{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Chapter%2012 - Petrophysics MSc Course Notes 12 Spectral...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
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.)
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon