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

GEOPHYSICS DEFINED 19
the surrounding rock, au indirect approach is frequently necessary.
Thus oil is not hunted directly, but iustead the geologic
structural conditions are songht which will permit oil, if present,
to segregate into wbat are commonly known as pools. That is, a
structural trap is bunted which involves a displacement of one or
more "key" geologic horizons which are known from well data
to bear a definite relation to oil reservoir sands. This same sort
of technique is also used in locating the position of artesian water
sands since it is difficult to work with the water directly except
in special cases. Of course the information about the geologic
structure obtained by geophysical methods can also be obtained
from bore hole and well data, but it is generally economically
impossible to sink a muffieient number of deep test wells or bore
holes in an mmxplored area to show what is beneath the surface
far even a small area--let alone a large one--and as a result this
method is geuerally reserved to test geophysical indications or to
develop known mineralized areas.
CHOICE OF _,/[ETHODS ]:_EST ADAPTED TO EACFI PROBLEM
In choosing the geophysical nlethod to be applied to any geological
problem the general nature of the problenl has to be considered
first. For instance, if a survey were being carried out to
determine the possible iron ore reserves in the New Jersey Highlands,
the magnetic method of prospecting would immediately
suggest itself since these ores are known to be highly magnetic.
They could also be mapped quite st_tisfactorily by using any of
the electrical methods since the ore is largely magnetite, a mineral
which is a good electrical conductor as compared with the
enclosing country rock. Also, since magnetite has a much higher
density thau the eomltry rock, these deposits might be slapped
by gravitational methods. Electro-magnetlc methods could probably
be used as well. In this area, though, the first choice would
be the msgnetie method, since the greatest difference in instru-.
ment readings would be expected with this method, For example,
the magnetic susceptibility of magnetite is 97,350 c.g.s, units,
while that for the average country rock is only around 1000. In
other words a factor of change of about 97 may be expected in
the readings as one passes from country rock over a magnetite
ore-body. In comparison, the factor of change for the electrical
method would be about 8 to i and for the other nlett_ods mentioned
it would be even less.
Consider now an artesian water supply problem: say that oi
finding the depth to a certain water horizon. Two geophysical
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