high attenuation rates also greatly reduce off-axis sen-
and L where the target appeared to be near the surface.
sitivity to targets so that full-diffraction signatures are
Most strikingly, the direction along which the stron-
rarely seen. We therefore recommend that the antenna
gest portions of the scattered field appear does not al-
towing speeds be kept very slow and data acquisition
ways run through the (mono-static, backscatter) obser-
rates very high to ensure recording of as much of the
vation point; or it does so, but not when the antenna is
diffraction responses as possible. We further recom-
directly over the center of the target. Thus these simu-
mend that an all-digital system eventually be developed
lations illuminate the ways in which interference pat-
so that extremely high rates (e.g., 5121024 traces/s)
terns and directional biases in the signal, dependent on
would further allow noise reduction by trace stacking.
surface location and target inclination, can produce de-
The most likely range of soil dielectric constants (5
ceptive returns on the surface.
14) is consistent with the measured moisture contents.
The average value of about 10 predicts a narrow
CONCLUSIONS AND RECOMMENDATIONS
beamwidth in the plane parallel with the antenna polar-
ization axis. This means that for normal towing opera-
tions, wherein the antennas are polarized perpendicu-
Soil attenuation at JPG was extremely high, as evi-
lar to the transect direction, the system is sensitive to
denced by the high amount of gain required to display
the signals received from within only the first meter of
ground mainly beneath the antennas. We therefore rec-
ommend that coverage be increased by using a multi-
depth. The gain values of 5065 dB that we applied are
channel radar system. Such operation is standard with
consistent with the general values of round-trip attenu-
present commercial systems, but its implementation
ation for 1 m of depth predicted by our propagation
compromises the data acquisition rate. Therefore, this
models. GPR bandwidths should be centered no higher
is further reason for development of a real-time digi-
than 600 MHz because the round-trip attenuation per
tizing GPR system above 300 MHz that could acquire
meter of depth above this frequency may exceed the
data at 5121024 traces/s.
dynamic range of the radar. This fact precludes the use
Horizontal filtering, normally an effective means of
of any microwave system for detection of UXO. These
|Ez|
1.5
0.100
0.093
0.086
1
0.079
0.071
0.064
0.5
0.057
0.050
0.043
0
X
0.036
0.029
0.5
0.021
0.014
0.007
1
0.000
1.5
1.5
1
0.5
0
0.5
1
1.5
Z
a. 45 from the left.
Figure 20. Scattered |Ez| values, when the incidence direction and incident E
field orientations are in the (x,z) plane, for various incidence angles.
28
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