I'm still unclear as to what Trevor's real question is: do you REALLY want to
know the depth of fill, or do you really want to know the origin of the
"trenches", whether natural, just from erosion of a right of way, or mining of a
vein?
In the conditions Trevor describes, looks like resistivity is best if he only
wants depth, magnetics or an e-m method if he wants to know the origin of the
features. The best way would be to use both.
Everybody's comments were very pertinent. I would like to add that, in wet
conditions, the depth penetration of GPR is very slight ( much less than a
meter?) whatever wavelength you use.
Martin Roe's comments were particularly relevant: geophysical methods pick up
anomalies (i.e. variations in various rock properties), so the method you choose
depends on the type of anomaly you think the phenomenon you are looking for will
give rise to under the conditions at the site. It helps to have a geologist
tell you what he would expect the local geology to do, and how its influence
might be mitigated. I heard a paper at the Amer. Inst. of Archaeol. meetings
in Philadelphia early this month in which a student used magnetic gradiometry in
central Italy to find building foundations. Since the area had been
deep-ploughed, she got a strong magnetic lineation from the furrows, probably in
part because she was a little closer to the ground over the ridges than over the
furrows. In her case she should have been able to estimate the depths to her
buildings, but made no attempt to do so. She also had a strong geological
signature (volcanic rocks), and some miss-ties due to diurnal drift.
It is best to get a competent non-seismic geophysicist out to the site to have a
look, listen to your questions, and help you to design the survey.
John
PS: Trevor - I'd be very interested in what you decide to do and your results -
there is a mountain-top site in Italy which has minimal soil thickness, but has
yielded much metal debris as well as large pieces of pottery that might be
interpreted as fragments of furnaces (could also be fragments of pottery kilns).
A trench that looks suspiciously like the outcrop of a mined-out vein runs
across the site. I may be asked to run geophysics over it, so knowledge of your
success or otherwise might be helpful. I have also been peripherally involved
with resitivity and GPR surveys on another, wetter site in Italy. GPR
penetration was minimal (20 cm), and resistivity anomalies were misleading: the
archaeologist's summary of the contribution of geophysics was "worse than
useless", because it made the site appear less interesting than it has since
turned out to be. So be warned. Also, try to do your work in a drought, if
they ever happen in North Devon!
John
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