Time-reversal acoustic focusing of waves produced by seismic vibrators

Abstract

A land mine detection system has been developed based on excitation of seismic waves by four mechanical shakers and measurements of ground vibration by a 16-beam, scanning laser Doppler vibrometer (LDV). Time-reversal acoustic (TRA) focusing was applied to concentrate seismic waves using the above system. Linear swept frequency signals and orthogonal initial signals in the frequency band of 100–500 Hz were used in outdoor testing. The system impulse response from any shaker to the LDV output was computed by cross correlating the original and recorded signals. With orthogonal signals, the system impulse responses between multiple shakers and the LDV were measured simultaneously, thus reducing the overall measurement time. Each shaker then reradiates the time-reversed impulse responses focusing the seismic energy to a point on the ground. Experiments demonstrate that the TRA focusing provided a high concentration of elastic wave energy in the test area with typical focusing spots about 20 cm in diameter. [Work supported by U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate.]