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- TIIE G,\~I )IA RAY AND SPGCTRAI. CAM )IA RAY LOCS - The relative contributions of the three radioisotopes are therefore measured by the tool. These can be related to the abundance of each element by comparison with known spectra, since the measured, composite spectrum, is a simple addition of the spectra from the three different sources. Using the known abundances of the radioiso- topes in normal elemental mixtures (Table 7.4), the actual abundances of each element can be derived. Thus, the log results from the spectral tool are the quantitative, elemental abundances of thorium, uranium and potassium (Section 7.4, Figure 7.6). i 7.4 Log characteristics Calibration, tog presents(ion, units and scales Simple gamma ray log -The accepted unit for radio- activity logging is the API (American Petroleum Institute) I unit. The API unit is defined in a reference well in the grounds of the University of HOns[On, Texas. The well contains specially mixed, high-radioactivity concrete surrounded by equally special low-radioactivity concrete. I An API unit is 1/200 of the difference between the two radioactivities (Belknap er al., 1959). [f a particular gamma ray tool is tested, the API unit for that tool is 1/200 of the difference between the low and high values. Thus, not only does the Houston pit serve as a standard for the r API unit, it also serves to calibrate a tool. The simple gamma ray log is usually recorded in track I along with the caliper. Scales are chosen locally, but (A) GAMMA RAY LOG San 0-100 or 0-150 API are common (Figure 7.6a). The tool is small and can be combined with practically any other tool be it a resistivity or porosity device. Speclrnl gnrntnn ray log -The essential results of [he spectral gamma ray tool are elemental abundances, derived as described above. The calibration facility for the spectral tools is the same pit in the University of Houston that is used for the simple gamma ray tool. This is because the high activity cement of this pit has known quantities of uranium (13.1 ppm), thorium (2-3.4 ppm) and potassium (4.0790) which contribute to the overall radioactivity (Belknap et al., 1959). The individual chan- nels of the spectral tool can be empirically calibrated. There are several common presentation (orma[s for the gamma ray spectral log. The simplest, and probably best, is a straightforward plot of elemental abundances across tracks 2 and 3 on arithmetic scales (Figure 7.6b). Thorium and uranium are given in ppm while potassium is given in per cent. Track I of this (Schlumberger) presentation shows two curves, the CGR and SGR. The SGR, or standard gamma ray, is the total contribution of the three elements in API units. That is, i[ is the same as the simple gamma ray log, but re-conswcted from the elemental values plotted on tracks 2 and 3. To arrive at this value (for the Schlumberger NGT-A) the following multipliers are used: Ippm U = g.09 API units, I ppm Th = 3.93 API units, 19c K = 16.32 API units. Thus, 3 ppm of uranium in other logs ~ E other logs (SP, caliper etc.) i y (resistivity, sonic, density neutron etc.) GAMMA RAY t n (AP 11 m 9 ~,p IG4.0 1100 gamma-ray log ~ rea ing a out 70 API) I I (B) SPECTRAL GAMMA RAY LOG CGR 1 PP 11 a TH(IR (PPnI PpTA 111 _- ---_ --_- M1.p 1GU.U c ....-.. _. _ i,.0 ...--.4i,.0u 0.0 U. tUVV SGR IRP 11 o VR{1N _lPPnl___ ___ r~.V IuU.O o 0.u 14. JU :•G. GU 3U.0i~ 1650 .; t I ~ .'i ,~~ i ~ I ll ~ ~~ 1 I ~ Imo-ray ith t f t am otal ma-ra '. thorium ~~' uranmm p pm polar sium % ou anium I g y I ppm _ I is ~ ~ ~ I 1 Figure 7.6'fypical gamma ray and spectral gamma ray log headings. 71