Geology Reference
In-Depth Information
Figure 13.7
(a) Effect on travel times of a bedrock depression. The arrivals
at G
3
of energy from S
1
and S
2
are delayed by approximately the same amount
(a
/
V
1
or b
/
V
1
). As in all the diagrams in this chapter, vertical exaggeration
has been used to clarify travel paths. (b) A more realistic, although still
vertically exaggerated, picture of the likely relationships between geophone
spacing and refractor depth and gradient.
The approach described above generally works far better than the very
qualitative 'proof' (and the rather contrived positioning of the geophones in
Figure 13.7) might suggest. Changes in slopes of difference lines correspond
to real changes in refractor velocity, so that zones of weak bedrock can be
identified.
The importance of long shots is obvious. Short-shot difference lines can
generally not be used, because the part of the spread over which the first-
arrivals from both shots have come via the refractor will be limited and
may not even exist. However, it is sometimes possible, especially when
there are centre shots, for the differencing technique to be also applied to an
intermediate refractor.
Difference lines are plotted using an arbitrary time-zero line placed where
it will cause the least confusion with other data (see Figure 13.9). Computers
can plot difference times as well as arrival times, but if graph paper is being
used instead, differences are easily measured graphically and can then be
transferred directly onto the T-D plot using dividers, or a pencil and a
straight-edged piece of paper.
13.2.5 The Generalised Reciprocal method
In Example 13.1 the reciprocal-time method, or Generalised Reciprocal
method, is used to interpret a four-shot refraction 'shoot'. This was the best
method available before computers capable of tomographic modelling (see
Section 13.2.6) became common in field camps. It still provides insights
that are hard to obtain from a computer package.
The
reciprocal time
,
t
R
, is defined as the time taken for seismic energy to
travel between long shots positioned beyond the opposite ends of a spread.
