SAGE 2010 – INTRODUCTION TO MAGNETIC METHODS
INTRODUCTION TO MAGNETIC METHODS
THE EARTH’S MAGNETIC FIELD
1) The Earth’s Magnetic Field
a. What is the Total Magnetic Field Intensity (Figure 1)
b. Approximates a dipole (Figure 2)
c. Geomagnetic Elements (Figure 3)
d. Geomagnetic Intensity (Figure 4)
e. Geomagnetic Inclination (Figure 5)
f. Secular Variations in the Inclination and Declination (Figure 6)
g. Diurnal Variations (Figure 7)
h. Micropulsations (Figure 8)
i. Magnetic Storms (Figure 9)
j. Correcting for Diurnal Variations (Figure10)
2) Rock Magnetism
a. Average Magnetic Susceptibilities (Figure 11)
b. Measured susceptibilities of rock materials (Table 1)
c. Induced versus Remanent Magnetization
i. Thermoremanent (Curie Point)
ii. Detrital (or depositional)
iii. Chemical
3) Geologic Models of common magnetic anomaly sources (Figure 12)
a. Dipoles, monopoles, pole arrays (Figure 13)
b. Total intensity induced dipole signatures (Figure 14)
c. Anomalies for geologic bodies at various orientations and field
inclinations (Figure 15)
d. Remnant dipole signatures. (Figure 16)
e. Effect of Depth on Anomaly width (Figure 17)
f. Depth-Amplitude behavior of dipole anomalies (Figure 18)
g. Nomograph for estimating anomaly for typical objects (Figure 19,
Table 2)
h. Typical anomalies of archaeological features (Figure 20)
4) Planning a Survey
a. Ground Search (Figure 21)
5) Examples
a. Bill’s Crater (Figure 22)
b. Sensitivity of Survey (Figure 23)
c. Aeromagnetic Map of Field Area (Figure 24)
References: Sheldon Breiner, APPLICATIONS Manual for Portable Magnetometers
www.georentals.co.uk/ampm-opt.pdf
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SAGE 2010 – INTRODUCTION TO MAGNETIC METHODS
Figure 1. Total Magnetic Field Intensity with resulting vector from local disturbance.
Figure 2. The Earth’s Dipole Field.
Figure 3. The Geomagnetic elements.
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Figure 4. The Total Intensity of the Earth’s Magnetic Field.
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Figure 5. The Geomagnetic Inclination in Degrees of Arc from the Horizontal.
Figure 6. Secular Changes in Declination and Inclination at London Since 1540.
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Figure 7. Typical Diurnal Variations in Total Field Intensity.
Figure 8. Typical Micropulsations.
Figure 9. Typical Magnetic Storm.
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SAGE 2010 – INTRODUCTION TO MAGNETIC METHODS
CORRECTING FOR TIME VARYING FIELDS
F = f(x,y,z,t)
x = Easting (Longitude)
y = Northing (Latitude)
z = Elevation (very small effect)
t = time
FB = Field at Base Station (time dependendent)
FR = Field at Rover (space/time dependent)
FC = Time corrected field at Rover = FB- FC
GEOREF = Geomagnetic Reference Correction
Figure 10. Correcting for time varying fields
Figure 11. Average magnetic susceptibilities for surface samples and cores as measured
in the laboratory. ((Compiled by J. W. Peters, Mobil Oil Corp.)
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Table 1. Measured susceptibilities of rock materials.
Figure 12. Geological model representations of common magnetic anomaly sources.
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Figure 13. Dipoles, monopoles, and array of poles.
Figure 14. Total intensity signatures at various inclinations for induced magnetic
moments.
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Figure 15. Anomalies for geologic bodies at various orientations and different
inclinations of the field.
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Figure 16 Total intensity signatures at various inclinations of the field for remanent
magnetic moments.
Figure 17. Effect of depth on anomaly width.
Figure 18. Depth amplitude behavior of dipole anomalies.
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Figure 19. Nomogram for estimating anomalies from typical objects.
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Table 2. Anomalies of common objects.
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Fgure 20 . Typical magnetic anomalies of common archaeological features.
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SAGE 2010 – INTRODUCTION TO MAGNETIC METHODS
`PLANNING A SURVEY
Size of Target
Aerial extent of survey
Ground or Airborne
Resolution of Survey
Field Methods
Pitfalls
Examples
Figure 21 . Typical sequences of traverses during search procedures. Magnetic profiles
are drawn along respective traverse lines.
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Figure 22. Magnetic anomaly over Bill’s crater – Four Corners Area
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SAGE 2010 – INTRODUCTION TO MAGNETIC METHODS
ESTIMATION OF DEPTH TO TOP OF
ANOMALOUS BODIES USING MAXIMUM
SLOPE
Figure 23. Comparison of magnetic effect of lateral susceptibility change in basement
with effect of structural feature on basement surface.
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Figure 24. High resolution airborne magnetic anomaly map of the SAGE 2007
field area.
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