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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,

SLIDE 1

Mid-Frequency Bottom interaction measurements

ff the New Jersey continental shelf

Jee Woong Choi1) and Peter H. Dahl2)

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.

SLIDE 2

SW’06 experimental site

SITE: 39.0245 N, 73.0377 W WATER DEPTH : ~ 80 m (Flat bottom)

3 1 5 9 13

M

12 16 7 11 15 2 6 10 14

300o

50m

100m 200m 300m

SLIDE 3

X × Y : size of 1st Fresnel zone (m)

F E D C B A 15 × 5 20 × 5 8 × 4 11 × 4 3 × 3 4 × 3 20 kHz 64 × 21 86 × 21 35 × 17 47 × 17 13 × 12 17 × 12 1 kHz 17° 12.5° 25° 19.5° 43.5° 35°

Grazing angle

X Y

0m 100m 200m 300m A B C D E F 80m 40m 25m 50m

BOTTOM REFLECTION

MORAY

SLIDE 4

2 4 6 8 10 12 14 16 18 20 22 24

MORAY1+ Power Spectal Density Frequency (Hz) Signal (dB V

2/Hz)

2 4

TIME (ms)

Direct path signal at r=50 m

Source Signals

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 CW (superimposed) signal

SLIDE 5

r=100 m, 10 kHz

50 100 150 200

0.3 TIME (ms)

MF output (8-16 kHz)

20 40 60 80 100 10 20 30 40 50 60 70 RANGE (m)

SD: 40 m RD: 25 m

RANGE (m)

SD: 40 m RD: 50 m

20 40 60 80 100 10 20 30 40 50 60 70 50 100 150 200

0.3 TIME (ms)

MF output (8-16 kHz)

14901510 1530

SOUND SPEED (m/s)

DEPTH (m)

SLIDE 6

10 20 30 40 50 60 70 80 0 50 100 150 200 250 RANGE (m)

SD: 40 m RD: 25 m

300 10 20 30 40 50 60 70 80 0 50 100 150 200 250 RANGE (m)

SD: 40 m RD: 50 m

300

r=300 m, 10 kHz

180 200 220 240 260 280 300 320

0.5 TIME (ms) 180 200 220 240 260 280 300 320

0.5 TIME (ms)

SLIDE 7

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, ~1.0-1.3φ), 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

SLIDE 8

14901510 1530 10 20 30 40 50 60 70 80

SOUND SPEED (m/s)

DEPTH (m)

20 40 60 80 100 120 140 160 180 200

RANGE (m) D S BS SB SBS

RELATIVE INTENSITY (dB)

D S B BS SB SBS ~37° ~30° 1630 m/s 1740 m/s 1499 m/s 21.8m ~27° 0.2 m, 1680 m/s

6 kHz

D S B BS SB R SBS RS SR

2 kHz

Measurements

R ??

MEASUREMENTS

SLIDE 9

GEOACOUSTIC PARAMETER ESTIMATE

2. 2nd layer : from the travel time difference between B & R paths

Thickness (21.8 ± 1 m) Sound speed (1630 ± 20 m/s)

Ref.) K. L. Williams, et. al. (2002) IEEE J. Ocean. Eng. 27, 413-428

B path R path

1. 1st layer : 1680 m/s at 2 kHz, 1730 m/s at 65 kHz

(Dispersion effect for sediment sound speed)

SLIDE 10

3. Attenuation of 2nd 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 2nd layer is 87 m. Most attenuation in 2nd layer (20 m)

4. Densities for 1st & 2nd layers assumed to be 2.1 and 2.0 g/cm3, respectively.

Ref.) Goff et. al. (2007) JASA 122 2983

5. R reflector as Half-space with 1740 m/s, 2.2 g/cm3, 0.3 dB/m/kHz

Ref.) Jiang & Chapman (JASA, in press)

B path R path

ℜ

Τ Τ Τ Τ

87 m

GEOACOUSTIC PARAMETER ESTIMATE

SLIDE 11

120 130 140 150 160 170 180 190 200 210

TIME (ms)

D S B BS SB R SBS RS SR

RELATIVE INTENSITY (dB)

PE Simulation 2 kHz

DEPTH (m)

WATER-SEDIMENT INTERFACE R-REFLECTOR THIN SURFICIAL LAYER 10

120 100 80 60 40 20

dB

PE Simulation

D S B BS SB R SBS RS SR

2 kHz

Measurements

MEASUREMENTS VS. SIMULATION

SLIDE 12

Range 200 m, RD 50 m, GA 19.5°

Frequency (kHz)

20log|R13|

1 2 3 4 6 8 10 12 14 16 18 20

Range 100 m, RD 25 m, GA 43.5°

Frequency (kHz)

1 2 3 4 6 8 10 12 14 16 18 20

5 10

Range 300 m, RD 25 m, GA 17°

20log|R13|

1 2 3 4 6 8 10 12 14 16 18 20

Range 100 m, RD 50 m, GA 35°

1 2 3 4 6 8 10 12 14 16 18 20

5 10

Range 300 m, RD 50 m, GA 12.5°

20log|R13|

1 2 3 4 6 8 10 12 14 16 18 20

Range 200 m, RD 25 m, GA 25°

1 2 3 4 6 8 10 12 14 16 18 20

1730 m/s 1650-1728 m/s (for 1 – 20 kHz) Surficial layer α = 0.2 f 1.6 dB/m, f: kHz BOTTOM LOSS FOR FIRST TWO LAYERS

SLIDE 13

MOVING SOURCE SD: 40 m

C4 -> C16

RELATIVE INTENSITY (dB)

D S B BS SB R SBS RS SR

2 kHz

120 130 140 150 160 170 180 190 200 210

D S B BS SB SBS R

TIME (ms) TIME (ms) RANGE (m) 0m 100m 200m 300m

RD: 25 m MEASUREMENTS

SLIDE 14

MOVING SOURCE SD: 75 m depth

D S B BS R

DEPTH (m) DEPTH (m) RANGE (m) TIME (ms) TIME (ms) TIME (ms)

D S B BS R Range: 50 m

RD: 50 m MEASUREMENTS PE SIMULATED ACOUSITC FIELD

SLIDE 15

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.(R13) measurements (for 1-20 kHz) show a thin (~20 cm) higher sound speed layer

verlaying a thicker (~20 m) lower sound speed layer

ending at a high-impedance reflector (R-reflector)

SLIDE 16