A segment of the subject 625-304 Applied Geophysics

A map of magnetic field in the Australian region shows much detail which often continues where surface geological observation is impossible, or simply reports Recent deposits. The detail is generally a response to magnetite content, so that magnetics information can readily be incorporated into geological interpretations as a kind of monominerallic geochemistry map. Where the ability of magnetics to add a third dimension to rock variations, the geological interpretation must be extended to account for these data.

This page is an edited version of a class presentation, not a full set of subject notes.

Home | ES304 magnetics | Overview | Gravity | Electrical | Seismic | Top

Parasnis is a standard textbook. Don't forget to look at AGSO, or the AGCRC page for more on large-scale geophysics data.

-- looks at applications of magnetics in geological mapping
Main objective
-- to enable the eventual integration of magnetic data with other geological data by giving rules of thumb which make the data more accessible.

• Describe physical basis of the Earth's magnetic field and its variations
• Discuss simple theory
• Show responses for simple models
• Case history

Magnetic field responses

• Rock materials have magnetic properties which cause them to act as magnetic objects.
• These properties principally centre on the magnetite concentration in the rock.
• This is the key to the importance of magnetic-field mapping.

• Rock bodies, magnetized, act as individual sources of magnetic field.
• The local magnetic fields contribute to the variation in the main field.
• The shape of the field variations depends on the rock body geometry (and other things).

Many of the concepts are identical to those discussed in the Gravity section.

• Anomaly
• Halfwidth
• Separation
• Inversion
• Interpretation

• The source of the field
  • What is it?
• The rock properties
  • What are they?
• The actual anomaly shapes
  • What do they look like?
• How do we measure the fields?
  • and this is notably easier than Gravity!

Susceptibility and Remanence

• These are the two important magnetic properties.
• Consider material immersed in ambient magnetizing field
  • The material will become magnetized
  • The magnetization depends on the susceptibility

• Susceptibility k may be positive, negative, "large", or "small"
• For most materials, k is small (and negative)
• For a very few minerals, k is large and positive
  • k is large for Iron, too, but that isn't a natural mineral.

If the ambient magnetizing field is removed, but the material retains its magnetization, it is said to be remanently magnetized .

Some rocks exhibit this property.

• The notable case is ocean-floor basalts, magnetized on formation

• The remanent magnetization in (old) rocks is a memory of the magnetic field at their formation.
• Study of these fields (palæomagnetics) has been a major contributor to learning how the Earth works.

Experiment shows that susceptibility k for rocks is generally directly dependent on magnetite concentration

These are very general suggestions!

The magnetic dipole moment of a volume V (cubic metres), filled with material with susceptibility k, in a magnetic field of strength F (Tesla), is given by

M = V k F

Using the guide shown, find the anomalous magnetization for a 100 m sphere of basalt in sediments.

Magnetic fields, like gravity fields, are observable "anywhere" near the Earth's surface.

• They are vector fields
• The direction varies with position
• The magnitude varies with position
• The direction and magnitude vary with time.

• The standard unit for magnetic fields is the Tesla
• The practical unit is the nanoTesla
  • Older use is "gamma", numerically the same
• The Earth's main field ranges from 25 000 nT to 75 000 nT
• Typical resolution is 0.1-1 nT
  • 0.001 nT resolution is often used (especially in airborne work).

The Earth's Main Magnetic Field

• General shape long known
• "North" end of magnet seeks North
• Field lines show dip as well as dir'n
• Dip varies with latitude.

• Source appears to be magnetic dipole within Earth.
• Not now thought to be a literal magnet, but electrical currents circulating in Earth's Core.

The force between two magnetic poles has the same shape as the gravity field:

(µ is the permeability of free space; p, P are magnetic pole strengths)

However, isolated poles are unknown.

Magnetic fields appear to arise from

• Pairs of (opposite) magnetic poles
  or
• loops of electrical current
  • (probably the true cause everywhere)

So, the normal field shape is more complicated than the gravity field.

for a magnetic dipole (a N-S pair of poles - or a loop of electric current)

p is the dipole moment ("strength" of the source)

• The dipole axis is approximately the rotation axis.
• H-tangential is approximately the horizontal component of the magnetic field.
• H-radial is approximately the vertical component of the field.
  
  
• The most common actual measurement is total-field amplitude (not a component).

• The field depends on the inverse cube of distance
  • more rapid decrease than gravity

  On the Earth's surface (r constant) :

  • There will always be a radial (outward) component (except . . .)
  • The field on the equator will be smaller than the field at the Poles.

Generally:

• Varies smoothly
  • the source must be near the centre
• Has a vertical component
  • why don't compass needles point up?
• Is about twice as strong (61000+ nT) at the poles as at the equator (25000- nT)
• Probably arises from deep, dipole source

A Closer Look

• Viewing the Earth's magnetic field in detail shows, almost everywhere, variations over "short" distances.
• These are most evident in amplitude, which varies by several % from simple dipole prediction.
• These variations generally occur over distances of few km or less.

The "local" variations must arise near the surface:

• Their halfwidths are small
• Their amplitudes are significant
  
  How do these observations lead to the conclusion in the first line?

Start with a simple "dipole" source:

The halfwidth rules, OK!.

• Local variations suggest local sources.
• Deeper sources are only seen if their strength is great.
  
  
• Sources are mainly in the Earth's crust.

Rules of thumb can be generated for magnetic anomalies, just as for gravity anomalies.
In general,

• "width at half of maximum is approximately the depth"

is a useful rule for isolated anomalies. Note that it is different from the gravity rule, because of the difference between the basic field laws.

What is a source?

• The Earth's main field magnetizes all (crustal) rocks.
• Some rocks have higher magnetic susceptibility than others.
• These rocks will be more strongly magnetized.
• These rocks will act as individual sources of magnetic field.

Suppose the Earth's main field is horizontal (at the magnetic equator)
All materials will be magnetized.
If a volume has higher k, it will be anomalously magnetized.

• This will be the vector sum of the Main and anomalous fields.
  • This is where the modelling gets tough, because the field directions change!
  • even with a spherical source!!
    
    
• The rules of thumb become even more important, because the anomaly shapes change.

• This is the total-field anomaly.
• There are +ve and -ve parts.

This is a S-N profile over a simple body at Melbourne latitudes.
The anomaly has both positive and negative portions!

• You should be able to explain why, now.

• The NS asymmetry can be seen.
• The maximum isn't over the source.

• At magnetic latitude 30°S
• The maximum is not over the source.

• The main (inducing) field direction changes with latitude.
• The local (anomalous) field direction changes with position.
  
  
• Even for a simple source (sphere shown) the magnetic response shapes vary widely.
  
  
• Most anomalies observed are dipolar.

Only the direction of dip changes from model to model in these profiles.

Some aspects are common to each profile:

for sources at the same depth:

• The "width" (aka halfwidth) is about the same
• The maximum slope on the profile (or map) is about the same

• The maximum slope is often associated with the edge of a magnetically-anomalous body
• The rule-of-thumb thus gives depths and locations to edges
  
  
• We can then interpret qualitatively (for shapes) but keep depth information in mind.

The summary, so far

• Magnetic properties of rocks can vary widely, even in adjacent beds.
• The variation affects the local magnetic field, measurably.
  
  
• The response can be complicated, but the locations and approximate depths of anomalous bodies can be discerned (in qualitative or "structural" interpretation).

• Magnetic images are very uniform, consistent views of geology.
• Presently used as one of most basic information sources, especially where Recent or unconsolidated material covers "real" geology.
• Interpretation resembles airphoto interpretation.

• Detailed measurements of magnetic fields can be inverted and interpreted as with gravity data.
• Calculations are more complicated.
• Rules of thumb are useful as starting point.
  
  
• Most inversion now done in software.

Some notes on data acquisition

• Magnetic mapping also favoured because airborne (light plane, helicopter) vehicles can be used.
• Recent large airborne surveys
  • 200m, 400m line spacing
  • sample spacing 7m along line
  • 300-1000 samples per square km
  • 10-30 ¢/sample (1994)

• Line orientation?
• Line spacing?
• Sample spacing?

  (These questions apply to ground surveys, too.)

  • Use geological concepts, plus simple models, rules-of-thumb, to provide answers.

Case Histories

• Discovery with magnetics:
  • Elura Zn-Pb-Ag deposit
    • Exploration Geophysics v11, #4
• Mapping sedimentary basin depths

  • Jacobson, in Geophysics, v26, #3.

• "Non"discovery
  • Woodlawn magnetic anomalies
    • "Woodlawn" volume (ed. Whiteley)

Also, there are many others in common texts...

• Base-metal orebody near Canberra, Australia
• Cu-Pb-Zn Sulphide Deposit
• Discovery by joint use of geology, geochemistry, geophysics
• Used as test site for geophysical methods for search for further volcanogenic, stratiform targets.

• Gossan is surface expression
  • Body dips vertical-to-W
• Ore is not magnetic
• What is the origin of the magnetic anomaly?
  • Is it irrelevant?
  • Is it cogenetic?

Target is very shallow (12 m)

Target is very shallow (12 m)

• Location, depth, trend, susceptibilities indicate anomalies associated with dolerite intrusions
• Dolerites postdate mineralization
  
  
• Magnetic anomalies thus unrelated to mineralization in this case
  
  
  • Analysis does help with structural studies of environment

Next: electrical methods

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