Laser Doppler Multi-Beam Differential Vibrometers for Detection of Buried Objects

The laser-acoustic detection of buried objects, such as landmines, is based on excitation of elastic waves in the ground and creating a vibration image of the ground surface by using a laser Doppler vibrometer (LDV). The technique provides high probability of detection and low false alarm rate. However, traditional LDVs require operation from a stable stationary platform due to their sensitivity to the motion of the vibrometer itself. Recently developed laser Doppler multi-beam differential vibration sensor has low sensitivity to the motion of the sensor itself, while measuring vibration velocity difference between points on the object with interferometric sensitivity. Low sensitivity to the sensor motion allows for vibration measurements from a moving vehicle. Two configurations of the developed sensor: the linear array and the 2D array sensors, are discussed in the paper. The linear array sensor measures velocity difference between points on the object illuminated with a linear array of 30 laser beams, and creates a vibration image of the object by scanning the array of beams in a transverse direction. The 2D array sensor employs an array of 34 x 23 laser beams and measures velocity difference between corresponding points on the object over the whole illuminated area simultaneously. Simultaneous measurements at all points allow for the fast recording of the vibration image of the area of interest, and makes possible calculation of the vibration phase and instantaneous velocity images. Description of the sensors and the experimental results are presented in the paper.