PERMFILE128636

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: OBJECTORS EXHIBITS
• Media Type: D
• Archive: No

4 • •
ih• Gamma iay Log /10
THE GAMMA RAY LOG
The Gamma Ray Log is a measurement of the natural
radioactivity of the formations. The log is therefore useful
in detecting and evaluating deposits of radioactive minerals
such as potash or uranium ore.
In sedimentary formations the Gamma Ray Log nor-
mally reflects the shale content of the formations. This is
because [he radioactive elements tend to concentrate in clays
and shales. Clean formations usually have a very low level of
radioactivity, unless radioactive contaminants such as vo]-
canicash or granite sash are present, or when the formation
waters con[ain dissolved potassium salts.
The Gamma Ray Log can be recorded in cased wells,
which makes it very useful in completion and workover op-
erations. It is frequently used as a substitute for the SP in
cased holes where the SP is unavailable or in open holes
where the SP is unsatisfactory. In both cases it is useful for
the location of the non-shaly beds and for correlation.
PROPERTIES OF GAMMA RAYS
Gamma rays are bursts of high-energy electromagnetic
waves which are emitted spontaneously by some radioactive
elements. Nearly all of the gamma radiation encountered in
the earth is emiaed by the ndioaaive potassium isorope of
atomic weight 40 and [he radioactive elements of the ura-
nium and thorium series.
Each of these elements emits gamma rays, the number
and energies of which are distinctive of each element. Fig.
10-1 shows the energies of the gamma rays emitted: Pons-
sium (KA0) emits gamma rays of a single energy at 1.46
DfeV, whereas the ruo radioactive series emit many gamma
ra}'s of various energies.
in passing through matter, gamma rays experience suc-
cessive Compmn-scutering collisions (Chaprer 1-S) with the
atoms of the Formation, losing energy with each collision.
Finally, af[er [he gamma ray has los[ enough energy, it is ab-
sorbed via the phoroelectric effect. (In the phoroelectxic e6
feet, low energy gamma rays are completely absorbed b}'
atoms of the formation material, resulting in the ejection of
electrons (ram the absorbing aroms.)
EFFECT OF FORMATION DENSITY
Gamma rays are gradually absorbed and their energies
degraded as they pass through formations. The amount of
absorption varies with formation density. Tw-o formations
having the same amount of radioactive material per unit vol-
ume but having different densities will show, on the Gamma
z
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a P07A SSIUM
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u
w
_z
N
O
w
a
THORIUM SERIES
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o_
N
VI
w
0
r
URANIUM-RADIUM SERIES
m
a
m
0
a G N t ~..t .... .t .
0 .5 I I.5 2 2.5 -
G4MM4-RAY ENERGY IMEV)
Fig. 10-1 -Gan:n:a ray emisrion r¢ectra of radio-
actir•e rrrineralt (Ref. 4).
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