Geology Reference
In-Depth Information
instruments had visual displays, on which the left-hand edge of the screen
defined the shot or impact instant and the time range was switch or key-pad
selected. Travel times were measured directly on screen, using a cursor that
could be moved across the screen while the time corresponding to its position
was displayed. Noise levels could be monitored by observing the trace in
the absence of a source pulse. Hard-copy was rarely generated but a digital
version of the signal could usually be stored in solid-state memory. A repeat
signal could either replace this or be added to it. Any num be r n of signals
could be summed ( stacked ) in this way, for a theoretical n improvement
( enhancement ) in signal/noise ratio.
Instruments that allowed signals to be displayed and summed were ob-
viously superior to mere timers, and could be used to study events other
than first arrivals. However, they were only useful in shallow refraction
work, since it is almost impossible to distinguish between direct waves, re-
fractions and reflections on a single trace. Hammer sources were universal,
since it would have been expensive and inefficient to use an explosive charge
to obtain such a small amount of data.
11.4.2 Multi-channel seismographs
Seismographs with 12 or 24 channels are generally used in shallow surveys,
compared to the 96 or more channels typical of deep reflection work. With
multiple channels, refractions and reflections can both be studied, and ex-
plosives can reasonably be used since the cost per shot is less important
when each shot produces many traces. Powerful microcomputers are incor-
porated into most modern instruments, with high-capacity flash drives for
data storage. Digital recording is virtually universal, display formats can be
varied and individual traces can be selected for enhancement, replacement
or preservation. Traces can be amplified after, as well as before, storage in
memory, and time offsets can be used to display events that occur after long
delay times. Filters can be used to reduce high-frequency random noise and
also the long-period noise, of uncertain origin, that sometimes drives the
traces from one or two geophones across the display, obscuring other traces.
Bewildering numbers of acquisition and processing options are available via
menu-driven software, and it is sometimes difficult, and time-consuming,
to persuade these instruments to carry out routine, straightforward survey
work. In practice, conditions in the field seldom allow full use to be made
of all the options available, nor is it an efficient use of the machine to have
it occupied with processing when it could be collecting fresh data. Digital
storage allows the traces to be preserved for further processing on ordinary
PCs, either at the field base or back in the office.
 
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