On Tue, 17 Mar 1998, William Moss wrote:
 
> Has anyone any experience using a proton magnetometer in an urban setting,
> particularly over roadbeds?
> We have recently used ground penetrating radar on several different sites.
> The results can be useful, but they are generally difficult to interpret
> without test trenching. We would like to supplement the ground-penetrating
> radar with other means of geophysical prospecting in zones where it is not
> feasible to do trenching, notably on high-traffic city streets. A proton
> magnetometer survey would seem to be one possibility.
 
Lyle Browning has recommended a fluxgate magnetometer in place of the
proton magnetometer, and I would agree that the fluxgate would be a better
choice.  However, I would recommend two other things.  First, make sure
that you use a gradiometer, no matter what mag model you choose.  This
will insure that you diminish any extraneous magnetic disturbances from
things such as cars, solar storms, a.c. lines, etc, and it will help
"isolate"  the features of interest.  Second, I would recommend using an
alkali-vapor model (cesium, for instance) because of their higher
sensitivity.  If your targets are large, then the manufacturers often
claim that their machines will pick them up through concrete floors and
what-not.  You can run this instrument with iterative (and cross-checking)
surveys over the area of interest in far less time than required by the
proton model.
 
I don't know what size of features you are expecting, but if they are too
small or too deep, the disruption of the urban environment and roadbed may
be too strong for much of any mag work.  We recently used a cesium mag
(not gradiometer) in a relatively urban setting in San Francisco, and we
had only limited results.  We had streets blocked off and everything to
cut down on vehicular disturbance.  I think the addition of the second
sensor would have made a world of difference. However, we were in a grassy
field underlain by utility lines rather than over a roadbed....
 
Andrew Edwards suggested a higher success rate with GPR and resistivity,
and there is truth to that for urban settings.  However, resistivity
won't work over a roadbed given the need to insert probes into the ground,
and electromagnetic conductivity might work better.  You can use it at
different heights to try to get a conductivity profile of sorts.  It can
be a bear to keep calibrated, though, and it would probably be quite
disturbed by electrical and magnetic sources nearby.  If your sources are
deep, you may want the coils quite a distance apart (perhaps the 3 m on
the Geonics EM-31 rather than the ca. 1 m on the EM-38).
 
Hope this helps,
 
Steve
 
 
Stephen Silliman
Department of Anthropology
232 Kroeber Hall
University of California
Berkeley, CA  94720
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