Experimental configuration: the 30 and 60 cm deep devices are SAW resonators soldered to a long dipole, buried in clay with a conducting wire located in the hole but neither electrically connected to the sensor nor to the interrogation unit. The 80 cm deep resonator was soldered to an RG174 coaxial cable protruding from ground as an open-feed.
Evolution over more than two years of the temperature of buried sensors at depths between 30 and 80 cm. The sensors survived this environment for the duration of this experiment, with no noticeable drift or loss in rf link quality, while providing data consistent with surface temperatures. Only relative temperatures are provided by the sensors since no calibration was performed prior to the experiment: the buried sensor temperatures have been shifted with respect to the averaged air temperature for clarity, while qualitatively exhibiting similar trends after processing the mean air temperature through a nine days running average (thick solid line, maximum and minimum daily temperature obtained from the website referenced in the main text. Data quality is assessed through the standard deviation of the 20 s data set gathered during each measurement: a few unsuitable data with excessive deviation are displayed for demonstration purpose (days 230 or 500, for example).
100 MHz GPR scans of the ice-rock interface: the signal is detected for an interface deeper than 100 m. The raw radar signal were processed using Aslak Grinsted’s PROCESSRADAR.M MATLAB tool. Data acquired on the Austre Lovénbreen glacier (Spitsbergen, Norway).
Frequency domain (top) and time domain (bottom) characterization of a 100 MHz, dual mirror delay line. The network analyzer measurements were performed using a Rohde & Schwartz ZVCE network analyzer under a probe station, with the time domain signal obtained as the inverse Fourier transform of the frequency domain characterization. The RADAR signal obtained in the time domain plot (bottom figure, bottom curve) is observed when locating a sensor 50 cm away from the receiving antenna. The RADAR pulse spectrum (top figure) in the frequency domain is obtained by Fourier transform of the emitted pulse: although the central frequency is dependent of the dielectric environment of the emitting antenna, a large fraction of the emitted pulse overlaps the frequency region of the delay line. Top-right inset: dimensions of the delay line, transducers and mirror position (all dimensions in micrometers). One mirror is located to the left of the transducer, two mirrors are located on the right. Each side of the interdigitated transducer is connected to one branch of a dipole antenna through silver-epoxy bonding.
Top left: the raw color-coded time evolution of the recorded radar echo magnitude between 1.0 and after the excitation pulse was emitted. The sampling is performed at 500 MHz, or five times the frequency of the signal of interest. Top-right: identification of the frequency component (index) representative of the delay line, here visible as a maximum of the magnitude of the Fourier transform of the points from 0.9 to (first echo) and 1.2 to (second echo). We observe that this frequency component of interest does not change with temperature (i.e., is independent on the trace number). Bottom right: time evolution of the unwrapped phase of the Fourier transform at frequency abscissa 25 as identified from the top-right graph. Bottom-left: time evolution of the phase difference between the first and second echoes, after scaling and translation to match the reference temperature curve recorded with a Pt100 probe located next to the delay line. During this whole experiment, the receiving antenna is located 1 m from the emitting antenna, and the sensor is 50 cm from the receiving antenna away from the emitting antenna.
The graph sequence and analysis is the same than the one described in the caption of Fig. 5. Here, however, the sensor is first located 1 m from the receiving antenna, away from the emitting antenna, and brought closer to 50 cm of the receiving antenna at trace number 400. This distance change is observed as an increase in the magnitude of the signal of interest (top-right graph, magnitude of the Fourier transform of the echo), a phase shift in the bottom graph affecting both echoes in the same way, and a decrease in the temperature estimate standard deviation (bottom-left graph).
Signal acquired while scanning a 100 MHz GPR unit over a sensor buried 2.20 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 echoes, the first three being used to extract the physical quantity under investigation. The absolute phase with respect to the emitted pulse is dependent on antenna position and constantly rises as the radar is brought close to the sensor but the phase difference is independent on antenna position and is representative of the physical quantity under investigation. As expected, each echo is made of 21 oscillations, which is equal to the number of electrode pairs in the transducer. The “inverted” hyperbola shape of the echoes is an aliasing artifact when displaying the data.
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 echoes, the first three being used to extract the physical quantity under investigation.
Fourier transform of the returned echoes 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 .
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