environmental and exploration geophysics i
DESCRIPTION
http://www.geo.wvu.edu/~wilson/geo252/lect12/mag2.pdf. Environmental and Exploration Geophysics I. Magnetic Methods (I). tom.h.wilson [email protected]. Department of Geology and Geography West Virginia University Morgantown, WV. Anomaly associated with buried metallic materials. - PowerPoint PPT PresentationTRANSCRIPT
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Environmental and Exploration Geophysics I
Department of Geology and GeographyWest Virginia University
Morgantown, WV
Magnetic Magnetic Methods (I)Methods (I)
http://www.geo.wvu.edu/~wilson/geo252/lect12/mag2.pdf
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Anomaly associated with buried metallic materials
Bedrock configuration determined from gravity survey
Results obtained from inverse modeling
Computed magnetic field produced by bedrock
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Where are the drums and how many are there?
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Locating Trench Boundaries
Theoretical modelExamination of trench for internal magnetic anomalies. actual field data
Gilkeson et al., 1986
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Trench boundaries - field data
Trench Boundaries - model dataGilkeson et al., 1986
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Locating abandoned wells
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From Martinek
Abandoned Wells
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From Martinek
Abandoned Well - raised relief plot of measured magnetic field intensities
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Falls Run Coal Mine Refuse Pile Magnetic Intensity
Wire Frame
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Gochioco and Ruev, 2006
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Magnetic monopoles
1 212 2
12
1
4m
p pF
r p1
p2
r12Fm12 Magnetic Force Magnetic Permeability
p1 and p2 pole strengthsCoulomb’s Law
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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).
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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
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monopole vs.
dipole
Toxic Waste
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The earth’s main magnetic field
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Steve Sheriff’s Environmental Geophysics Course
Proton Precession Magnetometers
Tom Boyd’s Introduction to Geophysical Exploration Course
Measuring the Earth’s magnetic field
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Source of Protons and DC current source
Proton precession generates an alternating current in the surrounding coil
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22
GFF
L
Mf
Proton precession frequency (f) is directly proportional to the main magnetic field intensity F. L is the angular momentum of the proton and G is the gyromagnetic ratio which is a constant for all protons (G = 0.267513/ sec). Hence -
fF 4874.23
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Magnetic Elements
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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.
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Date
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53000
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F (
nano
tesl
as o
r ga
mm
as)
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68
69
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72
Incl
inat
ion
(deg
rees
)
1900 1920 1940 1960 1980 2000
Date
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-9
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decl
inat
ion
(deg
rees
wes
t)
W
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Magnetic Elements for your location
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Magnetic Field Variations
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Long term drift in magnetic declination and inclination
Magnetic field variations generally of non-geologic origin
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Magnetic fields like gravitational fields are not constant. Their variations are much more erratic and unpredictable
http://www.earthsci.unimelb.edu.au/ES304 /MODULES/ MAG/NOTES/tempcorrect.html
Diurnal variations
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In general there are few corrections to apply to magnetic data. The largest non-geological variations in the earth’s magnetic field are those associated with diurnal variations, micropulsations and magnetic storms.
The vertical gradient of the vertical component of the earth’s magnetic field at this latitude is approximately 0.025nT/m. This translates into 1nT per 40 meters. The magnetometer we have been using in the field reads to a sensitivity of 1nT and the anomalies we observed at the Falls Run site are of the order of 200 nT or more. Hence, elevation corrections are generally not needed.
Variations of total field intensity as a function of latitude are also relatively small (0.00578nT/m). The effect at Falls Run would have been about 1/2 nT from one end of the site to the other.
International geomagnetic reference formula
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The single most important correction to make is one that compensates for diurnal variations, micropulsations and magnetic storms. This is usually done by reoccupying a base station periodically throughout the duration of a survey to determine how total field intensity varies with time and to eliminate these variations in much the same way that tidal and instrument drift effects were eliminated from gravity observations.
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Anomalies - Total Field and Residual
The regional field can be removed by surface fitting and line fitting procedures identical to those used in the analysis of gravity data.
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Magnetic susceptibility is a key parameter, however, it is so highly variable for any given lithology that estimates of k obtained through inverse modeling do not necessarily indicate that an anomaly is due to any one specific rock type.
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Opposites attract
S
N
S
N
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SN
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Magnetic fields are fundamentally associated with circulating electric currents; thus we can also formalize concepts like pole strength, dipole moment, etc. in terms of current flow relationships.
pl = n iA+
-
l
n turns
Cross sectional area A
pl is the dipole moment
Units of pole strength
niAp ampere meter
l
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I=kF
EkFI
I is the intensity of magnetization and FE is the ambient (for example - Earth’s) magnetic field intensity. k is the magnetic susceptibility.
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The intensity of magnetization is equivalent to the magnetic moment per unit volume or
V
MI
and also, EkFI . Thus
M plI
V V E
pkF
Aand yielding
Ep kAF
Magnetic dipole moment per unit volume
M plwhere
The cgs unit for pole strength is the ups
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Ep kAF
Recall from our earlier discussions that magnetic field intensity
2 or
pH F
r
2p Fr
so that
Thus providing additional relationships that may prove useful in problem solving exercises.
2r
AkFF EFor example,
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What does this tell us about units of these different quantities?
2 2 (or )
p upsH F
r cm
We refer to the magnetic field intensity as H or, more ambiguously, as F
Force
pole strength
dyneH
ups
1 an Oersted
dyne
ups
2thus 1 Oersted 1
ups
cm
2 2 yields Oersted-cmp Fr p
5Note also that 1 Oersted = 10
&
1 nT = 1
nT
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Force varies inversely as the square of the distance between charges, masses or poles. It has the general form
221
r
mmF
drr
mmFdrV
221
Potential on the other hand refers to the energy available to do work and is the integral of the force times displacement.
What is this integral?
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drr
mmFdrV
221
Remember the general power rule for integration
Crn
drr nn
1
1
1
Since n is -2, n+1 = -1 so that the potential V (per unit pole) is simply
r
m
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r
mV
As we have done repeatedly with the force, we divide it by unit mass, charge or pole to obtain
2""
r
mF an acceleration, electric or magnetic field intensity.
Doing the same with the potential yields a potential per unit pole strength, or just
Most importantly, working with potentials offers us some simplification since the denominator is in r and not r2. It offers
useful simplification when characterizing the dipole field.
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Basic Magnetic Unit and Vector Concepts
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Problem - At a point 20 cm from the center of a thin magnetized rod 40 cm long and equidistant from its ends, the magnetic field is 500 nT. What is the pole strength in Oersted-cm2?
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Sign conventions imply that the test pole is positive.
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HR=2Hx=500nT
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UNITS - nanoteslas, ups, Oersteds …..
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105
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HRX = 500nT
x
Given 2
what is H ?R xH H
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Then, what is H+ or H-? Once we know this, we can then determine the pole strength.
H = p/r2
so p = Hr2
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Bring questions to class Tuesday after Thanksgiving break –
November 28th
We will meet in the 310
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