I used infrared on a project about thirty years ago and found the different  
densities of soils from native landscape to prehistoric archaeology sites in  
agricultural fields gave off different colors. You could also see old test 
pits  as a slightly darker color. This was less useful with dense vegetation. I 
would  expect this technology has improved much since that time. Comparison 
with  historic photos (1928, 1930s, 1950s etc) created very good information on 
the  various cultural features on the land.
 
Ron May
Legacy 106, Inc.