Geology Reference
In-Depth Information
+
(e)
Combined effects
of both poles
Negative
pole
420
000
440
000
(Warwickshire coalfield)
Hinckley
Basin
Northern section
of central block
0
Distance
Positive
pole
260
000
?
?
?
?
Central section
of central block
-
Magnetic North
Withycombe
Farm B.H.
240
000
-
?
B
?
?
+
?
?
Southern section
of central block
Southern section
of central block
?
Fig. 7.18
The total field magnetic anomaly of an elongate body
approximated by a dipole.
HINGE
POINT
Steeple Aston B.H.
Bicester B.H.
220
000
(Fig. 7.19).The poles of the magnets are negative on the
surface of the body where the magnetization vector en-
ters the body and positive where it leaves the body.Thus
any uniformly-magnetized body can be represented by a
set of magnetic poles distributed over its surface. Con-
sider one of these elementary magnets of length
l
and
cross-sectional area
d
A
in a body with intensity of
magnetization
J
and magnetic moment
M
. From
equation (7.5)
420
000
440
000
Fig. 7.17
Continued
respectively. The boundaries implied by the solutions
have been used to construct the interpretation shown in
Fig. 7.17(e).
MJ
=
l
(7.15)
d
7.10.3 Indirect interpretation
Indirect interpretation of magnetic anomalies is similar
to gravity interpretation in that an attempt is made to
match the observed anomaly with that calculated for a
model by iterative adjustments to the model. Simple
magnetic anomalies may be simulated by a single dipole.
Such an approximation to the magnetization of a real
geological body is often valid for highly magnetic ore
bodies whose direction of magnetization tends to align
with their long dimension (Fig. 7.18). In such cases the
anomaly is calculated by summing the effects of both
poles at the observation points, employing equations
(7.10), (7.11) and (7.9). More complicated magnetic
bodies, however, require a different approach.
The magnetic anomaly of most regularly-shaped
bodies can be calculated by building up the bodies from
a series of dipoles parallel to the magnetization direction
If the pole strength of the magnet is
m
, from equation
(7.4)
m
=
M
/
l
, and substituting in equation (7.15)
m
=
J
d
A
(7.16)
If
d
A
¢ is the area of the end of the magnet and
q
the angle
between the magnetization vector and a direction
normal to the end face
d
A
=
d
A
¢ cos
q
Substituting in equation (7.16)
m
=
J
d
A
¢ cos
q
thus
