Magnetic Field 5

Applications – magnetic surveys

Measurement of the magnetic field - Test magnet is attached to a horizontal fiber so that it can rotate in a vertical plane If the magnetometer is inclined both vertical and horizontal components of the earth’s field act to rotate it In horizontal position of the shaft only the vertical field intensity component can act to rotate it - Measures relative changes in the vertical component of magnetic field intensity Torsion magnetometer (Askania) – the most convenient to operate, precision ~ 1 gamma (1 nT) Robinson&Coruh, 1988

Measurement of the magnetic field - The design is based on the principle 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 component of the magnetic field intensity Flux-gate magnetometer, precision ~ 1 gamma (1 nT) Robinson&Coruh, 1988

Measurement of the magnetic field Designed for operation in aircraft, ships, other moving vehicles Hand-carried instruments for ground surveying Sensitive only to the total field intensity Proton precession magnetometers, precision ~ 1/2 gamma Robinson&Coruh, 1988

Measurement of the magnetic field Proton precession magnetometers - principle Spin Magnetometer principle Precession w = g F p p g - gyromagnetic ratio of the proton Robinson&Coruh, 1988 http://www.earthsci.unimelb.edu.au/ES304/MODULES/MAG/NOTES/proton.html F – magnetic field, w - angular velocity

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 - types 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 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 Visual inspection of the shape and trend of the magnetic anomalies 2. Delineation of the structural trends 3. 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 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 v h Approximation by a dipole magnet Modeling of a dipole magnet: Vertical (v), horizontal (h) and total (t) intensity

Comparison of total intensity anomalies over a simple dipole magnet (different cases) Different depth of the dipole magnet Different angle of the dipole magnet Different oriented dipole magnets Different strength of the dipole magnet

Total intensity anomalies over plates of infinite horizontal extend (different cases) Plates inclined at different 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 the Grand 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 the subcrop of the dome. low density negative magnetic susceptibility

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 over chalk 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