Passive Cooperative Targets for Subsurface Physical and Chemical Measurements: A Systems Perspective

We investigate the use of a commercially available ground-penetrating radar (GPR) for probing the response of subsurface sensors designed as passive cooperative targets. Such sensors must meet two design criteria: introduce a signature unique to the sensor which will hence be differentiated from clutter, and introduce in this signature a response characteristic allowing for recovering the physical quantity under investigation. Using piezoelectric substrates for converting the incoming electromagnetic pulse to an acoustic wave confined to the sensor surface (surface acoustic wave transducer) 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. In all cases, the delay introduced by the physical quantity variation induces a phase rotation of the returned signal of a few periods at most, to be measured with a resolution of a fraction of a period: the GPR receiver sampling time reference must exhibit a long term stability at least as good as the targeted phase measurement needed to recover the physical quantity. We show that the commercial GPR exhibits excessive time-base drift, yielding a loss in the sensing capability, while a quartz-oscillator-based alternative implementation of the classical stroboscopic sampling receiver compensates for such a drawback.