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.