1.0
0
20
0.8
40
0.6
60
0.4
80
0.2
100
Figure 24. Spectra of two
0
120
0
50
100
150
200
0
50
100
150
200
center elements of vertical
1.0
0
geophone subarray for an
M60 tank moving at 4.5 m
20
0.8
s1. Spectra are strongly
40
0.6
peaked at 11 and 29 Hz.
60
0.4
80
0.2
100
0
120
0
50
100
150
200
0
50
100
150
200
Frequency (Hz)
Frequency (Hz)
shows the vertical-geophone power spectra of el-
Interference effects may also be present between
ements 1 and 2 from Figure 22. The spectra are
acoustic energy coupled near to the geophone and
strongly peaked at 11 and 29 Hz. Plane-wave
surface waves excited near the source. This hy-
modeling at these two frequencies with a phase
pothesis is partially supported by the presence of
velocity of 220 m s1 and the Grayling array geom-
a strong spectral peak at 29 Hz in both the micro-
etry indicates that signal interference may be the
phone and vertical-geophone spectra (Figures 24
cause of the amplitude variation across the array.
and 25).
1.0
0
20
0.8
40
0.6
60
0.4
80
0.2
100
0
120
0
50
100
150
200
0
50
100
150
200
Figure 25. Spectra of micro-
1.0
0
phone subarray response to
20
an M60 tank moving at 4.5
0.8
m s1. Spectra are strongly
40
0.6
peaked around 3 Hz and 29
60
0.4
Hz.
80
0.2
100
0
120
0
50
100
150
200
0
50
100
150
200
1.0
0
20
0.8
40
0.6
60
0.4
80
0.2
100
0
120
0
50
100
150
200
0
50
100
150
200
Frequency (Hz)
Frequency (Hz)
21
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