One thing that the Bosnian/Serbian conflict/genocide [0] taught authorities is how to find hidden grave sites. Ground penetrating radar comes in a convenient device that is sized & shaped like a typical American lawnmower.

Soil comes in layers and it would be best to spread ground cloths to capture different soils from different layers and to replace them at the appropriate levels in the ground. While this won't defeat ground penetrating radar, it will make the burial site more closely resemble surrounding soil to make the "signal" harder for the less skilled operators to detect.

Links:

0 - https://en.wikipedia.org/wiki/Bosnian_War