Geophysical Properties of the Near Surface Earth: Electrical Properties

Abstract

The electrical properties of rocks and minerals vary over 20 orders of magnitude, yet can be measured in the laboratory to an accuracy of one part per billion. This remarkable combination makes electrical techniques extremely sensitive to the composition and texture of geomaterials. Furthermore, electric currents can be generated in rocks remotely, and the flow of currents can also be measured remotely. This is a key advantage that opens up the possibility that the electrical properties of rocks deep in the Earth may be measured. For example, remote monitoring is implemented at a small scale in downhole induction tools and deeper in the case of the magnetotelluric method. The recent rapid development of electrokinetic research produces the possibility that remote electrical measurements will be able to infer fluid flow deep in the Earth, such as encroachment of water towards a productive oil well or monitoring the emplacement of waste CO2 in carbon capture and storage reservoirs. Perhaps, the most impressive example of the importance of the electrical properties of rocks is the implementation of Archie's law. This law, which was published in 1942, has been used to measure and monitor all of the Earth's oil and gas reserves since its invention. Thanks to Archie's law, over 1.5 trillion barrels of oil reserves and 7.5 trillion standard cubic feet of gas have been quantified and produced, running the global economy for the last 70 years. In this chapter, the basic electrical theory of geomaterials is set out together with sufficient experimental and modeling results to show what controls their electrical properties. For the purposes of this chapter, we have taken electrical properties to cover steady-state and frequency-dependent electrical transport and polarization as well as those electrokinetic processes that link electrical flow to hydraulic flow. After a brief introduction, the basic theory of steady-state electrical flow in rocks is considered, including Archie's laws and other mixing models, but restricting ourselves to near-surface temperatures and pressures and hence to conduction occurring through the pore fluids that occupy the pores of the rock rather than by electronic conduction through the rock matrix. However, surface conduction is often significant in the near-surface regime, especially for shaley rocks and some soils. Hence, we consider the source, measurement, and modeling of surface conduction in the third section of the chapter. Theoretical advances that have been made since 1994 in the understanding of steady-state electrical transport are also fully covered. A significant section considers electrical polarization, electric permittivity, and the frequency-dependent properties of rocks. Finally, the steady-state and frequency-dependent electrokinetic properties of geomaterials are considered including factors affecting the streaming potential of near-surface rocks.