Acoustic seafloor mapping systems. September 14, 2010

Transcription

1 Acoustic seafloor mapping systems September 14, Delft Vermelding Institute onderdeel of Earth organisatie Observation and Space Systems

2 Acoustic seafloor mapping techniques Single-beam echosounder (since 190s) sidescan sonar (since 1960s) multi-beam echosounder (since 1970s)

3 The single-beam echosounder (SBES) H ct Signal duration T : ~0.1-1 ms Beam width (angular aperture) : ~10 Frequency f : ~ khz Source level SL : db 3

4 Transducer directivity pattern Beam width Circular: 65 d Rectangular: 1, 50 L 1, 4

5 Modern digital single beam sounder: effect of frequency choice (38 khz or 00 khz) 5

6 Single-beam echo sounder Transmission sequence T R H c T T or T R nh c T T 6

7 7 Vertical resolution:, Horizontal resolution: Single-beam echo sounder Measurement resolution ct z B c z H H x tan 1 ) ( z x x z z

8 Horizontal resolution shallow water 8

9 Horizontal resolution - continued shallow water = 10 9

10 Horizontal resolution - continued deeper water - same beam width = 10 10

11 Single-beam echo sounder Echo formation Assumptions: A flat and horizontal seafloor A conical directivity pattern Only echo from water-seafloor interface a) t 0 H / c : impact point b) From t t 0 to t t 0 T : A re R H ) Hc( t t ) ( 0 c) From t t T to : 0 t max A ( r e r i ) R ct R HcT d) After t max t0 1 tan 8 A ( r e r, max i ) / t 1 0 with 11 r e, max H tan( / ) H /

12 Single-beam echo sounder Echo formation, continued short-pulse or pulse-limited regime: t max t 0 T long-pulse or beam-limited regime: t max t 0 T Modified expressions for active area, see lecture notes! 1

13 Exercise handed out last lecture Consider the following situation: We are taking measurements with a multi beam echo sounder system. The sound speed profile in the water column is as follows: 0 m 1460 m/s Sound speed (m/s) 100 m 1500 m/s Depth (m) Take for the density in water column of 1g/cm3 The sediment is a clay sediment (sound speed of 1470 m/s, density in the sediment of 1. g/cm3) a. Calculate for a sound ray emitted at an angle of.1 degrees with the horizontal, the angle of the ray at the bottom. b. What is the reflection coefficient? c. What happens to the sound if it arrives at the bottom, taking your above answers into account. September 14,

14 Exercise handed out last lecture, contn d Step 1: make a sketch of the situation 1 =.1 c 1 = 1460 m/s, 1 = 1 g/cm 3 c = 1500 m/s, 1 = 1 g/cm 3 3 Step : Determine angles c 3 = 1470 m/s, 1 = 1. g/cm 3 c 1500 a cos cos1 a cos cos c c a cos cos a cos cos c 1500 c or a cos cos1 a cos cos c September 14,

15 Exercise handed out last lecture, contn d Step 3: make a sketch of the reflection coefficient What is the situation, do we have an angle of intromission or a critical angle? -> The sound speed in the sediment is lower than that in the water column directly above the sediment: no critical angle -> c c3 1470x , i.e., we have an angle of intromission c 3 1 c 0 arctan c 3 1 c September 14,

16 Exercise handed out last lecture, contn d Step 4: What is the reflection coefficient Based on the previous analysis we expect a reflection coefficient of 0. Check: R c c 1.x c3 c 1.x sin sin sin 1.1 sin sin3 sin sin 1.1 sin September 14,

17 Exercise SBES with an opening angle of 10 degrees 4000 m of water depth What is the horizontal resolution? September 14,

18 H 4000xx10 re x180 x=698 m A= r 0.4 km e 349 m Beam footprint September 14,

19 Single-beam echo sounder Maximum operating range Water depth: 5 km Dominant noise source: self noise Receiving bandwidth: 500 Hz Ship speed: 15 knots Echo-sounder: circular transducer with an area of 400 cm SNR should be at least 10 db Required specifications for the echo sounder, its frequency f in particular? 19

20 Single-beam echo sounder Maximum operating range, continued SNR EL BGL with EL SL PL BGL NL W DI DI 10 log 10 4 A, with A = 400 cm, DI = 19.5 db at 0 khz SL log P DI NL W For P = 50 Watt: SL = 07 db re 1 Pa at 0 khz. NL log W with NL NL w amounts to 8.6 db at 0 khz v 0 log 1 10 f 0

21 Single-beam echo sounder Maximum operating range, continued PL log( H) H BL is 4.1 db/km BL 0 10 log c c c c 1 Clay Coarse sand : BL = db : BL = 7.7 db 1

22 Single-beam echo sounder Maximum operating range, continued

23 The sidescan sonar system Signal duration T : ~less than 0.1 ms Beam width (angular aperture) : ~1 in horizontal plane, ~85 in vertical plane Frequency f : several 100 khz Transmission sequence: T R R c max Footprint: signal footprint beam footprint 3

24 Sidescan sonar Image reconstruction Measurements consist of backscattered intensity as a function of time. For a flat seafloor: y R H c t 4 H 4

25 Sidescan sonar Echo formation t f t Obstacle height: h H t 5 t F

26 Sidescan sonar resolution Along track x Across track y Across track y: y ct sin y ct / y HcT for 90 for = 0 Along track x: x R 6

27 Sidescan sonar coverage To ensure 100 % coverage: x H x vt R (insonified strip is narrowest near vertical) (sonar distance between two pings) T R R c max v ch c cos R max max (i.e., 100 % coverage condition independent of H) 7

28 Sidescan sonar Examples 8

29 Multi-beam echo sounders (MBES) simultaneous measurement of bathymetry and backscatter 9

30 MBES MBES types Deep water systems: operating at f ~ 1 khz. The large dimensions of the transducer array limits their installation to deep-sea vessels. Shallow water systems: operating at f ~ khz. Mapping of the continental shelf. High-resolution systems: operating at f ~ khz. Local studies (shipwreck location, inspection of underwater structures). Small size allows for installation on small ships, tow fishes or autonomous underwater vehicles (AUV). 30

31 MBES transmission and reception arrays Transmission (determines along-track resolution x) Reception (using beamforming) (determines across track resolution y) 31

32 MBES bathymetry (topography) measurement Joint measurement of time t and angle y R( t)sin z R( t)cos ct sin ct cos (when ssp is constant, otherwise ray tracing is necessary) 3

33 MBES imaging Combining time signals after beamforming: Compared to SSS: backscatter pixels are now at the correct position because of bathymetry measurement capability!!! 33

34 MBES bathymetry measurement errors Basis: y R( t)sin z R( t)cos ct sin ct cos Errors in range R: Errors in angle : z y R R z y R cos R sin Rsin Rcos H tan H Prediction of bathymetry measurement accuracy with MBES is an important research item!!! Types of errors: Errors in the acoustic measurement itself (acoustic noise, non-stable backscattered signal); Movements of the support platform (finite accuracy of the attitude sensors); Inaccuracies in sound speed corrections. 34

35 MBES Refraction correction 35

36 MBES Refraction correction, continued c( z) c g z with c(h) = c 1 t ds c(z), with ds dy dz and dz tan dy : ds dz /sin Snell s law: cos1 cos( z) cos( z) c c( z) c gz 1 1 dz c 1 g sin d cos 1 t 1 1 c 1 cos 1 cos 1 c1 sin g sin cos 1 d 1 g 1 1 cos d 1 g / tan ln / 1 tan 36

37 MBES Refraction correction, continued 37

38 MBES resolution Along track resolution x R L for bathymetry & imaging Across track resolution For imaging: y ct sin y HcT 0 ( for ) For bathymetry: H y T cos - y ct sin 38

39 MBES Repetition period and coverage v c L cos max Global mapping of the deep oceans (H = 4 km) max = 75, L = 1.5 v: 5 m/s or 10 knots (100 % coverage) Swathe: L H tan max 30 km Area covered per second: vl 0.15 km / sec Surface of Earth s deep oceans: 350x10 6 km km per day Required for global mapping:.7x104 days = 74 years With 50 deep water MBES systems (situation in 00): ~1 year 39

40 A typical MBES result 40