VẬT lý địa CHẤN magnetic

Magnetic Surveying The investigation of the subsurface ‘geology’ on the basis of anomalies in the Earth’s magnetic field resulting from the magnetic properties of the causative body •A broad range of applications •Magnetic surveys can be performed on land, at sea, in the air, and on ice •Very cheap to perform Within the vicinity of a bar magnet a magnetic flux is developed which flows from one end of the magnet to the other From Kearey, Brooks, and Hill, 2002 Basic Concepts •Poles are where the flux lines converge •The pole of the compass/magnet which points in the direction of the Earth’s north pole is called the northseeking pole, or positive pole •This is balanced by a south-seeking or negative pole of identical strength at the opposite end of the magnet •The Earth’s magnetic field can be crudely modeled as a bar magnet with it’s south pole at the Earth’s north magnetic pole From Mussett and Khan, 2000 •Mapped from the directions assumed by a small compass needle, or bar magnet suspended within the field Basic Concepts The force F between two magnetic poles of strengths m1 and m2 separated by a distance r is given by: µ mm F = 22 4πµ R r Where μ0 and μR are constants corresponding to the magnetic permeability of a vacuum and the relative magnetic permeability of the medium separating the poles The force is attractive if the poles are of different sign, and repulsive if they are of like sign The magnetic field B due to a pole of strength m at a distance r from the pole is defined as the force exerted on a unit positive pole at that point: µ0 m B= 4πµ R r The magnetic field can be defined in terms of the magnetic potential in a similar manner to gravitational fields For a single pole of strength m, the magnetic potential V at a distance r from the pole is given by µm V= 4πµ R r The magnetic field component in any direction is then given by the partial derivative of the potential in that direction Basic Concepts In the SI (what is this??) system of units, magnetic parameters are defined in terms of the flow of electrical current •If a current is passed through a coil consisting of several turns of wire, a magnetic flux flows through and around the coil annulus which arises from a magnetizing force H •The magnitude of H is proportional to the number of turns in the coil and the strength of the current, and inversely proportional to the length of the wire H is expressed in A m-1 •B is proportional to H The constant of proportionality μ is know as the magnetic permeability From Mussett and Khan, 2000 •The density of the magnetic flux, measured over an area perpendicular to the direction of flow, is known as the magnetic induction, or magnetic field, B Basic Concepts •Lenz’s law of induction relates the rate of change of magnetic flux in a circuit to the voltage developed within it, so B is expressed in V s m-2 (Weber (Wb) m-2) •The unit of the Wb m-2 is designated the tesla (T) •Permeability is expressed in Wb A-1 m-1 or Henry (H) m-1 •The tesla is too large to express the small magnetic anomalies on the Earth’s surface Consequently, the nannotesla is used (1 nT = 10-9 T) Basic Concepts •The magnetic moment of a dipole with poles of strength m a distance l apart is: M = ml •The magnetic moment of a current carrying coil is proportional to the number of turns in the coil, its cross sectional area, and the magnitude of the current The magnetic moment is expressed in A m-2 •When a material is placed in a magnetic field it may acquire a magnetization in the direction of the field which is lost when it is removed •This is called Induced Magnetization and results from the alignment of elementary dipoles within the material in the direction of the field From Kearey, Brooks, and Hill, 2002 •Magnets exhibit a pair of pole – dipoles •The intensity of induced magnetization Ji of a material is defined as the dipole moment per unit volume of material: M Ji = LA •Where M is the magnetic moment of a sample of length L and cross-sectional area A Ji is expressed in A m-1 •The induced intensity of magnetization is proportional to the strength of the magnetizing force H of the inducing field: J i = kH Where k is the magnetic susceptibility of the material As Ji and H are both measured in A m-1, k is dimensionless From Kearey, Brooks, and Hill, 2002 Basic Concepts Basic Concepts •In a vacuum the magnetic field strength B and magnetizing force H are related by: B = µ0 H Where μ0 is the permeability of a vacuum (4π*10-7 H m-1) •As air and water have very similar permeabilities to μ0 the relationship can be taken to represent the Earth’s magnetic field when it is undisturbed •When a magnetic material is placed in the field, the resulting magnetization gives rise to an additional magnetic field in the region occupied by the material, whose strength is given by: µ0 J i Within the body the total magnetic field, B, is given by: B = µ0 H + µ0 J i Substituting the relationship with the magnetic susceptibility from the previous slide gives: B = µ H + µ kH = (1 + k ) µ H = µ R µ H Where μR is a dimensionless constant known as the relative magnetic permeability The magnetic permeability is thus equal to the product of the relative permeability and the permeability of vacuum Basic Concepts •All substances are magnetic at an atomic scale •Each atom acts as a dipole due to both the spin of its electrons and the orbital path of its electrons around the nucleus •Diamagnetic materials: All electron shells are full and no unpaired electrons exist When placed in a magnetic field the orbital paths of the electrons rotate so as to produce an opposing magnetic field Magnetic susceptibility is weak and negative •Paramagnetic materials: Electron shells are incomplete, creating a magnetic field from the spin of the unpaired electrons When placed in a magnetic field the dipoles corresponding to the unpaired electron spins rotate to produce a field in the same sense as the applied field The susceptibility is positive, but still weak •In small grains of certain paramagnetic substances whose atoms contain several unpaired electrons, the dipoles associated with the spins of the unpaired electrons are magnetically couples between adjacent atoms Such a grain is said to constitute a single magnetic domain This coupling may be either parallel or antiparallel Basic Concepts •Antiferromagnetism: Dipole coupling is antiparallel, with equal numbers of dipoles in each direction The magnetic fields of the dipoles cancel out Defects may give rise to a small positive magnetization (parasitic antiferromagnetism) Haematite •Ferrimagnetism: The dipole coupling is antiparallel, but the strengths in each direction are unequal Strong spontaneous magnetization, high susceptibility Magnetite Virtually all minerals responsible for the magnetic properties is common rock types fall into this category From Kearey, Brooks, and Hill, 2002 •Ferromagnetic: Dipoles are parallel Strong spontaneous magnetization which can exist in the absence of an external magnetic field Iron, cobalt, nickel Rarely occur naturally in the Earth’s crust Magnetic Anomalies This previous equation, with a couple of other steps, can be rewritten as: We can now calculate the anomaly caused by a small isolated magnetic pole of strength m, defined as the effect of this pole on a unit positive pole at the observation point This pole is at depth z, a horizontal distance x and radial distance r from the observation point The force of repulsion ΔBr on the unit positive pole in the direction r is given by: Cm µ ∆Br = where C = (assuming µ R = 1) r 4π If we assume that the profile lies in the direction of magnetic north so that the horizontal component of the anomaly lies in this direction, the horizontal (ΔH) and vertical (ΔZ) components can be computed by resolving in the different directions: Cm Cmx ∆H = cosθ = r r − Cm − Cmz ∆Z = sin θ = r r3 From Kearey, Brooks, and Hill, 2002 ∆B = ∆Z sin I + ∆H cos I cos α Magnetic Anomalies Cm Cmx cos θ = r2 r3 − Cm − Cmz ∆Z = sin θ = r r3 •The vertical field anomaly is negative as, by convention, the z-axis is positive downwards •The horizontal field anomaly is a positive/negative couplet and the vertical field anomaly is centered over the pole From Kearey, Brooks, and Hill, 2002 ∆H = By substitution, we can now find the total field anomaly ΔB, where α = If the profile is not in the direction of magnetic north, the angle α would represent the angle between magnetic north and the profile direction ∆B = ∆Z sin I + ∆H cos I cos α Magnetic Anomalies •A magnetic dipole produces a field shown by the dashed lines, whose directions and magnitudes at the surface are shown by the arrows •The anomaly is dependant on the direction of magnetization of the body From Mussett and Khan, 2000 •The actual field at the Earth’s surface is found by vector addition of the field due to the body and the Earth’s field Flux-Gate Magnetometer •Primary coils are wound around each bar in opposite directions Alternating current flows through the primary coils, producing a changing magnetic field •This induces a current in the secondary coil, which is wound around both bars •Because the primary coils are wound in opposite senses their fields are opposite in direction and cancel Therefore the induced current is zero •In the presence of an external field it will add to, then subtract from, the field if the magnetizing coil as the current alternated •The fields experienced by the two bars are no longer equal – the bar in which the field of the coil and the Earth add reaches saturation sooner The induced voltages are now out of phase •The magnitude of the voltage induced in the secondary coil is proportional to the amplitude of the external field From Kearey, Brooks, and Hill, 2002 •Sensor has identical bars of magnetic material Flux-Gate Magnetometer •Can measure Z or H by aligning the coils in that direction •Requires the orientation to be within 11 seconds of arc to achieve a reading accuracy of nT •This accuracy is hard to maintain in a mobile instrument •Instead, the total magnetic field is measured •Can be measured to an accuracy of nT with far less precise orientation as the field changes more slowly as a function of orientation about the total field direction •Airborne versions employ orienting mechanisms of various types to maintain the axis of the instrument in the direction of the geomagnetic field •Is not an absolute instrument, may require correction for drift and temperature effects Proton Precession Magnetometer •Coil wrapped around a container filled with a hydrogen atom rich liquid (water, kerosene) •The hydrogen nuclei (protons) act as small dipoles and align with the ambient geomagnetic field •A current is passed through the coil, generating a magnetic field 50-100 times larger than the ambient field •The protons align with the new field direction •When the current is switched off the protons return to their original orientation by spiraling, or precessing, in phase around the direction of the Earth’s ambient field From Kearey, Brooks, and Hill, 2002 •The most commonly used magnetometer Proton Precession Magnetometer •The frequency of precession is given by: γ p Be f = 2π Where γp is the gyromagnetic ratio of the proton, 26752.2 x 104 T-1 s-1 Therefore: Be = 23.4866 f Consequently, measurement of f provides a very accurate measurement of the total magnetic field f is determined by measurement of the alternating voltage of the same frequency induced in the coli by the precessing protons •Accuracy of 0.1 nT •Sensor does not have to be oriented •Can be towed behind a ship or aircraft Magnetic Surveys •Magnetic gradiometers: Typically two instruments separated by a short distance •Not prone to diurnal variation •Shallower magnetic bodies produce steeper gradients •May reveal boundaries not seen in a total field survey •Ground magnetic surveys: Small station spacing •Do not take readings near magnetic objects •Aeromagnetic and marine surveys: •In the air a sensor known as a ‘bird’ can be towed, isolating it from the magnetic field of the aircraft •Can be installed in a ‘stinger’ in the tail of an aircraft Coil installations compensate for the aircrafts magnetic field •At sea the sensor, or ‘fish’ is towed at least two ships’ lengths behind the vessel to remove its magnetic effect From Mussett and Khan, 2000 •Measures the gradient of the magnetic field Data Reduction •During quiet times, the diurnal variation changes smoothly Periodically returning to a base station and recording the Earth’s magnetic field allows corrections in a manner similar to drift correction in gravity surveys •Preferably, a magnetometer is set to continuously record at a base station while the survey is carried out The variations in the field at that location (where the field would ideally be fixed) can then be removed from the ‘mobile’ magnetometer •Use the records from a magnetic observatory – should be no more than 100 km away as the diurnal variations vary with location •Diurnal variation in land and airborne surveys can be removed with cross-over corrections From Kearey, Brooks, and Hill, 2002 •Diurnal variation correction: Data Reduction •Geomagnetic Correction: •Remove the IGRF (spherical harmonics) from the recorded field Very complex, must be done by computer •The magnetic field may also be approximated by a gradient – for example in the British Isles the gradient is approximately 2.13 nT km-1 N; 0.26 nT km-1 W •All these corrections vary with time •Alternatively a regional gradient can be removed by fitting a trend surface through the data •Terrain correction: •Fourier methods exist to removed the effects of terrain From Kearey, Brooks, and Hill, 2002 •Magnetic equivalent of the latitude correction in gravity data reduction Forward Modeling •Simple anomalies can be simulated by a single dipole •The magnetic anomaly of most regularly-shaped bodies can be calculated by building up the bodies from a series of dipoles parallel to the magnetixation direction From Kearey, Brooks, and Hill, 2002 •The poles of the magnets are negative on the surface of the body where the magnetization vector enters the body, and positive where it leaves the body Forward Modeling •In the example below, building a sill out of dipoles results in negatives along the top, and positives along the bottom From Kearey, Brooks, and Hill, 2002 •These cancel out – in a sill or lava flow the anomaly will only be present where the horizontal structure is truncated Direct Interpretation •Magnetic anomalies caused by shallow bodies have a higher frequency nature From Kearey, Brooks, and Hill, 2002 •The log-power spectrum of the anomaly has a linear gradient whose magnitude is dependant upon the depth of the source Potential Field Transformations •A consequence of the similar laws of attraction governing gravitating and magnetic bodies is that the two main equations have the variable of inverse distance (1/r) in common •Elimination of this term between the two formulae provides a relationship between the gravitational and magnetic potentials know as Poisson’s equation •Magnetic fields can be transformed into gravity fields and vice versa, for bodies in which the ratio of intensity of magnetization to density remains constant •Pseudogravity anomalies: •Transforming a magnetic anomaly to a gravity anomaly simplifies interpretation •If the pseudogravity and gravity anomalies are the same, then the body responsible for the magnetic anomaly is the same as that responsible for the gravity anomaly From Kearey, Brooks, and Hill, 2002 Applications •Finding metalliferous deposits – iron ore (must have high abundance of magnetite) •Delineate fault zones •Finding man made objects – pipelines, aircraft, etc •Volcanic studies – delineating volcanic vents •Large-scale crustal studies •Seafloor age •Sediment age •Etc [...]... temperature ferromagnetic and ferrimagnetic materials loose their magnetization Interatomic distances are increased to separations which preclude electron coupling and the material behaves as if paramagnetic •Magnetite has a Curie temperature of 578oC •Why might the curie temperature be important? Magnetic Domains: Grains may subdivide into domain, where all the dipoles are aligned •When a weak magnetic field... and a horizontal component H in the direction of magnetic north •Inclination = Dip of B •Declination = angle between magnetic north and true north B varies in strength from 25,000 nT in equatorial regions to 70,000 nT at the poles Geomagnetic Field From Mussett and Khan, 2000 •About 90% of the Earth’s magnetic field can be represented by a theoretical magnetic dipole at the center of the Earth and inclined... the International Geomagnetic Reference Field (IGRF) •The IGRF defines the theoretical undisturbed magnetic field at any point on the Earth’s surface •The geomagnetic field cannot in fact result from a series of superimposed bar magnets Why? •The dipolar magnetic moments are far greater than is realistic •The prevailing temperatures are far in excess of the Curie temperatures of any magnetic material... behind a ship or aircraft Magnetic Surveys Magnetic gradiometers: Typically two instruments separated by a short distance •Not prone to diurnal variation •Shallower magnetic bodies produce steeper gradients •May reveal boundaries not seen in a total field survey •Ground magnetic surveys: Small station spacing •Do not take readings near magnetic objects •Aeromagnetic and marine surveys: •In the air a... from intense solar activity and the arrival in the ionosphere of charge solar particles •Makes magnetic surveying difficult if not impossible The normal geomagnetic field can be described by a vector with vertical and horizontal components: From Kearey, Brooks, and Hill, 2002 Magnetic Anomalies B2 = H 2 + Z 2 A magnetic anomaly is now superimposed on the Earth’s field causing a change ΔB in the strength... 2002 ∆H = By substitution, we can now find the total field anomaly ΔB, where α = 0 If the profile is not in the direction of magnetic north, the angle α would represent the angle between magnetic north and the profile direction ∆B = ∆Z sin I + ∆H cos I cos α Magnetic Anomalies •A magnetic dipole produces a field shown by the dashed lines, whose directions and magnitudes at the surface are shown by the... magnetic due to their high magnetite content •Magnetite content decreases with increasing acidity •Granite is generally less magnetic than basalt •Lots of overlap, impossible to interpret lithology Geomagnetic Field From Kearey, Brooks, and Hill, 2002 From Mussett and Khan, 2000 At any point on the earth a freely suspended magnet will assume a position in space in the direction of the ambient geomagnetic... ‘bird’ can be towed, isolating it from the magnetic field of the aircraft •Can be installed in a ‘stinger’ in the tail of an aircraft Coil installations compensate for the aircrafts magnetic field •At sea the sensor, or ‘fish’ is towed at least two ships’ lengths behind the vessel to remove its magnetic effect From Mussett and Khan, 2000 •Measures the gradient of the magnetic field Data Reduction •During... 20-80 nT (maximum at the poles) •Caused by magnetic field induced by the flow of charged particles within the ionosphere towards the magnetic poles as both the circulation patterns and diurnal variations vary in sympathy with the tidal effects of the Sun and Moon •On disturbed days the diurnal variation is irregular, with short term disturbances of up to 1000 nT Magnetic storms resulting from intense... magnetization has two parts: •Induced magnetization exists only while a magnetic field exists and is aligned in the direction of the field The strength of magnetization is proportional to the strength of the field and to its magnetic susceptibility •Remnant magnetization can exists largely irrespective of the direction of the magnetic field It may have a direction very different to the field of today ... in V s m-2 (Weber (Wb) m-2) •The unit of the Wb m-2 is designated the tesla (T) •Permeability is expressed in Wb A-1 m-1 or Henry (H) m-1 •The tesla is too large to express the small magnetic. .. vacuum (4π*1 0-7 H m-1) •As air and water have very similar permeabilities to μ0 the relationship can be taken to represent the Earth’s magnetic field when it is undisturbed •When a magnetic material... exist When placed in a magnetic field the orbital paths of the electrons rotate so as to produce an opposing magnetic field Magnetic susceptibility is weak and negative •Paramagnetic materials: