Acoustic Transducers as Passive Cooperative Targets for Wireless Sensing of the Sub-Surface World: Challenges of Probing with Ground Penetrating RADAR

Abstract

Passive wireless transducers are used as sensors, probed by a RADAR system. A simple way to separate the returning signal from the clutter is to delay the response, so that the clutter decays before the echoes are received. This can be achieved by introducing a fixed delay in the sensor design. Acoustic wave transducers are ideally suited as cooperative targets for passive, wireless sensing. The incoming electromagnetic pulse is converted into an acoustic wave, propagated on the sensor substrate surface, and reflected as an electromagnetic echo. According to a known law, the acoustic wave propagation velocity depends on the physical quantity under investigation, which is then measured as an echo delay. Both conversions between electromagnetic and acoustic waves are based on the piezoelectric property of the substrate of which the sensor is made. Investigating underground sensing, we address the problems of using GPR (Ground-Penetrating RADAR) for probing cooperative targets. The GPR is a good candidate for this application because it provides an electromagnetic source and receiver, as well as echo recording tools. Instead of designing dedicated electronics, we choose a commercially available, reliable and rugged instrument. The measurement range depends on parameters like antenna radiation pattern, radio spectrum matching between GPR and the target, antenna-sensor impedance matching and the transfer function of the target. We demonstrate measurements at depths ranging from centimeters to circa 1 m in a sandbox. In our application, clutter rejection requires delays between the emitted pulse and echoes to be longer than in the regular use of the GPR for geophysical measurements. This delay, and the accuracy needed for sensing, challenge the GPR internal time base. In the GPR units we used, the drift turns out to be incompatible with the targeted application. The available documentation of other models and brands suggests that this is a rather general limitation. We solved the problem by replacing the analog ramp generator defining the time base with a fully digital solution, whose time accuracy and stability relies on a quartz oscillator. The resulting stability is acceptable for sub-surface cooperative sensor measurement.

Capteurs acousto-électriques radiofréquences – Modes d'interrogation

“…L'inconvénient de ces méthodes rapides de mesure est de nécessiter une grande bande passante qui se traduit par une consommation électrique importante généralement peu compatible avec les applica-tions embarquées. Une alternative bien connue dans la mise en oeuvre des RADARs à pénétration de sol (Ground Penetrating RADAR -GPR) -RADAR impulsionnel classiquement exploités en géophysique et parfaitement adaptés pour sonder la réponse de capteurs passifs enterrés [30], [31], [32], [33], est le récepteur stroboscopique. Dans cette approche, une impulsion radiofréquence est émise, et un unique échantillon mémorisé par un échantillonneur-bloqueur rapide pour permettre une conversion lente grâce à un convertisseur analogique-numérique de bande passante et consommation électrique réduites et de résolution supérieure à celle accessible pour les convertisseurs radiofréquences.…”

WiFi‐based imaging for ground penetrating radar applications: fundamental study and experimental results

“…Therefore, in order to apply WiFi signal into practical GPR applications, we propose to attach a passive sensor designed by SAW delay lines on the underground or inside‐wall targets. In the sensor, a piezoelectric substrate converts an incoming electromagnetic signal into an acoustic signal whose velocity depends on the surrounding physical properties of the transducer [12]. The acoustic wave is reflected by mirrors patterned on the piezoelectric substrate back to the electrodes that convert the acoustic wave back to the electromagnetic wave, yielding an echo signal which has enough delay to be distinguished from the clutters.…”

“…However, we show the practical implementations of WiFi radar imaging in this paper. Being aware of the fact that the range solution is insufficient for GPR subsurface shallow target imaging (such as concrete monitoring), a passive and wireless cooperative sensor designed by surface acoustic wave (SAW) delay lines [12] is also tested. Our experiments show that cars and metallic plates located within 10 m can be effectively detected and imaged.…”

A Half-Cut Compact Monopole Antenna for SFCW Radar-Based Concrete Wall Monitoring With a Passive Cooperative Target

“…However, the temperature of the concrete cannot be directly measured by the GPR technique. To solve this problem, a wireless and passive (no power supply needed) sensor designed by surface acoustic wave (SAW) delay lines [10][11][12] is used as the cooperative target. Since the piezoelectric substrate of the SAW sensor will convert an incoming electromagnetic signal into an acoustic signal whose velocity depends on the temperature of transducer, the phase of the echoes of the SAW sensor will be changed by the temperature variation of the concrete.…”

“…As indicated by [11], the SAW sensor should be connected to an antenna to convert the electromagnetic wave into an acoustic wave by polarizing the piezoelectric substrate. To make the size of the SAW sensor together with the antenna small enough to be buried inside the concrete, a half-cut compact monopole…”