Active signal application requires the use of a signal transmitter designed to produce from battery power an a.c. voltage of known frequency and ‘Signature’, and the means of applying it to the target buried conductors.
The output AC voltage from the signal generator is connected directly to the pipe or cable at an access point such as a valve, meter or end of the conductor, and the circuit is completed by a connection to a stake or other ground connection point.
The signal will be detectable along the line over a distance dependent upon the type and size of the line and soil conditions as discussed in the Practical Implications of Capacitance, Ground Conductivity and AC Frequency article. The presence of insulated pipe joints will of course significantly reduce or inhibit the signal, while the choice of frequency will also have an important influence. Note also that any lines sharing a common ground point with the connected line will also carry the same signal to a greater or lesser extent, depending largely on how well coupled to ground they are.