FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

BAR
MAGNET
IDEAL
FIELDS
MEASURED
FIELDS
N
Figure 2: Pipelines are constructed typically from 7 m joints welded together.
Each individual joint behaves rather like a bar magnet. There are
north and south magnetic poles located a little way in from the
ends. The external magnetic field lines run from north to south.
If a magnetometer scan is made parallel to the pipe axis and at a
constant distance outside it this gives radial and axial field logs
as shown ideally and as measured above a real pipe.