Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1939-11-11

ing and Metallurgy, the official or
gan of the American Institute oil
Mining and Metallurgical Engineers
When geophysical methods were
first employed in the search for
ore deposits and oil accumulations,
it was hoped that they would pro-
vide a direct means of locating
such concentrations. Maenetized
needles were used long ago in pros
pecting for magnetic iron ore, and
soon after the discovery of galvan-
ism in about 1890 attempts were
made to find concentrations OF val
liable minerals electrically Evil
dence indicating that the early
geophysical instruments We're re-
garded merely as improved divin-
ing or dowsing rods Is to be seen
in the names mineral finders,
lore finders" etc, that were ap-
piled to them Today. appellations
of this kind raise serious doubts
in informed minds as to the intelli-
gence and integrity of individuals
advocating the use Of such euphem-
istically named methods or instru-
ments, for experience and research
have shown that. geOphysically. 1t
is never a question OF directly To-
eating a given mineral. but merely
of delimiting its possible occurrence.
The fields of geophysics and geol-
ogy overlap to a considerable ex-
tent: the former is primarily con-
cerned with the physical properties
of rocks and minerals. and the
- a
stitutes a large part Of the latter.
However, the geologist is concerned
only with the physical properties
Of rocks and minerals which mani
fest themselves to his eyes: in this
respect he is a highly specialized
type of geophysicist The scope of
his work is confined to his range of
vision, although he may extend its
limits by means of the microscope
and by such expensive expedients
as drilling, test-pitting, and strip
ping Among the tools of his pro-
fession are the hand lens, micro
scope. transit and level, compass
and plane table The geophysicists,
on the other hand. investigate the
magnetic, electrical. and elastic pro-
perties of rocks and minerals as
we'll as their densities. These pro-
perties have no visible physical
manifestations but their character
istics may be studied by means OF
suitable methods to varying, and
often considerable depths beneath
the land surface The apparatus
used in this work consists OF var
ious types of magnetometers,
electrical instruments, seismographs,
and gravimeters as well as devices
to create or induce artificial energy
fields and to propagate impulses
within the ground.
At first it was thought that, by
adequate experimentation and the :
amassing Of sufficient Information.
geophysics could be developed into
an exact science so that the cor
rect Interpretation of geophysical
anomalies observed in the field /
could be determined by the appli- I
cation of suitable mathematical for
mulas The physical properties
of minerals. rock. and ore samples
were therefore carefully investigated
and accurately measured. At the |
same time countless experiments /
were carried out on small-scale !
models in which different geological /
and combinations of con- !
ditions were duplicated, and the
geophysical phenomena thus arti-
ficially reproduced were studied.
Methods which functioned perfectly
in these experiments were develop
ed as well as criteria for the inter-
probation of the results obtained
by their use. However. when the
scene of experimentation was trans
ferred to the field, perplexing re-
suits were often obtained and con-
experimental work on
rocks and ores in their natural
states was necessary before the
reason for the anomalous geophysi-
cal reactions became apparent
Highly accurate determinations
of the nhysical properties of min
erals and mineral combinations. car
ried out upon specimens in labora-
tories, were discovered to be mis-
leading for the purpose of applied
Such measurements
stress the physical contrasts be-
tween various materials beyond the
limits actually occurring in the field.
In practice. the physical properties
Of minerals and rock samples are OF
less Importance than
those of whole rock formations or
bodies of rock in stu; the accuracy
OF individual property determina-
tons is less important than a large
number OF approximations which
give good average values for rock
as they occur in nature Until
recent times. the study of the ply
steal properties of large geological
units. in place, has been difficult
to accomplish.
Dame Nature is a creature of I
such infinite variety that attempts
10 reproduce her moods artificially 1
can never be wholly successful. For s
example, experimentation has shown I
that the conductivities of rocks vary
with the amount and composition Of
the moisture they contain. However, !
The number of conditions affecting
these two factors was not appre- !
slated until considerable field work /
had heeu carried out. The compo- :