I have permission to dig in an old railroad yard so I tried probing it out and kept hitting something hard, impenetrable not far below the surface. Turns out there is asphalt in most places about six inches below the dirt. I found a privy on an old map and can narrow it down to a very small area but slam probing seems like it may be the only way to find this/these things (hopefully there are more than one there). How would I build a slam probe? How exactly do they work? I know that there is a heavy weight on the handle. Can I drive the probe right through asphalt fairly easily? Is is just a weight on a normal steel spring probe?