Properties of the non-propulsive ship noise field as measured from

1. Properties of the non-propulsive ship noise field as measured from a research vessel moored in the Yellow Sea and relation to sea bed parameters Peter H. Dahl1, Jee Woong Choi2, and David Dall’Osto1 1Applied Physics Laboratory University of Washington 2Hanyang University, Ansan, Korea

2. Photo of the R/V Shi Yan 3 taken in the East China Sea during ASIAEX, June 2001 Length 104 m Beam 14 m Draft 5 m 16 element VLA, separation 4 m Depth to Lowest Channel: 64 m Depth to Seabed: ~75m

3. KEY POINTS OF TALK • 1. The non-propulsive noise field beneath the vessel has effective radiation length L • L increases with decreasing frequency • 2. Vertical Spatial Coherence of this noise field shows influence of seabed • sensitive to ~10 % changes in r c • The seabed loading and scale L are often not included in far-field interpretations • of ship noise measurements made in shallow water

4. 16 m 32 m 48 m 64m 18 0 2 4 6 8 10 12 14 16 Stationary Noise from the ship’s service diesel generator Time (s)

5. Harmonics every 5-6 Hz: very typical of Ship’s Service Diesel Generator

6. Estimation of the vertical component of the intensity vector At 64 Hz Harmonic, 4 m separation between hydrophones is about 1/6 wavelength A finite difference approximation yield quantity proportional to vertical component of particle acceleration : av = -1/rDp/Dz This is time integrated for vertical component of particle velocity, v.The Average pressure p is taken between adjacent sensors Active Intensity =<p v > Reactive Intensity =<p av>/(2pf)

7. ACTIVE ~ Im (F12(w))/w REACTIVE ~ 1/2 (F11(w) –F22(w))/w ANALYSIS AT 64 Hz, BW < 0.5 Hz Fahy’s algorithm, 1989 0 -5 kZ* ~9 -10 -15 Spherical Spreading ACTIVE AND REACTIVE INTENSITY (dB, arb ref) -20 -25 -30 -35 -40 0 2 4 6 8 10 12 14 16 18 20 NORMALIZED RECEIVER DEPTH FROM KEEL (kz), 64 Hz

8. ANALYSIS OF HIGHER FREQUENCY HARMONICS finite difference analysis not possible, but still can identify kZ* 446 Hz and 476 Hz kZ* ~45 75 446 Hz and 476 Hz kZ* ~45 70 65 60 p p* (dB, arb ref) 55 74 Hz and 79 Hz kZ* ~12 50 45 0 10 20 30 40 50 60 70 80 90 100 110 NORMALIZED RECEIVER RANGE FROM KEEL (kz)

9. Summary of measurements of kz* mapped to an effective radiation scale L= effective radiation length scale along the hull kz*= pL2/l2 L (m) Frequency (Hz)

10. Ray based modeling source (SSDG) air-coupled to the hull postulate an effective radiating source consisting of elemental sources along an effective aperture direct path + single bottom bounce path reduced in amplitude by about 80% (bottom loss) Length of aperture depends on frequency ~36 m at 60 Hz to ~10 m at 600 Hz 75 m Sea bed with estimated geoacoustic model (Dahl & Choi, JASA, Dec 2006)

11. OBSERVATIONS: VERTICAL SPATIAL COHERENCE, D = 4 m Im Re

12. Re Model Im Model FREE FIELD (NO SEA BED)

13. Re Model Im Model ADDITIONAL LOADING FROM SEA BED

14. Re Model Im Model INFLUENCE OF SEABED DENISTY

15. ACTIVE ~ Im (F12(w))/w REACTIVE ~ 1/2 (F11(w) –F22(w))/w

16. ACTIVE REACTIVE Model: active reactive REACTIVE INTENSITYW/O SEABED LOADING Spherical wave effects (courtesy DJ Tang’s code)

17. SUMMARY • 1. The non-propulsive noise field beneath the vessel has effective radiation length L • L increases with decreasing frequency (damping by internal structures) • 2. Vertical Spatial Coherence of this noise field shows influence of seabed • sensitive to ~10 % changes in r c • 3. Spherical wave effects are non-observable with this data –likely will be for • receivers closer to seabed, and frequencies < ~ 50 Hz • The seabed loading and scale L are often not included in far-field interpretations • of ship noise measurements made in shallow water

18. 100 Hz kZ* ~ 10 400 Hz kZ* ~60