Bulletin 54. Geophysical Methods of Exploration and their ...

GEOPHYSICAL METHOI_S OF EXPLORATION 29
Other factors controlling the nature of a magnetic disturbance
are the shape and orientation of the bodies of rock or ore
which cause the disturbance. Just as a magnetized needle orients
itself so as to parallel the lines of maguetic force surrounding a
magnet, whether it is the earth or some other magnetic body, so
the lines of magnetic _orce passing through a body tend to
polarize it. For example ,in the northern hemisphere the lines
of magnetic force teud to polarize near-surface bodies so that
they have a smith pole on their upper side and a north pole
beneath. This would be the orientation of a magnetized needle
(i.e. the dip needle would have the north pole dipping down).
Therefore, if a lenticular ore-body was standing essentially vertical
its polarization would be such that a south pole would be
induced on its upper end and a north pole on its lower end;
whereas, if the same ore-body were lying essel_tially horizontal,
the poles would be induced on the respective sides rather than
the euds. In this example there is no other difference than the
above orientation, yet the magnetic disturbance noted at the sur.
face of the ground would be entirely different. In the first case a
marked disturbance over a small area would be noted, in the
second a minor disturbance over a larger area would be found.
This difference in magnetic effects for the same body may be
explained as follows: Since the attraction of a magnetic pole
varies as the square of the distance, the attraction measured in
the first case would be primarily that of the upper pole since
the lower pole is so much further away that its effect would be
very small. In the second ease the poles are o11 the sides of the
body which means that they arc relatively close together and
although the upper one still has the predominant effect it is
largely nullified by that of the lower pole of opposite sign which
is only a short distance below. The difference iu areal distribution
of the magnetic disturbance in the two cases is a reflection
of the size of the pole areas. When in the horizontal position
the sides of the body were the poles and as a result a much larger
area was disturbed than when the ends were the poles. When
bodies occur in between these two positions the disturbance varies
in intensity between these extremes and sbuilarly in areal distribution.
Magnetic profiles across the body will show an assymetric
shape and it is possible from these to postulate the direction
of dip of the body.
MAGNETIC
CALCULATIONS
A number of corrections are necessary in the calculation of
NEW JERSEY GEOLOGICAL SURVEY