Trevor:
Where are these trenches? And what is the fill? These things determine the
best method to use. If in Devon, no geophysical method is likely to work
extremely well, due to the dampness of the climate, but a combination might
work. If in the Middle East or North Africa, almost any method works.
This is a summary of your choices and their limitations:
Ground Penetrating Radar (GPR). Relatively high rental cost. Provides 2D
cross-section. Excellent results in dry material. Post-processing of
data necessary.
Resistivity: A great variety of methods available, from cheap and simple to
expensive and very good. Works best when there is a great contrast
in conductivity between the bedrock and the fill. This requires great contrast
in water content or porosity or mineralogy. Doesn't work well if everything is
saturated, or if the trench is very narrow at depth. Post-processing
necessary. The more modern methods produce good 2-D cross-sections.
Induced Polarisation (IP), E-M, etc: These methods are similar to resistivity
in that they involve passing a current into the ground in various configurations
and receiving various types of returning signal. They are usually used for
deeper targets. In this case they could prove that the trenches are underlain
by veins. They will not give the depth of the trench as such. They work well
even in wet conditions if sulfide minerals are present. IP can distinguish
between oxidised and reduced material. If the slates are generally reduced (are
grey or contain pyrite) to shallow depths, then the method might be useful,
since the fill will certainly be oxidised.
Magnetics: Equipment rental relatively cheap. A detailed magnetic survey on a
tight two dimensional grid (preferably orthogonal to either the trenches or the
schistosity) should reveal whether the trenches are underlain by a vein at
depth: they will be likely to have a magnetic signature cross-cutting that of
the schist if a vein is present. If not, the mag signature will continue right
across the trenches. Magnetic fill in the trenches (lumps of hematite, perhaps
pyrrhotite, old iron tools) if present, would under optimal conditions enable
you to calculate a minimum depth of fill.
Gravity: An extremely detailed gravity survey MIGHT enable you to calculate
the depth to the bottom of the trench IF the fill were loose or contained voids.
Auguring: if the fill is not full of large rocks you may be able to augur down
to bedrock. The nature of the lowest fill might tell you what the original
purpose of the trench was.
Trevor Dunkerley wrote:
Date: Wed, 23 Jan 2002 22:36:23 +0000
From: Trevor Dunkerley <[log in to unmask]>
Subject: Survey of Mining Trenches
Problem:
Suggested mining trenches from antiquity which have become unmetalled lanes
and cut through the bedding plane in Devonian Slates. Current depth 13m x
4.6m wide and up to 900m long. Known to have been filled up by 3m in 20th
century.
Suggestions as to best method of survey to reveal original depth please.
Regards,
Trevor.
John Berry Assocs - Remote Sensing Services
5013 Westview Drive, AUSTIN, TX 78731
Ph: +1-512-452-8068 Fx: +1-512-452-8068
Mo: +1-512-293-8068
e-mail: [log in to unmask]
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