“Thumper” was the nickname given to a large, box-shaped machine that was used on Dr. Alan Grant’s fossil excavation site near Snakewater, Montana. It was designed to fire a soft lead slug into the ground (within the vicinity of fossilized remains), creating shock waves to be registered by a nearby computer. The computer would then assemble an X-ray-like image of the area, allowing the paleontologists to determine the size of the trench dug to control water runoff. This procedure was known as ‘computer-assisted sonic topography,’ or CAST.
Sources: Jurassic Park, by Michael Crichton, page 56 (Fifth reprint)