A Comparison of Electrode Arrays in IP Surveying
Presented at the Society of Mining Engineers AIME Annual Meeting, San Francisco, California, February, 1972.
John S. Sumner, Professor of Geophysics, Laboratory of Geophysics and College of Mines, University of Arizona, Tucson, Arizona, U.S.A.
Paper – [pdf] ElectrodeArraysIP_Sumner_1972
The induced polarization (IP) method of geophysical exploration is capable of detecting even 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 has been 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, and line spacing. With uncertain target characteristics as encountered in the real world, geological guidance must be used to help direct an optimized IP survey.
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. These can 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 in differing 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 time-domain 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.
Electrode arrays can be chosen to take advantage of the subsurface situation, surveying conditions, and the kind of equipment being used.