Madeleine Griselin

Surface acoustic wave devices as passive buried sensors

Surface acoustic wave (SAW) devices are currently used as passive remote-controlled sensors for measuring various physical quantities through a wireless link. Among the two main classes of designs—resonator and delay line—the former has the advantage of providing narrow-band spectrum informations and hence appears compatible with an interrogation strategy complying with Industry-Scientific-Medical regulations in radio-frequency (rf) bands centered around 434, 866, or 915 MHz. Delay-line based sensors require larger bandwidths as they consists of a few interdigitated electrodes excited by short rf pulses with large instantaneous energy and short response delays but is compatible with existing equipment such as ground penetrating radar (GPR). We here demonstrate the measurement of temperature using the two configurations, particularly for long term monitoring using sensors buried in soil. Although we have demonstrated long term stability and robustness of packaged resonators and signal to noise ratio compat...

Deriving ice thickness, glacier volume and bedrock morphology of the Austre Lovénbreen (Svalbard) using Ground-penetrating Radar

The Austre Lovenbreen is a 4.6 km2 glacier on the Archipelago of Svalbard (79°N) that has been surveyed over the last 47 years in order of monitoring in particular the glacier evolution and associated hydrological phenomena in the context of nowadays global warming. A three-week field survey over April 2010 allowed for the acquisition of a dense mesh of Ground-penetrating Radar (GPR) data with an average of 14683 points per km2 (67542 points total) on the glacier surface. The profiles were acquired using a Mala equipment with 100 MHz antennas, towed slowly enough to record on average every 0.3 m, a trace long enough to sound down to 189 m of ice. One profile was repeated with 50 MHz antenna to improve electromagnetic wave propagation depth in scattering media observed in the cirques closest to the slopes. The GPR was coupled to a GPS system to position traces. Each profile has been manually edited using standard GPR data processing including migration, to pick the reflection arrival time from the ice-bedrock interface. Snow cover was evaluated through 42 snow drilling measurements regularly spaced to cover all the glacier. These data were acquired at the time of the GPR survey and subsequently spatially interpolated using ordinary kriging. Using a snow velocity of 0.22 m/ns, the snow thickness was converted to electromagnetic wave travel-times and subtracted from the picked travel-times to the ice-bedrock interface. The resulting travel-times were converted to ice thickness using a velocity of 0.17 m/ns. The velocity uncertainty is discussed from a common mid-point profile analysis. A total of 67542 georeferenced data points with GPR-derived ice thicknesses, in addition to a glacier boundary line derived from satellite images taken during summer, were interpolated over the entire glacier surface using kriging with a 10 m grid size. Some uncertainty analysis were carried on and we calculated an averaged ice thickness of 76 m and a maximum depth of 164 m with a relative error of 11.9%. The volume of the glacier is derived as 0.3487±0.041 km3. Finally a 10-m grid map of the bedrock topography was derived by subtracting the ice thicknesses from a dual-frequency GPS-derived digital elevation model of the surface. These two datasets are the first step for modelling thermal evolution of the glacier and its bedrock, as well as the main hydrological network.

High-overtone bulk acoustic resonator as passive ground penetrating RADAR cooperative targets

RAdio-frequency Detection And Ranging instruments—RADARs—are widely used for applications aimed at measuring passive target velocity or ranging for various metrology applications such as ground position and localization. Within the context of using piezoelectric acoustic passive sensors as cooperative targets to RADARs probed through a radiofrequency link, this paper reports on investigating the compatibility of narrowband resonator architectures with the classical operation mode of wideband RADAR instruments. Since single mode resonators are hardly compatible due to the limited bandwidth of their spectrum, the investigation has been extended to High-overtone Bulk Acoustic Resonator (HBAR) whose comb of modes appears appropriate to the use with RADAR instruments. This analysis leads to consider HBARs as delay lines providing a comb of echos in the time domain rather than through the usual frequency comb considerations. Experimental measurements of HBAR responses are demonstrated using Ground Penetrating RADAR instruments fitted with a variety of antennas, and thus, operating in various frequency ranges, as well as the identification of the device temperature through the echo time delay computed as the cross correlation maximum position. Finally, the use of such cooperative targets for single reflector identification within a clutter of reflectors is theoretically considered with the proposal of a Finite-Difference Time-Domain-based simulation method encompassing both passive dielectric reflectors and the contribution of buried passive acoustic sensors.

Using a small COTS UAV to quantify moraine dynamics induced by climate shift in Arctic environments

ABSTRACT Arctic regions are known to be places where climate shift yields the most visible consequences. In this context, glaciers and their environment are highly subject to global warming effects. New dynamics are observed and the behaviour of arctic systems (such as glaciers, moraines, beaches, etc.) changes at rates visible over yearly observations. According to recent works on climate change impacts on the cryosphere, short/violent events are recently observed and are one characteristic of these changes. As a consequence, an accelerating rate of glacial and pro-glacial activity is observed, especially at the end of each hydrological season (early fall). As an example, many phases of streamflow increase/decrease are observed, transforming glacier outflows, moraine morphology, and re-organizing intra-moraine processes. Within only a few days, the morphology of some parts of the moraine can be completely changed. In order to observe and quantify these processes, reactive methods of survey are needed. That is why the use of commercial off the shelf – DJI Phantom3 Professional – unmanned aerial vehicle (UAV) for aerial photography acquisition combined with structure from motion analysis and digital elevation model computation were chosen. The robust architecture of this platform makes it well suited as a reliable picture acquisition system for high resolution (sub-decimetre) imaging. These increasingly popular methods, at a convergence of technologies including inertial guidance systems, long lasting batteries, and available computational power (both embedded and for image processing), allow to fly and to acquire data whatever the conditions of cloud cover. Furthermore, data acquisition is much more flexible than traditional satellite imagery: several flights can be performed in order to obtain the best conditions/acquisitions at a high spatiotemporal resolution. Moreover, the low-flying UAV yielding high picture resolution allows to generate high-resolution digital elevation models, and therefore, to measure accurately dynamics on the field with decimetre resolution in all three directions. Our objective is to show an experimental campaign of small UAV data acquisition in an arctic basin (Austre Lovén glacier, Svalbard, 78°N) separated by a few days. Knowing the changing conditions at this period, similar UAV flights have been reiterated in order to catch moraine dynamics. This allowed us to select two sets of images whose processing highlights and quantifies morphological changes into the moraine while a rain event occurred between two cold periods.

Piezoelectric radiofrequency transducers as passive buried sensors

We demonstrate that single-piezoelectric substrate-based acoustic transducers act as ideal sensors for probing with various RADAR strategies. Because these sensors are intrinsically passive devices working in the radiofrequency range, they exhibit improved interrogation range and robustness with respect to silicon-based radio frequency identification tags. Both wideband (acoustic delay lines) and narrowband (acoustic resonators) transducers are shown to be compatible with pulse-mode and frequency-modulated continuous-wave RADAR strategies, respectively. We particularly focus on the ground-penetrating RADAR (GPR) application in which the lack of local energy source makes these sensors suitable candidates for buried applications in roads, building or civil engineering monitoring. A novel acoustic sensor concept – high-overtone bulk acoustic resonator – is especially suited as sensor interrogated by a wide range of antenna set, as demonstrated with GPR units working in the 100 and 200 MHz range.