It depends on the ground potential around the container, which takes into consideration ionic mineral content, moisture, density of material, area/volume etc. In addition, the emf field strength determines the effective penetration of the field.

Bascially, it is how deep you bury it, how conductive the material is you buried it in, and how far the conductive material spreads out without encountering a less-conductive area. Also, how big the EMP is and the proximity to it. Under ideal conditions, underground burial can be highly effective at protecting electronics. However, if the equipment is still hooked up to an outside supply source that the EMP could get to, the equipment and it's components can still be compromised.