M.S. Thesis Presentation by Fabien Codron

(Dr. Yves Berthelot, advisor)

"Detection of Surface Waves in the Ground Using an Acoustic Method"

Abstract

This thesis investigated a new technique for detection of surface waves in the ground. This technique uses two reciprocal ultrasonic electrostatic transducers: one sends sound at 50kHz to the ground and the second is used as a receiver of the reflection. As the sound reflects from the ground, the soil displacement phase-modulates it. An analog phase demodulation is performed to get the vertical displacement of the soilís surface. The demodulation process can detect phase shifts down to 10-6 radian. This great sensitivity gives to the system a vertical resolution approaching one nanometer. The use of the natural focusing of the transducers produces a spot size of the order of a square centimeter. This spot size makes it possible to get a spatial resolution less than a quarter of the surface wavelength in the ground up to one kilohertz. The relatively low cost of this device makes large sensor arrays practical. Such arrays would permit real time imaging of surface waves in the ground.

This device offers an alternative to a radar-based sensor, which has been developed for land mine detection at Georgia Tech. In this application, an acoustic source creates waves in the soil over the frequency range of 100Hz to 800Hz. The waves are observed using a synthetic aperture radar array that scans the area where the presence of a mine is suspected. The very different acoustic properties of the mines and the soil cause seismic wave interactions in the vicinity of a mine to be quite distinctive. These effects are visualized by the radar imaging and provide a method to localize land mines.