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