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BASIC PRINCIPLES OF ELECTROMAGNETIC PROSPECTING
1.1 Definition, Concept and Historical Background
In Geology, Prospecting is the first stage of geological analysis; which entails the search for mineral deposits, especially by drilling and excavation. Elsewhere, in Electricity, Electromagnetism is the science of charges as well as the forces and fields associated with the charge.
However, Electromagnetic Prospecting seeks the study and application of electromagnetic methods to the idea of prospecting (I.E in the search for mineral deposits). By definition, It is a geophysical method employing the generation of electromagnetic waves at the Earth’s surface, such that when the electromagnetic waves penetrate the Earth and impinge on a conducting formation or ore-body, currents are induced into the conductors, which forms new waves that are radiated from the conductors for which detectors can be used to record the waves at the surface. (Mindat.org, 2017).
The pioneer method used for the study of EM prospecting was the electrical method; it was one of the first geophysical exploration techniques widely used at the end of the 1920s and at the beginning of 1930s in oil, gas, and mineral-deposit exploration. The earth’s interior was being explored by practical use of electrical currents which was as a result of pioneering work of the Schlumberger brothers, Conrad and Marcel, who went on to engineer one of the world’s most successful geophysical service companies.
The1D model of a layered earth or the 2D models were the basic models of interpretation for so many years. However, during the last 20 years, geophysicists have developed and begun the use of 3D models for interpretation as well. This advancement has required developing the corresponding mathematical methods of interpretation, based on modern achievements of EM theory and computer science in numerical modeling and inversion.
The two main applications of electrical and EM methods are mostly for petroleum and other stratigraphic studies, and searching for discrete conductors in base-metal exploration.
The first successful application of electrical and EM methods was successfully applied first in the exploration of highly conductive metallic ores. Electromagnetic methods were applied in the exploration for massive sulfide ore bodies and disseminated metal ores. Even with the recent development of other methods of metallic ore body exploration, the use of EM methods in the search for metallic ores remains one of the most important commercial applications. The fact is that in the big picture of expenditures in exploration geophysics, EM methods are still small compared to seismic methods; and within EM methods, the search for minerals has held the dominant position. Electromagnetic methods in geophysics are distinguished by:
- Use of differing frequencies as a means of probing the Earth (and other planets), more so than source-receiver separation. Think “skin depth”. Sometimes the techniques are carried out in the frequency domain, using the spectrum of natural frequencies or, with a controlled source, several fixed frequencies (FDEM method —“frequency domain electromagnetic”). Sometimes the wonders of Fourier theory are involved and a single transient signal (such as a step function) containing, of course, many frequencies, is employed (TDEM method – “time domain electromagnetic”). The latter technique has become very popular.
- Operate in a low frequency range, where conduction currents predominate over displacement currents. The opposite is true (i.e., has to be true for the method to work) in Ground Penetrating Radar (GPR). GPR is a wave propagation phenomenon most easily understood in terms of geometrical optics. Low frequency EM solves the diffusion equation.
- Relies on both controlled sources (transmitter as part of instrumentation) and uncontrolled sources. Mostly the latter is supplied by nature, but also can be supplied by the Department of Defense.
Some Electromagnetic (EM) methods include:
- Frequency domain EM methods, such as EM induction, EM utility locator/metal-detection methods, very low frequency (VLF) EM, and
- Controlled source audio-frequency magnetotellurics (CSAMT), as well as time domain EM methods (TDEM).
NB: GEO-Vision geophysicists have successfully utilized a wide variety of EM methods for hydro-geologic, engineering, and environmental investigations.
1.2 Early History
As early as 1882, Dr. Carl Barus conducted experiments at the Comstock Lode, Nevada, which convinced him that the method could be used to prospect for hidden sulfide ores. To Barus goes the credit for introducing the non-polarizing electrode. Conrad Schlumberger put the method on a commercial basis in 1912. The first plan map of self -potential over a metallic deposit was prepared by Conrad Schlumberger in 1913 and published in 1918; it pertained to the pyrite mines at Sain-Bel, France. Roger C. Wells, of the U.S. Geological Survey, in 1914 contributed the first chemical understanding of the passive self- potential phenomena. Kelly (1957) introduced the self-potential method to Canada and the United States in 1924.
Fred H. Brown, in the era from 1883 to 1891, and Alfred Williams and Leo Daft in 1897 first attempted to determine differences in earth resistivity associated with ore deposits and were granted patents on their methods. In 1893 James Fisher measured the resistivity of copper bearing lodes in Michigan (Broderick and Hohl, 1928), while in 1900, N. S. Osborne did equipotential work in the same district. The first practical approach to utilizing active electrical methods, wherein the earth is energized via a controlled source and the resulting artificial potentials are measured, was due to Conrad Schlumberger in 1912. At that time he introduced the direct current equipotential line method (Schlumberger, 1920).
The concept of apparent resistivity was introduced about 1915 by both Wenner (1912) of the U. S. Bureau of Standards and by Schlumberger (1920). The field techniques for apparent resistivity were then developed by O. H. Gish and W. J. Rooney of the Carnegie Institution of Washington and by Marcel Schlumberger, E. G. Leonardon, E. P. Poldini, and H. G. Doll of the Schlumberger group. Wenner used the equi-spaced electrode array which today bears his name while the Schlumberger group standarized on an electrode configuration in which the potential electrodes are sufficiently close together that the potential gradient, i.e. the electric field, is measured midway between the current electrodes (the Schlumberger array). The earliest attempt to understand telluric currents is generally credited to Charles Mateucci (1867) of the Greenwich Observatory. It was not until 1934 that Conrad Schlumberger (1939, p. 272-3) made commercial use of the method.
The three Geophysical Prospecting volumes of the Transactions of the American Institute of Mining and Metallurgical Engineers (AIME), in 1929, 1932, and 1934, provided forums for dissemination of knowledge of this rapidly growing field of electrical geophysical prospecting. These volumes were followed by Geophysics, 1940, Vol. 138 and Geophysics, 1945, Vol. 164 of the Transactions of AIME. The Society of Exploration Geophysicists published six papers on electrical methods in Early Geophysical Papers. Most of these papers dealt with oil and gas exploration. The first paper on electrical methods to appear in Geophysics (Vol. 1) was by Statham (1936). From these beginnings, the electrical methods slowly developed, with most of the development taking place after World War II. Highlights of these developments follow. (Bleil , D. F., 1953)
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