TL;DR: Does burying a cable provide shielding? I.e., is a shielded cable in air equivalent to an unshielded cable buried ~ 6 inches in soil?
Long:
I'm a technician at a university. I don't have formal training in electrical engineering, but one of my tasks is designing an environmental monitoring system for the field. Analog voltage sensors (thermocouples) will be read by a device in the field, and the signal will be passed ~100 m to a datalogger via shallow buried cables. In selecting these cables, I was concerned about voltage drop (since thermocouple signal is a very low voltage to begin with) and cable shielding (since there are power transmission lines in the area, and AC power supply for the datalogger in the same enclosure box). I asked a more experienced technician for advice, and he advised that in this application he would be concerned about both things.
However, when I approached the project manager about this, I was told not to worry about voltage drop (no reason given), and that burying an unshielded cable was "the same as shielding it". I can order unshielded cable and spend no further time on the issue… but I'm concerned that the manager is just trying to save a buck, and that this could come back to bite us (i.e., I'll be creating more work for myself) in the future.
I'm also generally curious. Is he correct in saying that a shielded cable and an unshielded cable buried less than 6 inches in soil are equivalent?
*Edit to add detail: the site is a pine forest with well-drained (dry), predominantly sandy soil. I don't have much experience to go on, but I would say it's fairly quiet for EM. There's one high-power line ~200 m from the thermocouples, and 120V AC running through outdoor extension cords – the 120V lines are co-located, and will run alongside the TC's in some cases.
**Edit to clarify: I'm not concerned about voltage drop on the thermocouple itself. I'm concerned about drop on the 100 m cable that will carry the signal from the thermocouple voltage-reader (a Campbell Scientific AM16/32B multiplexer) back to the datalogger (a Campbell CR1000).
Best Answer
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.