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Unformatted text preview: Petrophysics MSc Course Notes Spectral Gamma Ray Log Dr. Paul Glover Page 111 12. THE SPECTRAL GAMMA RAY LOG 12.1 Introduction The spectral gamma ray log measures the natural gamma radiation emanating from a formation split into contributions from each of the major radio-isotopic sources. Analysis of the sources of the natural gamma radiation give us added information concerning the composition and likely lithology of the formation. The spectral gamma ray log is commonly given the symbol SGR . A typical spectral gamma ray tool is shown in Fig. 12.1. 12.2 Principles It will be remembered that the amplitude of the output from the gamma ray sensor is proportional to the energy of the incident gamma ray. We can use this information to measure the proportion of the total gamma radiation coming from each of potassium-40, the uranium-radium series, and the thorium series for a particular formation. Figure 10.1 shows that the energy distributions from each of the major contributors to the natural gamma radiation from a formation are different. The labeled peaks in Figure 10.1 are the energies that are dominant (most gamma rays have this energy). When the radiation has traveled through the rock and drilling fluid to the sensor, the energy distributions shown in Figure 10.1 are added together because the rock contains different amounts of each of radio-isotopes. The energy distributions are also spread out in energy space as the result of compton scattering. However, the energy peaks from each of the major contributors to the gamma radiation are still recognizable (Fig. 12.2). Figure 12.1 The Spectral Gamma Ray Sonde (SGS). (Courtesy of Reeves Wireline Ltd.) Petrophysics MSc Course Notes Spectral Gamma Ray Log Dr. Paul Glover Page 112 The spectral gamma ray tool uses the same sensor as the total gamma ray tool. The output from the sensor is fed into a multi-channel analyzer that calculates the amount of radiation coming from the energies associated with each of the major peaks. This is done by measuring the gamma ray count rate for 3 energy windows centred around the energies 1.46 MeV for potassium-40, 1.76 MeV for the uranium-radium series, and 2.62 MeV for the thorium series. These readings represent the gamma ray radioactivity from each of these sources. Their sum should be the same as the total gamma ray value measured by the total gamma ray tool, and is coded SGR if measured with a spectral gamma ray tool. Any combination of the three components can be summed and analyzed. However, the most important is the sum of the potassium-40 and thorium radiation, which is called the computed gamma ray response ( CGR ). Figure 12.2 Measured gamma ray energy spectrum from a formation after compton scattering....
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- Winter '11