Acoustic wave transducers as Ground Penetrating RADAR cooperative targets for sensing applications

Acoustic wave transducers act as cooperative targets for passive, wireless sensing by converting an incoming electromagnetic pulse to an acoustic wave whose velocity is dependent, by design, with a given quantity under investigation. The acoustic wave is delayed and emitted back to the measurement electronics as an electromagnetic pulse by direct piezoelectric effect. In the context of sub-surface sensing, we address the challenges of diverting a classical geophysical instrument, a Ground Penetrating RADAR (GPR), for probing the cooperative target response. While the GPR provides both the electromagnetic pulse source and the means of recording the echo delays, representative of the acoustic velocity on the transducer and hence of the sensor environmental properties, the measurement range is dependent on the radiation pattern, emitted pulse spectra matching the transfer function of the target, and sensor antenna impedance matching properties. Furthermore, recovering the physical quantity under investigation assumes the timebase of the GPR to be more stable than the variation of the acoustic velocity induced by the environment of the sensor: we observe excessive timebase drift in the commercial GPR unit we use in experiments, and provide a solution by replacing an analog ramp generator with a stable, digital solution.