1. Geology 5660/6660 Applied Geophysics 30 Mar 2016
Last Time: Magnetic Methods
• Magnetic field strength decays as 1/r3
• Total intensity of magnetization:
where remanent magnetization IR is in the direction
of HE at the time of magnetization… i.e., when the rocks
cooled through their Curie temperature
• Must know strength & direction of the Earth’s
main field!
For Fri 1 Apr: Burger (§ )
© A.R. Lowry 2016

2. Geology 5660/6660 Applied Geophysics 30 Mar 2016
Last Time (Cont’d): Earth’s Main Magnetic Field
• Earth’s Main Field derives from “core dynamo”…
convective flow of Ni-Fe outer core + coriolis forces ( +
feedback)  electrical current flow  magnetic field
( + feedback) + single-crystal Fe inner core
( + feedback)
• Mostly dipolar, oriented ~ (and precesses around) rotation
axis; varies nonlinearly through time…
• Express the vector field at a point as either intensity HE,
inclination i, declination or local Hx, Hy, Hz components:
For Fri 1 Apr: Burger (§ )
© A.R. Lowry 2016

3. Problem: If both Earth’s main field and crustal field are
determined from measurements, how do we separate them?
Core field dominates long wavelengths (small n of spherical
harmonics). We describe core field only out to n = 14–15
where it dominates the total field.

4. Measurement: Fluxgate magnetometer: Wire coils wound in
opposite directions;
these cancel &
produce zero current
in secondary coil
in absence of external
magnetic field, but
if aligned with a field,
one core reinforces,
other counteracts
external field resulting
in a current.
Gives intensity in the
orientation of the
coils.
(Direction?)

5. Measurement: Proton precession magnetometer:
Bottle containing a hydrogen-rich
fluid (distilled water or hydrocarbon)
is surrounded by a wire coil. Current
through the coil produces a strong
magnetic field; protons align with
field… Current is shut off & as protons realign with ambient
magnetic field, they precess at a frequency determined by
magnetic field strength ( Hz/nT). So, measure
frequency of the induced AC current and convert to a total
field strength.
(Lots of other types but these two are most commonly used
for terrestrial geophysics!)

6. Remove/avoid all metal objects when collecting data!!!
Data Reduction:
Remove/avoid all metal objects when collecting data!!!
Keep magnetometer high off the ground to reduce “noise”
Sensitive to variations in ionosphere, magnetosphere:
Perform looping and correct for drift; don’t bother
measuring during solar storms!
Correct for elevation if > a few hundred m (~0.03 nT/m)
Horizontal position correction: Use WMM if latitude change
is > a few hundred m (correction here ~6 nT/km)
From
Component Field Value Secular Variation
Declination degrees °/year
Inclination degrees °/year
Horizontal Intensity nT nT/year
North Component (x) nT nT/year
East Component (y) nT nT/year
Vertical Intensity (z) nT nT/year
Total Intensity nT nT/year

7. Aeromag measurements
of magnetic anomalies
over Nevada…
Most magnetic anomaly
maps on scales < 500
km use magnetometer
data collected by flying an
airplane in a grid pattern
over the target region

8. Commonly use in combination with gravity, e.g. this kimberlite
prospect in Botswana

9. Satellite
measurements of
induced + remanent
magnetization of
the Earth’s crust
(scalar total field
anomaly relative
to Earth’s main,
i.e. core, field)

10. Satellite data are more self-consistent and useful for
regional-scale tectonics studies– Here depth-integrated
magnetic susceptibility shows strong relationships to
Proterozoic accretion history of the mid-continent.

11. Combination of global aeromag and CHAMP satellite crustal
magnetic anomalies… Note merging problems!

12. Conterminous US satellite/aeromag merge is better…
(albeit badly aliased in this image)

13. Modeling Magnetic Anomalies:
Laplace’s equation for magnetism (analogous to gravity) is:
where is magnetic field, a is magnetic potential,
 is magnetic permeability,
is dipole moment per unit volume
If we assume a magnetic “monopole” (i.e., -m part of the
dipole is so far away we can ignore it) at the origin, then
the potential (A is cross-sectional area)
and