H. SAIBI October 16, 2014 Slide 2 Schema illustration of the
attractive and repulsive magnetic forces (F M ) generated between
two magnetic poles by Coulombs 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) Slide
3 MLA style: Earth: geomagnetic field. Video. Encyclopdia
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
Earths core generate a magnetic field according to a principle
known as the dynamo effect. Slide 4 History, Lodestone, Magnetism
Slide 5 - 200 BC. The Chinese first used lodestone (magnetite-rich
rock) in direction-finding. - William Gilbert (English physicist):
first European scientist to analyze the Earths magnetic field in
1600. - In 1870, Thalen and Tiberg developed instruments to measure
various components of the Earths magnetic field. - In 1960s,
optical absorption magnetometers were developed which provided the
means for extremely rapid magnetic measurements with very high
sensitivity. Slide 6 Chinese known to use the Lodstone compass for
navigation (12 th Century). Western European by 1187. Arabs by
1220. Scandinavians by 1300. Slide 7 Slide 8 Slide 9 Slide 10
Lodstone = magnetite 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/I o ): 2.53(1), 1.483(0.85),
1.614(0.85), Fe 3 O 4 What is a Mineral ? Slide 11 Magnetism
Magnetic Force field: The region around a magnetic object in which
its magnetic forces act on other magnetic objects. Slide 12
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. Slide 13
Magnetic field strength: Strongest at the poles. Weakest at the
midpoint. Slide 14 Sir William Gilbert (15401603) made the first
investigation of terrestrial magnetism De Magnete. He showed that
the earths magnetic field can be approximated by the field of a
permanent magnet lying in a general NS direction near the earths
rotational axis. Slide 15 Origin of the Earths geomagnetic field
What is the source of Earths magnetic field? Does the magnetic
field has effect on our life? Slide 16 Inner Core Outer Core Core
Mantle Radioactive heating Chemical differentiation Radioactive
heating Chemical differentiation Outer core in a turbulent
convection Outer core in a turbulent convection Natural electrical
generator Kinetic energy converted to electric and magnetic energy
The motion of electrical conducting iron in the presence of
magnetic field induces electric current Generate their own magnetic
field Slide 17 Earths 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. Slide 18
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 earths crust
Slide 19 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! We can
visualize the Earths magnetic field as being produced by a giant
bar magnet within the Earth. Slide 20 If you know your longitude
and latitude (33.58N/130.4W for Fukuoka) you can calculate the
local magnetic declination at:
http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp Slide 21
Above the Curie Point, atoms within crystals vibrate randomly and
have no associated magnetic field. Slide 22 Below the Curie Point
the magnetic fields of the minerals act like tiny compass needles:
they become aligned to the Earth's magnetic field. Slide 23 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 The
geomagnetic field Slide 24 Applications of geomagnetic surveys
Slide 25 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 Slide 26 Basic concepts and units of geomagnetism Slide
27 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.
Slide 28 Magnetic Field Strength Flux density (teslas): is a vector
quantity. In geophysics we use nanotesla as unit (nT)=10 -9 T.
Magnetic field strength (Amperes per meter): is a force field
produced by electric current: is a current. Slide 29 Absolute
Magnetic Permeability( ) Flux density Magnetising field strengh In
water and air ( 0 ) = 4 10 -7 Wb A -1 m -1 Relative permeability r
: rock Water, air Slide 30 Susceptibility (k) A measure of how a
material is to becoming magnetised. In vacuum: k=0; r =1. Intensity
of magnetisation Slide 31 Area (m 2 ) Magnetic moment (A.m 2 )
Volume of the magnetised body (m 3 ) Length of the dipole Intensity
of the induced magnetisation in rock susceptibility Earth magnetic
field Permeability of free space Pole strength (A.m) Slide 32
Remanent magnetization Total magnetization = induced + remanent
Amplitude and direction Depends on magnetic history of rock NRM
Causes of NRM 1.Thermoremanent magnetization (TRM) when magnetic
material is cool below the curie point in the presence of external
field. 2.Detrital remanent magnetization (DRM) occurs during the
slow setting of fine grained particles in the presence of an
external field. 3.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. 4.Isothermal remanent
magnetization (IRM) residual left after removing the external
field. 5.Viscous remanent magnetization (VRM) produce by long
exposure to an external field. Slide 33 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 580C for magnetite. Slide 34 olivine cobalt, nickel
and iron hematite magnetite, titanomagnetite, and ilmenite Slide 35
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 Slide 36 Elements of the
magnetic field Total Field F Our main target Magnetic north Slide
37 Magnetic Instruments Slide 38 Torsion and Balance Magnetometers
(Obsolete) Magnetometers measure the total magnetic field F T or
the horizontal and/or vertical components of magnetic field, F H
and F Z 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 Earths field. (Use compass?) Calibrated to read vertical
magnetic field component. Slide 39 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. o
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. Slide 40 Principle of Operation of
Fluxgate Magnetometer Principle behind operation is Faradays 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. Slide 41 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 Earths magnetic field or an
applied field. Measures precession as protons reorient to Earths
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. Slide 42 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 Earths field,
causing all protons to align. When current turned off protons
process back to orientation of Earths 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 Earths total magnetic field as it is proportional to
precession frequency. Measuring current frequency to 0.004 Hz gives
field to 0.1nT. Slide 43 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
ships 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. Slide 44 Magnetic
Gradiometers Gradiometers use two sensors separated by fixed
distance to measure gradient of the Earths 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. Slide 45 Magnetic Surveying
Slide 46 Ground 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 Earths surface. Slide 47 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. Slide 48 Reduction of Magnetic Survey
Data 2 Geomagnetic Correction Similar to latitude correction in
gravity: produces "anomaly" data Earths total magnetic field varies
from 25,000 nT at equator to 69,000 nT at poles Three possible
correction methods: 1) Subtraction of IGRF: Earths theoretical
magnetic field is removed from survey data by subtracting IGRF 2)
Linear approximation to IGRF: Earths 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. Slide 49 Shape
of magnetic anomalies Earths magnetic field is dipolar: single body
can appear as peak and trough. Example: Vertical component of
magnetic field induced in body inclined at 60 o parallel to Earths
magnetic field (no remnant magnetisation) Fig. The magnetic field
generated by a magnetised body inclined at 60 parallel to the
Earths field (A) would produce the magnetic anomaly profile from
points A-D shown in (B). Slide 50 Qualitative Interpretation of
Magnetic Anomalies 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. General inferences can be made from
magnetic anomaly shapes Fig. Two magnetic anomalies arising from
buried magnetised bodies. Slide 51 Summary of Qualitative
Interpretation of magnetic profiles and maps Slide 52 >J r as
anomaly not distorted"> 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 J i >>J r as anomaly not distorted Slide 53
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. Slide 54 Effect of Change of Position on Magnetic
Profile 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 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. Slide 55 Depth
Determination Sphere or half-cylinder: Depth to centre of body w is
roughly equal to width of anomaly peak at half its maximum value dF
max /2. Dipping Sheet or Prism: Depth to centre of body is roughly
width of linear segment of anomaly d. Can get very approximate
depth to the top of a magnetised body from magnetic anomalies.
Peters 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 Slide 56 Subsurface structure Libya- Saadi
N., Watanabe K., Imai A., Saibi H.: Earth Planets Space, 60,
539547, 2008 Slide 57 2.5D magnetic model 3D magnetic model Case
study: Aynak in Afghanistan Slide 58 Power Spectrum Analysis of
Aeromagnetic data of Afghanistan Slide 59 Aeromagnetic Map of
Afghanistan Slide 60 Curie-point depth map of Afghanistan Slide 61
Geothermal Gradient Map of Afghanistan Slide 62 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. Slide 63 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.
