Does burying a cable shield it?
It's largely down to moisture content. 

Put an unshielded cable in a few inches of dry sand with still more inches of dry sand below it and it will look largely like an unshielded cable placed above ground with respect to electric fields it might generate. Maybe dielectric losses will reduce high frequency E-field values.

As for magnetic fields they'll be hardly diminished at all.

Water content makes life difficult for E-fields and salt water is much better at shielding than tap water. This is why it's so darn difficult to use radio waves under water. Penetration is very poor except at profoundly low frequencies.