Can't say if its this, but they might be using arrayed drones with magnetometers to detect magnetic abnormalities.
This might be a search for mining sites. Cheaper than flying fixed wing aircraft.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5191148/
https://www.tandfonline.com/doi/full/10.1080/10095020.2017.1420508
A specific purpose might be to locate meteorites. If you find a meteorite on Federal Land it belongs to the Federal governement. If I remember the drone searches were conducted in Colorado and Nebraska, probably in the plains areas that are public land, so you are more likely to get to keep the meteorite.
https://www.scientificamerican.com/article/how-to-find-a-meteorite-in-5-steps/
Anyway, this is the semi-prosaic suggestion regarding what is being done. Maybe if if we had more information it might be easier to provide a more accurate suggestion.
One more....A possible areal agricultural survey of farm or livestock areas.
https://twitter.com/LauraLaura1650/status/1207140523292708864
Not related to your post Pete, yours was just the most recent when I went to post.
All you mention are possibilities. And I think it is something along those lines in general. I like a good mystery and learning the eventual outcome.
Good possibilities.