Transcript What is radiometrics?

ETS 306
EXPLORATION GEOPHYSICS 1
LECTURE ONE
Radiometrics – Objectives of this
study???
Define and relate the terms radioactive
decay and nuclear radiation.
 Describe the different types of radioactive
decay and their effects on the nucleus.
 Define the term half-life, and explain how
it relates to the stability of a nucleus.
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RADIOMETRICS
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What is radiometrics?
Radiometrics is a measure of the natural
radiation in the earth’s surface, which can
tell us about the distribution of certain soils
and rocks. Geologists and geophysicists
routinely use it as a geological mapping tool
to tell them where certain rock types
change. Radiometrics is also useful for the
study of geomorphology and soils.
RADIOMETRICS
Radiometrics is also known as Gamma-Ray
Spectrometry. A radiometric survey
measures the spatial distribution of three
radioactive elements (potassium-K, thoriumTh and uranium-U) in the top 30-45 cm of
the earth’s crust. The abundances of K, Th
and U are measured by detecting the
gamma-rays produced during the natural
radioactive decay of these elements.
How is radiometrics related to rock
and soil type?
Radioactive elements occur naturally in the
crystals of particular minerals. The
abundance of minerals changes across the
earth’s surface with variations in rock and
soil type. Because the energy of gamma rays
is related to the source radioactive element,
they can be used to measure the abundance
of those elements in an area. So by
measuring the energy of gamma rays being
emitted in an area, we can infer the presence
of particular minerals in the earth’s surface.
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There are many hundreds of naturally
occurring radioactive isotopes, but the
majority are rare or are only weakly
radioactive and thus are of little interest
for mineral exploration. There are 45
minerals that exhibit strong radioactivity,
and a further 225 that are very strong and
to which exposure should be limited for
health reasons.
USES OF RADIOMETRICS
Radiometric method have been and
continue to be used for hydrocarbon and
mineral exploration.
 They are currently also being used for
shallow archaeological and environmental
investigations.
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How are gamma rays
measured?
Gamma rays can be measured on the
ground or from a low flying aircraft. The
gamma rays are detected by a
spectrometer.
Some common terms used in
radiometrics
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Isotope: an element whose atoms have a
common number of protons and
electrons (i.e. Same atomic number) but
which vary in the number of neutrons in
their nucleus. E.g. Hydrogen exists in
three isotopic forms: hydrogen (one
proton, no neutron),
deuterium (one proton, one neutron),
Tritium (one proton, two neutrons).
Some common terms used in
radiometrics
Nucleus: the centre of an atom
comprising of protons (+ve charge) and
neutrons (neutral).
 Atom: made up of nucleus and its
surrounding electrons.
 Spectrometer: An instrument that
measures the abundance of gamma rays
with different energy values.
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Some common terms used in
radiometrics
Radioactive decay: The process where
an unstable "parent" element loses
(emits) particles from its nucleus and
becomes a stable "daughter" element.
 Radiation: Energy that is transmitted, or
radiated, in the form of rays, waves or
particles eg. sound, heat or the
electromagnetic spectrum (including
light).
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Types of Radioactive Decay
• A nuclide’s type and rate of decay depend on the
nucleon content and energy level of the nucleus.
• Alpha Emission
• An alpha particle (α) is two protons and two
neutrons bound together and is emitted from the
nucleus during some kinds of radioactive decay.
4
2
He
• Alpha emission is restricted almost entirely to
very heavy nuclei.
•Beta Emission
• A beta particle (β) is an electron emitted from the
nucleus during some kinds of radioactive decay.
• To decrease the number of neutrons, a neutron can
be converted into a proton and an electron.
n 
1
0
1
1
p + -10 β
• The atomic number increases by one and the mass
number stays the same.
• Gamma Emission
• Gamma rays () are high-energy electromagnetic
waves emitted from a nucleus as it changes from an
excited state to a ground energy state.
Half-Life
• Half-life, t1/2, is the time required for half the atoms
of a radioactive nuclide to decay.
• Each radioactive nuclide has its own half-life.
• More-stable nuclides decay slowly and have longer
half-lives.
Potassium-40 Half-Life
Half-Lives of Some Radioactive Isotopes
WORKED EXAMPLE – HALF
LIFE
Radioactivity of Rocks
BOREHOLE GEOPHYSICS
What is borehole geophysics: the
science of recording and analyzing
measurements made in wells or testholes.
 Borehole geophysics utilizes boreholes or
wells to make geophysical measurements.
 Probes that measure different properties
are lowered into the borehole to collect
continuous or point data that is
graphically displayed as a geophysical log.
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Geophysical Logging System
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The geophysical logging system consists of
probes, cable and draw-works, power and
processing modules, and data recording
units.
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Borehole-geophysical logging can provide
a wealth of information that is critical in
gaining a better understanding of
subsurface conditions needed for groundwater and environmental studies.
Reasons for Logging:
Delineation of hydrogeologic units: can
be used to determine the character and
thickness of the different geologic materials
penetrated by wells and test holes.
 Definition of groundwater quality: with
water-quality sampling logging provides a
more complete picture, whether the
objective is to develop a water-supply well
or remediate a contaminated aquifer.
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Reasons for logging
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Determination of well construction
and conditions: The location and
condition of casing and screen can be
rapidly evaluated with geophysical logging
common geophysical logs
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Common geophysical logs include caliper,
gamma, single-point resistance,
spontaneous potential, normal resistivity,
electromagnetic induction, fluid resistivity,
temperature, flowmeter, television, and
acoustic televiewer.
Geophysical borehole logging techniques can be
divided into two:
 Static techniques: measure the resident
energy field.
 Spontaneous potential, calliper, fluid velocity,
temperature.
 Dynamic technique: records the response to
injected flux.
 Formation resistivity, fluid resistivity, point
resistivity, gamma gamma, fluid velocity, neutron
gamma.
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Applications – Radiometric
Surveys
Mineral Exploration – Radiometric
surveys have been used successfully to map
uranium deposits.
 Engineering applications – in contrast to
mineral exploration, the use of surface
radiometric measurements in geotechnical
and archaeological applications has been
extremely limited.
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However it has been demonstrated that some
structures buried within 10 cm of the ground
surface can be identified using radio-elemental
ratios of Th/K and Th/U to within 0.1m
horizontal resolution.
 Soil mapping – Air-borne and ground
radiometric surveys with high resolution can
be used to map soils up to a scale of 1:25,000
and even 1:10,000. This info can be used by
land managers in efficient land management.
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More detailed examination of the
radiometric results can yield additional
information about the soil types such as
soil texture, horizon changes and
homogeneity in the top 40 cm.
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Nuclear waste disposal investigations –
Radiometric surveys has been used in the
USA to monitor levels of radioactivity at a
nuclear waste dump in an area of about
1400km². Some 47,800 large containers
with low-level radioactive waste were
dumped between 1946 and 1970. There was
a need to monitor radioactive levels in the
area because of commercial and sporting
activities in the same area.
Geophysical Techniques In
Hydrogeology
Electrical Methods:
 No other surface geophysical methods
have been used more widely than
electrical and electromagnetic methods in
the study of ground water.
 Here, electrical applies to methods in
which electrical currents are injected into
the ground by the use of direct contact
electrodes.
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Electrical Methods
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Electrical methods operate using direct
current (DC).
Types of Electrical Methods:
The three major types of electrical
methods are DC electrical resistivity and
induced polarization (including complex
resistivity), which involve artificial field
sources, and self-potential, which involves
the measurement of natural electrical
currents in the subsurface.
Sub-surface properties that
are measured:
1. Resistivity (or reciprocal –
conductivity): the amount of current that
moves through rock material when a
specified potential difference is applied.
 2. Electrochemical activity: which is
caused by chemical activity in ground
water and charged mineral surfaces. This
provides the basis for self-potential and
induced polarization methods.
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3. Dielectric constant: a measure of the
polarizability of a material in an electric
field, and gives information on the capacity
of rock material to store an electric
charge. This property is important in the
use of induced polarization.
Units of Measurement
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The unit for measurement in electrical
resistivity is ohm-meter. Note that 1 ohmmeter = 1000 milliSiemens/meter.
Electrical Resistivity Methods
Electric resistivity method measures the
resistance to flow of electricity in
subsurface material.
 Involve the placement of electrodes,
called current electrodes, on the
surface for injection of current into the
ground. The current stimulates a potential
response between two other electrodes,
called potential electrodes.
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Electrical Resistivity Methods
Resistivity (measured in ohm-meters) can be
calculated from the geometry and spacing of
the electrodes, the current injected, and the
voltage response.
 These methods are identified according to
the arrangement of the current and
potential electrodes, called array.
 Some of these methods include
schlumberger, wenner, dipole-dipole, poledipole, among others.
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Diagram showing basic concept of
resistivity measurement .