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

GEOPHY_qICAL METHODS OF EXPLORATION 45
silence emmot always be used directly to indicate the direction of
an ore-body. There are numerous modifications of these methods,
one of which is used to locate points of equal potential and thus
outline the subsurface body much as with electrical potential
measurements. The methods have met with their greatest success
in locating sulphide ore-bodies, al_d another common use has been
in locating buried pipe lines.
RADIO-ACTIVE
MEASUREMENTS
The radioactive methods all depend upon natural radioactive
emanations, particularly gamma rays, by eertaiu minerals or
collections of minerals which are more radioactive either naturally
or through secondary concentration than the adjacent material.
The instrument used has been the G4iger Counter which is
an ionized chamber eoutainiag two electrodes between which
there is a discharge whose frequency wuqes according to the external
ionization caused by the proximity of radioactive materiM.
For instance, in prospecting for radioactive ores such as the
earnotite deposits in the petrified forests of Arizona, the Counter
will diseharge more frequently as a mass of this mineral is approached,
It has been used similarly in prospecting for the pitchblende
deposits in the northern part o£ Canada. Another more recent
use has been in locating the position of sands and also old
erosion surfaces in wells by lowering such an instrument inside
the easing. In this respect it has proven superior to electrical
logging since cased wells cannot be exanfined by the latter
method. This technique is commonly referred to as Gannna Ray
logging and in general it is found that sands are characterized by
a tow gamma ray count as compared with shales and this permits
their definition as well as with a potential log'.
CONSIDERATIONS WHICH DETERMINE ]'HE CHOICE OF
GEOPHYSICAL
METHODS
The methods described are the ones most generally used in subsurface
geological exploration. It, frequently happens though that
the geophysical data obtained in areas where there are no well
data cannot be adequately interpreted without such data and the
cost of the bm'e holes needed to clarify the geophysical findings
has to be added to the exploration cost. This cost is trivial
though as compared to that of a complete drilling program and
such a combination of drill hole and geophysical data gives a
knowledge of a large area which would not be economically pos-
NEW JERSEY GEOLOGICAL SURVEY