Lecture 15 - Radiometric Surveys PDF

GLGY3764 Lecture #15 Radiometric Surveys Dr. John Carranza Professor of Economic Geology Radiometrics Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which...

GLGY3764 Lecture #15 Radiometric Surveys Dr. John Carranza Professor of Economic Geology Radiometrics Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of radioactive decay are alpha decay (α-decay), beta decay (β-decay), and gamma decay (γ-decay), all of which involve emitting one or more particles. 2 Radiometrics Properties of radioactive energy 3 Radiometrics Because of their properties, only γ radiation is relevant.  While many naturally occurring elements have radioactive isotopes, only K and the U and Th decay series have radioisotopes that produce gamma rays of sufficient energy and intensity that can be measured by gamma-ray spectrometry. 4 Radiometrics  γ radiation is caused by 238U, 235U , 232Th and 40K decay.  γ-rays from each decay system have different energies, which can be measured by a spectrometer. 5 Radiometrics 6 Radiometrics Gamma-ray radiation, the gamma-ray spectrum Energy spectrum (not wavelength spectrum as used in spectral remote sensing) 7 Radiometrics  eU = equivalent uranium  means that the amount of uranium isotopes that were actually detected were adjusted to account for the other uranium isotopes (i.e., all of the uranium present is included).  U concentration estimates are based on the measurement of 214Bi and 214Pb isotope abundances. These occur far down in the radioactive decay chain and may not be in equilibrium with U.  Estimates of U concentration are therefore usually reported as eU as these estimates are based on the assumption of equilibrium conditions. 8 Radiometrics  eTh = equivalent thorium  concentration of Th estimated by gamma ray spectrometry under the assumption that the Th decay series are in equilibrium 9 Radiometrics Mean radioelement concentration in various rocks (Killeen, 1977) 10 Radiometrics Survey techniques Radiometric techniques may be both ground and airborne, but the latter is by far the most common today. 11 Radiometrics Survey techniques 50% of γ- radiation is absorbed by detector at 200 m and 75% at 50 m Therefore, airborne surveys should be between these heights. 12 Radiometrics Survey techniques Flight heights h are typically 80 to 100 m. Flight lines should be 2h apart. 13 Radiometrics Data Processing Corrections must be made for: Height Cosmic rays Aircraft radiation Atmospheric radon Spectrometers are then calibrated so that the γ-radiation can be directly converted into U, Th and K concentrations. 14  Radiometrics Ternary image representation of data K Th U …results in a K-Th-U image. 15 Radiometrics Applications – geological mapping Both bedrock and surficial sediments are possible to map 16 Radiometrics Applications Mineral exploration for U, granite-related elements such as Sn, W etc. rare elements such as Be, Zr, Y, carbonatites which are commonly sources of REEs, P, Nb Mapping of alteration patterns around precious and base metal mineralizations Data processing typically involves looking at ratios of the concentrations, which can be diagnostic for hydrothermal alteration Environmental applications of radiometrics include radon hazard, and nuclear accidents 17 Radiometrics Applications – mineral exploration 18 Total count (K + Th + U) Map of South Africa Radiometric Map of Australia