Passive cooperative targets for subsurface physical and chemical measurements

We investigate the development of passive cooperative targets as sub-surface sensors interrogated by Ground Penetrating RADAR (GPR). Using piezoelectric substrates for converting the incoming electromagnetic pulse to an acoustic wave confined to the sensor surface (Surface Acoustic Wave transducer - SAW) allows for shrinking the sensor dimensions while providing sensing capability through the piezoelectric substrate acoustic wave velocity dependence with the physical quantity under investigation. Two broad ranges of sensing mechanisms are discussed: intrinsic piezoelectric substrate velocity dependence with a quantity - restricted to the measurement of temperature or strain and hence torque or pressure - and extrinsic load dependence on the sensor, allowing for the measurement of variable capacitive or resistive loads. We demonstrate, using readily available surface acoustic wave filters diverted from their original use to become reflective cooperative targets, how commercially available GPR hardware can be used, with minor software addition, to probe such sensors with no hindrance to the shallow subsurface structure detection capability by defining multiple time windows - some for sub-surface monitoring and others for sensing capability.