"WHAT CAN GPR FIND?
US Radar GPR Systems are designed to display differences in material composition. They can be used to locate any object that has a different composition than it's surrounding materials. For example, a PVC pipe will have a different composition than the surrounding soil. Voids and excavations that have been filled in will also have different compositions than the surrounding soil. However, GPR does not know what the actual materials are that it is imaging.
For this reason, it is not suited to locating gold, precious gems, and treasure".
"Radar is sensitive to changes in material composition, detecting changes requires movement. When looking through stationary items using surface-penetrating or ground-penetrating radar, the equipment needs to be moved in order for the radar to examine the specified area by looking for differences in material composition. While it can identify items such as pipes, voids, and soil,
it cannot identify the specific materials, such as gold and precious gems. It can however, be useful in providing subsurface mapping of potential gem-bearing pockets, or "vugs." The readings can be confused by moisture in the ground, and they can't separate gem-bearing pockets from the non-gem-bearing ones".
https://en.wikipedia.org/wiki/Ground-penetrating_radar
It is a fairly sensitive method, but it cannot distinguish gold (so a pebble sitting in clay would look the same as a gold nugget sitting in clay).
As for the "ionising" and other detail - well......