SEISMIC APPARATUS 327
The amplifier is very frequently combined with a filter. This
is desirable because it is found that much clearer reflections are
indicated when a selected frequency range is amplified. Usu-
ally the desirable reflections are in the range of 25 to 60 cycles per
second. Lower frequencies, particularly the "ground roll,"
which is the result of a slow-speed surface wave, are often present
and cause undesirable impulses on the records unless removed.
The frequency characteristics of the reflections vary with the
nature of the underground materials so that different filtering
may be desirable in different areas. Therefore, some filter sys-
tems are made adjustable to pass the most favorable frequencies
for each field condition. The over-all frequency characteristics
of the system depend on the frequency response of the detector
itself as well as the filter. Therefore, the electrical design of an
efficient amplifier-filter system must take into account the elec-
trical and mechanical characteristics of the associated detector
with which it is intended to be connected. In fact, in some
instruments the chief filtering action is in the detector itself,
rather than in the electrical system.
Variable Gain Control.—In recent years, it has become com-
mon practice to use some sort of variable gain, or volume, control
in the amplifier. The variable gain may be an "expander"
by which the amplification is increased with time after the first
impulse is received, to compensate for the lo\\er amplitude of
later reflections. The alternative volume control is similar to
the "automatic volume control/7 commonly used on radio
receivers to give a nearly constant output (loudness) with a
widely varying strength qf the received signal. Such control is
desirable on the seismic amplifier so that strong impulses from
shallow reflections can be read on the same record with very
weak impulses from deep reflections.1 Without such control, it
may be necessary to fire several shots of different magnitude and
make different records to show shallow and deep reflections.
Oscillographs.—The output of each amplifier is connected
to an oscillograph, or galvanometer unit. Two types of unit
have been widely used.
One type uses the principle of the string galvanometer in
which a very fine wire (or metal-coated quartz fiber) is lightly
stretched in a strong magnetic field. When a current passes
1 Norman, 1939, p. 13.