Electromagnetic Frequency (EMF) locating is one of the most common forms of locating, due to its speed and accuracy. A transmitter is used to induce a specific frequency into the required "target", and then a receiver matches this frequency and is able to follow the target line with an accurate depth.
Generally, these materials need to be conductive to work, however in some instances we can employ special trace rods to share the same opening as the services, and can trace that instead.
Ground Penetrating Radar
Ground Penetrating Radars (GPR's) are compact rover units, that pulse out radio waves into the ground at a set interval. Simply put, these signals then get reflected back into the receiver unit whenever they hit something in the ground. Depending on the congestion of the ground (as well as a number of other factors) patterns can appear that signify that there is a buried pipe, cable or conduit in the ground.
This is useful for locating things such as water or gas pipes, as they aren't typically able to be located electronically.