Franklin,
I assume you are asking about the exploration depth of ground penetrating radar (GPR). This is dependent on numerous factors, including:
1. The center frequency of the GPR antenna.
2. The electrical properties of both the target you are trying to find and the surrounding material you are 'looking' through.
3. The size, orientation, and depth of bury of the target.
With all that said, I will tell you that I have used GPR to find buried objects, map geology, detect and map caverns/former mine workings, and determine other subsurface conditions up to 100 feet depth. However, that is in my regular job. I'm a licensed engineer with over 20 experience years as a geophysicist. As part of my treasure hunting hobby, however, I've only used it to search for items up to about 5 feet deep.
Let me know if you have other questions.
- Alan406
