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

GEOPHYSICS DEFINED 17
can get water for a hundred dollars or whether it will cost them
thousands. Others sink thousands of dollars in local oil wells with
no real knowledge as to whether they have a chance of getting
their money back or not. States set up heavily financed councils
and commissions to entice industry to build plants within their
borders without even knowing .what potential mineral wealth they
have to offer or if they can furnish the newcomer with a water
supply. To be sure they tell of the railways and rivers serving
various localities, the abundant labor, the cheap fuel, the low
taxes; but there arc few areas where the geologic factors which
also enter into whether a venture will be a success or not are
known with the same degree of certainty. These examples show
the present need for a knowledge of what is beneath the surface
of the ground as well as what is at the surface.
Formerly mineral prospects were found on the basis of surface
outcrops and their subsurface extension had to be followed by a
very expensive program of shafting and drilling. Similarly oil
and coal deposits were located by gas leaks and outcrops, although
structural geologic intelTretations of surface outcrops
were later applied iu locating possible oil pools. Occasionally
then, as now, promoters persuaded people to drill for oil purely
on a hunch and New Jersey has quite a few such "oil" wells
which cost thousands of dollars. Contrast these methods, however,
with the prcse_lt ones used by the oil industry; with the
methods of development used on mineral prospects in the mining
areas of Canada, and on the great gold deposits of South Africa.
The work is still being done at the surface, but without drilling
except as a final step. The deposits studied are from a few hundred
feet to thousands of feet beneath the surface, and there are
few if any surface indicatim_s of their presence. These investigations
are being done cheaply and economically--in fact they cost
less than the few drill holes that are sunk to verify the indications
obtained by instruments which in 1900 or even 1920 were
not thought of in connection with geology. These instruments
have all been borrowed from the various branches of geophysics
and have been adapted to the problems of geology. Magnetometers
are being used to study not the broad relations of the earth's
magnetic field as in Terrestrial Magnetism, but to study the many
local magnetic disturbances in order to determine whether they
'are due to a sheet of trap rock, an area o_ serpentine, a gabbro
mass, or a potential iron ore deposit. Seismographs are used not
to study earthquakes, but rather the speed of sound through rock
from man-made earthquakes caused by the explosion of charges
NEW JERSEY GEOLOGICAL SURVEY