Search

There is considerable interest in detecting objects such as landmines shallowly buried in loose earth or sand. Various techniques involving microwave, acoustic, thermal and magnetic sensors have been used to detect such objects. Acoustic and microwave sensors have shown promise, especially if used together. In most cases, the sensor package is scanned over an area to eventually build up an image or map of anomalies. We are proposing an alternate, acoustic method that directly provides an image of acoustic waves in sand or soil, and their interaction with buried objects. The INEEL Laser Ultrasonic Camera utilizes dynamic holography within photorefractive recording materials. This permits one to image and demodulate acoustic waves on surfaces in real time, without scanning. A video image is produced where intensity is directly and linearly proportional to surface motion. Both specular and diffusely reflecting surfaces can be accommodated and surface motion as small as 0.1 nm can be quantitatively detected. This system was used to directly image acoustic surface waves in sand as well as in solid objects. Waves at frequencies of 16 kHz were generated using modified acoustic speakers. These waves were directed through sand toward partially buried objects. The sand container was not on a vibration isolation table, but sat on the lab floor. Interaction of wavefronts with buried objects showed reflection, diffraction and interference effects that could provide clues to location and characteristics of buried objects. Although results are preliminary, success in this effort suggests that this method could be applied to detection of buried landmines or other near-surface items such as pipes and tanks.

The INEEL has developed a photorefractive ultrasonic imaging technology that records both phase and amplitude of ultrasonic waves on the surface of solids. Phase locked dynamic holography provides full field images of these waves scattered from subsurface defects in solids, and these data are compared with theoretical predictions. Laser light reflected by a vibrating surface is imaged into a photorefractive material where it is mixed in a heterodyne technique with a reference wave. This demodulates the data and provides an image of the ultrasonic waves in either 2 wave or 4 wave mixing mode. These data images are recorded at video frame rates and show phase locked traveling or resonant acoustic waves. This technique can be used over a broad range of ultrasonic frequencies. Acoustic frequencies from 2 kHz to 10 MHz have been imaged, and a point measuring (non-imaging) version of the system has measured picometer amplitudes at 1 GHz.

A new approach to the measurement of ultrasonic vibrations has been developed at the INEEL. This system utilizes heterodyne interferometric detection in a photorefractive material to provide real time, full field images of ultrasonic vibration amplitude and phase without scanning. The INEEL Laser Ultrasonic Camera has linear response for ultrasonic displacement (xi) < (lambda) divided by 4(pi) , approximately 45 nm displacement at a laser wavelength of 532 nm. In addition, the system exhibits flat response and narrow band detection for a broad range of vibration frequencies. The system is very robust, and has the potential for operation even in noisy industrial settings. A description of the system, representative data and several potential applications will be presented.