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survey equipotential
lines or potential profiles

produce inductive
methods and refraction methods

supply formation
boundaries

apply alternating
current

induce high
radio frequency

radiate potential
difference

furnish frequency
bands

observe point
electrodes

compare emission
characteristics

detect
potential drop

56. Read the text, do the exercises. Electrical Methods

Mineral
deposits and geologic structures may be mapped by their reaction to
electrical and electromagnetic fields. These are produced by either
direct or alternating
current,
except where ore bodies spontaneously furnish
their own electrical field (self-potential
methods).
Electrical energy may be supplied to the ground by contact or by
induction. Three main groups of electrical methods may be
distinguished: (1) self-potential, (2) surface-potential, and (3)
electromagnetic methods. Frequently the first two groups are combined
into one group of potential methods; the electromagnetic methods are
usually subdivided into galvanic-electromagnetic and
inductive-electromagnetic.

Four
frequency
bands
may be used in connection with alternating current electrical
prospecting: (1) low frequencies of from 5 to about 100 cycles; (2)
the audio-frequency
range of from 200 to 1000 cycles; (3) high frequencies of from 10 to
80 kilocycles; and (4) radio
frequency
of from 100 kilocycles to several megacycles. The low frequency range
is applied in most potential methods; the audio-frequency range is
used in some potential and most electromagnetic methods; the
high-frequency range in the high-frequency electromagnetic methods;
and radio-frequency in the radio methods of electromagnetic
prospecting. The application of high radio frequencies is limited
owing to their lack of depth penetration; of greatest importance are
the audio frequencies and the low frequencies. In a number of
respects, electrical methods are similar to seismic methods;
comparable to refraction
methods
are resistivity
and the potential-drop-ratio
methods;
inductive
methods as applied to the mapping of horizontal beds are comparable
to reflection
methods
but lack their resolving power.

Self-potential
method.
The self-potential
method
is the only electrical method in which a natural field is observed;
its causes are spontaneous electrochemical phenomena. These phenomena
occur on ore bodies and on metallic minerals and placers;
they are produced by corrosion of pipe lines and on formation
boundaries
in wells by differences in the conductivity of drilling fluid and
formation
waters.
Ore bodies whose ends are composed of materials of different solution
pressure
and are in contact with solutions of different ion concentration, act
as
wet cells
and produce an electrical field which can be detected by surveying
equipotential
lines
or potential
profiles.
For the mapping of equipotential lines, a high-resistance
milliammeter is connected to two unpolarizable
electrodes
are used. One is kept stationary
and the other is moved until the current vanishes. At that point the
electrodes are on an equipotential line.

Equipotential-line
and potential-profile methods.

Equipotential
lines of the current

When
a source of electrical energy is grounded at two points, an
electrical field is produced. Distortions
of this field result from the presence of bodies of different
conductivity; good conductors will attract the lines
of flux,
and vice versa. As it is difficult to survey these lines of flux,
lines of equal potential, that is, lines along which no current
flows, are mapped instead. In practice power is supplied to two
grounded electrodes from an alternating current
generator.

Resistivity
methods.

Equipotential-line
methods, while useful for the mapping of vertical or steeply
dipping
geologic bodies, are not suited to the investigation of horizontally
stratified ground. Conversely, resistivity
methods
are applicable to depth determinations of horizontal strata and the
mapping of dipping
formations.

In
resistivity procedures not only the potential difference between two
points but also the current in the primary
circuit
is observed. The ratio of potential
difference
and current, multiplied by a factor depending on electrode spacing,
gives the resistivity of the ground.

Potential-drop-ratio
methods.
The essential feature of the resistivity methods is a determination
of the potential difference between two points at the surface and a
measurement of the current in the external
circuit.
In potential-drop-ratio
methods current measurements in the external circuit are not made and
the potential drops in two successive ground intervals (represented
by three
stakes
arranged in a straight line, radiating
from
one of the power electrodes) are compared. The potential-drop-ratio
method is best suited for the location of vertical formation
boundaries (faults, dikes, veins, and the like).

Electromagnetic-galvanic
methods.
Electromagnetic methods of electrical prospecting differ from
potential methods in that the electromagnetic field of ground
currents
and not their surface potential (electric field) is measured. They
fall into two major groups: (1) electromagnetic-“galvanic”
methods in which the primary energy is supplied
by
contact as in the potential methods; (2) electromagnetic-“inductive”
methods in which the ground is energized by inductive
coupling
(with insulated
loops).
To
supply
electrical energy to the ground by contact, line electrodes are laid
out at right angles to the strike,
point
electrodes
parallel with the strike.

Electromagnetic-inductive
methods.
In inductive procedures power is supplied to the ground by insulated
loops which will cause induction currents to flow in subsurface
conductive bodies. An advantage of inductive methods is the ease with
which power may be transferred into the ground when the surface
formations are poor conductors. Since currents induced
in the subsurface conductors are dependent on frequency,
interpretative advantages may be gained by regulating the frequency.

Radio
methods.
Since radio methods employ frequencies still higher than the
high-frequency-inductive methods, they are subject to the same
limitations. In one group of radio methods the effect of subsurface
conductors on the emission
characteristics
of a transmitter
is
observed. In a second group a receiving arrangement is employed in
addition to the transmitter, and the variation of field
intensity with
location is measured. In the category of radio methods belong the
so-called “treasure-finders.” These are portable instruments for
the location of shallow metallic objects, pipe lines, and the like.

(C.A.
Heiland. Geophysical Exploration. New York, 1940)

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