Photo Sharing. Your Photos Look Better Here. | SmugMug : ohio : The thumper trucks always travel in threes, and travel together. They are obviously very gregarious. At every stop, which is often, a heavy plate is lowered to the road surface. Most of the weight of the truck is placed on the plate. A low frequency vibration is sent into the ground via the plate. The shock waves are reflected by changes in density underground, and are tailored to reflect off pockets of liquid. The reflected sound waves are picked up by the geosensors, transformed into electrical signals, and sent down the orange wire to a data collection point. Different liquids have different densities, so the collected data can regularly show gas pockets deep underground.