Mahmoud,
A notoriously tricky problem. The geophysical method of choice is a
combined magnetic (measuring total field and vertical gradient) and
electromagnetic survey with possible use of Ground Penetrating
Radar. However, detection can be almost impossible at the kind of
depths you talk of and especially when the shafts are of old
construction and infilled with arisings providing little or no variation in
the geophysically detectable properties (magnetic, electrical
properties mainly) between the fill and surrounding rock.
That said there are plenty of examples where geophysics has
worked, for example when there has been metal placed over to
cover the mine, where the mine has been capped, where the
arisings have been sufficiently permeable compared with the
competent rock to provide an area of preferred drainage detectable
using electrical/electromagnetic methods in the correct conditions or
where the shaft has been infilled with arisings and whatever else is
lying around. Finally where the mineshaft is brick lined, providing it is
not buried too deep, it can be detected using high resolution
magnetometry (vertical gradient) surveys that pick up the change in
magnetisation of the fired bricks compared to the surrounding
ground.
A little more information is available in the latest CIRIA publication
C562 - Geophysics in Engineering Investigations.
Bare in mind non of these techniques are 100 per cent reliable for
the reasons detailed above, but used correctly and in conjunction
with a thorough desk study they can speed up the process of drilling
many investigation holes over an area.
Health and Safety is, as ever, an important aspect and should be
part of a full pre-site hazard and risk assessment.
I hope this is useful and await other responses (perhaps on the
engineering options)
Steve
On 12 Mar 2002 at 17:31, Mahmoud I Alsayed wrote:
> Dear all,
>
> We are investigation several sites to locate old mineshafts. The
> mining information available confirms the existence of the shafts but
> the exact location, method of treatment on abandonment and current
> condition are unkown. To complicate matters, some of the sites have
> been landscaped and in some cases the original ground surface is
> situated below railway embankments up to 8m high. We are considering
> using a combination of intrusive ground investigation and geophysical
> techniques but are not sure of how successful we will be.
>
> If you have been involved in this type of work and would like to
> share
> your experiences (good or bad) then please let me know.I would
> appreciate any comments you may have.
>
> Regards,
>
> Mahmoud I Alsayed
>
> First Consulting, Quay Road, Rutherglen, Glasgow G73 1LD, UK
> Tel. +44 (0)141 335 5300
> Fax +44 (0) 141 335 5301
> Email [log in to unmask]
Dr Steve Taylor
Alta Geophysics Research Unit
Birmingham Research & Development Ltd
Research Park
Edgbaston
Birmingham
Bxx xxx
Tel : 0121 414 6134
Fax : 0121 414 4942
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