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

GEOPHYSICAL METHODS OF EXPLORATION 41
quires a spread of over half a mile for the geophones for most
work, and waste land in which to explode the dynamite. Despite
this handicap, however, it has been possible to carry out investigations
in nlost areas.
ELECTRICAL
M_.THODS
The term electrical methods embraces so many different techniques
and kinds of apparatus that they have to be divided into
groups. Some of the first used were the self-potential or natural
current methods. Their operation depends on the existence of a
natural galvanic field resulting from the action of ground water
ou certain metallic sulphide ore-bodles which form a naturM
battery and reveal the presence of the ore-body by the resulting
potmJtial field (see Fig. 11). Other methods take into accmmt
the fact that many ore-bodies are good conductors and if current
is introduced into the ground so as to flox_ between a pair
of point or line electrodes, the presence of the buried conductor
can be determined by plotting the eqnipotential lines to see if the
field between the e)ectrodes is disturbed or not. If only homogeneous
material is present a symmetrical potential field is obtained
betweeu the electrodes; but if a conductor is present, the
equipotential lines deflect away from the conductor and its presence
is immediately discernable. (See Figs 12 and 13).
Other methods make use of resistivity measurements: that is,
current is introduced into the ground through a pair of electrodes
and the difference in potential between another pair of electrodes
is noted. From these data the apparent resistivity of the ground
is computed by using the relation that resistance is equal to the
potential drop divided by the current. In making resistivity measurements
it has been found that best results can be obtained by
certain arrangements of electrodes. In one o£ these, the Wenner
configuration as it is called, the four electrodes are placed in a
line and spaced at equal distances from each other. The two current
electrodes are in the end positions and the potential electrodes
in the center position (see Fig. 14). By moving this string
of electrodes along a line with an overlap each time, it is possible
to detect suhsurface ebaz_gcs due to changes in the apparent
resistivity. Since the depth of penetration of the electric
field is about equal to the electrode spacing, the method is also
useful for determining the depths to geologic horizons, This is
done by keeping the spread centered at one spot and progressively
increasing the electrode spacing m_til a "break" is obtained
ia the resistivity curve. The spacing at this point is then
NEW JERSEY GEOLOGICAL SURVEY