Geology Reference
In-Depth Information
resistance, since contact resistances are nearly always very much higher
than cable resistance. Steel reinforcement may be needed for long cables.
In virtually all DC surveys, two of the four cables will be long, and the
good practice in cable handling described in Section 1.4.2 is essential if
delays are to be avoided. Multicore cables that can be linked to multiple
electrodes are becoming increasingly popular because, once the cable has
been laid out and connected up, a series of readings with different com-
binations of current and voltage electrodes can be made using a selector
switch.
Power lines can be sources of noise, and it may be necessary to keep
the survey cables well away from their obvious or suspected locations. The
50 or 60 Hz power-line frequencies are very different from the 5 to 0.5 Hz
frequencies at which current is reversed in most DC and IP surveys, but
can affect very sensitive modern instruments, particularly in time-domain
IP (see Section 7.3). Happily, the results produced are usually either absurd
or non-existent, rather than misleading.
Cables are usually connected to electrodes by crocodile clips, since screw
connections can be difficult to use and are easily damaged by careless
hammer blows. Clips are, however, easily lost and every member of a field
crew should carry at least one spare, a screwdriver and a small pair of
pliers.
6.2.4 Generators and transmitters
The instruments that control and measure current in DC and IP surveys
are known as
transmitters
. Most deliver square-wave currents, reversing
the direction of flow with cycle times of between 0.2 and 2 seconds. The
lower limit is set by the need to minimise inductive (electromagnetic) and
capacitive effects, the upper by the need for an acceptable rate of coverage.
Current levels must be either preset or monitored, since low currents may
affect the validity of the results.
Power sources may be dry or rechargeable batteries, or motor generators.
The hand-cranked generators (
Meggers
) once used for DC surveys are now
very rare. Outputs of several kVA may be needed if current electrodes are
more than one or two hundred metres apart, and the generators used are
then not only not very portable but also supply power at levels that can be
lethal. Stringent precautions must then be observed, not only in handling
the electrodes but in ensuring the safety of passers-by and livestock along
the whole lengths of the current cables. In at least one case (in Australia),
a serious grass fire was caused by faulty insulation on a time-domain IP
transmitter cable.
