Environmental Engineering Reference
In-Depth Information
Figure
11.15
plots the relative counting rate on the
Y
-axis and the time (micro-
seconds) since the initial burst of fast neutrons on the
X
-axis. Note that after a few
hundred microseconds a straight-line portion of the decay curve develops. Note also
how the water line has a steeper slope than the oil line. Note, too, that at late times
the background gamma ray count rate remains substantially constant. The
Y
-axis on
Fig.
11.15
is logarithmic, but the time scale (
X
-axis) is linear. Thus, the straight-line
portions of the curve represent exponential decays. If
N
is the number of gamma
rays observed at time
t
and
N
o
is the number observed at
t
= 0, then
o
e
/ t
where ˄ (Greek letter tau) is the time constant of the decay process. Tau is measured
in units of time. It is convenient to quote values of tau in microseconds (1 ʼs = 10
−6
s).
The capture cross section of the formation, the property of interest, is directly
related to tau by the equation
NN
t
-
=
= 4 55, / ,
where ʣ (Greek letter sigma) is the capture cross section measured in capture units
(cu). Thus, the essence of measuring ʣ is to irst ind the straight-line portion of the
capture gamma ray decay, and then to measure its slope. This is accomplished in
different ways by the various commercially available tools.
S
t
Log Presentations
On a typical pulsed neutron log, there may be up to nine curves displayed. The curves
are illustrated in the Thermal Decay Time (TDT) log presentation of Fig.
11.16
Fig. 11.16
TDT log presentation. Courtesy Schlumberger
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