Geology Reference
In-Depth Information
EXPONENTIAL DECAY
1.0
Decay constant = λ
= log
e
2/
m
=
m
o
e
-λ
t
t
λ
Half-life =
1
2
ATTENUATION
Attenuation constant = α
Skin depth =
δ = 1/α
I
=
I
o
e
-α
d
0.5
1/e = 0.368
1/e
0.25
0.125
δ
t
2
t
3
t
2
2
2
Time or depth
Figure 1.6
The exponential law, illustrating the parameters used to char-
acterise radioactive decay and radio wave attenuation.
Attenuation rates are alternatively characterised by
skin-depths
,whichare
the reciprocals of attenuation constants. For each skin depth travelled, the
signal strength decreases to 1
/
e
of its original value, where
e
(
=
2.718) is the
base of natural logarithms. Radioactive decay rates are normally described
in terms of the
half-lives,
equal to log
e
2(
=
0.693) divided by the decay
constant. During each half-life period, one half of the material present at its
start is lost.
1.3 Geophysical Survey Design
1.3.1 Will geophysics work?
Geophysical techniques cannot be applied indiscriminately. Knowledge of
the material properties likely to be associated with a target (and its burial
setting) is essential to choosing the correct method(s) and interpreting the
results obtained.
Armed with such knowledge, the geophysicist can assess feasibility and,
where possible, select a geophysical method to meet the survey objectives.
Table 1.2 lists some of the more important physical properties, for some
of the commoner rocks and minerals. Inevitably, the values given are no
more than broad generalisations, but the table does at least indicate some of
the circumstances in which large contrasts in physical properties might be
expected, or at least be hoped for.
