James Gerrard, Liz Caldwell and Alisa Kennedy (two of them from University of Newcastle Classics Dept, one from a commercial geophys firm) have published a short three-page text to gladden the heart of many a detectorist: 'Green Waste and Archaeological Geophysics' Archaeological Prospection Volume 22 Issue 2 April/June 2015, pp 139-42. Here's the abstract:
Environmental concerns, supported by regulatory frameworks, have encouraged the conversion of organic and biological waste into fertilizers and soil conditioners (so-called green waste) that are being increasingly used on arable fields. Recent work has shown that the level of ferrous contaminants within this waste can have a detrimental impact on shallow geophysical prospection methods that use the principles of magnetism. This paper highlights the negative impact of this new agricultural practice on the historic environment and calls for tighter regulation of green waste.The geophysical techniques are metal detecting and magnetometry. Of course in the UK until now, geophysicists had had it easy, there never was at any time in recent history any process whatsoever that would put modern ferrous material into the ploughsoil over ancient sites. The beer cans, bottle tops, tractor parts, trouser buttons, dropped tools, crashed aeroplane parts etc were always deposited by some magical process well away from any site that would ever be investigated by geophysical means. So there was no need to develop statistical 'levelling' techniques in magnetometry for example to deal with this effect. Now there is.