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Hi,

Harold's question on DIY magnetometers got me thinking
about the various sensors I know of and the feasability
of using some very sensitive differential sensors (to null
out the constant background field of the earth) from the
surface.  Without the need of a submercible fish the cost
would be a fraction of that associated with a towable device.

However this seems totally unfeasible unless the vessel
on which the sensor is located doesn't generate magnetic
"noise".  A motor or horrors, alternator/ignition coils
would generate lots of high frequency magnetic noise
that one would have to either get away from or filter
out.

I realize a few of you have or have used magnetic sensors
and I'm wondering about the usefulness of these sensors
close to or on board the towing vessel and if there is
a rule of thumb regarding the relative distances between
the sensor and target and sensor and towing vessel imposed
on their use ?

Land based sensing appears most sensitive when the sensor
array is positioned for vertical sensing.  I'm getting
some info sent to me on sensors meant for use in water
but I wonder if the compliactions associated with a
tall towable fish (vertical sensing) is just a waste
of effort and if not two sensors displaced in the
direction of travel would be just about as efficient.

Thanks,
John

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