Keyline ploughing (plowing) How deep is too deep

I suppose the Soil fourm is the  correct place for this...  Aerating the soil such as P. A. Youman  advocated is obviously a positive thing.  How deep is too deep, though?  Are there any lower limit constraints to the soil?  Generally where does diminishing returns kick in?

A heavy subsoiler has about a 24" shank.  Most rippers have at most 18 inches.  Equipment weight and cost of material have a lot to do with the design of those implements.  But if one was not limited by available equipment, how deep would one aerate?  If a person could, say, run a ditch witch and make a 48" cut and backfill with the loose soil, would the be any advantage?  

Would a very deep 'rip' like this be advantageous or detrimental?   Thoughts or experiences?

There is more to Keyline ploughing than depth.
The process is more wholelistic than just depth of plough.
https://www.mbequineservices.com/equine-permaculture-soil-part-2/
This site explains it clearly.
Basically a number of steps and design aspects need to be considered during the process of rehabilitation of compacted soild.

Jack Edmondson wrote: Would a very deep 'rip' like this be advantageous or detrimental?   Thoughts or experiences?

Well, that depends on whether there is something in the fields that you do NOT know about.
What about old clay tiles and newer plastic tile? Are there any there?

You should check with your local AG 'expert' and the County officials where you live to see what has been done over the last 50 years that might help with your first decision:  to plow or not to plow.

Thank you John Daley for posting that link. It really helped me to better understand keyline plowing.

Darin Doherty talked about that here, but I can’t find the thread.

Basically, going deeper than the roots will reach in the next growing season is a waste of fuel. The most efficient use of time and fuel is to go 2-4 inches below the hard pan to break it up, then let roots do the rest.  Hard pan is usually 8-10 inches down for most former ag land, but you need to check.

When running a ripper you can tell when you are at optimum depth because you can see the swell in front of the shank and it going wide around it. If you are too deep or shallow the ground only moves right next to the shank