I guess in this case you need to make an earthing system with very low resistance to provide zero potential to the instrument. For electronic & telecom. instruments and systems it is always required 0.5 ohms, and its maximum acceptable value in 1 ohm.
One of the best techniques is making the earth electrode from a plate usually made of coper as it has high contact area with the soil. Otherwise you can use at least 3 or 4 rods, if the rod length is L then the distance between 2 rods shall be at least 2L.
Also you can use a copper strip, or chemically activated rods. Or you can make any means of soil treatment.
For soil treatment we (in our company working in telecom.) use normal slat (NaCl) and charlcoal (or coke) in layers, but I guess there are some disadvantages for using this. Some other materials are used as bentonite.
I was making some studies about earthing in the last few months for nearly the same reason, If you need as much links as you want about earthing I'll be glade to send it to you.
take a look at: Art of electronics by paul horowitz
it's a really nice book and also contains some helpful informations about grounding considerations and low noise devices.
