Yea I think the rod idea might be a good way to start. As long as your soil is soft enough (I don't mean frozen or the frost line)... I mean soft enough. In MI. you could push a shovel into the soil and pull out a full shovel full so probing was easy. But here in TN. I'd bend the rod in two after the first 1/2" (there's no such thing as shoveling here). If the soil is soft... maybe make the probe 4-5 ft. because you'll be pushing it a lot it appears horizontally. Grind tip to point and spot weld a handle to other end. Shouldn't cost much at all (so I say). Maybe mark the rod at 6" increments with magic marker or paint with each foot its own color or just count and map.
Ok.... gotta go get more coffee and fix breakfast now. You take day off work and get to probing... WE ALL WANNA KNOW!!!!!!!!! That's all we want for Christmas... well some of us... maybe? Later, Brad