radioactive properties of rocks
TRANSCRIPT
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Summary of Lecture 3 Chapter 5
Simplest resistivity measurement and calculationof geometric factor to convert resistance intoresistivity
Normal device
Lateral device
Boundary and thin bed behavior
Focused devices Induction devices
Corrections to estimate true resistivity
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Radioactive Properties of Rocks
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Reading Assignment
Bassiouni Ch. 2 Radioactive Properties of Rocks
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Where are we heading?
What reservoir properties can be inferred by
radioactive measurements?
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Atomic StructureCartoon only
Electrons occupy orbitals, many ofwhich are not spherical
Size of nucleus is much smaller Size of atom ~ 10-10 = angstrom
Size of proton ~ 1 femtometer ~ 10-15 m~ .00001 size of atom, i.e., tiny
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Atomic Structure
Cartoon only
Electrons occupy orbitals,many of which are notspherical
Size of nucleus is muchsmaller Size of atom ~ 10-10 = angstrom
Size of proton ~ 1 femtometer~ 10-5 angstrom
Courtesy: C.C. Lin,
University of
Wisconsin
Conventional units
Na = Avogadro's number
= 6.023 x 1023
Mass:
1 amu = (1 g/Na)
me = .5 MeV/c2
Energy = eV
Emitted light is visible
to ultraviolet
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Nuclear Structure
The nucleus also has energy levels
Conventional units
mp
~ mn
~ 1 amu ~938 MeV/c2
Note: masses are often expressed as
energy/(speed of light)2 to make it
easier to calculate the energy of
emitted photons since E = h = hc/Energy = MeV
Emitted light is gamma radiation
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Types of Natural Radioactive Decay Alpha particle emission
two protons + two neutrons = He nucleus
Beta particle emissionone electron from the nucleus not the atomicorbitals
Proton conversion to neutron Positron emission
Electron capture (K shell capture)
16 12 4
8 6 2 (which is alpha particle)O C He +
14 14
6 7 (which is beta particle)
n p e
C N e
+
+ +
+
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Decay Rates
0 0
0
ln 2
0
0
ln 2(constant)
ln 2
ln 2
ln 2ln
When then2
hl
hl
hl
N t
hlN
hl
t
hl
dN Ndt Ndt
dNdt
N
dN
dtN
N t
N
N N e
Nt N
= =
=
=
=
=
= =
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Natural Gamma Radiation in Rocks
Clean sandstones emit very little radiation
Clays attract radioactive minerals
So for real rocks
Shales are about 3% K, of which about 0.01% isradioactive K40 (or about 300 ppm) + 6 ppm U + 20ppm Th
Sandstones contain about 100 ppm K40 (about 1/3 of
shale concentration) + 6 ppm U + 20 ppm ThSummary: shale emits about 3X the gamma rays of
sandstone
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Significance of Natural Gamma Rays
For sandstone-shale sequences, simply
recording the nature gamma ray emissions from
the formation provides a shale indicator
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Absorption of Gamma Rays
Photoelectric effect low energy photons
Depends on atomic number Z of atoms
Compton scattering medium energy photons
Independent of Z, just saps energy from
Pair production high energy photons
Independent of Z, just saps energy from
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Photoelectric Effect
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Compton Scattering
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Gamma Ray Absorption Logging
Litho-density tool
Photoelectric effect absorption is a function of Z
so absorption can be calibrated to estimate
lithology (types of rocks and minerals)
Compton scattered just depends on density of
electrons which in turn depends on density of
rock so absorption due to Compton scattering can
be used to estimate density
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Neutron Scattering
Fast neutrons are produced by bombarding
targets with accelerated positive ions
Neutrons are neutral bullets
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Inelastic scatteringNeutron hits nucleus, excites it, the nucleus emits a thatis detected. The has a particular frequency that isassociated with a particular element.
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Elastic Scattering
Hydrogen is the most effective moderator
because the mass of a hydrogen nucleus is
about the same as the mass of a neutron
Where is most of the hydrogen in a reservoir?
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Why is H the most effective
moderator?
An effective neutron moderator is a nucleus to which a neutron will
transfer a lot of energy in each elastic collision. What should the mass of
an effective neutron moderator be?
You have all the tools to derive this result. Dont get lost in the weeds.
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1. Physics
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2. Algebra to Obtain Velocities
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3. Algebra to Obtain Energy Transfer
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4. Calculus to Obtain
Maximum Energy Transfer
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4. Calculus to Obtain
Maximum Energy Transfer (continued)
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Summary of Lecture 4 Chapter 2
Types of natural radioactive decay
Units
Decay rates and half-life Natural gamma radiation as shale indicator
Gamma ray absorption
Preview of gamma ray absorption logging
Elastic and inelastic neutron scattering