O.k. My own father had me sign a waiver to do work on his house. It wasn't about him so much as it was for the insurance company. Its funny 'cause I have broken numerous bones while trespassing to fish in quarries and never sued anyone. Maybe my dad is a "Richard", but "honor thy father and mother", and the work needed doing, so I signed. No harm done.
The owner of the property you would like to detect is just trying to protect his interests. Why should you gain everything while he might be liable?
For a contract include that damages to him and also damages to you will be excluded from litigation, and that any damages to him must be settled our of court through impartial arbitration. Include everything discussed along with any split of prizes offered.
Be sure to thank him. The last thing you want is to sue a property owner for you getting hurt while they are being kind to the sport.