Supplementary material
Fig. 1 (supp. material) depicts the experimental setup for measuring the response of SAW
delay lines buried 5 m deep in a snowdrift. The sensor and the associated 70 cm long dipole
antenna are located in a 3 cm diameter tube filled with snow. The tube is inserted about
1.5 m deep in snow while the GPR scans this area and records both dielectric refections and
echos from the SAW sensor as a function of antenna position. Although each absolute echo
phase with respect to the emitted pulse is dependent on the antenna position, the difference
of the phases of the echos is independent on the antenna position and only depends on
the acoustic velocity, or in this case the temperature through the temperature coefficient of
frequency of the piezoelectric substrate. The data displayed in Figs. 7 and 1 (supp.material)
were processed using the Seismic Unix package (http://www.cwp.mines.edu/cwpcodes)
with the application of a normalization step and bandpass filtering in the 100±50 MHz
band.
Analyzing the signal to noise ratio of the echos returned – starting 1 µs after the emitted
pulse and for a duration of 3.3 µs – from the sensor buried 5 m deep in snow, one can
estimate the depth at which the minimum signal will be detectable. Considering a signal
to noise ratio above 1.7, the maximum readout distance should be 8 times further than the
current position (since 1.74 ' 8), consistent with the 40 m maximum depth estimated from
the classical GPR link budget presented in the main text.
Figures
1
distance (a.u.)
0
1
echo2
time (us)
echo1
2
3
echo3
echo4
4
500 cm
1m
10
8
phase (rad)
45 cm
6
4
2
echo 1
echo 2
echo 3
0
-2
5
10
15
20
25
30
25
30
35
40
trace number
4
3.5
phase (rad)
3
2.5
2
1.5
echo 1-echo 2
echo 1-echo 3
1
5
10
15
20
35
40
trace number
FIG. 1: Experimental setup for recording signals from a sensor while scanning a 100 MHz GPR
unit over a sensor buried 5 m deep in snow. The emitted pulse exhibits ringing due to impedance
mismatch, a condition degrading depth resolution but favorable to efficiently load the acoustic
delay line. The recorded signal clearly displays four echos, the first three being used to extract the
physical quantity under investigation.
2
0.012
0.01
power (a.u.)
0.008
max(=0.01)/1.7
0.006
0.004
0.002
0
0
100
200
300
400
frequency (MHz)
500
600
FIG. 2: Fourier transform of the returned echos for a sensor buried 5 m deep in snow: the spectrum
is given in linear arbitrary unit, exhibiting a signal to noise ration above 1.7. This measurement
indicates that the echo detection should be possible at a distance between the GPR unit and the
sensor of 40 m. Indeed, following the RADAR equation, and assuming only propagation loss, the
returned power decreases as the fourth power of the distance, and 81/4 ' 1.7.
3