db. This is a rough approximation but a good rule of thumb. 1

22 Francois & Garrison (198) Figure 5- shows the variation of the absorption coefficient (α) as a function of frequency from 0.1 to 1000 khz at zero depth (surface) for a salinity of 35 and ph of 8.0. The accuracy of the predicted absorption coefficients is estimated as ±5% for the ranges of 0.4 to 1000 khz, -1.8 to 30 o C, and 30 to 35.

23 Speed of Sound in the Sea Speed of sound in water has been determined theoretically and experimentally. Leroy equation ( ) 3 ( ) ( ) ( ) ( )( ) c= T T T S T 18 S 35 + Z/61 where c is sound velocity, m/s;t is temperature, o C at the depth; S is salinity, ppt; Z is depth, m. MacKensie (1981) ( ) ( ) c = T 5.304x10 T +.374x10 T S x10 d x10 d x10 T S x10 Td where c is sound speed (m/s), T is temperature ( o C) at the depth, S is salinity (ppt), and d is depth (m). The range of validity for the MacKensie (1981) equation is: 0 o C T 30 o C, 30 S 40, and 0 m d 8000 m. The MacKensie equation is good for practical work and shows that sound speed increases with temperature, salinity, and depth. 3

24 Velocity Structure in the Ocean Surface Layer - sound velocity subject to daily and local changes in heating and cooling, and wind action. Seasonal thermocline - negative thermal or velocity gradient that varies with season. Summer-fall - near surface waters are warm and it is well defined. Winter-spring - it tends to merge and be indistinguishable from the surface layer. Main thermocline - affected only slightly by seasonal changes. Here the major decrease in temperature occurs. Deep isothermal layer - nearly constant temp of 39 o F. Sound velocity increases due to depth. 4

25 Sound Ray Propagation in Ocean 5

26 Ray Tracing Spreadsheet 6

27 Common Sources of Ambient Noise in Deep Water I. Tides and hydrostatic effects of waves - Pressure fluctuations resulting from tides and waves - very low frequency - not too important at frequencies of interest in underwater sound. Tidal currents can cause flow induced noise. II. Seismic disturbances - results from earth's constant seismic activity - low frequency < 100 Hz. III. Oceanic turbulence - caused by turbulence a) induces motion of transducer and causes self noise. b) pressure changes associated with turbulence may be radiated. c) turbulent pressure fluctuations - most significant at low frequency. IV. Ship traffic - dominant source at 100 Hz; principal noise source Hz. V. Surface waves - ambient noise between 500 Hz - 5 khz correlates well with sea state or wind speed. Causes - breaking white caps flow noise - wind blowing over rough sea surface. cavitation - collapse of air bubbles. Rough sea surface is dominant noise source at 1-30 khz. VI. Thermal Noise - results from molecular agitation in the sea. Important at high frequencies (750 khz). 7

28 Average Deep Water Ambient Noise Spectra 8

29 Intermittent Sources of Ambient Noise Do not persist over periods of hours or days. a) Biological sounds - whales, porpoises, dolphins shellfish Hz Snapping shrimp Hz - 0 khz Commercially important fish don't make noise b) Rain 30 db increase 5-10 khz heavy rain 10 db increase 19.5 khz steady rain c) Seismic explosions - seismic surveys If shipping and biological noise are absent and wind is the primary contributor, then shallow and deep water noise levels are nearly the same. In general shallow water is a noisy and highly variable environment for most underwater acoustic operations. 9

30 Sounds from the Ocean Google sounds in ocean 30

31 QUESTIONS 31