A Brief History of Soil Electromagnetics Prior to 1980

Given the common use of electricity and electromagnetic waves in our modern society, it is perhaps difficult to imagine that knowledge of them in ancient times was very limited. In ancient Greece it was known that rubbing amber would allow it to attract straw (Kirby et al., 1990). This observation is credited to Thales of Miletus (640-546BC), who also knew that lodestone (i.e. magnetite) could attract iron (Cajori, 1935). The attraction of straw to amber may appear simple, but it is from the term amber (known as electron in ancient Greece) that the word electricity originates. Although there is some evidence that small earthenware pots were used as batteries in Iraq as early as 2000 years ago (Staubach, 2005), Shadowitz (1998) describes the intervening time between the work of ancient Greece, and the birth of modern scientific investigation, as "Twenty-two centuries of standing still!". However, Shadowitz may have been a little unfair in this comment, as the related branch of optics was studied by Al Hasen (c.965-1038) who extended Greek knowledge of reflection angles (an early form of Snells Law) and studied reflections from mirrors.

The ending point for Shadowitz's period of standing still may be ascribed to William Gilbert who, in 1600 AD, published his findings that amber was not the only material that could be used to attract other materials, and that it was also possible to cause a repulsive force (Shadowitz, 1988). However, in terms of our ability to utilise electricity for the generation of electromagnetic signals, potentially the most important milestone is the invention of the electric battery, then termed a pile, by Alessandro Volta and announced in 1800 (Kirby et al., 1990). This provided a power source for many ensuing experiments, including those of Humphrey Davy (Cajori, 1935) and subsequently of his apprentice Michael Faraday (Faraday, 1861).

In the early 19th century electricity and magnetism became an important part of natural philosophy. In 1819 Hans Christian Oersted carried out his famous experiment to deflect a magnetic needle using a current carrying wire, and the field of electromagnetics was born (Cajori, 1935). Experiments were later carried out in an attempt to show rotation of current carrying wires around a permanent magnet (Morus, 2004) and the great experimental scientist Michael Faraday is credited as the first scientist to demonstrate this (Kirby et al., 1990), a significant milestone that, in fact, almost ended his career (Morus, 2004). This led to the development of the electric motor, but Faraday also realised that the reverse was true: a moving magnetic field could be used to generate electricity and that it was the relative motion of the two that produced this effect (Kirby et al., 1990). This was the birth of the alternating, or oscillatory, electrical potential and so it could be argued that Faraday's 1820 experiments (Figure 1), in taking us beyond direct current electricity, provided the concept of the sinusoidal signal and its frequency.

Figure 1. Faraday's 1820 rotating wire experiments (Morus, 2004).

Soon afterwards, a further fundamental propagation parameter can be argued to have come from the work of Georg Simon Ohm who, in 1826, published experimental proof of Ohms law (Cajori, 1935). Ohms law states that the resistance of a material to electricity causes proportionality between the applied voltage and the resulting current flow, giving rise to the principle of conductivity which will, in subsequent sections, be seen to be an important concept in electromagnetic signal propagation.

Although Ohms law relates to the direct current electricity relevant to Volta's batteries, it paved the way for the concept of impedance in alternating current flow, which is important in determining the strength of electromagnetic reflections at boundaries. This was obviously an exciting time in the development of electromagnetics as, within a decade, Faraday had realised that electric charges were not unaffected in the propagation of their force by intervening matter, and so he coined the term 'dielectrics' to describe this phenomenon (Cajori, 1935). Using a version of a Leyden Jar, Faraday showed that the attractive or repulsive forces were scaled by the properties of a material used in place of air within the jar. In using air as a reference for this scaling effect, Faraday essentially conceived of the dielectric constant, or relative permittivity, that is also now a fundamental parameter for electromagnetic signal propagation.

Through the above achievements, his development of the concept of lines of force, and his speculations that light, electricity and magnetism are related (Cajori, 1935), Faraday provided a significant foundation for later scientific investigations in electromagnetics.

THE TIME OF MAXWELL

One of the most important scientists involved in that later work was James Clerk Maxwell who, in contrast to the great experimental skills of Faraday, was a great theorist. His theoretical skills allowed him to translate the experimental work of Gauss and Faraday into mathematical terms (Cajori, 1935), from which he was able to significantly expand on them. The famous Maxwell Equations describing the propagation of electromagnetic energy still provide the basis for our classical understanding, which is significant when one considers that Maxwell worked at a time when the nature of electromagnetic waves was unknown and that he considered propagation to be within an 'all pervading ether'. As described by Everitt (1975), Maxwell's ether (Figure 2) was made up of minute vortices and "Each vortex is separated from its neighbours by a layer of minute particles, identified with electricity, counter-rotating like the idle wheels of a gear train." Science has since largely dismissed the ether, but Maxwell provided a theory that fitted with real world observations and so is still of great use to this day.

Figure 2. Maxwell's 1861 concept of the ether (Everitt, 1975).

While Maxwell's work provided a significant advancement in knowledge of electromagnetic propagation, it was considered unwieldy for general use. His equations were therefore simplified, and adapted to describe electric and magnetic field vectors, by Oliver Heaviside at the end of the 19th Century (Crease, 2009). It should be noted that Heaviside was also responsible for incorporating imaginary numbers into electrical analysis (Crease, 2009).

Maxwell's Equations are described by Equations 1 to 4 (Fleisch, 2008). As well as describing the motion of an electromagnetic signal, Maxwell's Equations were also used to show that even a vacuum has an impedance (377 ohms - Hayt and Buck, 2006) and from this can be determined the velocity of light in-vacuo. Of particular significance, however, is that Maxwell's Equations provided an insight into the connected nature of electric and magnetic fields: i.e. that a magnetic field can give rise to an electric field, and vice versa (Fleisch, 2008).

where E is the electric field (v.m-1), B is the magnetic field (webers.m-2), rho is the charge density (C.m-3), t is time (s), J is the current density (A.m-2), and mu0 and epsilon0 are respectively the magnetic permeability and permittivity of free space (e.g. a vacuum or, in Maxwell's time, the ether).

DISCOVERING ELECTROMAGNETIC WAVES

Despite the brilliance of his work, Maxwell's Equations were always purely theoretical during his life. Although a number of scientists came close to discovering electromagnetic waves, this proof came from Heinrich Rudolf Hertz who, in 1888, developed a means of detecting electromagnetic waves caused by sparks in Leyden jars (Cajori, 1935). Hertz was thus able to show that electromagnetic waves exist, that they may be reflected and transmitted, and that they can interfere constructively, and destructively, to create maxima and minima in interference patterns. The ability to detect electromagnetic waves sparked a whole new area of study, that of using them to transmit information: in 1901 Marconi made the first trans-Atlantic radio transmission, making him famous and marking a turning point in our ability to control and utilise electromagnetic waves.

It was also around this time that advances were made in other aspects of electromagnetic propagation that are important to soils research. Newton had already undertaken experiments on dispersion of light through prisms and it was known that materials have a refractive index, i.e. light may travel through them at different velocities to those in a vacuum. Also, Fox Talbot, in the 19th century, knew of anomalous dispersion where the dispersion of light is different to that of a prism, and that some wavelengths may be absorbed more than others (Cajori, 1935). Drude (1902) made advances in this area and helped define our modern understanding of anomalous dispersion, which he says would occur only where the investigation covered natural periods (i.e. of oscillation) of ions. Further work culminated in the theories of Debye (1929) who defined the frequency dependence of water from which the associated variations in electromagnetic wave velocity and attenuation may be determined. Debye's theories also considered the effects of ions in solution on the permittivity of water and this is significant given the work between c.1879 and c.1924 on diffuse double layer theory, which can be attributed to the work of Helmholtz, Nernst, Gouy, Chapman and Stern, amongst others. That work provided a theoretical description of the distribution of mineral surface charges and ions within soil pores, which can be considered an important aspect of anomalous dispersion in soil pore water. It is this anomalous dispersion that we now term simply electromagnetic dispersion when referring to differing electromagnetic properties in soils at different frequencies. Further refinements were made to the theories of Debye in subsequent years, including the work of Cole and Cole (1941), but the basic principle that water has a relaxation frequency, around which the velocity and attenuation of electromagnetic waves vary, was by then well established.

THE ELECTROMAGNETIC PROPERTIES OF SOILS

Given the widespread adoption of electromagnetic waves for communications using radio, it is not surprising that researchers started to consider the electromagnetic properties of soils. Of particular note is the work of Smith-Rose (1935) who found electromagnetic dispersion in soils, and Cownie and Palmer (1951) who advanced the theory that more than one form of water may exist in a soil: bound water and free water. This hypothesis was based on the fact that the permittivity of some soils, at low water contents, does not increase in proportion to the amount of water added, but at higher water contents significant increases occur. However, it also brings forward another aspect of soil electromagnetics: that the different types of water may have different relaxation frequencies, causing a great deal of complexity in determining the frequency dependence of electromagnetic properties in electromagnetically dispersive soils. Over the subsequent decades research into soil electromagnetic properties steadily progressed. Of note during those times is the work of Hoekstra and Delaney (1974) who used a time-domain reflectometry (TDR) technique (with inversion to the frequency domain) to measure the electromagnetic properties of both sand and clay. Their data showed that the soils exhibited dispersion of a Debye (1929) type as well as illustrating low water content variations due to bound water and free water. However, their system was limited to gravimetric water contents up to 15%, thus limiting our interpretation of their data.

Of particular significance was that, during this time, other researchers were also using TDR for determination of soil water content and their equipment was based on commercial cable testers. That fact, small as it may seem, allowed the widespread testing of soil electromagnetic properties, and their application to soil state monitoring. This work culminated in the seminal paper of Topp et al. (1980) and the work they described has become the basis for the highly popular field of soil TDR. Although this work faced initial strong opposition (Topp et al., 2003), it also provided a model for relating an apparent permittivity (essentially a representation of signal velocity) to volumetric water content, thus helping ensure its subsequent success. The work of Hoekstra and Delaney (1974) and Topp et al. (1980) also demonstrates significant difficulty associated with soil electromagnetic measurements: different researchers often use different measures of water content.

QUANTUM CONSIDERATIONS

In concluding this introduction we must understand that the equations of Maxwell, and others, associated with the propagation of electromagnetic waves, are a classical representation of what is, in effect, a quantum system. Feynman (1985) quite clearly states that electromagnetic energy does not exist in the form of waves, but rather as packets (or quanta) of energy known as photons, which have rotational properties giving rise to apparently sinusoidal variations in intensity with time and distance. Through his work on Quantum Electro-Dynamics, he also showed that our classical concepts of the constant velocity of light in a vacuum, and even the laws of reflection and refraction, are actually only based on the fact that photons not following those rules cancel each other out, due to their incompatible phases (Feynman, 1985). Furthermore, Einstein's theory of relativity showed that even the magnetic field is illusory, it being a representation of the transformation of an electric field for a moving observer, and that the apparent discrepancy this holds for permanent magnets can be explained by the relativistic effects caused by electrons having spin (Shadowitz, 1988). While an understanding of these quantum effects is not necessary in order to understand the classical rules for electromagnetic wave propagation, it is important to understand that it is just that: a representation that allows us to dispense with much of the overwhelming complexity we would otherwise have to work within.

ABOUT THIS PAGE

The text of this page is based on a PhD Thesis by Andrew Thomas, and citation should be based on the full text of that thesis as figure, equation and page numbers will then be correct. The full copy of the thesis can be found in paper form at the University of Birmingham Library, and an e-text version is available through their eThesis repository.

REFERENCES

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Cownie, A. and Palmer, L.S. “The Effect of Moisture on the Electrical Properties of Soil” Proceedings of the Physical Society, 1951, 295-301.

Crease, R.P. "The Great Equations: The Hunt for Cosmic Beauty in Numbers", Constable and Robinson Ltd., London, 2009.

Drude, P. ‘The Theory of Optics’, Longmans, Green and Co., New York, 1902.

Debye, P. ‘Polar Molecules’, The Chemical Catalog Company Inc, New York, 1929.

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Hoekstra, P. and Delaney, A. "Dielectric Properties of Soils at UHF and Microwave Frequencies", Journal of Geotechnical Research, 1974, Vol. 79, No. 11, 1699-1708.

Kirby, R.S., Withington, S., Darling, A.B. and Kilgour, F.G. "Engineering in History", Dover Publications Inc., New York, 1990.

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Shadowitz, A. ‘The Electromagnetic Field’, Dover Publications Inc., New York, 1988.

Smith-Rose, R.L. “The Electrical Properties of Soil at Frequencies up to 100 Megacycles Per Second; With a Note on the Resistivity of Ground in the United Kingdom” Proceedings of the Physical Society, 1935, Vol. 47 923-931.

Staubach, S. "Clay: The History and Evolution of Humankind's Relationship with Earth's Most Primal Element", The Berkley Publishing Group, New York, 2005.

Topp, G.C., Davis, J.L. and Annan, A.P. “Electromagnetic Determination of Soil Water Content: Measurements in Coaxial Transmission Lines” Water Resources Research, 1980, Vol. 16 No. 3, 574-582.

Topp, G.C., Davis, J.L. and Annan, A.P. “The Early Development of TDR for Soil Measurements” Vadose Zone Journal, 2003, 2:492-499.