Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 94

weaker than the Earth ' s

field. For the case where the

Inclination = +/-90°

Earth is magnetism is also

inclined and the anomaly is asymmetrical and dipolar.
Where the Earth

' s

field is inclined, the body

'

a)

s field is horizontal the dominant
response is a counter-intuitive decrease in TMI above the
body, even though it is more magnetic than its surround-
ings. This occurs because the two fields are opposed in the
region above the body where the TMI is measured. Only
when both the Earth

'

E a s t in g

N o r t h in g

Inclination = +45°

s magnetism are
predominantly vertical does a simple response comparable
to gravity occur.
Remanent magnetism, magnetic anisotropy and self-
demagnetisation have the effect of changing the strength
and direction of the body

' s field and the body '

b)

E a s t in g

N o r t h in g

s total magnetism. The myriad of
possible strengths and directions of the total magnetism
allows for a great many possible anomaly shapes and
amplitudes. Remanent magnetism is often a signi

'

Inclination = 0°

c)

cantly
complicating factor in the analysis of magnetic anomalies.
The issue is discussed in detail in Section 3.10.1.2 .
Anomalies measured below the surface can be envisaged
in a similar manner to those in Fig. 3.9 , but they can be
more complex. Similar complications may arise in very
rugged terrain since the magnetic geology may not all be
below the survey point (see Section 2.4.1.1 ). Measuring the
components of the magnetic field in one or more direc-
tions provides more information about the source than just
a TMI measurement. This is standard practice in downhole
surveys. As with gravity data, full tensor measurements
fully define the field. Nevertheless, it is currently normal
practice just to measure TMI, primarily because this can be
done very quickly and easily, and sufficient information is
obtained for most needs.

E a s t i n g

N o r th i n g

Inclination = -45°

d)

E a s t i n g

N o r t h i n g

N

e)

E a s ti n g

N o r t h i n g

S

Source

Depth

Figure 3.8 Total magnetic intensity measured on a horizontal surface
above a sphere (e) magnetised by induction for inducing

3.3 Measurement of the Earth's gravity field

fields with

different inclination. (a) Polar, inclination ¼ 90º. (b) Northern

The vast majority of the Earth is mass is contained within

the core and the mantle, the mass of the crust being a tiny
fraction of the total (about 0.4%). It is the interior com-
ponents of the Earth that are mostly responsible for its
gravity For this reason variations in the gravity field related
to geological features in the crust are very small compared
with the absolute value of gravity of about 9,800,000 gu.
Note that the Earth

'

hemisphere, inclination ¼ +45º. (c) Equator, inclination ¼ 0º. (d)

Southern hemisphere, inclination ¼ - 45º.

magnetic fields of the Earth and the sphere, i.e. the vector
addition of the two fields (see Appendix 1 ) .
Figure 3.9 shows variations in TMI along a traverse
across the centre of a source. In some locations the eld
due to the sphere is in roughly the same direction as the
Earth

s gravity field has no part in producing
anomalies of interest; it is an external interference that
must be compensated in order to resolve the anomalies.
This is unlike the magnetic field whose presence is funda-
mental in producing magnetic anomalies.

'

'

s field, so the strength of the resultant field (TMI) is
greater than the Earth

s field alone. Elsewhere the two
fields are in opposite directions, so the resultant field is

'