Geophysics foundations:    Physical properties:  

Electrical resistivity of geologic materials

Introduction

Understanding how electrical resistivity (or conductivity) relates to the actual geologic properties of the earth is important. The following are questions it can help answer:

If interpretation of a geophysical survey suggested that there is a 10m thick layer of overburden with a resistivity of 11,000 Ohm-m overlying a "basement" with a resistivity of 140 Ohm-m, what geological materials would be consistent with these two layers of different resistivity?

What if a resistivity profile gathered over an ore body in Australia revealed apparent resistivities ranging from 40 to 600 Ohm-m, while analysis of borehole cores showed that true bulk resistivities range from 80 to >1000 Ohm-m. Are these results consistent, and do they indicated the presence of an economic ore body?

If bulk resistivity of a deeply buried sandstone is 1000 Ohm-m, can details about the matrix (rock units in which fluids reside) and/or fluid resistivities be extracted? This is of particular interest in hydrogeology, oil and gas exploration and environmental (contaminant) studies.

In this chapter electrical properties of geologic materials are discussed separately for metallic minerals, rocks, soils, and electrolytes (ground fluids).

What is resistivity?

Electrical conductivity (or resistivity) is a bulk property of material describing how well that material allows electric currents to flow through it.

Resistance is the measured voltage divided by the current. This is Ohm's Law. Resistance will change if the measurement geometry or volume of material changes. Therefore, it is NOT a physical property.

Resistivity is the resistance per unit volume. Consider current flowing through the unit cube of material shown to the right: resistivity is defined as the voltage measured across a unit cube's length (Volts per metre, or V/m) divided by the current flowing through the unit cube's cross sectional area (Amps per meter squared, or A/m2). This results in units of Ohm-m2/m or Ohm-m. The greek symbol rho,   , is often used to represent resistivity.

Conductivity, often represented using sigma,  , is the inverse of resistivity:   = 1/  . Conductivity is given in units of Siemens per metre, or S/m. Units of millisiemens per metre (mS/m) are often used for small conductivity values; 1000mS/m = 1S/m. So 1mS/m = 1000 Ohm-m, since resistivity and conductivity are inversely related.

The electrical conductivity of Earth's materials varies over many orders of magnitude. It depends upon many factors, including: rock type, porosity, connectivity of pores, nature of the fluid, and metallic content of the solid matrix. A very rough indication of the range of conductivity for rocks and minerals is in the following figure.

Figure 2.

The reminder of this section describes factors affecting electrical conductivity of minerals, rocks, fluids in the ground, soils

Electrical conductivity of metallic minerals

Metallic ore minerals are relatively uncommon compared to other crustal materials. However, they are often the target of mineral exploration. Even in small quantities, they can significantly affect the bulk resistivity of geologic materials. Most metallic ore minerals are electronic semiconductors. Their resistivities are lower than those of metals and highly variable because the inclusion of impurity ions into a particular metallic mineral has a big effect on the resistivity. For example, pure pyrite has a resistivity of about 3x10-5 Ohm-m but mixing in minor amounts of copper can increase the resistivity six orders of magnitude to 10 Ohm-m. Conductivity properties of some important minerals can be summarized as follows:

Pyrrhotite (FeS) is consistently highly conductive mineral. Graphite ( C ) is a true conductor like a metal (i.e. not a semiconductor like ore minerals),

and it is very conductive, even in very low concentrations. It is also chargeable (a different physical property - see the separate chapter on Chargeability), and it is notoriously difficult to distinguish from metallic ore minerals.

Pyrite (FeS2) is the most common metallic sulfide and has the most variable conductivity. Its conductivity is generally higher than porous rocks.

Galena (PbS) and magnetite (Fe3O4) are conductive as minerals, but much less conductive as ore because of their loose crystal structures.

Other conductive minerals include Bornite (CuFeS4), chalcocite (Cu2S), covellite (CuS), ilmenite (FeTiO3), molybdenite (MoS2), and the manganese minerals holandite and pyrolusite.

Haematite and zincblende are usually nearly insulators.

Although metallic minerals (particularly sulfides) may be conductive, there are at least two reasons why ore-grade deposits of these minerals may not be as conductive as expected.

Sulfide deposits can be either desseminated or massive. In the first type the mineral occurs as fine particles dispersed throughout the matrix, and in the second, the mineral occures in a more homogeneous form. Desseminated sulfides may be resistive or conductive, whereas massive sulfides are likely to be conductive.

Chemical and/or thermal alteration can convert metallic minerals into oxides or other forms that are not as conductive as the original minerals.

Electrical Properties Of Rocks

Of all the geophysical properties of rocks, electrical resistivity is by far the most variable. Values ranging as much as 10 orders of magnitude may be encountered, and even individual rock types

can vary by several orders of magnitude. The next figure is a representative chart (adapted from Palacky, 1987) illustrates very generally how the resistivities of important rock groups compare to each other. This type of figure is given in most texts on applied geophysics.

Figure 3.

Soils and rocks are composed mostly of silicate minerals, which are essentially insulators, meaning that they have low electrical conductivity. The most common exceptions include magnetite, specular hematite, carbon, graphite, pyrite, and pyrrhotite. Therefore, conduction is largely electrolytic, and conductivity depends mainly upon:

Porosity, hydraulic permeability, which

describes how pores are interconnected,

moisture content, concentration of dissolved

electrolytes, temperature and phase of pore

fluid, amount and composition of

colloids (clay content).

Figure 4.Pore space and pore geometry are the most significant factors. Porosity exists mostly in joints, fractures, vugs (dissolved pockets in limestones and dolomites), and intergranular voids in sedimentary rocks. The figure above and tables below (from Geonics TN5, 1980) give some idea of the complexity, and range of porosities possible.

The "Ratio" column is bulk resistivity divided by electrolyte resistivity (see Archie's law below).

Vuggy porosity (composed of larger discrete voids) may have very low permeability making for low resistivity when measured using galvanic (DC current) techniques. However, inductively measured resistivity (using electromagentic induction methods) may be higher because currents induced by oscillating electromagnetic fields do not have to flow over large distances. See "Foundations => Survey methods" and "Foundations => Geophysical surveys" for details about these surveying techniques.

Resistivity may be anisotropic in layered rocks, especially for shales where the coefficient of anisotropy (ratio of transverse resistivity to longitudinal resistivity) can be as high as 4. See the "Anisotropy" section below for more details.Much of our understanding about resistivity of porous rocks comes from the oil/gas well-logging industry. The effect of fluids other than water, Archie's law, formation factor, etc. are detailed in the next few sections.

Electrolytes in the ground

Conductivity of fluids depends upon quantity and mobility (velocity) of charge carriers. Mobility depends on viscosity of fluid (hence temperature) and diameter of charge carriers. Temperature dependence is significant. For sodium chloride solutions, change of conductivity is roughly 2.2% per degree C. So a change of 40oC doubles the conductivity. In the illustration showing conductivities of waters in the Great Lakes (below), compare conductivities in igneous (western) versus sedimentary (eastern) regions, and note the dependence of conductivity on temperature of these lake waters.

Typical conductivities of electrolytes, and examples from the Great Lakes.Natural source mS/m

Meteoric waters (from precipitation)

1 to 30

Surface waters (lakes & rivers)

0.3 for very pure waters  10,000 for salt lakes  2 to 30 in igneous regions  10 to 100 in sedimentary regions

Soil waters Up to 10,000  average around 10

Ground water 6 to 30 in igneous regions  1,000 in sedimentary regions

Mine waters (copper, zinc etc. i.e. sulphates)

not usually less than 3,000

Note that Lake Superior is the westernmost lake and therefore in an igneous region, while Lake Ontario is the easternmost or sedimentary region. This may contribute to the generally more conductive waters of the eastern lakes.

Figure 5.

The dependence of fluid conductivity on salinity (concentration of ions) for a variety of electrolytes is illustrated to the right. Tap water is usually a minimum of around .01 S/m (i.e. 100 Ohm-m) with a salinity of about 40 ppm, and sea water is roughly 3.3 S/m with a salinity of 30,000 ppm. Compare these values to thoes of lake water shown above.