In this part, we review some of the basic physics of the magnetic method, in a generally-qualitative manner, and draw some conclusions about the scale of magnetic anomalies which might arise from crustal geology.

At this site you can find contour maps of the Earth's magnetic field intensity and other attributes. The field looks very smooth! and it is not until we measure and display the field at much higher resolution that we see geologically-linked variations.

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

Outline of Magnetics Segment (this page)

• Basic Background - magnetisation
• The source of the Earth's main magnetic field
• How magnetic anomalies arise
• Simple interpretation approaches

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.

Basic Introduction

• 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).

Ground hazard example

• Abandoned mineshafts were plugged with iron grid, then backfilled to surface
• Shafts on road easement pose collapse hazard during (and after) construction
• Magnetisation of Iron is physical target
• Although not "rock", iron grid is located/distinguished by magnetic properties.

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 magnetic susceptibility

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.

Remanence and ...

• 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...

...Palæomagnetism

• 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.
• see Figure 17.11 of Clark and Cook "Perspectives of the Earth" to recall the association of magnetic anomalies with sea-floor spreading.

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Susceptibility and magnetite concentration

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

Common values and rock types   

• These are very general suggestions!
• individual rock samples can vary widely.
• Closer study coming later.

Observing the fields

• Magnetic fields are observable "anywhere" near the Earth's surface.
• They are vector fields
• The direction varies with position
  • Declination information is given on topographic maps
• The magnitude varies with position
  • and is easily measured and displayed
  
  
• The direction and magnitude vary with time.

Field magnitudes

• 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.

Airborne data acquisition

• Recent large airborne surveys
  • 200m, 400m line spacing
  • sample spacing 7m along line
  • 300-1000 samples/square km
    • (compare geological sampling)
  • 10-30 ¢/sample
    
• Government initiative projects typically resell the data at 90% discount, or more.

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Introduction to Magnetic Fields

Magnetic Dipole Fields | The Earth's Field | Anomalous Fields

The Earth's Main Magnetic Field

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

Earth's Main Magnetic Field

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

Magnetic Forces

• The force between two magnetic poles follows the inverse-square law:

  
  
  (µ is the permeability of free space)
  
  

• However, isolated poles (p, P) are unknown in nature.

Magnetic Field Sources

• Magnetic fields appear to arise from
• Pairs of (opposite) magnetic poles
• loops of electrical current
  • (probably the true cause everywhere)
    
• So, the normal field shape is more complicated than expected.

Magnetic Field Law

d is the dipole moment ("strength" of the source) For a magnetic dipole (a N-S pair, or a loop of electric current) represented by the red arrow, the field at P is given by a vector whose components are:

Vectors for Field on a sphere

For the Earth,

• 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).

Generalizations

• The field depends on the inverse cube of distance
• because of the dipole source form
  
• 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.

World Magnetic Model 1995

Note that this is the main (core) field intensity variation - it is notably smooth at this scale! The parent page shows the main field variation in all of the various components of the magnetic field, as well as the secular variation (the change in the components with time).

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Earth’s Main Field

Generally, the main field:

• 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)
  
  so, the main field
  
  
• 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.
  
  For example:

Elura airborne response

What are these Variations?

• The "local" variations must arise near the surface, because:
  • Their widths are small
  • Their amplitudes are significant
    
• How do these observations lead to that conclusion?
  
  If we start with a simple "dipole" source, and do some simple calculations of the field along a profile:

Effect of depth (1)

The amplitude falls quickly with depth. Removing this factor by normalising the profiles:

Effect of depth (2)

The halfwidth rules, OK! (approx) - the width of the curve, as well as the amplitude, depends on the depth.

Which means that

• 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

• "Rules of thumb" can be generated to find depth indicators from magnetic anomalies.
• In general,
  
  
  • "width at half of maximum is approximately the depth"
    
    is a useful rule for isolated anomalies.

Unravelling the Maps

• Most magnetic maps are a complicated superposition of contributions from different sources
• It’s possible to treat an image "like an airphoto" for geological interpretation, but
• Understanding a few basics helps!

What is a magnetic 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.

Using the most simple model (a simple cartoon)

The measured local field

• 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.

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The equator example

This cartoon shows how a source at the equator interacts with the main magnetic field

The anomaly in the total field magnitude

is the change in the magnitude, and is shown in this cartoon.

At the North Pole

the Earth's main field is vertical, so the induced, anomalous field interacts to give a different result, shown here.

In mid-latitudes

there is an asymmetric response, as shown here.

Map views of sphere anomaly

• View 1, at 60° S
• View 2, at 30° S

Here are three samples from the Orbost area, east of Melbourne.

• Note that the negative (blue) anomalies lie to the south of the Mt Elizabeth pluton in the west of the airborne magnetic image (and also south of other anomalies, generally).
• In this version, mapped geological boundaries are superposed (but you will need to reference a paper map to identify the units). How do the boundaries and the magnetic units correspond?
• And for comparison, here is the radiometric image of the identical area.
  • (Images from Geological Survey of Victoria distributed data)

A dipping dyke at 45°S

Complexity!

• 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.

Making order from confusion

Some aspects are common to each profile:

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

Ordering II

• 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 info 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).

Processing can help

• Because the physics and maths are well understood
• Images can be processed to remove the asymmetries caused by main-field variations
• Reduced to the Pole images are a good attempt to simplify the magnetic map
  • Look back to the "pole" example to see why.

Quantitative interpretation of magnetic data

• Models of magnetisation distribution (induced and/or remanent) can be used to predict anomalous magnetic fields
• Comparison with observed values leads to adjustment of model parameters ("inversion" or quantitative interpretation)
  
• Most inversion now done in software.

Quantitative Interpretation Example

Modelling of basalt-filled grabens in West Siberia is demonstrated at Dick Gibson's Grav-Mag Mall. You could also look at his primer for an alternative approach.

The role of qualitative interpretation

• As implied earlier, images are also useful in discerning structure: this is qualitative interpretation
• Value comes from uniformity, density of coverage
• Value comes from insignificant contribution to anomalies from non-magnetic rocks (most cover rocks)

Now, move on to a look at the geological influences on the magnetic properties of rocks.

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