Environmental and Exploration Geophysics II
Magnetic methods
tom.h.wilson
tom.wilson@mail.wvu.edu
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
We’ll pick up with Magnetic Methods on Thursday
Tom Wilson, Department of Geology and Geography
Magnetic polarity reversals on the sea floor
provide
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
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
We are also interested in local induced magnetic fields
Gochioco and Ruev, 2006
Tom Wilson, Department of Geology and Geography
Data
Acquisition
Tom Wilson, Department of Geology and Geography
Measuring the Earth’s magnetic field
Proton Precession Magnetometers
water kerosene &
alcohol
Steve Sheriff’s Environmental Geophysics Course
Tom Boyd’s Introduction to Geophysical Exploration Course
Tom Wilson, Department of Geology and Geography
Magnetic Fields – Basic Relationships
Source of Protons and
DC current source
Proton precession generates
an alternating current in the
surrounding coil
Tom Wilson, Department of Geology and Geography
M
GF
f 
F
2L
2
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 -
F  23.4874 f
Tom Wilson, Department of Geology and Geography
Locating Trench
Boundaries
Theoretical model
Tom Wilson, Department of Geology and Geography
Examination of trench for
internal magnetic anomalies.
actual field data
Gilkeson et al.,
1986
Trench boundaries - field data
Trench Boundaries - model data
Tom Wilson, Department of Geology and Geography
Gilkeson et al.,
1986
Abandoned Wells
From Martinek
Tom Wilson, Department of Geology and Geography
Locating abandoned wells
Tom Wilson, Department of Geology and Geography
Abandoned Well - raised relief plot of
measured magnetic field intensities
Tom Wilson, Department of Geology and Geography
From Martinek
Magnetic Fields – Basic Relationships
Magnetic monopoles
Fm12
r12
Fm12 Magnetic Force
 Magnetic
Permeability
p1 and p2 pole
strengths
Tom Wilson, Department of Geology and Geography
p1
1 p1 p2

4 r122
Coulomb’s Law
p2
Magnetic Fields – Basic Relationships
Fm12
1 p1 p2

4 r122
Fo
1 po
Ho 

pt 4 r 2
F
1 pE
" FE" 

pt 4 r 2
Force
Magnetic Field
Intensity often
written as H
pt is an isolated test
pole
The text uses F instead of H to represent magnetic
field intensity, especially when referring to that of the
Earth (FE).
Tom Wilson, Department of Geology and Geography
Magnetic Fields – Basic Relationships
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
Tom Wilson, Department of Geology and Geography
The earth’s main magnetic field
Tom Wilson, Department of Geology and Geography
The Earth’s magnetic field
http://www.windows2universe.org/physical_science/magnetism/earth_magnet_dipole_interactive.html
Tom Wilson, Department of Geology and Geography
Location of north magnetic pole
Tom Wilson, Department of Geology and Geography
Location of north magnetic pole
http://www.compassdude.com/compass-declination.shtml
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
Magnetic Fields – Basic Relationships
monopole
vs.
Toxic Waste
dipole
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
The compass
needle points to
the magnetic
north pole.
Tom Wilson, Department of Geology and Geography
Geomagnetic north pole: pole associated with the
dipole approximation of the earth’s magnetic
field.
Magnetic Intensity
61000
F (nanoteslas or gammas)
60000
59000
11/15/2012
Total intensity =
52580nT
58000
57000
56000
55000
54000
53000
1900
1920
1940
1960
Date
Tom Wilson, Department of Geology and Geography
1980
2000
Magnetic Inclination
11/15/2012
Inclination =
67.13o
Tom Wilson, Department of Geology and Geography
Magnetic Inclination
Inclination (degrees)
72
71
11/15/2012
Inclination =
67.13o
70
69
68
1900 1920 1940 1960 1980 2000
Date
Tom Wilson, Department of Geology and Geography
Magnetic Declination
11/15/2012
declination = 9.08o
Tom Wilson, Department of Geology and Geography
Magnetic Declination
declination (degrees west)
W -9
-8
-7
-6
-5
-4
-3
Today, the compass
points about 9
degrees west of GN
Tom Wilson, Department of Geology and Geography
-2
1900 1920 1940 1960 1980 2000
Date
Declination to the west as in Morgantown
rotate compass dial clockwise to the east
To the east, rotate to the west
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
Magnetic Elements
http://www.ngdc.noaa.gov/geomag/magfield.shtml
Today’s Space Weather
http://www.swpc.noaa.gov/today.html
Tom Wilson, Department of Geology and Geography
Another site of interest
http://spidr.ngdc.noaa.gov/spidr/
Tom Wilson, Department of Geology and Geography
Introduction to the magnetics computer lab
Anomaly associated
with buried metallic
materials
Computed magnetic
field produced by
bedrock
Results obtained
from inverse
modeling
Bedrock configuration
determined from gravity
survey
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 –
Tom Wilson, Department of Geology and Geography