Doppler radar detection of mechanically resonating objects

The limitations of existing counter-mine technology have motivated the development of an acousto-electromagnetic technique for detecting landmines and other buried objects (Lawrence, D. and Sarabandi, K., IEEE Trans. on Antennas and Propag., vol.49, p.1382-92, 2001). This hybrid method exploits the acoustic resonance of an object to discriminate it from soil, plant matter, rocks, and other clutter that cause false alarms for GPR. An object is distinguished based on the Doppler signature imparted to radar signals scattering off the object´s vibrating surface. We investigated whether small vibrations in objects can be detected by a sensitive Doppler radar. Objects with known acoustic resonant frequencies (tuning forks) were excited (by a speaker), and their Doppler signatures measured using a CW radar system. In the resulting Doppler return, the carrier signal is frequency modulated by a pure tone. Our Doppler radar succeeded in detecting resonant tuning forks even when the vast majority of the receive power was clutter. The primary limitation on our system´s performance was not the sensitivity of the radar, but rather the ability to couple acoustic energy into the forks from a remote source.