Reverberation clutter from combined internal

wave refraction and bottom backscatter

Dajun (DJ) Tang and Frank S. Henyey

Applied Physics Lab

University of Washington

Thorsos/Yang for PE support

ONR funding support

H

DAHL 1-20 k m

Some candidate clutter sources

Rough

surface

Rough Bottom A wreck

IWFish

Motivation:

A widely studied mechanism for clutter in recent years: a long-tailed

distribution of scattering intensity from a single “pool” of scatterers.

(Abraham and Lyons, 2002)

Proposed hypothesis: one alternate mechanism for clutter observed in

shallow water reverberation measurements is due to the combined effect

of forward scatter and subsequent backscatter.

Non-linear internal wave is examined as source for forward scatter.

• Get Stratification from New Jersey shelf data

• Calculate Solitary Waves

• Calculate Sound Speed Field with a Wave

• Trace Acoustic Rays through that Sound Speed Field

• Calculate reverberation time series using

1. PE for two-way forward scatter

2. Generate a single bottom rough surface for backscatter

3. Perturbation theory for single backscatter

4. Fourier synthesis to obtain time-domain reverberation

Procedure

Acoustic problem studied:Narrow-band source and receiver at 38 m to measure reverberation

fc = 250 Hz

Sandy bottom with “typical” bottom roughness

NLIW present

SW06 web site has SWARM Stratification

http://4dgeo.whoi.edu/swarm-bin/view_ctds.pl

Station 7

Stratification

Source

Buoyancy Frequency

Dubreil-Jacotin, Long Equation

• Solitary Wave (unchanging form)

• Nondissipative

• Two Dimensions

• Arbitrarily Large Amplitude

• Solve by variational method of Terkington et al.

• One parameter family of solutions

V2 2 = No2(z– )

Ray Tracing

• Source at x = – 800 m, z = –38

m, close to the minimum

sound speed

• 41 rays with initial slope

between –6o and + 6o

The Rays

and the wave

2 222 22

1 0 '

'

( ) ( ' | ) 1( , ) ' ( ') ( 1) ( ' | ) 1 ( ' | )

4 'x

z H

kQ f Gp f dx x k G G

z

r rr r r r r

Procedure to calculate time-domain reverberation

1.PE to calculate two-way forward scatter, , on the rippled seafloor2.Add to the seafloor a realization of small scale roughness3.Use 1st order perturbation theory (formula below) to calculate reverberation at individual frequencies.4.Fourier synthesis to obtain time domain reverberation

( ' | )G r r

Small-scale rougness parameters

P = h2 KL/[p*(KL 2+ Kx

2)]

h = 0.316 m

KL = 2.5*1e-3

10-6

10-5

10-4

10-3

10-2

10-1

100

101

-150

-100

-50

0

50

Kx (1/m)

P (

dB

re

. m

)

0 200 400 600 800 1000-0.5

0

0.5

1

x (m)

f1(x

) (m

)

0 2 4 6 8 10 12 14-130

-120

-110

-100

-90

-80

-70

-60

-50

t (sec.)

RL

re

. 1

Pa

Wave at 5000 m Range

0 2 4 6 8 10 12 14-130

-120

-110

-100

-90

-80

-70

-60

-50

t (sec.)

RL

re

. 1

Pa

Wave at 5000 m Range

Clutter

No Wave

1 2 3 4 5 6 7 8 9 10

0

20

40

60

80

-25

-20

-15D

ep

th(m

)

Wave @ 5.0 km

1 2 3 4 5 6 7 8 9 10

0

20

40

60

80

-25

-20

-15

Range (km)

Wave @ 5.5 km

1 2 3 4 5 6 7 8 9 10

0

20

40

60

80

-25

-20

-15

Forward scatter viewed as mode conversion by NLIW

Predictions/Conclusions

• Solitary wave deflects the sound out of the

sound channel through mode conversion.

• Target-like reverberation just beyond the

wave, moving with the speed of the wave.