Magnetic Surveying - Stanford University · PDF fileMagnetic Surveying Instruments Three types...
Transcript of Magnetic Surveying - Stanford University · PDF fileMagnetic Surveying Instruments Three types...
Magnetic Surveying
Magnetic Surveying
Basic concepts
Magnetic field strength, flux density, and permeability
B = µ H
B = magnetic flux density (flux per unit area)
H = field strength
µ = absolute permeability of the medium (in a vacuum µ = µ0 = 4πx 10-7)
Units: of flux density B is Tesla (T), we commonly use nanoTesla in surveying due to size of anomalies (1nT = 10-9T)
Magnetic Surveying
Basic concepts
Relative permeability, susceptibility and magnetization
In a medium other than vacuum we write µ = µr µ0
B = µ H
= µr µ0 H
= µ0 H + µ0 (µr – 1) H
= µ0 H + µ0 κ H
µr = relative permeability of the medium
κ = magnetic susceptibility
Magnetic Surveying
Basic concepts
Susceptibility and magnetization
M = κ H
Μ = intensity of magnetization induced by H
κ = magnetic susceptibility
Magnetic Surveying
Causes of Magnetic Susceptibility
• At the atomic level, materials have a net magnetic moment due to:
- rotation of electrons in various shells around the nucleus
- the spin of the electrons
- number of electrons in each shell
• Each atomic nucleus can be thought of as a small magnetic dipole with its own moment
Magnetic Surveying
Magnetic Susceptibility
Magnetic Surveying
Classification of Magnetic Materials
• Diamagnetic
- all electron shells are full, thus there is no net moment
- in presence of external field, the net moment opposes the external field
• Paramagnetic
− material contain unpaired electrons in incomplete electron shells
- however magnetic moment of each atom is uncoupled from others so they all behave independently
- weakly magnetic
Magnetic Surveying
Classification of Magnetic Materials
• Ferromagnetic
- material contain unpaired electrons in incomplete electron shells
- magnetic moment of each atom is coupled to others in surrounding ‘domain’ such that they all become parallel
- gives rise to a spontaneous magnetization even in absence of an external field
- magnets are ferromagnetic
Magnetic Surveying
Classification of Magnetic Materials
• Anti-ferromagnetic
- almost identical to ferromagnetic except that the moments of neighboring sub lattices are aligned opposite to each other and cancel out
- thus no net magnetization is measured
- example: hematite
• Ferrimagnetic
− similar to above but the sub lattices have unequal magnetic moments
- high magnetic susceptibility
- example: magnetite
Magnetic Surveying
Classification of Magnetic Materials
Magnetic levitation:
Researchers at the
University of Nijmegen in
the Netherlands
demonstrated levitating
a small frog in a powerful
magnetic field.
Magnetic Surveying
Classification of Magnetic Materials
Hysteresis
Magnetic Surveying
TRMDRM
CRMBiological
Remanant Magnetization
Magnetic Surveying
Geomagnetic Field
Origin:
99% from the Earth
94% dipole field
5% non-dipole field
1% current in ionosphere
diurnal variations
magnetic storms
Magnetic Surveying
Magnetic Surveying
Geomagnetic field can be described by the declination D, the inclination I, and total field vector F.
Geomagnetic Field
Magnetic Surveying
Secular Variations
Magnetic Surveying
Diurnal variations:
Daily changes in field due to changes in currents of charged particles in the ionosphere.
Smooth variations, amplitude 20 - 80 nT.
Magnetic Storms:
Short term disturbances in magnetic field associated with sun spot activity and streams of charged particles from sun. Can be up to 1000 nT in magnitude, and make magnetic surveying impossible.
Magnetic Surveying
Magnetic Surveying
Instruments
Three types of magnetometers are frequently used in magnetic surveying:
• Proton magnetometer
• Cesium vapor magnetometer (optically pumped)
• Fluxgate magnetometer
Magnetic Surveying
Fluxgate Gradiometer
Cesium Vapor Gradiometer
Magnetic Surveying
Surveying
• Establish a base station to incorporate drift
should be in flat terrain, away from electromagnetic field sources, and easy to reoccupy, return to base at least every hour or continually record data using separate magnetometer
• Wherever possible, conduct surveys perpendicular to strike
• You cannot be too obsessed with magnetic cleanliness
belt buckles, glasses, spiral bound notebooks, etc
power lines, wire fences, field vehicles, buildings with metal beams
keep sensor at least 1 m from ground else soil variations might dominate signal
Magnetic Surveying
Data Reduction
Can be separated into ‘corrections’ and ‘data enhancements’
• Diurnal Correction
• Normal Field Correction
essentially a correction for variations in field with latitude and longitude – use IGRF to correct on large scale surveys
• Elevation and Terrain Corrections
vertical gradient is a maximum at poles (0.03nT/m) minimum at equator (-0.015nT/m)
elevation correction not required for ground surveys
terrain correction difficult since we need to know magnetic properties in surrounding terrain
Magnetic Surveying
Magnetic Surveying
Data Reduction
Can be separated into ‘corrections’ and ‘data enhancements’
• Diurnal Correction
• Normal Field Correction
essentially a correction for variations in field with latitude and longitude – use IGRF to correct on large scale surveys
• Elevation and Terrain Corrections
vertical gradient is a maximum at poles (0.03nT/m) minimum at equator (-0.015nT/m)
elevation correction not required for ground surveys
terrain correction difficult since we need to know magnetic properties in surrounding terrain
Magnetic Surveying
Derivatives
• Emphasizing shorter wavelength features.• First vertical derivative emphasizes near surface features. It can be measured with gradiometer, or derived from corrected data• Second vertical derivative emphasizes boundaries of target zones
Magnetic Surveying
Magnetic Surveying
Gaffney et al 2000
Magnetic surveys over the Roman city of Wroxeter , UK (left) and the Roman fort Rainau-Buch, Germany (right)
Osten-Woldenburg 2005
Magnetic Surveying
Magnetic Surveying
Magnetic Surveying
Magnetic Surveying
Magnetic Surveying
Magnetic Surveying