Mid-Frequency Bottom interaction measurements off the New Jersey continental shelf Jee Woong Choi 1) and Peter H. Dahl 2) 1) Dept. of Environmental Marine Sciences, Hanyang Univ., KOREA 2) Applied Physics Laboratory, University of Washington, USA SHALLOW WATER 2006 1) APL-UW (Applied Physics Laboratory, Univ. of Washington) Mid-to-High frequency Bottom reflected signal measurements Mid-to-High frequency Sea Surface scattered signal measurements Mid frequency propagation measurements Sediment sound speed measurements Sediment roughness measurements Supported by ONR, with partial support from Hanyang Univ.
SW’06 experimental site SITE: 39.0245 N, 73.0377 W WATER DEPTH : ~ 80 m (Flat bottom) 300 o 14 300m 10 200m 100m 6 2 50m M 15 11 7 3 1 5 9 13 4 8 12 16
BOTTOM REFLECTION 0m 100m 200m 300m MORAY 25m 40m 80m 50m X Y A B C D E F X × Y : size of 1 st Fresnel zone (m) A B C D E F Grazing 35 ° 43.5 ° 19.5 ° 25 ° 12.5 ° 17 ° angle 17 × 12 13 × 12 47 × 17 35 × 17 86 × 21 64 × 21 1 kHz 4 × 3 3 × 3 11 × 4 8 × 4 20 × 5 15 × 5 20 kHz
Source Signals Direct path signal at r =50 m 1-4 kHz(1, 2, 3, 4 kHz), 5 ms 4-20 kHz(4, 6, 8, 10, 12, 14, 16, 18, 20 kHz), 3 ms 0 2 4 CW (superimposed) signal TIME (ms) MORAY1+ Power Spectal Density -50 -55 -60 -65 2 /Hz) -70 Signal (dB V -75 -80 -85 -90 -95 -100 2 4 6 8 10 12 14 16 18 20 22 24 Frequency (Hz)
r =100 m, 10 kHz SD: 40 m RD: 25 m 0 0.3 10 20 DEPTH (m) 30 40 50 60 MF output (8-16 kHz) 70 -0.3 14901510 0 20 40 60 80 100 50 100 150 200 1530 TIME (ms) RANGE (m) SOUND SPEED (m/s) SD: 40 m RD: 50 m 0 0.3 10 20 30 40 50 MF output 60 (8-16 kHz) 70 -0.3 0 20 40 60 80 100 50 100 150 200 TIME (ms) RANGE (m)
r =300 m, 10 kHz SD: 40 m RD: 25 m 0.5 0 10 20 30 40 50 60 70 80 0 -0.5 50 100 150 200 250 300 180 200 220 240 260 280 300 320 RANGE (m) TIME (ms) SD: 40 m RD: 50 m 0 0.5 10 20 30 40 50 60 70 80 0 -0.5 50 100 150 200 250 300 180 200 220 240 260 280 300 320 RANGE (m) TIME (ms)
Sediment structure : SW’06 experiment site N. R. Chapman and Y.-M. Jiang, "Geoacoustic inversion results from SW06 data ― long range vs. short range," ONR SW06 Workshop, Fort Lauderdale, FL, USA, 2008. 65 kHz in situ probes: Surficial sediment � medium-coarse sand, ~1.0-1.3 φ ), 1720-1740 m/s Ref.) Goff et. al., Mar. Geol., 209, 147-172, (2004) Coring: Thickness of surficial sediment ~20 cm lower layer � very clay rich (~1630-1660 m/s) sediment Ref.) Fulthorpe and Austin, Geology, 32, 1013-1016, (2004) Two Observations : 1) Reflection from R-reflector 2) Reflection from the surficial layers
MEASUREMENTS 0 10 20 S DEPTH (m) 30 1499 m/s D 40 SBS 50 ~37 ° 0.2 m, 1680 m/s BS B 60 SB 21.8m 70 ~27 ° 1630 m/s 80 ~30 ° 14901510 0 20 40 60 80 100 120 140 160 180 200 1530 1740 m/s RANGE (m) SOUND SPEED (m/s) R 0 RS SR RELATIVE INTENSITY (dB) B -20 2 kHz S BS SB SBS D -40 Measurements -60 0 6 kHz B -20 R ?? S BS SB SBS D -40 -60
GEOACOUSTIC PARAMETER ESTIMATE 1. 1 st layer : 1680 m/s at 2 kHz, 1730 m/s at 65 kHz (Dispersion effect for sediment sound speed) Ref.) K. L. Williams, et. al. (2002) IEEE J. Ocean. Eng. 27, 413-428 2. 2 nd layer : from the travel time difference between B & R paths Thickness (21.8 ± 1 m) Sound speed (1630 ± 20 m/s) B path R path
GEOACOUSTIC PARAMETER ESTIMATE 3. Attenuation of 2 nd layer : α = 0.05 ± 0.01 dB/m/kHz From the amplitude ratio between B & R path for freq. 1-4 kHz. (assumption: ℜ , Τ are constant for 1-4 kHz and Total path length within 2 nd layer is 87 m. Most attenuation in 2 nd layer (20 m) B path R path Τ Τ Τ Τ 87 m ℜ 4. Densities for 1st & 2nd layers assumed to be 2.1 and 2.0 g/cm 3 , respectively. Ref.) Goff et. al. (2007) JASA 122 2983 5. R reflector as Half-space with 1740 m/s, 2.2 g/cm 3 , 0.3 dB/m/kHz Ref.) Jiang & Chapman (JASA, in press)
MEASUREMENTS VS. SIMULATION R RS 0 SR -20 2 kHz S BS SB SBS D B -40 Measurements -60 10 0 0 20 dB -10 40 DEPTH (m) WATER-SEDIMENT INTERFACE -20 60 -30 80 THIN SURFICIAL LAYER -40 100 R-REFLECTOR -50 PE Simulation -60 120 R INTENSITY (dB) 0 SR RS RELATIVE 2 kHz -20 -40 SBS D S B BS SB PE Simulation -60 120 130 140 150 160 170 180 190 200 210 TIME (ms)
BOTTOM LOSS FOR FIRST TWO LAYERS Range 300 m, RD 50 m, GA 12.5 ° Range 200 m, RD 25 m, GA 25 ° -20log|R 13 | 5 5 0 0 -5 -5 1 2 3 4 6 8 10 12 14 16 18 20 1 2 3 4 6 8 10 12 14 16 18 20 Range 300 m, RD 25 m, GA 17 ° Range 100 m, RD 50 m, GA 35 ° 5 10 -20log|R 13 | 0 5 -5 0 1 2 3 4 6 8 10 12 14 16 18 20 1 2 3 4 6 8 10 12 14 16 18 20 Range 200 m, RD 50 m, GA 19.5 ° Range 100 m, RD 25 m, GA 43.5 ° -20log|R 13 | 5 10 0 5 -5 0 1 2 3 4 6 8 10 12 14 16 18 20 1 2 3 4 6 8 10 12 14 16 18 20 Frequency (kHz) Frequency (kHz) 1730 m/s Surficial layer 1650-1728 m/s (for 1 – 20 kHz) α = 0.2 f 1.6 dB/m, f : kHz
MOVING SOURCE SD: 40 m 0m 100m 200m 300m RD: 25 m D S B BS SB SBS R RANGE (m) C4 -> C16 MEASUREMENTS TIME (ms) R INTENSITY (dB) 0 RS SR RELATIVE -20 2 kHz D S B BS SB SBS -40 -60 120 130 140 150 160 170 180 190 200 210 TIME (ms)
MOVING SOURCE SD: 75 m depth RD: 50 m D S R B BS RANGE (m) MEASUREMENTS TIME (ms) Range: 50 m D B BS DEPTH (m) DEPTH (m) S R TIME (ms) TIME (ms) PE SIMULATED ACOUSITC FIELD
SUMMARY Acoustic bottom-interacting measurements from SW06 in New Jersey Continental Shelf, WD 80 m (1) Direct measurements of the reflection from the R-reflector (for < 6 kHz) (2) Reflection Coeff.( R 13 ) measurements (for 1-20 kHz) � show a thin (~20 cm) higher sound speed layer overlaying a thicker (~20 m) lower sound speed layer ending at a high-impedance reflector (R-reflector)
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