PERMFILE128524

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: GAMMA RAY AND SPECTRAL GAMMA RAY LOG
• Media Type: D
• Archive: No

- TIfE GA,H 1tA RAF AND SPECTR.{L Gri,tl tl.{ RAT' LOCS -
7.2 Natural gamma radiatiml
Natural radiation in rocks comes essentially from only
three elemental sources: the radioactive elements of the
thorium family, of the uranium-radium family and of
the radioactive isotope of potassium '0K (Adams and
`.Vaver, 1958).
Quantitatively, potassium is by far the most abundant of
the three elements (Table 7.2) but its contribution to the
overall radioactivity in relation to its weight is small. In
reality, the contribution to the overall radioactivity of the
three elements is of the same order of magnitude, the abun-
dance seeming to be the inverse of [he contribution in
' energy: a small quanury of uranium has a large effect on the
radioactivity, a large quantity of potassium a small effect.
Each of the three sources emits gamma rays sponta-
neously. That is, they emit photons with no mass and no
charge but great energy (this being the definition of a
gamma ray). The energy in the case of uranium, thorium
and potassium emissions occurs in the spectrum from 0 -
3MeV (million elecvon volts).
Table 7.2 Abundance and relative radiation activin~ of the
natural radioactive elements.
K Th U
f Relative abundance
in the eart)t's crust 2.599c -T2ppm -3ppm
'Gamma rays par
unit weight 1 1300 3600
tSerra (1979), Serra et a(., (1980)
'Adams and Weaver (1958)
1 <6 MeV
POTA66iUM
The radiation fromJOK is distinct, with a single energy
value of 1.46 MeV (Figure 7.2). Both thorium and
uranium emit radiations with a whole range of energies,
but with certain peak frequencies. These peaks are espe-
ciallydistinct al the higher energy levels of 2.62 MeV for
thorium and 1.76 MtV (or uranium (Figure 7.?).
The spectra and the energy levels illustrated are those
at the point of emission. One of the characteristics of
gamma rays is that when they pass through any material
their energy is progressively absorbed. The effect is
known as Compton scattering, and is due to the collision
between gamma rays and electrons which produces a
degrading (lowering) of energy (Figure 7.3). The higher
the common density through which the gamma rays pass,
the more rapid the degradation or loss of energy (in real-
ity it depends on the material's electron density, which is
very similar to common density).
In borehole logging, when radiations are observed by
the tool, they have already passed through the formation
and probably also the drilling mud, both of which cause
Compton scattering. Thus, the discrete energy levels at
which gamma rays are emitted become degraded. and a
continuous spectrum of values is observed (Figure 7.4).
When each of the radioactive minerals is present, their
radiations become mixed and the resulting spectrum is
very complex. However, a glance at the original spectra
(Figure 7.2) will show that the final complex, mixed spec-
trum, even after Compton scattering, will still contain
diagnostic peaks, especially in the I-3 h1eV region. The
original distinct peaks of potassium at 1.46 MzV, uranium
dense less Genes
source
URANIUM-RADIUM SERIES
1.r6 MeV
E
- THORIUM SERIES
~ 3 63 MeV
0 0.5 1.0 I6 3.0 3.6 JO
gamma rdy <nergy tMeV)
Figure 7.2 The gamma ray emission specva of naturally
radioactive minerals. The principal peaks used to identify each
source are indicated. (After Tittman er al., 19ti5, re-drawn
from Schlumberger, 1972).
Figure 7.3 Schematic drawing o(the Compton scattering of
gamma rays. The effect is more marked m denier matter Lcf.
Lavenda, 1950.
gamma ray energy MeV
631
Figure 7.4 Complex specwm obsen~ed from a mdioactice
source containing potassium, thorium and uranium, after
Compton scauering. (After Hassan er nl., 1976).
69