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- THE CE~IC,4L INTERPRF. T.4TIOt9 OF R~ELL LOGS• <br />Table 7.3 Typical modem gamma ray tools <br />Name Symbol Company <br />Gamma ray log GR all <br />Spectralog SL Western Atlas <br />Nmural gamma ray <br />spectrometry NGS Schlumberger <br />Specval gamma ray SGR, CSNG Halliburton <br />Spectral gamma <br />sonde SGS B.EB. <br />Table 7.4 Ratios of radioactive to non-radioactive material in <br />normal elemental mixt ures (Serra er al., 1980). <br /> mK v-~ neU xtsU lpU <br />9c radioactive isotopes <br />in normal mixtures 0.0199 100 99.27 0.72 0.0057 <br /> '0K ='sU "'U ~'U <br /> - All - <br /> K,ow U,wi Uaw Umw <br />at 1.76 hieV and thorium at 2.62 MeV still exist and can <br />be used to identify the original source o(radia[ions. This <br />is the principle used in the spectral gamma ray tool <br />(Section 7.3). <br />they cause a flash. These are collected by the photo-mul- <br />tiplier and stored in the attached condenser over a set <br />period of time, the time constant (Table 7.5). The energy <br />accumulated during the time constant is the detector <br />value at that depth for that time constant. The tool liter- <br />ally 'counts' the gamma rays. <br />Spectral gamma ray too! <br />The spectral gamma ray tool, like the simple tool, <br />consists of a scintillation counter and photo-multiplier. <br />However, in the spectral tool, the sodium iodide crystal <br />has a much greater volume, typically 5 cm in diameter <br />and 20 cm long and so gives the tool a much better <br />'counting' sensitivity. When a gamma ray passes through <br />a scintillation crystal, it not only causes a flash, but the <br />intensity of that flash depends on the energ}• of the <br />incident gamma ray. This characteristic is used by the <br />specval gamma ray tool, with its large scintillator crystal, <br />to identify the gamma radiations in several, pre-defined <br />energy bins or windows. These windows are desiened to <br />separate the distinctive energy peaks of the individual <br />radioactive elements discussed above (Figure 7.4), <br />namely bracketing the energies of 2.62 MeV for thorium, <br />1.76 MeV for uranium and 1.46 MeV for potassium. In <br />most tools the lower energy counts are also used and <br />'allocated' [o each element. <br />Table 7.5 L,oe4ing speed v. time-constant - simplt ¢amma <br />rav tool. <br />7.3 Tools <br />Simple gamma ray tool <br />The simple gamma ray tool is a sznsitive gamma ray <br />detector consisting of a scintillation counter and a pho[o- <br />multiplier (Figure 7.5). The scintillation counter is <br />typically a sodium iodide crystal, 2 cm in diameter and 5 <br />cm long in the simple tool, with minor impurities of <br />Thallium. When gamma rays pass through the crystal, <br />gamrr <br />ray <br />Time-constant Logging speed Formation logged <br />(seconds) (m/h) in time-constant <br />(cm) <br />1080 30.0 <br />2 550 30.5 <br />4 275 30.5 <br />6 140 31.0 <br />Figure 7.5 Schtma of a gamma ray tool (re-drawn from Sena, 1979 alter a Lane R'ells document). <br /> <br />70 I <br />