Magnetic Surveying

Question 1

What is magnetic permeability?

Answer 1

The constant of proportionality between density of a magnetic flux and magnitude of the magnetising force

Question 2

What 3 factors is the magnetic moment of a current-carrying coil proportional to?

Answer 2

The number of turns in the coil, its cross-sectional area, and the magnitude of the current

Question 3

What is induced magnetisation?

Answer 3

The phenomenon of a material being placed in a magnetic field and acquiring magnetisation in the direction of the field, which is lost when the material is removed

Question 4

Why are all substances magnetic at an atomic scale?

Answer 4

Each atom acts as a dipole due to the spin of its electrons and the orbital path of electrons around the nucleus

Question 5

What are paired electrons?

Answer 5

Electrons that exist in the same shell with spins in the opposite directions

Question 6

What are diamagnetic materials? What happens when they are placed in a magnetic field?

Answer 6

Materials in which all electron shells are full and no unpaired electrons exist. Orbital paths of the electrons in a magnetic field rotate to produce a magnetic field in opposition to the applied field

Question 7

What are paramagnetic substances? What happens when they are placed in a magnetic field?

Answer 7

Materials in which electron shells are incomplete and a magnetic field results from the spin of their unpaired electrons. When placed in a magnetic field, dipoles corresponding to unpaired electrons spin to produce a field in the same sense as the applied field

Question 8

What are ferromagnetic materials? What happens when they are placed in a magnetic field?

Answer 8

Materials with parallel dipoles that give rise to a very strong spontaneous magnetisation that can exist in the absence of a magnetic field

Question 9

What are antiferromagnetic materials? What happens when they are placed in a magnetic field?

Answer 9

Materials with antiparallel dipole coupling of equal strength that do not experience magnetisation due to self-cancelling effect

Question 10

What are ferrimagnetic materials? What happens when they are placed in a magnetic field?

Answer 10

Materials with antiparallel, unequal strength dipoles that can exhibit strong spontaneous magnetisation

Question 11

What are the 3 types of remanent magnetisation?

Answer 11

Primary, which can be acquired as an igneous rock solidifies and cools or as magnetic particles of a sediment align with the Earth’s field
Secondary, which can be impressed later as magnetic minerals recrystallise or grow
Remanent magnetisation may also develop slowly in a rock standing in an ambient magnetic field

Question 12

What is the Königsberger ratio? How does its vector control amplitude and shape of an anomaly?

Answer 12

The ratio of relative to induced remanent magnetisation intensities
Magnitude influences amplitude and orientation influences shape

Question 13

Which rock types (igneous, sedimentary, and metamorphic) are magnetic?

Answer 13

Basic igneous rocks, high ppO2 metamorphic rocks, and few sedimentary rocks

Question 14

What is the shape (angle and tilt) of Earth’s magnetic field?

Answer 14

80% dipolar with a 9.6° tilt

Question 15

What are inclination and declination of a field?

Answer 15

Inclination is the dip of the total field vector, and declination the horizontal angle between geographic and magnetic north

Question 16

How does the magnetic field generally dip in the northern and southern hemispheres?

Answer 16

Northern: downwards to the north, becoming vertical at the north magnetic pole
Southern: dips upwards towards the north

Question 17

What is the magnetosphere?

Answer 17

The elongated shape produced by the effect of solar wind carrying a plasma of charged particles

Question 18

What causes diurnal variations and how are they corrected for?

Answer 18

Magnetic effects of external origins, such as (on an extreme level) magnetic storms
On land, diurnal variation is corrected for by taking periodical readings at a fixed base
In the air, crossover points are calculated into the flight route

Question 19

How are sedimentary basins and igneous and metamorphic terrains represented on an aeromagnetic contour map?

Answer 19

Sedimentary covered areas are represented by relatively smooth contours
Igneous and metamorphic terrains generate more complex magnetic anomalies

Question 20

How do the Rule of Thumb and Peter’s Method differ?

Answer 20

The rule of thumb uses a rough estimate of limiting depth, the most linear segment, and the horizontal extent of the segment
Peter’s method uses a rough estimate of limiting depth, the gradient of the steepest point of the curve, 2 points on the curve, and the distance between these points

Question 21

How is indirect interpretation simulated for 2D and 3D models?

Answer 21

2D: cross-sectional forms of the body are approximated by polygonal outline
3D: causative body is approximated by a cluster of right rectangular prisms or horizontal polygonal slices

Question 22

What is the standard method for magnetic survey data interpretation?

Answer 22

1. The magnetic field is defined and calculated
2. The magnetic anomaly is separated through calculations
3. Diurnal and geomagnetic variations are accounted for
4. Contour maps are produced and studied
5. Limiting depth is calculated
6. Magnetic anomaly models are created and fine tuned