Montana oil and mining journal. [volume] (Great Falls, Mont.) 1931-1953, 1941-09-27
MODERN PROSPECTOR FINDS
ORE BODIES WITH ELECTRIC
AND MAGNETIC INSTRUMENTS
Continued from Page TWO

Continued from Page TWO
work in a similar manner excepting
that positive potentials are observed
above the deposit.
The POtentiOmeter
The instrument for the spontaneous
polarization method Is quite simple
The method requires an accurate po-
tentiometer which does not draw any
power from the ground. These poten-
tiometers are usually built to the
specifications of the owner, although
Leeds Northrup make one which is
very satisfactory. 1t should be porta-
BILE and light. The disadvantage of the
conventional potentiometers is that
they contain a Weston standard cell
that is easily damaged by frost or by
a mistake in closing the keys. For
electrodes one must use non-polar-
izing type It is usual to use unglazed
pots containing a saturated solution
of copper sulfate with copper cleo
trodes Contact with the ground Is
made by setting the electrode on
bare spot or by scraping the dry soil
away. In very dry places it IS oc-
caSionally necessary to wet the ground.
The wire is of the stranded variety
and should be rubber insulated. 1t Is
kept on a spool equipped with a slip
ring for contact. High resistance or
open circuits are the common troubles.
They are easily tested since the po-
tentiometer appears to be balanced at
any setting If the circuit is open. One
thousand feet of wire is usually SUI
ficient. The apparatus we have here
for demonstration has value Of about
$3.50.
The spontaneous polarization meth
cd is often difficult to use in mines
which are working or which have been
worked on a large scale. The direct
current installations give interference
with the natural earth currents II
there are steel rails or other metals
present in the mine or near 1t they
will produce earth currents and there
is no way of telling for sure which
currents are produced by the ore body
and which currents are produced arti-
ficially. Electric railroads often pro-
duce interferences over large areas.
Estimating the cost of geophysical
surveys is very difficult. 1t probably
will cost $50 per day for a Self-poten-
tial outfit and crew with costs higher
for resistivity work. The number Of
acres which can be covered per day
depends upon the amount Of detail
work to be done and upon the ease
with which the apparatus can be
moved from station to station. 1t Is
probably a safe estimate to say that
so to 100 stations can be covered In
a day with the higher number OF
stations applied to detail work. Mag
netometer work is less costly per day.
In oil geophysics approximately one
dollar per station with stations mile
apart is a fair estimate. For placer
work the cost per station is probably
less. However, in the oil survey the
man and instrument go from station
to station in a car, while in placer
work it is necessary to carry the in
strument through brush and over
rough terrain. In the past the usual
rule for custom surveys has been all
that the traffic would bear. The con-
tract prices depend upon the competi-
ton and the ability of the customer
to pay. A geophysicist with a large
amount of experience will make an
apparently greater charge, but his cost
win probably be less in the long run.