Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1941-09-27

make use of some other physical prop
erty or measurement as a check on
the Interpretation. Geophysical meth
eds are usually best adapted to the
location Of structures which are favor
able to particular types of deposits and
they cannot guarantee to accurately
outline the deposit nor can they give
an assay of the minerals present. They
are often more accurate in outlining
areas with a negative reaction and
thereby giving savings by cutting down
on the more expensive methods Of
prospecting than they are in indicat-
ing positive reactions In many min
ing regions a geophysical survey will
give the most favorable positions for
core drilling and make savings Of
thousands of dollars in the drilling
cost. The same is true in the inves-
tigation of damsites-more informa-
ton can be obtained If the core drill
holes are favorably placed
Geophysical methods are necessarily
limited in their scope. 1t is obvious
that we must have suitable differ
ence between the sought body and its
surroundings to make measurable diff
ferences, the sought body must be
favorably positioned, surface effects
may mask the effects of the deeper
structures, and the measurements
must be skillfully made and inter-
preted. It is possible to have several
good interpretations of the observed
facts and then to make tests which
will give a confirmation Of which in
depreciation is correct. The determine
ton of the depth at which the sought
body lies is one of the most difficult
and unsatisfactory problems. A small
shallow structure may give the same
physical effect at the surface as
larger one at greater depth.
Our geophysical methods are CLASH
fled into two large groups, those IN
which natural fields of force are meas
ured, and those in which the effects
of artificially applied forces are meas
ured. In the first group the gravita-
tional field of the earth, the magnetic
field of the earth, and naturally oc-
curing electric currents are the most
often used. In the second group we
use elastic waves produced by explo-
sions and electrical fields most often.
Depth control Is possible with applied
forces, but we are limited in our OPP
erations because of the power require
ments if great depth is to be reached.
A Few Methods
Since it is impossible to discuss all
of the geophysical methods in the time
allotted, I shall try to discuss few
of the commonly applied ones and to-
save time for questions. We shall first
discuss magnetic methods briefly.
Strange as it may appear, magnetic
is useful in the search for
placer gold. Stream action will often
leave big deposits of magnetic (black
sand) along old channels. This black
sand is of high specific gravity and
may have gold associated
The magnetic field of the earth will
be stronger above these deposits. Thus
we may find gold if it happens to be
associated with the magnetic. The dip
needle is a simple instrument which
S known to almost all prospectors and
4 has been used to find many black
and deposits. A more sensitive IN
;trument will do better work. One OF
.hese is the HotchkisS Superdip, sort
Of glorified dip needle. Another one
S the Schmidt magnetic balance. The
magnetic balance or magnetometer
lses a magnet which is balanced hor-
zontally for work with the vertical
of the earths magnetic
'ield. 1t is oriented with its axis IN
IN east-west direction. The region Is
raversed and readings taken. When
he constants of the instrument are
,nown the vertical component OF the
,arth's field can be computed with