Geology Reference
In-Depth Information
25
Base refractor lines
20
15
Intermediate
refractor line
10
t
i
Direct
wave lines
05
0
W
-10
E
10
20
30
40
metres
Figure 13.3
Time-distance plot for the record shown in Figure 13.2. There
appears to be an additional refractor to the west of of the shot-point, con-
firming this as a location at which there is a rapid change in subsurface
conditions. The 'intercept time',
t
i
, for the refractor mapped in the east is
given, to a first approximation, by the point where the 'best-fitting' straight
line through the refracted arrival times cuts the vertical line through the
shot-point. This point has then been used to decide which of the western
arrivals have come via which refractor.
Centre and other additional shots are used much less than they deserve.
The extra effort is generally trivial compared to the work done in laying
out the spread, and the additional and possibly vital information is cheaply
obtained.
13.1.6 Annotation of field records
The several dozen records produced in a day's work that includes repeats,
checks and tests as well as the completion of a number of different spreads
must be carefully annotated if confusion is to be avoided. Annotations
(
metadata
) should obviously include the date and the name of the observer-
in-charge, along with the survey location and spread number. Orientation
should be noted, and the position of Geophone 1 should be defined. Unless
the geophone spacing is absolutely uniform, a sketch showing shot and
geophone locations should be added. If the interval between timing lines on
the records can be varied and/or variable time offsets can be applied, these
settings must also be noted. Modern instruments store data, including the
instrumental settings, digitally but hard-copy records can be (and should be)
generated in the field by most of the seismographs now used for shallow
refraction surveys.
Other items are optional. Amplifier gains and filter settings are not often
recorded in notebooks but such information can be useful. The number of
