Electrodearraysip

A COMPARISON OF ELECTRODE ARRAYS IN IP SURVEYING

John S. Sumner
Professor of Geophysics
Laboratory of Geophysics
and College of Mines
University of Arizona
Tucson, Arizona

This paper is to be presented at the AIME Annual Meeting –
San Francisco, California – February 20-24, 1972

INTRODUCTION
The induced polarization (IP) method of geophysical exploration is capable of
detectingeven small amounts of metallic luster minerals in a rock mass. Consequently,
in the years since discovery, IP surveying has become the most popular ground
geophysical survey method.
It is not too difficult to understand many applications of IP surveying as used in
the search for mineral deposits. However, the basic theory of the IP phenomenon is not
well developed or understood, and there hasbeen some disagreement on fundamental
concepts.
In order to most effectively apply the IP method in the field it is necessary to
know the physical characteristics of the sought-for deposit including its size, shape,
depth, and electrical properties. With this information an optimum IP search arrangement
can be devised and one could use the best possible electrode interval, type of array, andline spacing. With uncertain target characteristics as encountered in the real world,
geological guidance must be used to help direct an optimized IP survey.
SUMMARY AND CONCLUSIONS
Geometric arrangements of the grounded contacts, or electrodes, can be classified
according to the shape of the electrical field that is being measured. In general there are
three basic families of arrays. Thesecan be described as approximately either a parallel
electric field, the field about a point current electrode, or a dipolar electric field.
The ratio of the signal voltage to the external disturbing noise voltage is an
important consideration in designing equipment, in making a survey layout, and in the
interpretation of field data. Signal-to-noise ratio can be determined for each array indiffering conditions and this ratio is helpful in selecting the best array for particular
conditions of a given survey.
Electromagnetic (EM) coupling between the IP transmitter and receiver circuits is
also an important matter in the choice of an electrode array. As a generalization the timedomain IP method has less difficulty with EM coupling and more problems with handling
noise disturbances,while the converse is true with the frequency domain method.

Society Of Mining Engineers of AIME

2

Electrode arrays can be chosen to take advantage of the subsurface situation,
surveying conditions, and the kind of equipment being used.
ARRAY FUNDAMENTALS
Ohm’s law for three-dimensional materials can be applied to derive the electric
field about a point current electrode. The fieldfrom a second current source can be
superimposed on the first, which can be illustrated by curved current flow lines as shown
on Figure 1.
Measurement of the resulting electric field is carried out by using two potential
electrodes in contact with the ground, which give the voltage potential difference
between the detecting points. This voltage difference V can be written
V=
where

ρI
2π⎛1 1
1
1⎞
⎟
⎜−−
+
⎜r
rB R A R B ⎟
⎠
⎝A

(1)

ρ is the resistivity of the medium, I is current in amperes, r is the distance from

one potential point to current points A and B, and R is the distance from the other
potential point. Equation (1) is the general voltage relationship for any electrode array
and it can be simplified for specific arrays, such as the Wenner (rA = a,rB = 2a, RA = 2a,

RB = a), whereupon ρ = 2π a

V
I

Figure 2 shows several of the common resistivity arrays. These can be classified
according to the type of electric field being measured, whether parallel, hemispherical, or
dipolar. Figure 3 illustrates these different types of electric fields from the standpoint of
array geometry, that is, the location of the potential measuring...