Development of a time-frequency representation for acoustic detection of buried objects

A recently developed apparatus permits the detection of buried objects, such as landmines, by remotely sensing the variations of ground vibration that occur over the buried object with a laser Doppler vibrometer (LDV), when the ground is insonified by means of acoustic-to-seismic coupling. As it is currently implemented, the LDV scans individual points on the ground. The technique shows much promise, but it is slow when compared to some other techniques. This work demonstrates that mines can be detected as the LDV beam moves continuously across the ground, by using an optimal time-frequency representation of the LDV signals. This improvement has the potential to significantly increase scanning speeds. The vibrometer output signal is analyzed by means of time-frequency representations, which exhibit characteristic acoustic “signatures” when scanning over buried objects. The most efficient representation appears to be the smoothed spectrogram weighted by the time-frequency coherence function. It detects the searched signal energy enhancements while filtering out most features due to speckle noise. Detection is then efficiently achieved by searching simultaneous extrema of the marginals and moments of this representation. Experiments show that scanning speeds up to 3.6 km/h can be achieved for successful detection of buried landmines in outdoor ground.