Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 147

Gamma Ray Spectroscopy
Each radioactive decay produces a gamma ray that is unique. These various gamma
rays have characteristic energy levels and occur in characteristic abundance, as
expressed in counts per time period. The simple method of just counting how many
gamma rays a formation produces can be carried a step further to count how many
gamma rays from each energy group it produces. The spectrum produced when the
number of occurrences is plotted against the energy group is characteristic of the
formation logged. Figure 11.9 shows such a spectrum, where energies from 0 to
approximately 3 MeV have been split into 256 specific energy bins. The number of
gamma rays in each bin is plotted on the Y -axis. This spectrum can be thought of as
a mixture of the three individual spectra belonging to uranium, thorium, and potas-
sium. Some unique mixture of these three radioactive families would have the same
spectrum as the observed one. The trick is to find a quick and easy method of dis-
covering that unique mixture. Fortunately, on-board computers in logging trucks are
capable of quickly finding a “best fit” and producing continuous curves showing the
concentrations of U, Th, and K.
Fig. 11.9 Gamma ray energy spectrum and elemental “windows”
Figure 11.10 illustrates a gamma ray spectral log. Both total gamma ray activity
(SGR) and a uranium-free version of the total activity are displayed in Track 1. Units
are API. In Tracks 2 and 3, the concentration of U, Th, and K are displayed. Depending
on the logging service company, the units may be in counts/s, ppm, or percent.