Geology Reference
In-Depth Information
shots or impacts combined in a single record can also be important. And, of
course, features such as the use of S-wave geophones at some points or pe-
culiarities in the locations of some of the geophones should always be noted.
Much of the metadata listed above will be printed on any hard-copy record
generated, provided that they have first been entered into the machine. This
is often a more tedious, and more error-prone, process than simply writing
them on each record by hand, but is usually vital for identifying the individual
records in digital memory.
13.1.7 Picking refraction arrivals
Picking first-arrivals on refraction records is still more reliably done by hand
than by computer program, but may be difficult if the signal-to-noise ratio is
poor. Some of the later peaks and troughs in the same wave train are likely
to be stronger (Figure 13.2), and it is sometimes possible to work back
from these to estimate the position of the first break. However, because high
frequencies are selectively absorbed in the ground, the distance between the
first break and any specific later peak gradually increases with increasing
distance from the source. Furthermore, the trace beyond the first break is
affected by many other arrivals as well as by later parts of the primary wave
train, and these will modify peak and trough locations. Using later features
to estimate first-arrival times is always a poor substitute for direct picking.
13.1.8 Time-distance plots
The first step in any refraction interpretation is to enter the arrival times
(usually first-arrival times) on to a T-D plot (Figure 13.3). Times on these
plots are measured along the vertical axes and distances along the horizontal
axes, and the gradient of any line is equal to the reciprocal of a velocity.
If the arrival times lie on a number of clearly defined straight-line seg-
ments, best-fit lines may be drawn, defining velocities. These are not actually
necessary if the Generalised Reciprocal interpretation method (see Section
13.2.5) is being used, and will be difficult to define if, as in Figure 13.3,
the arrival times are irregularly distributed because of variations in refractor
depth. It is often best to draw lines through only the direct-wave arrivals
(which should plot on straight lines), leaving refracted arrivals either un-
joined or linked only by faint lines between adjacent points.
In Generalised Reciprocal interpretation, all the data for a single spread are
plotted on one sheet that has a working area covering only the ground where
there are actually geophones (see Figure 13.9, accompanying Example 13.1).
It is not necessary to extend the plot out to the long-shot positions. Since
at least four sets of arrivals have to be plotted, as well as a set of time
differences, different colours or symbols are needed to distinguish between
data sets.
