Note the flat pad suspended under the middle of the truck.
Once the truck reaches the specified point, this pad is used
to raise the 20,000 kg truck entirely off the ground. The hydraulics
then shake the truck up and down on this central piston for
a specified time and over a precisely controlled frequency
band (e.g., eight 14 second sweeps from 10 to 56 Hz ). Typical
LITHOPROBE surveys use four trucks operating together, linked
by one vibration control system.