Prior to drilling, gas developers study the specific geologic conditions beneath the ground, and whether it has the potential to produce natural gas. Geophysicists use both two- and three-dimensional seismic testing approaches to learn about underground rock formations such as the Marcellus and Utica Shale. They use seismic trucks, often called 'thumper trucks' to generate geologic images of the sub-surface. Such testing cannot determine what's in the rock. For that they have to do exploratory drilling.