Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1937-06-26
GEOPHYSICS ARRIVES IN MONTANA

tiOn discussed the various types of geo.
physical instruments. While the several
instruments find use in particular dis
tricts, 1t is believed that the reflection
method is by far the most important here
in Montana. Briefly, this is the prin-
ciPle and method, as we understand it:
shallow hole is drilled to approx
mate surface water level. In this hole is
placed a charge of explosives. Record
ing machines which will measure vi
bratiOns, are set up at various intervals
These recording machines are similar
to the device used to record earthquakes
and the earthquake principle is applied
When a shot Is fired, the vibrations tray
ell downward until they strike a hard
surface from which they are reflected
upward.
The recording instruments register the
instant OF the explosion and thereafter
measure the speed with which the vdbra-
tons are reflected to the surface of the
earth. The speed of wave transmission
is proportional to the density of the
various earth strata, as well as the
depth. The rate of travel through gran
lite and solid limestone is very rapid;
through shales and sandstOnes it is very
slow: not much faster than the travel
Of sound through the air.
A telemeter illustrates the principle:
II a stop watch is started at the instant
of a flash of lightning and stopped when
the crash of thunder sounds, it will
show how far away the lightning flash
occurred. The speed Of light is instan-
taneOus, TO all intents and purposes. 1t
takes about four and four-fifths seconds
for sound to travel a mile.
Scientists know how long IL takes for
a vibration to travel through different
geologic formations. BY electrical corr
ing they determine a great deal about
the formations of given district. With
modern instruments they can lower a
an
(a) resistivity OF the rocks (b) rock
pOrOsity (c) location of faults (d) direc-
tion of dip of strata and (e) water in
cursiOn. Thus, modern geOphysical crews
take advantage of all known data through
drilled wells and thereby learn how to
interpret the vibrations set up ON the
recording Instruments.
The recording instrument works on the
principle of radio. The vibrations are
very slight, but they are transmitted
through radio tubes which ansplify them,
Just as radio vibrations are amplified to
provide volume The signals so am
plified by the seismOgraph, as a record
ing instrument is known, are recorded on
Photographic paper by tiny electric
beam which swings back and forth at a
greater or lesser rate Of speed, depending
on the rate of the vibrations.
AS we understand the Sweetgrass Arch
the Rocky Mountains started to come
up in this Arch, before the surface Of the
earth gave way over i'm Idaho and created
the great Lewis Overthrust and the main
range of the Rockies. Had Idaho held
out a few million years longer, there
might have been a mountain sticking its
head up where Kevin-Sunburst is now
located. Another mountain would have
appeared at Pondera and another mOun-
tain at Bannatyne. There would have
been other mountains throughout the
Sweetgras Arch, but the relief Of the
strain ON the earth's surface provided by
the uprising OF the Rockies halted the
uprising granite masses i'm the Arch at
considerable depth, but some of the
bumps" now show on the surface of the
ground, as at Kelvin:
So it is possible today for the geo.
physicist TO cause vibrations which will
bounce off the tops of these granite
masses-these unborn mOuntains-and
reveal a structural condition. Whether
a break i'm the lime, known as a fault,
viii be recorded on these delicate instru-
ment, remains to be seen. The old idea
on which we have been fed for the last
20 years is: ut won't work in Montana"
We talked with a geOlogist of the