Geology Reference
In-Depth Information
often energetic, waves usually prevent the use of any event other than the
first arrival of energy.
Noise that is not generated by the shot is termed
random
. Movements of
traffic, animals and people all generate random noise and can, to varying
extents, be controlled. It should at least be possible to prevent the survey
team contributing, by giving warning using a whistle or hooter.
Random noise is also produced by vegetation moving in the wind and
disturbing the ground. The effects can be reduced by positioning geophones
away from trees and bushes, and sometimes by clearing away smaller plants.
Significant improvements can often be achieved by moving particularly
noisy geophones a few centimetres. Placement is also important. It may not
be easy to push a spike fully home in hard ground but a geophone that is
just a couple of centimetres above the ground vibrates in the wind.
11.3.5 Seismic cables
Seismic signals are carried from geophones to recorders as varying electric
currents, in cables that must contain twice as many individual wires as there
are geophones. Wires are necessarily packed very closely and not only can
external current carriers such as power and telephone cables induce currents,
but also a very strong signal in one wire can be passed inductively to all the
others. This '
cross-talk
' can be particularly severe from the strong signals
produced by geophones close to the shot-point, and it may even be necessary
to disconnect these to obtain good records on other channels.
The amount of cross-talk generally increases with the age of the cable,
probably because of a gradual build-up of moisture inside the outer insulating
cover. Eventually the cable has to be discarded.
Cables and plugs are the most vulnerable parts of a seismic system and
are most at risk where they join. It is worth while being very careful. Re-
soldering wires to a plug with 24 or more connections is neither easy nor
interesting.
Most cables are double-ended, allowing either end to be connected to the
receiver. If a wire is broken, only the connection to one end will be affected
and the 'dead' channel may revive if the cable is reversed. All too often,
however, other dead channels are discovered when this is done.
11.4 Recording Seismic Signals
Instruments that record seismic signals are known as
seismographs.
They
range from timers that record only single events to complex units that
digitise, filter and store signals from a number of detectors simultaneously.
11.4.1 Single-channel seismographs
Rudimentary seismic 'timers', which simply displayed the travel time of
the first significant energy pulse, were once popular. More sophisticated
