Insulated conductors must be grounded at the far end. Otherwise,
current will have more resistance because it is contained
within the insulation barrier. (higher frequencies can overcome
this limitation to some extent; see the article on Frequencies
for more information).
If the conductor is not insulated, the signal will transfer
its energy into the surrounding soil as it travels down the
line. Higher frequencies "bleed off" to ground faster, limiting
the line's locatable distance.