A Review of Human Signatures in Urban Environments Using Seismic and Acoustic Methods

Techniques for sensing footstep vibrational frequencies, typically below 100 Hz, by seismic sensors are well- developed human detection methods. Walking styles (standard, soft, or stealthy) and the background noise floor limit the detection range of footsteps. Walking style changes the dynamic footstep force on the ground and the influences the footstep detection range. The seismic background noise floor is much higher in urban areas and in buildings than in rural areas, dramatically influencing detection range. Alternatively, high- frequency passive and active ultrasonic methods for human detection are being developed. High-frequency sound produced by friction forces between a foot and the ground/floor allow passive footstep detection in urban areas and in buildings. The active method utilizes continuous-wave Doppler ultrasound. Simultaneously collecting Doppler motion and footstep ultrasonic signals reveals correlated timing features between the footstep friction and the Doppler shift from the human motion. Discrimination between human and other moving targets is accomplished by analyzing the envelopes of footstep signatures.