Geology Reference
In-Depth Information
Figure 12.2
Enhancement seismograph record showing curved alignments
of reflections (black dashed line). The earlier, negative polarity, picked event
was produced by the direct wave. The variable area presentation used is
popular for reflection work since it emphasises trace-to-trace correlations,
although some information is lost where traces overlap.
where
t
n
is the transit time through the
n
th layer at velocity
V
n
,and
T
n
is
the total transit time to the base of the
n
th layer. Interval velocities can be
calculated from RMS velocities using the
Dix formula
:
V
DIX
=
V
n
−
1
T
n
−
1
−
V
n
T
n
/
(
T
n
−
1
−
T
n
)
The subcripts
n
−
1and
n
denote, respectively, the top and bottom of the
n
th layer.
RMS
velocities are normally slightly higher than true average
velocities, since squaring the high velocities increases their influence on the
average. Significant errors can arise if RMS velocities are used directly to
make depth estimates but if the interfaces concerned are not horizontal these
