1. Magnetic Field 5

2. Applications – magnetic surveys

3. Measurement of the magnetic field
- Test magnet is
attached to a horizontal
fiber so that it can rotate
in a vertical plane
If the magnetometer is
inclined both vertical and
horizontal components of
the earth’s field act to
rotate it
In horizontal position of the
shaft only the vertical field
intensity component can
act to rotate it
- Measures relative changes
in the vertical component of
magnetic field intensity
Torsion magnetometer (Askania) – the most convenient to operate, precision ~ 1 gamma (1 nT)
Robinson&Coruh, 1988

4. Measurement of the magnetic field
- The design is based on the
principle of magnetic induction
It can be mounted for continuous
operation on an aircraft, ship,
or any moving vehicle
- It is not affected by motion or
changes in elevation
To avoid the distortion of the
reading by the metal parts of the
aircraft it has to be mounted in a
streamline container called “bird”
which trailed on a long cable
behind the aircraft
Measures the vertical component
of the magnetic field intensity
Flux-gate magnetometer, precision ~ 1 gamma (1 nT)
Robinson&Coruh, 1988

5. Measurement of the magnetic field
Designed for operation in aircraft,
ships, other moving vehicles
Hand-carried instruments for
ground surveying
Sensitive only to the total
field intensity
Proton precession magnetometers, precision ~ 1/2 gamma
Robinson&Coruh, 1988

6. Measurement of the magnetic field
Proton precession magnetometers - principle
Spin
Magnetometer principle
Precession
w = g F p p g
- gyromagnetic ratio of the proton
Robinson&Coruh, 1988
F – magnetic field, w - angular velocity

7. Geomagnetic Survey - applications
Locating:
Pipes, cables, magnetic objects
Buried military ordnance (shells, bombs)
Buried metal drums of contaminated or toxic waste
Concealed mine shafts
Mapping:
Archeological remains
Concealed basic igneous dykes
Metalliferous mineral lodes

8. Geomagnetic Surveys - types
On the surface – used mostly to map anomalies that are
too narrow for adequate detection by airborne operations.
Use flux-gate, proton-precession, or torsion type
magnetometers
2. By airplane (aeromagnetic surveys) –
measure the magnitude but not the direction
of the magnetic field; they can cover big areas
Use flux-gate and proton-precession type magnetometers
Magnetometer surveys must include measurement of the
diurnal and secular variations of the magnetic field 
a time base must be designated.
Most portable magnetometers measure only vertical relative
intensity  a base station must be designated in this case
Sharma, 1976; Robinson&Coruh,1988

9. Interpretation of magnetic data
End product of magnetometer survey – contoured anomaly
map in isogams (or nT)
2. Interpretation of magnetic data – similar to gravity data
interpretation but more complicated because of:
a) dipolar nature of the magnetic field
b) the additional unknown parameter introduced by
the direction of the magnetization in rocks.
3. Three types of interpretation:
a) qualitative
b) quantitative – forward modeling and inversion
c) semi-quantitative
Sharma, 1976

10. Qualitative interpretation of magnetic data
Visual inspection of the shape and trend of the
magnetic anomalies
2. Delineation of the structural trends
3. Close examination of the characteristic features of each
individual anomaly:
- the relative locations and amplitudes of the positive and
negative parts of the anomaly
- the elongation and areal extend of the contours
- the sharpness of the anomaly as seen by the
spacing of contours
In many cases meaningful geological information can be
obtained directly by looking at the map, without making
any calculations.
Sharma, 1976

11. Magnetic map - Ontario

12. Depth to magnetic basement
Crystalline basement rocks, which are commonly more mafic than overlying
sedimentary deposits, are the main source of magnetic anomalies in the region.
(High ferromagnetic mineral content results in high level of magnetization)

13. Curie Depth
Minerals that exhibit strong (ferromagnetic) behaviour at low temperatures have
weaker (paramagnetic) properties when hotter than the Curie temperature (~600).
Areas with high geothermal gradient have a shallow bottom to magnetic basement,
compared to colder areas. The form of magnetic anomalies can thus be used
to map the approximate Curie depth.

14. Quantitative interpretation of magnetic data
From the relative spreads of the maxima and minima
of the anomaly the approximate location and horizontal
extend of the causative body can be obtained.
2. The form of the anomaly, its shape and depth may
be determined.
3. The geometrical parameters must be translated into
structural terms in the light of known geology.
4. From the amplitude of the anomaly, the magnetization
contrast can be determined.
Sharma, 1976

15. Geophysical Anomaly
Gravity Profile
Magnetic Profile

16. Geophysical Anomaly Plots
Profile
Map
3-D

17. Approximation by a simple dipole magnetv h
Approximation by
a dipole magnet
Modeling of a dipole magnet:
Vertical (v), horizontal (h) and total (t) intensity

18. Comparison of total intensity anomalies over a simple dipole magnet (different cases)
Different depth of
the dipole magnet
Different angle of
the dipole magnet
Different oriented dipole magnets
Different strength of
the dipole magnet

19. Total intensity anomalies over plates of infinite horizontal extend (different cases)
Plates inclined at
different angles,
magnetized in
different direction (i),
and oriented in
different directions (s)
relative to the
magnetic north.

20. Magnetic anomaly interpretation- simple graphical method
Reynolds, 1997

21. Igneous intrusion / Salt dome
Igneous intrusion – have large amounts of magnetite and high magnetization.
Salt – exhibits diamagnetic behaviour and a weak field opposite in direction to the ambient field.

22. Salt dome Gravity anomaly over the Grand Saline Salt Dome,Texas, USA
(contours in gravity units)
Magnetic anomalies over the Grand Saline Salt Dome,Texas, USA
(contours in nT)
The stippled area represents the
subcrop of the dome.
low density
negative magnetic
susceptibility

23. Modeling ore body deposits

24. Zinc ore deposit (Yukon)
Geochemical analysis of soils
Residual Gravity
Magnetic Field

25. Copper ore body (Finland)
Magnetic field
Gravity
Electrical resistivity (SLINGRAM) – caused by black shists

26. Magnetic Surveys for buried infill
Magnetic anomalies over
a clay-with-flints infill over
chalk
Shape of the profile across
the clay pipe infill

27. Magnetic surveys for buried pipes

28. Texts Lillie – all appropriate sections Fowler, p.43-67; 77-88.
Additional reading:
Campbell, W.H. (1997) Introduction to geomagnetic fields, Cambridge, 1997, p.51-58,
Robinson & Coruh (1988) Basic exploration geophysics, John Wiley & Sons, all appropriate sections
Sharma, 1976