Geology Reference
In-Depth Information
Cable piles can be made portable by dumping cables into open boxes,
and on many seismic surveys the shot-firers carried their firing lines in this
way in old gelignite boxes. Ideally, however, if cables are to be carried from
place to place, they should be wound on properly designed drums. Even
then, problems can occur. If a cable is being unwound by pulling on its free
end, the drum will not stop simply because the pull stops, and a free-running
drum is an effective, but untidy, knitting machine.
A drum carried as a back-pack should have an efficient brake and should
be reversible so that it can be carried across the chest and be wound from a
standing position. Some drums sold with geophysical instruments combine
total impracticality with inordinate expense and are inferior to garden-centre
or even home-made versions.
Geophysical cables exert an almost hypnotic influence on livestock, and
cattle have been known to desert lush pastures in favour of midnight treks
through hedges and across ditches in search of them. Not only can a survey be
delayed but a valuable animal may be killed by chewing on a live conductor.
Constant vigilance is essential.
1.4.3 Connections
Crocodile clips are usually adequate for electrical connections between sin-
gle conductors. Heavy plugs must be used for multi-conductor connections
and are usually the weakest links in the entire field system. They should be
placed on the ground very gently and as seldom as possible and, if they do
not have screw-on caps, be protected with plastic bags or 'clingfilm'. They
must be shielded from grit as well as moisture. Faults are often caused by
dirt, which increases the wear on the contacts in socket plugs, which are
almost impossible to clean.
Plugs should be clamped to their cables, since any strain will otherwise
be borne by the weak soldered connections to the individual pins. Inevitably,
cables are flexed repeatedly just beyond the clamps, and wires may break
within their insulated sleeves at these points. Any break there, or a broken or
dry
joint inside the plug, means work with a soldering iron. This is never easy
when connector pins are clotted with old solder, and is especially difficult if
many wires crowd into a single plug.
Problems with plugs can be minimised by ensuring that, when moving,
they are always carried, never dragged along the ground. Two hands should
always be used, one holding the cable to take the strain of any sudden pull,
the other to support the plug itself. The rate at which cable is reeled-in
should never exceed a comfortable walking pace, and special care is needed
when the last few metres are being wound on to a drum. Drums should be
fitted with clips or sockets where the plugs can be secured when not in use.
