Have you tried just attaching resistors to the amplifier, in place of the
cable and geophone?  If there are still spikes, then the cable is not
involved.

Have you tried a short cable between the geophone and the amplifier?
Don't worry about the local ground noise, just look for the spikes.  The
length of the cable may be a critical factor, but not necessarily.  If there
are spikes with a short cable but not with just resistors, then you
can experiment with the ground attachments and cable type with
short cables until you find something that works.

John


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