Magnetic Field 5

Applications – magnetic surveys

Measurement of the magnetic field

• Torsion magnetometer (Askania) – the most convenient to operate, precision ~ 1 gamma (1 nT)

- Test magnet is attached to a horizontalfiber so that it can rotate in a vertical plane-If the magnetometer isinclined both vertical andhorizontal components ofthe earth’s field act torotate it-In horizontal position of theshaft only the vertical field intensity component canact to rotate it- Measures relative changesin the vertical component of magnetic field intensity

Robinson&Coruh, 1988

Measurement of the magnetic field

• Flux-gate magnetometer, precision ~ 1 gamma (1 nT)

- The design is based on theprinciple of magnetic induction- It can be mounted for continuous operation on an aircraft, ship, or any moving vehicle- It is not affected by motion or changes in elevation-To avoid the distortion of the reading by the metal parts of the aircraft it has to be mounted in a streamline container called “bird” which trailed on a long cable behind the aircraft - Measures the vertical componentof the magnetic field intensity

Robinson&Coruh, 1988

Measurement of the magnetic field

• Proton precession magnetometers, precision ~ 1/2 gamma

- Designed for operation in aircraft, ships, other moving vehicles- Hand-carried instruments for ground surveying- Sensitive only to the total field intensity

Robinson&Coruh, 1988

Measurement of the magnetic field

• Proton precession magnetometers - principle

Robinson&Coruh, 1988http://www.earthsci.unimelb.edu.au/ES304/MODULES/MAG/NOTES/proton.html

Spin

Precession

Magnetometer principle

F p

F – magnetic field, angular velocity

p - gyromagnetic ratio of the proton

Geomagnetic Survey - applications

• Locating:- Pipes, cables, magnetic objects- Buried military ordnance (shells, bombs)- Buried metal drums of contaminated or toxic

waste- Concealed mine shafts• Mapping:- Archeological remains- Concealed basic igneous dykes- Metalliferous mineral lodes

Geomagnetic Surveys - types1. On the surface – used mostly to map anomalies that are too narrow for adequate detection by airborne operations. Use flux-gate, proton-precession, or torsion type magnetometers 2. By airplane (aeromagnetic surveys) – measure the magnitude but not the direction of the magnetic field; they can cover big areas Use flux-gate and proton-precession type magnetometers

Magnetometer surveys must include measurement of the diurnal and secular variations of the magnetic field a time base must be designated.Most portable magnetometers measure only vertical relative intensity a base station must be designated in this case

Sharma, 1976; Robinson&Coruh,1988

Interpretation of magnetic data

1. End product of magnetometer survey – contoured anomaly map in isogams (or nT)2. Interpretation of magnetic data – similar to gravity data interpretation but more complicated because of: a) dipolar nature of the magnetic field b) the additional unknown parameter introduced by the direction of the magnetization in rocks.3. Three types of interpretation: a) qualitative b) quantitative – forward modeling and inversion c) semi-quantitative

Sharma, 1976

Qualitative interpretation of magnetic data

1. Visual inspection of the shape and trend of the magnetic anomalies2. Delineation of the structural trends3. Close examination of the characteristic features of each individual anomaly: - the relative locations and amplitudes of the positive and negative parts of the anomaly - the elongation and areal extend of the contours - the sharpness of the anomaly as seen by the spacing of contours

In many cases meaningful geological information can be obtained directly by looking at the map, without making any calculations.

Sharma, 1976

Magnetic map - Ontario

Depth to magnetic basement

Crystalline basement rocks, which are commonly more mafic than overlying sedimentary deposits, are the main source of magnetic anomalies in the region. (High ferromagnetic mineral content results in high level of magnetization)

Curie Depth

Minerals that exhibit strong (ferromagnetic) behaviour at low temperatures have weaker (paramagnetic) properties when hotter than the Curie temperature (~600).Areas with high geothermal gradient have a shallow bottom to magnetic basement, compared to colder areas. The form of magnetic anomalies can thus be used to map the approximate Curie depth.

Quantitative interpretation of magnetic data

1. From the relative spreads of the maxima and minima of the anomaly the approximate location and horizontal extend of the causative body can be obtained.2. The form of the anomaly, its shape and depth may be determined.3. The geometrical parameters must be translated into structural terms in the light of known geology.4. From the amplitude of the anomaly, the magnetization contrast can be determined.

Sharma, 1976

Geophysical Anomaly

Gravity Profile

Magnetic Profile

Geophysical Anomaly Plots

Profile

Map

3-D

Approximation by a simple dipole magnet

Approximation bya dipole magnet

Modeling of a dipole magnet:Vertical (v), horizontal (h) and total (t) intensity

h

v

t

Comparison of total intensity anomalies over a simple dipole magnet (different cases)

Different depth ofthe dipole magnet

Different angle ofthe dipole magnet

Different strength ofthe dipole magnet

Different oriented dipole magnets

Total intensity anomalies over plates of infinite horizontal extend (different cases)

Plates inclined atdifferent angles, magnetized in different direction (i), and oriented in different directions (s) relative to the magnetic north.

Magnetic anomaly interpretation- simple graphical method

Reynolds, 1997

Igneous intrusion / Salt dome

Igneous intrusion – have large amounts of magnetite and high magnetization. Salt – exhibits diamagnetic behaviour and a weak field opposite in direction to the ambient field.

Salt dome

Gravity anomaly over theGrand Saline Salt Dome,Texas, USA(contours in gravity units)

Magnetic anomalies over the Grand Saline Salt Dome,Texas, USA(contours in nT)

The stippled area represents thesubcrop of the dome.

negative magneticsusceptibility

low density

Modeling ore body deposits

Zinc ore deposit (Yukon)

Geochemical analysis of soils

Residual Gravity

Magnetic Field

Copper ore body (Finland)

Magnetic field

Gravity

Electrical resistivity (SLINGRAM) – caused by black shists

Magnetic Surveys for buried infill

Magnetic anomalies over a clay-with-flints infill overchalk

Shape of the profile across the clay pipe infill

Magnetic surveys for buried pipes

Texts• Lillie – all appropriate sections• Fowler, p.43-67; 77-88.

Additional reading:Campbell, W.H. (1997) Introduction to

geomagnetic fields, Cambridge, 1997, p.51-58, 228-269.

Robinson & Coruh (1988) Basic exploration geophysics, John Wiley & Sons, all appropriate sections

Sharma, 1976