The avebury-web.co.uk information says they're based on aerial photography/parch marks.
Does anyone know if the NT published their survey results from 2003?
"A survey of these areas by the National Trust has revealed that at least 15 of the megaliths lie buried in the circle itself."
http://news.bbc.co.uk/1/hi/england/wiltshire/3257174.stm
or any results from EH's surveys in the same year:
"Ground penetrating radar and probing had suggested it existed to only about one-and-a-half to three feet below the surface."
http://news.bbc.co.uk/1/hi/england/wiltshire/2956995.stm
amd