4. SFGPR field tests
The output display of the SFGPR is quite similar to that of an impulse GPR. A bipolar
pulse can be synthesized as shown in the exam- ple of Fig. 4. A colour image can be displayed
with hot colours to show the negative swings of the synthesized received pulse and cool colours
to show the positive swings. However, the grey
Ž .
scale images in this paper e.g., Fig. 5 uses grey shades. The use of a logarithmic palette
allows the images to be displayed without the need for software sensitivity time control. The
grey scale bars enable the signal power of re- ceived reflections to be read directly from the
image.
The radar has been tested on several different occasions along the banks of a perched freshwa-
ter lake on a sand island near Brisbane, Aus- Ž
. tralia Stickley et al., 1997 . The sand is rela-
tively uniform and has very low attenuation. The underground water table extends under the
sand from the lake, and provides a strong though sometimes broken interface reflector.
Fig. 5 shows the ungated SFGPR image of the perched water table on the sand island as the
antennas were moved up an incline away from the lakeshore. Using the methodology presented
Ž .
by Noon et al. 1998 , it is possible to estimate the strength of the water table reflector. For this,
the following assumptions and estimations are made; antenna gain balances the antenna ineffi-
Ž ciencies; sand has a Q s 64 i.e., virtually loss-
. less ; reflection loss at the water table is 11 dB;
Ž reflector depth is 2.5l for 100 MHz centre
. frequency and 25 ns one-way . Using the rough
planar interface graph for the water table reflec- Ž
. tor Noon et al., 1998 , a received signal loss of
54 dB is estimated. This estimation is in good agreement with 55 dB loss determined in Fig. 5.
Fig. 6 shows the gated image obtained during a subsequent survey to that of Fig. 5. The power
level is adjusted to take into account the duty cycle loses of gating mode. The gate was set so
Fig. 7. Gated image of the water table taken along a 120 m survey of an elevated track. The variation in travel time mirrors the surface topography. The radar was stepped in frequency from 60 MHz to 180 MHz.
that all returns with delays less than 30 ns were rejected. The strongest target in the image is the
water table at 62 dB loss. The 7 dB discrepancy with the ungated image is due to the partial
overlap of the receiver gate and reflector delays.
An electromagnetic double bounce can be seen clearly in Fig. 6. It arises when the echo
from the water table reaches the surface and is reflected to make a second round trip. As a
result its phase is inverted relative to the origi- nal signal and it occurs at twice the delay time
of the main water table reflection. Both these features are seen in the image. The double
bounce signal overlaps the centre of receiver gate almost completely and hence reads at the
correct power.
Using the similar assumptions and methodol- ogy as with Fig. 5, the estimated signal loss of
84 dB for the double bounce in Fig. 6 compares well with the actual measurement of 85 dB in
the image. Fig. 7 shows a gated image of the water table
recorded on an elevated sand hill further away from the perched lake. The reflection depths are
much deeper, and the variation in travel time to the water table mirrors the surface topography.
At these depths, the water table is not as contin- uous as the previous shallow images. With the
use of gating, the cross-coupling signal has been reduced by more than 60 dB compared with
ungated mode. Using similar methodology as earlier, the estimated signal loss for the water
table reflection is 75 dB. This is in good agree- ment with Fig. 7, where the water table signal
loss varies between 70 dB and 80 dB.
The SFGPR has also been trialed at the Uni- versity of Queensland campus. The image in
Ž .
Fig. 8 was taken of a shallow f 50 cm deep rectangular culvert on the banks of the Brisbane
river. Although the signal losses from the cul- Ž
. vert are very high 62 dB , reflections are
produced at the edges and within the culvert.
Fig. 8. Radar image taken across a shallow culvert on the banks of the Brisbane River. The radar was stepped in frequency from 50 MHz to 350 MHz.
Fig. 9. Mean power of each frequency component for the data shown in Fig. 8. The straight-line approximation is
part of the frequency compensation procedure used to produce Fig. 10.
The highly attenuating soil in this area causes the higher frequencies to be significantly re-
duced in strength in comparison with the low frequencies. This is evident in Fig. 9, which
Ž .
shows the mean power at each frequency step taken across many depth profiles in Fig. 8. The
severe attenuation at higher frequencies causes a reduction in the resolving power, and the ap-
pearance of significant range-sidelobes in the image.
Because this data has abundant signal-to-noise ratio, the frequency domain data can be weighted
to form a white spectrum. Fig. 9 portrays a manually fitted straight line through the fre-
quency domain that is used to whiten the spec- trum. Weightings are applied to the frequency
domain data such that this line becomes hori- zontal. In order to keep power levels consistent
with the uncompensated image, the weighting applied to the centre frequency is constrained to
be unity. A Hamming window is also applied after the frequency correction step to reduce the
range-sidelobe level.
Fig. 10 is the resulting frequency compen- sated counterpart to Fig. 8. The resolution and
range-sidelobe levels are much improved.
Fig. 10. Frequency compensated version of Fig. 8. Note the increased resolution and reduced range-sidelobe levels.
Fig. 11. Gated radar image of a 500 lb bomb buried at 1.7 m. The bomb hyperbola is clear; as is the interruption by the trench of a natural layer in the ground.
The weakest confirmed underground target imaged with the gated SFGPR is a 500 lb bomb
buried 1.7 m in the ground. Fig. 11 shows a Ž
. radar image
with frequency compensation taken crossing the burial trench at right angles.
The bomb was buried over ten years ago and the trench was carefully backfilled such that it is
not visible from the surface. The peak signal strength taken from the bomb hyperbola of Fig.
11 is 100 dB below the transmitted power, with the signal from the bomb at least 40 dB above
the noise, and with the transmitter power still 15 dB below maximum.
The dynamic ranges of commercial impulse GPR systems are limited to 55 dB in the sam-
Ž .
pling head Daniels, 1996 . This dynamic range limits the weakest signal that can be seen in the
presence of a strong signal. At the bomb site of Fig. 11, the cross-coupling signal was 40 dB.
Hence the bomb would not have been observed Ž
with a commercial impulse GPR with similar .
antennas .
5. Conclusions
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