Magnetic Surveying Flashcards
What is magnetic permeability?
The constant of proportionality between density of a magnetic flux and magnitude of the magnetising force
What 3 factors is the magnetic moment of a current-carrying coil proportional to?
The number of turns in the coil, its cross-sectional area, and the magnitude of the current
What is induced magnetisation?
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
Why are all substances magnetic at an atomic scale?
Each atom acts as a dipole due to the spin of its electrons and the orbital path of electrons around the nucleus
What are paired electrons?
Electrons that exist in the same shell with spins in the opposite directions
What are diamagnetic materials? What happens when they are placed in a magnetic field?
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
What are paramagnetic substances? What happens when they are placed in a magnetic field?
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
What are ferromagnetic materials? What happens when they are placed in a magnetic field?
Materials with parallel dipoles that give rise to a very strong spontaneous magnetisation that can exist in the absence of a magnetic field
What are antiferromagnetic materials? What happens when they are placed in a magnetic field?
Materials with antiparallel dipole coupling of equal strength that do not experience magnetisation due to self-cancelling effect
What are ferrimagnetic materials? What happens when they are placed in a magnetic field?
Materials with antiparallel, unequal strength dipoles that can exhibit strong spontaneous magnetisation
What are the 3 types of remanent magnetisation?
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
What is the Königsberger ratio? How does its vector control amplitude and shape of an anomaly?
The ratio of relative to induced remanent magnetisation intensities
Magnitude influences amplitude and orientation influences shape
Which rock types (igneous, sedimentary, and metamorphic) are magnetic?
Basic igneous rocks, high ppO2 metamorphic rocks, and few sedimentary rocks
What is the shape (angle and tilt) of Earth’s magnetic field?
80% dipolar with a 9.6° tilt
What are inclination and declination of a field?
Inclination is the dip of the total field vector, and declination the horizontal angle between geographic and magnetic north
How does the magnetic field generally dip in the northern and southern hemispheres?
Northern: downwards to the north, becoming vertical at the north magnetic pole
Southern: dips upwards towards the north
What is the magnetosphere?
The elongated shape produced by the effect of solar wind carrying a plasma of charged particles
What causes diurnal variations and how are they corrected for?
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
How are sedimentary basins and igneous and metamorphic terrains represented on an aeromagnetic contour map?
Sedimentary covered areas are represented by relatively smooth contours
Igneous and metamorphic terrains generate more complex magnetic anomalies
How do the Rule of Thumb and Peter’s Method differ?
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
How is indirect interpretation simulated for 2D and 3D models?
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
What is the standard method for magnetic survey data interpretation?
- The magnetic field is defined and calculated
- The magnetic anomaly is separated through calculations
- Diurnal and geomagnetic variations are accounted for
- Contour maps are produced and studied
- Limiting depth is calculated
- Magnetic anomaly models are created and fine tuned
