1. Magnetic method
Magnetic force and field strength for pole strength m’ and m

2. Magnetic method Magnetic poles only exist in pairs  magnetic moment M
C = magnitude of couple

3. Magnetic method Intensity of magnetization J
k is the magnetic susceptibility which describes the degree to which a body is magnetized when put in an external field H. k is the fundamental parameter in magnetic prospecting.

4. Magnetic method
Diamagnetic minerals have low negative susceptibilities (quartz, feldspar)
Paramagnetic minerals have low positive susceptibilities (olivine, pyroxene)
Ferromagnetic material have strong susceptibilities (iron, nickel, cobalt)
Antiferromagnetic mineral have a net (almost) zero magnetic moment (hematite)
Ferrimagnetic minerals have a net magnetic moment (magnetite, illmenite)

5. Magnetic method

6. Magnetic method

7. Magnetic method
If a magnetic material is placed in an external field H, its internal poles line up to produce a field on their own H’ producing a total field B

8. Earth’s Magnetic field
Main field (core: mostly dipolar)
Small external field, changes rapidly with time
Variations of the main field due to local magnetic anomalies (targets)

9. Earth’s Magnetic field

10. Earth’s Magnetic field

11. Dipole equations

12. Measuring the magnetic field
Flux-gate magnetometer measures all components of the magnetic field. Approximately 1nT precision

13. Measuring the magnetic field
Proton-precession magnetometer only measures the intensity of the magnetic field. Approximately 1nT precision

14. TOTAL field anomalies

15. TOTAL field anomalies We measure FET which is FAT plus FEU
The Earth’s field is much stronger than that of the anomaly if low susceptibility
We define a body and calculate HA and ZA

16. Field procedures
Magnetic cleanliness (watches, pens, cars, power lines….)
Short-term variations in the external field of a few nT
Returning to base
Continuous recording at base
Storms!

17. Field procedures Elevation correction
Approximately 0.03 nT/m , normally neglected because lost in noise
Horizontal correction
Approximately 6 nT/km

18. Field procedures

19. Interpretation more difficult than for gravity
Positive and negative poles
Horizontal and vertical component

20. Magnetic effect of simple shapes: monopole

21. Magnetic effect of simple shapes: monopole

22. Magnetic effect of simple shapes: dipole

23. Fig. 7.20g top

24. Fig. 7.20g middle

25. Fig. 7.22g

26. Magnetic effect of simple shapes: sphere
Poisson’s relation, where
U gravitational potential,
w direction of magnetization

27. Magnetic effect of simple shapes: sphere

28. Fig. 7.25g

29. Fig. 7.27g

30. Fig. 7.28g top

31. Fig. 7.28g bottom

32. Fig. 7.30g top

33. Fig. 7.30g bottom

34. Fig. 7.30g bottom

35. Fig. 7.31g

36. Fig. 7.33g bottom

37. Fig. 7.33g top

38. INTERPRETATION of magnetic data
Difficulties
No unique solution additional information
Remnant magnetization
Large variability and non-uniform distribution of susceptibility
Total-field measurements only
Dependence of anomaly on direction of magnetization

39. INTERPRETATION of magnetic data
Advantages
Low cost - high precision
Orientation of Earth’s field is constant for given survey  compare to appropriate characteristic curves
Large anomalies due to few rock types with high susceptibility
Poisson’s relation can turn magnetic into pseudo gravity data
Similar techniques to gravity

40. Half-maximum technique
Less precise than with gravity even if we know the shape.
Example: thin vertical rod (monopole)
Sphere and cylinder: width at ZA2 = z
Semi-infinite sheet: (xmax-xmin)/2 = z

41. Fig. 7.34g bottom

42. Fig. 7.34g top

43. Fig. 7.35g
SLOPE methods
Peters: z=d/1.6 (prism depth ~ width << length, strike infinite // meridian)

44. Fig. 7.35g
Applications
Archaeology: often iron objects (high susceptibility) associated with ancient sites, high remnant magnetism in production of bricks etc.
Voids and well castings, steal objects, bombs
Landfill geometry
Geology

45. Fig. 7.38g

46. Fig. 7.40g