Geology Reference
In-Depth Information
Fieldworthiness:
Are the control knobs and connectors protected from ac-
cidental impact? Is the casing truly waterproof? Does protection from
damp grass depend on the instrument being set down in a certain way?
Are there depressions on the console where water will collect and then
inevitably seep inside?
Automation:
Computer control has been introduced into almost all the instru-
ments in current production. Switches have almost vanished, and every
instruction has to be entered via a keypad. This has reduced the problems
that used to be caused by electrical 'spikes' generated by switches but,
because the settings are usually not permanently visible, unsuitable val-
ues may be repeatedly used in error. Moreover, simple operations have
sometimes been made unduly complicated by the need to access nested
menus. Some instruments do not allow readings to be taken until line and
station numbers have been entered, and in extreme cases may demand to
know the distance to the next station and even to the next line!
The computer revolution has produced real advances in field geophysics,
but has its drawbacks. Most notably, the ability to store data digitally within
data loggers has discouraged the making of notes on field conditions where
these, however important, do not fall within a restricted range of options.
This problem is further discussed in Section 1.7.
1.4.2 Cables
Almost all geophysical work involves cables, which may be short, linking
instruments to sensors or batteries, or hundreds of metres long. Electrical
induction between cables (electromagnetic coupling, also known as
cross-
talk
) can be a serious source of noise.
Efficiency in cable-handling is an absolute necessity. Long cables always
tend to become tangled, often because of well-intentioned attempts to make
neat coils using hand and elbow. Figures of eight are better than simple
loops, but even so it takes an expert to construct a coil from which cable can
be run freely once it has been removed from the arm. On the other hand,
a seemingly chaotic pile of wire spread loosely on the ground can be quite
trouble-free. The basic rule is that cable must be fed on and off such piles in
opposite directions; that is, the last bit of cable fed on must be the first to be
pulled off. Any attempts to pull cable from the bottom will almost certainly
endindisaster.
Cable piles are also unlikely to cause the permanent kinks that are often
features of neat and tidy coils and that may have to be removed by allowing
the cable to hang freely and untwist naturally. Places where this is possible
with 100-metre lengths are rare.
