These tools incorporate an electrical insulator in the drillstring, but due to the challenges of receiving data through a good conductor (Salt Water) this approach is largely confined to onshore areas without shallow saline aquifers. To transmit data, the tool generates an altered voltage difference between the top part (the main drillstring, above the insulator), and the bottom part (the drill bit, and other tools located below the insulator of the MWD tool). On surface, a wire is attached to the wellhead, which makes contact with the drillpipe at the surface. A second wire is attached to a rod driven into the ground some distance away. The wellhead and the ground rod form the two electrodes of a dipole antenna. The voltage difference between the two electrodes is the receive signal that is decoded by a computer.