EXPLORATION GEOPHYSICS 1 LECTURE ONE
ETS 306 EXPLORATION GEOPHYSICS 1 LECTURE ONE Radiometrics Objectives of this study???
Define and relate the terms radioactivedecay and nuclear radiation. Describe the different types of radioactivedecay and their effects on the nucleus. Define the term half-life, and explain howit relates to the stability of a nucleus. RADIOMETRICS What is radiometrics?
Radiometrics is a measure of the naturalradiation in the earths surface, which cantell us about the distribution of certain soilsand rocks. Geologists and geophysicistsroutinely use it as a geological mapping toolto tell them where certain rock typeschange. Radiometrics is also useful for thestudy of geomorphology and soils. RADIOMETRICS Radiometrics is also known as Gamma-RaySpectrometry. A radiometric surveymeasures the spatial distribution of threeradioactive elements (potassium-K, thorium- Th and uranium-U) in the top cm ofthe earths crust. The abundances of K, Thand U are measured by detecting thegamma-rays produced during the naturalradioactive decay of these elements. How is radiometrics related to rock and soil type?
Radioactive elements occur naturally in thecrystals of particular minerals. Theabundance of minerals changes across theearths surface with variations in rock andsoil type. Because the energy of gamma raysis related to the source radioactive element,they can be used to measure the abundanceof those elements in an area. So bymeasuring the energy of gamma rays beingemitted in an area, we can infer the presenceof particular minerals in the earths surface. There are many hundreds of naturallyoccurring radioactive isotopes, but themajority are rare or are only weaklyradioactive and thus are of little interestfor mineral exploration. There are 45minerals that exhibit strong radioactivity,and a further 225 that are very strong andto which exposure should be limited forhealth reasons. USES OF RADIOMETRICS Radiometric method have been andcontinue to be used for hydrocarbon andmineral exploration. They are currently also being used forshallow archaeological and environmentalinvestigations. How are gamma rays measured?
Gamma rays can be measured on theground or from a low flying aircraft. Thegamma rays are detected by aspectrometer. Some common terms used in radiometrics
Isotope: an element whose atoms have acommon number of protons andelectrons (i.e. Same atomic number) butwhich vary in the number of neutrons intheir nucleus. E.g. Hydrogen exists inthree isotopic forms: hydrogen (oneproton, no neutron), deuterium (one proton, one neutron), Tritium (one proton, two neutrons). Some common terms used in radiometrics
Nucleus: the centre of an atomcomprising of protons (+ve charge) andneutrons (neutral). Atom: made up of nucleus and itssurrounding electrons. Spectrometer: An instrument thatmeasures the abundance of gamma rayswith different energy values. Some common terms used in radiometrics
Radioactive decay: The process wherean unstable "parent" element loses(emits) particles from its nucleus andbecomes a stable "daughter" element. Radiation: Energy that is transmitted, orradiated, in the form of rays, waves orparticles eg. sound, heat or theelectromagnetic spectrum (includinglight). Types of Radioactive Decay
A nuclides type and rate of decay depend on the nucleon content and energy level of the nucleus. Alpha Emission An alpha particle () 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 tovery heavy nuclei. Beta Emission A beta particle () 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: thescience of recording and analyzingmeasurements made in wells or test- holes. Borehole geophysics utilizes boreholes orwells to make geophysical measurements. Probes that measure different propertiesare lowered into the borehole to collectcontinuous or point data that isgraphically displayed as a geophysical log. Geophysical Logging System
The geophysical logging system consists ofprobes, cable and draw-works, power andprocessing modules, and data recordingunits. Borehole-geophysical logging can providea wealth of information that is critical ingaining a better understanding ofsubsurface conditions needed for ground- water and environmental studies. Reasons for Logging: Delineation of hydrogeologic units: canbe used to determine the character andthickness of the different geologic materialspenetrated by wells and test holes. Definition of groundwater quality: withwater-quality sampling logging provides amore complete picture, whether theobjective is to develop a water-supply wellor remediate a contaminated aquifer. Reasons for logging Determination of well constructionand conditions: The location andcondition of casing and screen can berapidly 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, andacoustic televiewer. Geophysical borehole logging techniques can bedivided into two:
Static techniques: measure the residentenergy field. Spontaneous potential, calliper, fluid velocity,temperature. Dynamic technique: records the response toinjected flux. Formation resistivity, fluid resistivity, pointresistivity, gamma gamma, fluid velocity, neutrongamma. Applications Radiometric Surveys
Mineral Exploration Radiometricsurveys have been used successfully to mapuranium deposits. Engineering applications in contrast tomineral exploration, the use of surfaceradiometric measurements in geotechnicaland archaeological applications has beenextremely limited. However it has been demonstrated that somestructures buried within 10 cm of the groundsurface can be identified using radio-elementalratios of Th/K and Th/U to within 0.1mhorizontal resolution. Soil mapping Air-borne and groundradiometric surveys with high resolution canbe used to map soils up to a scale of 1:25,000and even 1:10,000.This info can be used byland managers in efficient land management. More detailed examination of theradiometric results can yield additionalinformation about the soil types such assoil texture, horizon changes andhomogeneity in the top 40 cm. Nuclear waste disposal investigations Radiometric surveys has been used in theUSA to monitor levels of radioactivity at anuclear waste dump in an area of about1400km.Some 47,800 large containerswith low-level radioactive waste weredumped between 1946 and There wasa need to monitor radioactive levels in thearea because of commercial and sportingactivities in the same area. Geophysical Techniques In Hydrogeology
Electrical Methods: No other surface geophysical methodshave been used more widely thanelectrical and electromagnetic methods inthe study of ground water. Here, electrical applies to methods inwhich electrical currents are injected intothe ground by the use of direct contactelectrodes. Electrical Methods Electrical methods operate using directcurrent (DC). Types of Electrical Methods: The three major types of electricalmethods are DC electrical resistivity andinduced polarization (including complexresistivity), which involve artificial fieldsources, and self-potential, which involvesthe measurement of natural electricalcurrents in the subsurface. Sub-surface properties that are measured:
1. Resistivity (or reciprocal conductivity):the amount of current thatmoves through rock material when aspecified potential difference is applied. 2. Electrochemical activity: which iscaused by chemical activity in groundwater and charged mineral surfaces. Thisprovides the basis for self-potential andinduced polarization methods. 3. Dielectric constant: a measure of thepolarizability of a material in an electricfield, and gives information on the capacityof rock material to store an electriccharge. This property is important in theuse of induced polarization. Units of Measurement The unit for measurement in electricalresistivity is ohm-meter. Note that 1 ohm- meter = 1000 milliSiemens/meter. Electrical Resistivity Methods
Electric resistivity method measures theresistance to flow of electricity insubsurface material. Involve the placement of electrodes,called current electrodes, on thesurface for injection of current into theground. The current stimulates a potentialresponse between two other electrodes,called potential electrodes. Electrical Resistivity Methods
Resistivity (measured in ohm-meters) can becalculated from the geometry and spacing ofthe electrodes, the current injected, and thevoltage response. These methods are identified according tothe arrangement of the current andpotential electrodes, called array. Some of these methods includeschlumberger,wenner, dipole-dipole, pole- dipole, among others. Diagram showing basic concept of resistivity measurement .