Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics II
Department of Geology and GeographyWest Virginia University
Morgantown, WV
Magnetic methodsMagnetic methods
Tom Wilson, Department of Geology and Geography
We’ll pick up with Magnetic Methods on Thursday
Magnetic polarity reversals on the sea floor provide
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Protons and electrons in the solar wind crash into earth’s magnetosphere.
Tom Wilson, Department of Geology and Geography
Charged particles from the sun stream into the earth’s magnetic field and crash into the gasses of the atmosphere
The Earth’s magnetic field
Tom Wilson, Department of Geology and Geography
http://www.windows2universe.org/physical_science/magnetism/earth_magnet_dipole_interactive.html
Location of north magnetic pole
Tom Wilson, Department of Geology and Geography
Location of north magnetic pole
Tom Wilson, Department of Geology and Geography
http://www.compassdude.com/compass-declination.shtml
Tom Wilson, Department of Geology and Geography
The fundamental magnetic element is a dipole or combination of one positive and one negative magnetic monopole. The characteristics of the magnetic field are derived from the combined effects of non-existent monopoles.
Dipole Field
Magnetic Fields – Basic Relationships
Tom Wilson, Department of Geology and Geography
The earth’s main magnetic field
Tom Wilson, Department of Geology and Geography
Magnetic Elements
Tom Wilson, Department of Geology and Geography
Magnetic Elements
• three orthogonal strength components (X, Y, and Z);• the total field strength and two angles (F, d, i); or• two strength components and an angle (H, Z, d)
http://gsc.nrcan.gc.ca/geomag/field/comp_e.php
Tom Wilson, Department of Geology and Geography
monopole vs.
dipole
Toxic Waste
Magnetic Fields – Basic Relationships
Tom Wilson, Department of Geology and Geography
Magnetic Elements
Tom Wilson, Department of Geology and Geography
Magnetic Elements
Tom Wilson, Department of Geology and Geography
Magnetic Elements
Tom Wilson, Department of Geology and Geography
Magnetic north pole: point where field lines point vertically downward
Geomagnetic north pole: pole associated with the dipole approximation of the earth’s magnetic field.
The compass needle points
to the magnetic north
pole.
Tom Wilson, Department of Geology and Geography
Date
1900 1920 1940 1960 1980 2000
53000
54000
55000
56000
57000
58000
59000
60000
61000
F (
nano
tesl
as o
r ga
mm
as)
Magnetic Intensity
11/15/2012Total intensity =
52580nT
Tom Wilson, Department of Geology and Geography
Magnetic Inclination
11/15/2012Inclination =
67.13o
Tom Wilson, Department of Geology and Geography
68
69
70
71
72
Incl
inat
ion
(deg
rees
)
1900 1920 1940 1960 1980 2000
Date
Magnetic Inclination
11/15/2012Inclination =
67.13o
Tom Wilson, Department of Geology and Geography
Magnetic Declination
11/15/2012declination =
-9.08o
Tom Wilson, Department of Geology and Geography
1900 1920 1940 1960 1980 2000
Date
-9
-8
-7
-6
-5
-4
-3
-2de
clin
atio
n (d
egre
es w
est)
W
Magnetic Declination
Today, the compass points about 9
degrees west of GN
Tom Wilson, Department of Geology and Geography
Magnetic Elements for your location
http://www.ngdc.noaa.gov/geomagmodels/struts/calcPointIGRF
Tom Wilson, Department of Geology and Geography
Today’s Space Weather
http://www.swpc.noaa.gov/today.html
Magnetic Elements
http://www.ngdc.noaa.gov/geomag/magfield.shtml
Another site of interest
Tom Wilson, Department of Geology and Geography
http://spidr.ngdc.noaa.gov/spidr/
Tom Wilson, Department of Geology and Geography
Gochioco and Ruev, 2006
We are also interested in local induced magnetic fields
Tom Wilson, Department of Geology and Geography
Locating Trench Boundaries
Theoretical modelExamination of trench for internal magnetic anomalies. actual field data
Gilkeson et al., 1986
Tom Wilson, Department of Geology and Geography
Trench boundaries - field data
Trench Boundaries - model data
Gilkeson et al., 1986
Tom Wilson, Department of Geology and Geography
From Martinek
Abandoned Wells
Tom Wilson, Department of Geology and Geography
Locating abandoned wells
Tom Wilson, Department of Geology and Geography From Martinek
Abandoned Well - raised relief plot of measured magnetic field intensities
Tom Wilson, Department of Geology and Geography
Data Acquisition
Tom Wilson, Department of Geology and Geography
Steve Sheriff’s Environmental Geophysics Course
Proton Precession Magnetometers
Tom Boyd’s Introduction to Geophysical Exploration Course
Measuring the Earth’s magnetic field
water kerosene & alcohol
Tom Wilson, Department of Geology and Geography
Source of Protons and DC current source
Proton precession generates an alternating current in the surrounding coil
Magnetic Fields – Basic Relationships
Tom Wilson, Department of Geology and Geography
22
GFF
L
Mf
Proton precession frequency (f) is directly proportional to the main magnetic field intensity F and magnetic dipole moment of the proton (M). L is the angular momentum of the proton and G is the gyromagnetic ratio which is a constant for all protons (G = M/L = 0.267513/ sec). Hence -
fF 4874.23
Tom Wilson, Department of Geology and Geography
Magnetic monopoles
1 212 2
12
1
4m
p pF
r p1
p2
r12Fm12 Magnetic Force Magnetic Permeabilityp1 and p2 pole strengths
Coulomb’s Law
Magnetic Fields – Basic Relationships
Tom Wilson, Department of Geology and Geography
1 212 2
12
1
4m
p pF
r
2
1
4o o
ot
F pH
p r
Force
Magnetic Field Intensity often written as H
pt is an isolated test pole
2
1" "
4E
Et
pFF
p r
The text uses F instead of H to represent magnetic field intensity, especially when referring to that of the Earth (FE).
Magnetic Fields – Basic Relationships
Tom Wilson, Department of Geology and Geography
Anomaly associated with buried metallic materials
Bedrock configuration determined from gravity survey
Results obtained from inverse modeling
Computed magnetic field produced by bedrock
Introduction to the magnetics computer lab
Tom Wilson, Department of Geology and Geography
Where are the drums and how many are there?
Tom Wilson, Department of Geology and Geography
To do list …
• Hand in your paper summaries today
• Hand in the gravity lab today.
• Magnetic papers will be in the mail room tomorrow morning
• Magnetic paper summaries will be due Tuesday, December 4th
• The magnetics lab will be due December 6th. Stay tuned for specific instructions on this last lab report
• Continue reading Chapter 7 –
