EXPLORATION GEOPHYSICS 1 LECTURE ONE 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.

RADIOMETRICS 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, thorium- Th 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.

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.

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 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.

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).

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. 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. 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 test- holes. 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.

Geophysical Logging System The geophysical logging system consists of probes, cable and draw-works, power and processing modules, and data recording units.

Borehole-geophysical logging can provide a wealth of information that is critical in gaining a better understanding of subsurface conditions needed for ground- water 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.

Reasons for logging Determination of well construction and conditions: The location and condition of casing and screen can be rapidly evaluated with geophysical logging

common geophysical logs 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.

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.

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.

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.

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.

Electrical Methods 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.

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 The unit for measurement in electrical resistivity is ohm-meter. Note that 1 ohm- meter = 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.

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, pole- dipole, among others.

Diagram showing basic concept of resistivity measurement .