Lecture N. 3: Geomagnetic H. SAIBI October 16, 2014

Schema illustration of the attractive and repulsive magnetic forces (FM) generated between two magnetic poles by Coulomb’s Law. The unit magnetic dipole (top) consists of two fictious point poles of equal strengths (p), but opposite signs and separated by an infinitesimal distance (r)

MLA style: Earth: geomagnetic field. Video MLA style: Earth: geomagnetic field. Video. Encyclopædia Britannica Online. Web. 19 Oct. 2010 <http://www.britannica.com/EBchecked/topic-video/229754/148016/Currents-in-the-Earths-core-generate-a-magnetic-field-according>. Currents in the Earth’s core generate a magnetic field according to a principle known as the dynamo effect.

History, Lodestone, Magnetism

200 BC. The Chinese first used lodestone (magnetite-rich rock) in direction-finding. William Gilbert (English physicist): first European scientist to analyze the Earth’s magnetic field in 1600. In 1870, Thalen and Tiberg developed instruments to measure various components of the Earth’s magnetic field. In 1960’s, optical absorption magnetometers were developed which provided the means for extremely rapid magnetic measurements with very high sensitivity. that has magnetic properties Magnetite, a magnetic mineral form of iron(II), iron(III) oxide Fe3O4, one of several iron oxides. A piece of intensely magnetic magnetite that was used as an early form of magnetic compass. Only magnetite with a particular crystalline structure, lodestone, can act as a natural magnet and attract and magnetize iron. The name "magnet" comes from lodestones found in a place called Magnesia. In China, the earliest literary reference to magnetism lies in a 4th century BC book called “Book of the Devil Valley Master (鬼谷子): “

Chinese known to use the Lodstone compass for navigation (12th Century). Western European by 1187. Arabs by 1220. Scandinavians by 1300. A compass is an instrument containing a freely suspended magnetic element which displays the direction of the horizontal component of the Earth's magnetic field at the point of observation. Magnetic Compass The magnetic compass is an old Chinese invention, probably first made in China during the Qin dynasty (221-206 B.C.). Chinese fortune tellers used lodestones to construct their fortune telling boards. Eventually someone noticed that the lodestones were better at pointing out real directions, leading to the first compasses. They designed the compass on a square slab which had markings for the cardinal points and the constellations. The pointing needle was a lodestone spoon-shaped device, with a handle that would always point south. Magnetized Needles Magnetized needles used as direction pointers instead of the spoon-shaped lodestones appeared in the 8th century AD, again in China, and between 850 and 1050 they seem to have become common as navigational devices on ships. Compass as a Navigational Aid The first person recorded to have used the compass as a navigational aid was Zheng He (1371-1435), from the Yunnan province in China, who made seven ocean voyages between 1405 and 1433.

Comments: Black, opaque octahedral crystals of magnetite on matrix Comments: Black, opaque octahedral crystals of magnetite on matrix. Location: Isle of Ischia, near Naples, Compania, Italy. Scale: Not Given. © Lou Perloff / Photo Atlas of Minerals

Magnetite   Nordenskioldine Comments: Colorless to white bladed crystals of nordenski?ldine with black magnetite. Location: P?hla-Tellerh?user Gallery, P?hla, Schwarzenberg District, Erzgebirge, Saxony, Germany. Scale: Picture size 5 mm. © Thomas Witzke / Abraxas-Verlag

Magnetite   Chesterite Comments: Green platy chesterite in black, metallic magnetite. Location: Near Missoula, Missoula County, Montana, USA. Scale: See Photo. © Jeff Weissman / Photographic Guide to Mineral Species

Lodstone = magnetite What is a Mineral ? Fe3O4 Cell Dimensions Magnetite Crystallography Cell Dimensions a = 8.391, Z = 8; V = 590.80 Den(Calc)= 5.21 Crystal System Isometric – Hexoctahedral H-M Symbol (4/m 3 2/m) Space Group: F d3m X Ray Diffraction: By Intensity(I/Io): 2.53(1), 1.483(0.85), 1.614(0.85), A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes" (Nickel, E. H., 1995). "Minerals are naturally-occurring inorganic substances with a definite and predictable chemical composition and physical properties." (O' Donoghue, 1990). "A mineral is a naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement" (Mason, et al, 1968). "These minerals can be distinguished from one another by individual characteristics that arise directly from the kinds of atoms they contain and the arrangements these atoms make inside them" (Sinkankas, 1966). "A mineral is a body produced by the processes of inorganic nature, having usually a definite chemical composition and, if formed under favorable conditions, a certain characteristic atomic structure which is expressed in its crystalline form and other physical properties" (Dana & Ford, 1932). "Every distinct chemical compound occurring in inorganic nature, having a definite molecular structure or system of crystallization and well-defined physical properties, constitutes a mineral species" (Brush & Penfield, 1898).

Magnetism Magnetic Force field: The region around a magnetic object in which its magnetic forces act on other magnetic objects.

Magnetic field about a simple bar magnet: North pole attracts the south poles of magnetic objects within the field. South pole attracts the north pole of magnetic objects within the field. Magnetic field orientation:   Parallel to the magnetic axis at the midpoint of the magnet. Curves strongly towards the poles.

Magnetic field strength: Strongest at the poles.   Weakest at the midpoint.

Sir William Gilbert (1540–1603) made the first investigation of terrestrial magnetism “De Magnete”. He showed that the earth’s magnetic field can be approximated by the field of a permanent magnet lying in a general NS direction near the earth’s rotational axis.

Origin of the Earth’s geomagnetic field What is the source of Earth’s magnetic field? Does the magnetic field has effect on our life?

Self-sustaining process Mantle Inner Core Outer Core Radioactive heating Chemical differentiation Self-sustaining process Generate their own magnetic field The motion of electrical conducting iron in the presence of magnetic field induces electric current Outer core in a turbulent convection Natural electrical generator Kinetic energy converted to electric and magnetic energy

Earth’s Magnetic Field   Generated by the convective motion the fluid outer core about the solid inner core. Geodynamo: the conversion, within the Earth, of mechanical energy (convection of metals) to electrical energy which produces the magnetic field. A magnetic field produced by such fluid motion is inherently unstable and not as uniform as about a simple bar magnet.

Magnetic measurement Main Field External Field Local Field 90% of the field generated internally From the outer core Electric currents in the ionosphere, particles ionized by solar radiation Variations caused by local magnetic anomalies in the earth’s crust

We can visualize the Earth’s magnetic field as being produced by a giant bar magnet within the Earth. What we call the “North geographic pole” corresponds to the “south pole” of the imaginary bar magnetic so that the north needle on a compass points towards the north geographic pole!

If you know your longitude and latitude (33. 58°N/130 If you know your longitude and latitude (33.58°N/130.4W for Fukuoka) you can calculate the local magnetic declination at: http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

Above the Curie Point, atoms within crystals vibrate randomly and have no associated magnetic field.

Below the Curie Point the magnetic fields of the minerals act like tiny compass needles: they become aligned to the Earth's magnetic field.

The geomagnetic field The origin of the dipole field is in the liquid core. This field and its reversals have been simulated numerically by Glazmaire and Roberts [1995]. http://www.psc.edu/research/graphics/gallery/geodynamo.html

Applications of geomagnetic surveys

Locating Pipes, cables and metallic objects Buried military ordnance (shells, bombs, etc.) Buried metal drums of contaminated or toxic waste. Concealed mineshafts and adits Mapping Archaeological remains Concealed basic igneous dykes Metalliferous mineral lodes Geological boundaries between magnetically contrasting lithologies, including faults Large-scale geological structures

Basic concepts and units of geomagnetism

Force (F) between two magnetic poles Both gravity and magnetism are potential fields and can be derived by comparable potential field theory. m is pole strength;  is the magnetic permeability of the medium separating the poles; r is the distance between them. F = \frac{m_1 m_2}{4 \pi \mu r^2}

Magnetic Field Strength  Flux density  (teslas): is a vector quantity. In geophysics we use nanotesla as unit (nT)=10-9T. Magnetic field strength  (Amperes per meter): is a force field produced by electric current:  is a current. H = \frac{I}{2r}

Absolute Magnetic Permeability() Flux density Magnetising field strengh In water and air (0) = 4  10-7 Wb A-1 m-1 Relative permeability r: u = \frac{B}{H} u_r = \frac{u}{u_0} rock Water, air

Intensity of magnetisation Susceptibility (k) A measure of how a material is to becoming magnetised. In vacuum: k=0; r=1. B = uH u = u_0u_r \longrightarrow B = u_0 u_r H \;\;\; ; \;\;\; k = u_r -1 B = u_0 H (1+k) \;\;\;\; \mathrm{and} \;\;\;\;J = kH Intensity of magnetisation

Intensity of magnetisation Pole strength (A.m) Magnetic moment (A.m2) Length of the dipole Area (m2) Volume of the magnetised body (m3) Earth magnetic field Intensity of the induced magnetisation in rock susceptibility Permeability of free space

Remanent magnetization Depends on magnetic history of rock NRM Total magnetization = induced + remanent Amplitude and direction Causes of NRM Thermoremanent magnetization (TRM) when magnetic material is cool below the curie point in the presence of external field. Detrital remanent magnetization (DRM) occurs during the slow setting of fine grained particles in the presence of an external field. Chemical remanent magnetization (CRM) take place when magnetic grain increase in size or changing from one form to another as a result of chemical changes. Isothermal remanent magnetization (IRM) residual left after removing the external field. Viscous remanent magnetization (VRM) produce by long exposure to an external field.

Magnetic Minerals Align in Earth’s Field Temperature at which magnetic minerals “fix” the orientation and magnitude of the external field is called the Curie point, which is 580°C for magnetite.

cobalt, nickel and iron olivine magnetite, titanomagnetite, and ilmenite hematite

Susceptibilities of common minerals and rock While the spatial variation in density are relatively small (between 1 and 3 Kg m-3, magnetic susceptibility can vary as much as four to five orders of magnitude. Wide variations in susceptibility occur within a given rock type. Thus, it will be extremely difficult to determine rock types based on magnetic prospecting

Elements of the magnetic field Magnetic north Total Field F Our main target

Magnetic Instruments

Torsion and Balance Magnetometers (Obsolete) Magnetometers measure the total magnetic field FT or the horizontal and/or vertical components of magnetic field, FH and FZ respectively. First magnetometers devised in1640 essentially comprised: a magnetic needle suspended on a wire (Torsion type), or a magnetic needle balanced on a pivot (Balance type) Needle oriented in direction of magnetic field at station location. Adolf Schmidt Variometer Magnetic beam asymmetrically balanced on agate knife edge, and zeroed at base station. Different magnetic field at another station caused displacement of beam, which was measured using collimating telescope. Had to be oriented perpendicular to magnetic meridian to remove horizontal component of Earth’s field. (Use compass?) Calibrated to read vertical magnetic field component.

Fluxgate Magnetometer Measures component of magnetic field parallel to cores with accuracy of 1-10 nT. Comprises two parallel cores of high m ferromagnetic material. Primary coil wound on two cores in series in opposite directions. Secondary coils also wound, but in opposite direction to primary. Operation of Fluxgate Magnetometer An alternating current at 50-1000 Hz is passed through primary coils, producing magnetic field that drives each core to saturation through a magnetisation hysteresis loop. With no external magnetic field, cores saturate every half cycle. Voltages induced in secondary coils have opposite polarity as coils wound in opposite directions. So zero net voltage. In Earth's magnetic field, component of field parallel to cores causes one core to saturate before the other, and voltages in secondary coils do not cancel.

Principle of Operation of Fluxgate Magnetometer Principle behind operation is Faraday’s Law of Induction (twice) Voltage induced in secondary coil proportional to magnetic field generated in ferromagnetic core. When core saturated, magnetic field does not change, and no voltage is induced in secondary coil.

Proton Precession Magnetometer Uses sensor consisting of bottle of proton-rich liquid, usually water or kerosene, wrapped with wire coil. Protons have a net magnetic moment, and so are oriented by Earth’s magnetic field or an applied field. Measures precession as protons reorient to Earth’s field. Precession frequency proportional to total field strength. As sensor bottle 15 cm long, accuracy of measurement is reduced in areas of high magnetic field gradient. Measures total field strength, so instrument orientation not important, unlike fluxgate. Overhauser Effect adds electron-rich fluid to enhance polarisation effect, and increase accuracy.

Principle of Operation of Proton Magnetometer In ambient field, majority of protons aligned parallel to field, remainder antiparallel. Current in coil generates strong magnetic field at right angles to Earth’s field, causing all protons to align. When current turned off protons process back to orientation of Earth’s field. Protons are charged particles, and create magnetic field, which alternates as proton processes. Current induces alternating voltage in coil at precession frequency. Measuring frequency of current in coil gives magnitude of Earth’s total magnetic field as it is proportional to precession frequency. Measuring current frequency to 0.004 Hz gives field to ±0.1nT.

Airborne and Seaborne Magnetometers Proton precession magnetometers are used extensively in marine and airborne surveys: At sea: sensor bottle is towed in a "fish" 2-3 ship’s length astern to remove it from magnetic field of ship In air: sensor is towed 30 m behind aircraft or placed in a "stinger" on nose, tail or wingtip. Often active compensation for magnetic effect of aircraft is calculated. Effectiveness of compensation is called Figure of Merit (FOM). Advantage: Aeromag is rapid, cost-effective method for covering large areas.

Magnetic Gradiometers Gradiometers use two sensors separated by fixed distance to measure gradient of the Earth’s magnetic field: In airborne work, separation is 2-5 m for stinger, up to 30 m for bird. In ground work, separations of 0.5 m are common. Example of 3-axis gradiometer system: Advantages: No correction for diurnal variation required as measurement is difference off two magnetic sensors. Vertical gradient measurements emphasize shallow anomalies and suppress long wavelength features.

Magnetic Surveying

Ground Surveys Airborne Surveys Ideally lines should be perpendicular to strike, with a few along strike tie-lines. Establish base-station to monitor diurnal variations every 0.5-1.0 hours. Avoid readings near metal objects such as railway tracks, cars... Avoid wearing metal objects, such as watch, geological hammer.   Airborne Surveys Estimate line spacing to avoid significant signal aliasing for aircraft height. Approximate rule of thumb for maximum line spacing for particular application: Note that h is flight height above magnetic basement, not Earth’s surface.

Reduction of Magnetic Survey Data 1 Magnetics data reduction is usually simpler than with gravity, comprising: Diurnal Correction Geomagnetic Correction Elevation/Terrain Correction (occasionally) Diurnal Variation Similar to tidal correction in gravity Reading is recorded at base station during survey, and then corrections applied to survey data. Difficult to return to base station in airborne work: possible to estimate diurnal correction from line intersections especially with additional tie lines Fig. Tracks of a shipborne or airborne magnetic survey. Fig. Diurnal drift curve using a proton magnetometer.

Reduction of Magnetic Survey Data 2 Geomagnetic Correction Similar to latitude correction in gravity: produces "anomaly" data Earth’s total magnetic field varies from 25,000 nT at equator to 69,000 nT at poles Three possible correction methods: 1) Subtraction of IGRF: Earth’s theoretical magnetic field is removed from survey data by subtracting IGRF 2) Linear approximation to IGRF: Earth’s field is approximated by linear variation across survey area, and subtracted: For example, in UK IGRF is approximated by 2.13 nT/km north, and 0.26 nT/km west. 3) Regional correction: With large surveys, regional trend can be estimated and removed to leave residual anomaly, as with gravity data. Terrain Correction There are no elevation corrections (equivalent to Free-air and Bouguer corrections) with magnetic data as gradient is only 0.035 nT/m at poles, 0.015 nT/m at equator. Terrain corrections can be applied, but are complicated. Require estimate of ground susceptibility, and topography.

Shape of magnetic anomalies Earth’s magnetic field is dipolar: single body can appear as peak and trough. Example: Vertical component of magnetic field induced in body inclined at 60o parallel to Earth’s magnetic field (no remnant magnetisation) Fig. The magnetic field generated by a magnetised body inclined at 60 parallel to the Earth’s field (A) would produce the magnetic anomaly profile from points A-D shown in (B).

Qualitative Interpretation of Magnetic Anomalies General inferences can be made from magnetic anomaly shapes Anomaly B is same form as A, but has longer wavelength, so must be deeper. Amplitude of B greater than A, so B has greater magnetisation. Fig. Two magnetic anomalies arising from buried magnetised bodies.

Summary of Qualitative Interpretation of magnetic profiles and maps

Qualitative profile interpretation Identify zones with different magnetic properties: Zones with little variation, "magetically quiet", associated with rocks of low susceptibility Sources in subsurface in "magnetically noisy" areas. Example: Mineralisation in granite (Dartmoor, UK) Profile quiet except around mineralised zone. Negative on north side indicates direction Ji >>Jr as anomaly not distorted

Qualitative map interpretation Magnetic data acquired on ugrids can be displayed as maps Example: Shetland Islands, Scotland Elongate lows correspond to gneissified semipelites Can also identify fault at discontinuity Fig. Aeromagnetic map. Fig. Magnetic characteristics.

Effect of Change of Position on Magnetic Profile Fig. Total field anomalies over a 10 m wide vertical sheet-like body oriented east-west and buried at depths of 20 m, 60 m and 110 m; the position of the magnetised body is indicated. Fig. Total field anomalies over a thin dyke (5 m wide) dipping to the north at angles from =90 to =0 ; body strike is east-west. Change in Depth: Anomaly will broaden and decrease in amplitude with increase in depth. Total field over 10 m wide vertical dyke oriented E-W Change in Dip: Shape of anomaly is altered Total field over 5 m wide dyke with varying dips

Depth Determination Can get very approximate depth to the top of a magnetised body from magnetic anomalies. Peter’s Half-Slope Method (~theoretically-based) Draw tangent at point of maximum slope (line 1) Find two tangents to curve with half maximum slope (lines 3, 4) Depth to top of body is distance between tangent points Sphere or half-cylinder: Depth to centre of body w is roughly equal to width of anomaly peak at half its maximum value dFmax/2. Dipping Sheet or Prism: Depth to centre of body is roughly width of linear segment of anomaly d.

Subsurface structure –Libya- Saadi N., Watanabe K., Imai A., Saibi H.: Earth Planets Space, 60, 539–547, 2008

Case study: Aynak in Afghanistan 2.5D magnetic model 3D magnetic model

Power Spectrum Analysis of Aeromagnetic data of Afghanistan

Aeromagnetic Map of Afghanistan

Curie-point depth map of Afghanistan

Geothermal Gradient Map of Afghanistan

The World Digital Magnetic Anomaly Map (WDMAM) shows the variation in strength of the magnetic field after the Earth's dipole field has been removed. Earth's dipole field is generated by circulating electric currents in the planet's metal core. It varies from 35,000 nanoTesla (nT) at the Equator to 70,000 nT at the poles.

Homework Why gravity and geomagnetic methods are called Potential-field methods? What are the similarities and the differences between geomagnetic and gravity ? Deadline: next week.