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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.