Geology Reference
In-Depth Information
One disadvantage of analogue recording was that the range of amplitudes
over which data could be recorded with acceptable accuracy (the dynamic
range ) was limited at low amplitudes by tape noise and at high amplitudes
by magnetic saturation. The recorded signal amplitudes were therefore kept
roughly constant by applying automatic gain control (AGC) before record-
ing, but this always distorted the signal. Digital recording has eliminated the
need for AGC, because of the very large dynamic range available when data
are stored as fixed precision numbers plus exponents (Example 11.2).
Example 11.2 Dynamic range and digital recording
In digital systems, data are recorded as numerical values plus exponents ,
which are the powers of some other number by which the significant figures
in the numerical value must be multiplied. Thus, the values
46 789
and
0
.
000 046 789
which demand a very large dynamic range of an analogue storage system,
can be written in engineering notation, which uses powers of 10, as:
4 . 6789E + 4 d4 . 6789E 5
The percentage accuracy is the same in both cases. Digital data are usually
recorded in binary formats, the exponents being powers of 2 between 128
and + 127, i.e. between roughly 10 38
and 10 + 38 .
11.4.3 Seismograph records
Most modern seismographs can provide optional hard-copy as well as screen
displays. The refraction survey record in Figure 13.2 shows the signals
recorded by 24 geophones at points successively further from the source,
with the traces from distant geophones amplified more to compensate for
attenuation. Inevitably, amplifying the signal also amplifies the noise.
In the field, arrival times can be estimated from the screen but this is
never easy and seldom convenient. On the other hand, hard copies produced
directly from the instrument are often of rather poor quality. This is especially
true of dot-matrix outputs such as those in Figure 13.11, where the matrix
size causes irregularities in what should be smooth curves. It is worthwhile
having a reasonable printer at the field base, coupled to a laptop PC loaded
with processing software. It would be unwise, however, not to produce, and
preserve, the field hard-copy
 
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