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. Dahl 1, Jee Woong Choi 2, and David Dall’Osto 1 1 Applied Physics Laboratory University of Washington 2 Hanyang University, Ansan, Korea

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

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  c 3.The seabed loading and scale L are often not included in far-field interpretations of ship noise measurements made in shallow water KEY POINTS OF TALK

Stationary Noise from the ship’s service diesel generator

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

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 : a v = -1/   p /  z This is time integrated for vertical component of particle velocity, v. The Average pressure p is taken between adjacent sensors Active Intensity = Reactive Intensity = /( 2  f)

NORMALIZED RECEIVER DEPTH FROM KEEL ( kz ), 64 Hz ACTIVE AND REACTIVE INTENSITY (dB, arb ref) ACTIVE ~ Im (F 12 (  ))/  REACTIVE ~ 1/2 (F 11 (  ) –F 22 (  ))/  Fahy’s algorithm, 1989 ANALYSIS AT 64 Hz, BW < 0.5 Hz kZ* ~9 Spherical Spreading

446 Hz and 476 Hz kZ* ~45 74 Hz and 79 Hz kZ* ~ NORMALIZED RECEIVER RANGE FROM KEEL (kz) p p* (dB, arb ref) 446 Hz and 476 Hz kZ* ~45 ANALYSIS OF HIGHER FREQUENCY HARMONICS finite difference analysis not possible, but still can identify kZ*

kz*=  L 2 / 2 L = effective radiation length scale along the hull L (m) Frequency (Hz) Summary of measurements of kz* mapped to an effective radiation scale

Sea bed with estimated geoacoustic model (Dahl & Choi, JASA, Dec 2006) 75 m 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 Ray based modeling

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

FREE FIELD (NO SEA BED) Im Model Re Model

ADDITIONAL LOADING FROM SEA BED Im Model Re Model

INFLUENCE OF SEABED DENISTY Im Model Re Model

ACTIVE ~ Im (F 12 (  ))/  REACTIVE ~ 1/2 (F 11 (  ) –F 22 (  ))/ 

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

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  c 3. Spherical wave effects are non-observable with this data –likely will be for receivers closer to seabed, and frequencies < ~ 50 Hz 4. The seabed loading and scale L are often not included in far-field interpretations of ship noise measurements made in shallow water SUMMARY

400 Hz kZ* ~ Hz kZ* ~ 10