reverberation clutter from combined internal wave...
TRANSCRIPT
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.