Geology Reference
In-Depth Information
S
Ground surface
Shot
point
i
c
sin i
c
=
V
1
/V
2
cos i
c
= (V
2
2
-
V
1
2
)/ V
2
d
tan i
c
=
V
1
/ (V
2
2
-
V
1
2
)
d.tan i
c
P
Q
Figure 13.4
The 'magic triangle' for calculating intercept times.
V
1
and
V
2
are the P-wave velocities in the upper and lower layers respectively, and
i
c
is the critical angle. For critical refraction below more than one layer, the
same geometrical analysis can be applied repeatedly.
13.2 Interpretation
Because the success of a refraction survey depends on parameters, such as
line orientation, geophone spacing, shot positions and spread lengths, that
can be varied almost at will, rapid 'first-pass' interpretation is essential. Only
if analysis keeps pace with data collection will the right choices be made for
the next day's work. Field interpretation has been made easier by computer
programs that can be implemented on laptop PCs or on the seismographs
themselves, but most are based on very simple models and are no substitute
for actually thinking about the data.
13.2.1 Intercept times
The reason why the back-extrapolated refracted-arrival lines in Figure 13.3
do not pass through the t-x zero on the T-D plot is that it takes time for the
energy to get down to the refractor and also to rise from the refractor to the
detectors. For horizontal refractors these two delays are the same (Figure
13.1), and each makes up half of the total intercept time, because each is
equal to the time taken, in Figure 13.4, for the energy to travel from S to Q
at velocity
V
1
, minus the time it would have taken to travel from P to Q at
velocity
V
2
. Simple trigonometry shows that this time is equal to:
[
d
/
(
V
1
.
cos
i
c
)
−
d
.
tan
i
c
/
V
2
]
