Bulletin 54. Geophysical Methods of Exploration and their ...

GEOPHYSICAL METHODS OF EXPLORATION 57
There are several other differences in the two techniqnes. For
instance, it is eustmuary in reflection work to place the shot much
deeper than in refraction work, an effort being made to get below
what is commonly termed the weathered layer, or zone affected
by ground water and hence altered or weathered. This is done to
avoid possible errors introduced by the poor transmission characteristics
of this zone. Due to the short wave paths in reflection
work, explosive charges are generally only a few pounds of dynamite_
whereas in refraction work perhaps a hundred pouuds and
even on occasions tons of dynamite have to be used. For refraction
depth investigations involving horizons about 5,000 feet
deep, the writer's experience has been that about forty pomlds
of dynmnite is sufficient except in very abnormal eases where
the presence of peat or other high-energy absorbent material
makes very large charges necessary in order to get a record of
the explosion. The instruments and the arrangement used in
both methods arc very similar with the exception that it is necessary
in reflection work to have a filter hi the circuit which will
permit only certain frequencies to pass so that the reflections can
be recognize(] on the record. The time factor, which is so important,
is determined by recording both the instant of the explosion
and the arrivals of the waves at the geophones on a
photographic tape. Time lines are put on this tape by the breaking
of a reflected beam of light by the vibrations of a tuning
fork with a frequency of 200 vibrations per second. On these
are imposed traces from a series of galvanometers with mirrors
attached which reflect a beam of light onto the tape. Each
galvanometer is connected to a geophone which in reality is a
small electrical seisnmgraph and the small changes in current
caused by the arrival of the waves is amplified and passed on to
the galvanmneters which deflect accordingly and cause a visual
record of the arrival of the waves to be obtained. In refraction
work the instant of firing is obtained by breaking the circuit of
a radio transndtter at the point of firing and this break is picked
up by a radio receiver at the recording end and put on the tape
through another galvanometer. In reflection work a telephone
line is used to transmit the firing instant to the recording apparatus.
For diagrams of the apparatus and field layout see
Figures 7 and 8.
The arrival tbnes are taken off the photographic tape by counting
the tuning fork time lines from the shot instant to the wave
arrivals. These time data can then be plotted on a time-distance
graph and the velocities of the various horizons determined (see
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