Mine detector

     (sfx: explosion)

     OVER 80 MILLION UNEXPLODED LANDMINES LAY SCATTERED AROUND THE
WORLD.  LEFT BEHIND FROM AS LONG AGO AS WORLD WAR TWO, EFFORTS HAVE
RECENTLY BEGUN TO CLEAN UP THESE DEADLY EXPLOSIVES. BUT FINDING THE MINES
ISN'T SO SIMPLE. METAL DETECTORS DON'T WORK WELL, SINCE MANY MINES ARE MADE
COMPLETELY OF PLASTIC OR WOOD. SO PHYCISIST CHARLES DON AT THE UNIVERSITY
OF MELBOURNE IN AUSTRALIA IS BUILDING A DIFFERENT TYPE OF MINE DETECTOR. HIS
DETECTOR USES SOUND WAVES TO PROBE THE GROUND. DON AIMS A CLAP SOUND (clap)
INTO THE GROUND AND THEN USES TWO MICROPHONES TO LISTEN FOR THE ECHO. 

     Don: We can separate it. . . electronically and say it didn't come from the surface or some other
object but from the thing buried under the ground.

     THIS WORKS, DON SAYS, BECAUSE THE SOUND REFLECTS DIFFERENTLY OFF AN
OBJECT DEPENDING ON THE TYPE OF OBJECT. WHEN DON SENDS OUT THE SOUND
WAVE, PART OF THE WAVE REFLECTS OFF THE GROUND SURFACE (same clap fades in and
out, like its refelected off of something) BUT SOME OF THE SOUND KEEPS TRAVELLING
STRAIGHT INTO THE GROUND (clap just fading away)--IF IT HITS A MINE UNDERGROUND, OR
MAYBE A ROCK OR A TREE ROOT, THAT SOUND WILL ALSO BOUNCE BACK. BUT THE
SOUND REFLECTING OFF A MINE WILL BE NOTICABLY DIFFERENT THAN THE SOUND OFF
THE GROUND SURFACE.  MOREOVER, THE MINE'S SOUND WILL BE LOUDER IN ONE
MICROPHONE THAN IN THE OTHER. 
     
     Don: It requires some sophisticated electronics. . . but we can say our object is buried underneath
one of the microphones as opposed to the other one by recording and observing this little pulse.

     THE NEXT STEP IS TO MAKE SURE THAT THE DETECTOR CAN TELL WHAT
EXACTLY IS BURIED UNDERNEATH IT.  RIGHT NOW DON'S DETECTOR CAN USUALLY
TELL A MINE FROM A HARMLESS ROCK, BUT NOT ALWAYS.  WITH A LITTLE MORE
RESEARCH, DON HOPES THAT HIS ACOUSTICAL MINE DETECTOR MAY SOMEDAY WORK
WELL ENOUGH FOR SOLDIERS TO SLING OVER THEIR SHOULDERS AND USE IN THE
FIELD.