Physical properties - chargeabilitiy

Introduction

Note: before gaining an appreciation for what makes ground chargeable, it is useful to understand why the DC electrical resistivity of ground varies. There is an AGLO module devoted to a discussion about electrical resistivity of geological materials.

The chargeability of earth materials is essentially an electrochemical effect caused by many factors, not all of which are completely understood. If ground is chargeable, it responds as if resistivity was a complex quantity - it behaves somewhat like a leaky capacitor. Therefore the chargeability can be measured in a number of ways using time domain or frequency domain techniques (detailed in the section on measurements and data). Aspects affecting the chargeability of a sample include:

• the grain size of particles in the sample;
• the types of minerals involved;
• the type and mobility of ions within the pore fluids;
• the details of microscopic interactions between solid surfaces and fluids;
• the amount of surface area within a specific volume.

The surface area-to-volume ratio is an important factor. Clays tend to be chargeable while sandstones are not, and the images here illustrate one reason why this is true. In addition, the surface interactions between clay minerals and fluids enhance the ability of these materials to hold charges.

Illite (a clay mineral) with surface area-to-volume ratio of 100m2/g (1000 times greater than sandstone)

Quartz overgrowths in sandstone with surface area-to-volume ratio of 0.1m2/g

On the remainder of this page, some details about causes of chargeability are introduced.

Two microscopic effects cause macroscopic chargeability

There are two primary causes of chargeability. In both cases the re-distribution of charges takes some time to occur when an external DC electric field is applied. Equivalently, it takes the same time to revert to a balanced charge distribution once the electric field is removed. "Charging" is hard to measure in practice. "Discharging" is measured using time domain IP survey techiques. The effect of finite charging time on sinusoidal signals at different frequencies also can be measured using frequency domain or phase IP surveys. The two types of polarization are called "membrane polarization" and "electrode polarization."

Membrane polarization

	Membrane polarization occurs when pore space narrows to within several boundary layer thicknesses (which is the thickness of ions adsorbed to a surface).
	Charges cannot flow easily, so they accumulate when an electric field is applied.
	The result is a net charge dipole which adds to any other voltages measured at the surface.

A second form of membrane polarization is similar to the first:

This occurs where clay particles partially block ionic solution paths, as in the adjacent figure. Upon application of an electric potential, positive charge carriers pass easily, while negative carriers accumulate. There is an "ion-selective membrane."

A surplus of both cations and anions occurs at one end of the membrane, while a deficiency occurs at the other end. The reduction of mobility is most obvious at frequencies slower than the diffusion time of ions between adjacent membrane zones; i.e. slower than around 0.1 Hz. Conductivity increases at higher frequencies.

Electrode polarization

	Electrode polarization occurs when pore space is blocked by metallic particles. Again, charges accumulate when an electric field is applied.
	The result is two electrical double layers which add to voltages measured at the surface.

Comments on electrode polarization Some remarks are appropriate here in order to provide some sense of the complexity of the chargeability phenomenon. At an interface between ionic and metallic conduction (for example, an ore grain in pore water), there is an impedance involved in getting current to flow across the barrier. These interfaces look like the top figure and have the simplified circuit analogue shown in the bottom figure. Current can flow via charge transfer (or ion diffusion), which involves electrochemical processes, or via a capacitive effect (no charge transfer), involving diffusion currents. Ion diffusion is not easy to model with circuit elements. The process is called the Warburg impedance. Its magnitude varies as approximately 1/frequency.

Note that, while it is useful to understand simplified models of the relevant electrical behaviour of surface-electrolyte interactions, all rocks are, in fact, "dirty" in the sense that they are not simply pure "electrodes" (semiconducting mineral grains) and electrolytes (pore solutions).  There are other materials and particles affecting ionic behaviour within and outside the diffuse layer, and some of the sample's constituents will affect the behaviour of the fixed layer near and on the liquid-solid interfaces.

Summary of what affects the chargeability of material

• Induced polarization is greater when there are larger regions of adsorbed anomalous charge (adjacent to an interface); i.e. when there is a large surface area-to-volume ratio.
• Non-ionic fluids (such as contaminants) can markedly change the behaviour of surface-electrolyte interactions.
• Changes in ion concentration (such as increased salinity) will also affect both types of polarization.
• Both effects (membrane and electrode polarization) are related to grain size as much as material type. Therefore, discrimination of mineral type on the basis of chargeability alone is not recommended.

Typical chargeabilities for materials

The following tables (from Telford et al, 1976) provides a very general guide to possible chargeabilities of materials. One reason that in-situ chargeabilities tend to appear lower than laboratory values is that large volumes of mixed materials are involved in field measurements.

These examples show that a wide range of variability can be expected, implying that it is difficult to use values of intrinsic chargeability (in models obtained by inversion of IP data) to determine exactly what type of rock or material is in the ground. However, this is an ongoing topic of research.

Table 1: Charging and integration times were about 1 minute each, which is much longer than field survey systems; therefore, values are larger than field measurements.

Material type	Chargeability (msec.)
20% sulfides	2000 - 3000
8-20% sulfides	1000 - 2000
2-8% sulfides	500 - 1000
volcanic tuffs	300 - 800
sandstone, siltstone	100 - 500
dense volcanic rocks	100 - 500
shale	50 - 100
granite, granodiorite	10 - 50
limestone, dolomite	10 - 20

Table 2: The values below involved more realistic charging and integration times of 3 seconds and 0.02-1.0 seconds respectively.

Material type	Chargeability (msec.)
ground water	0
alluvium	1 - 4
gravels	3 - 9
precambrian volcanics	8 - 20
precambrian gneisses	6 - 30
schists	5 - 20
sandstones	3 - 12
argilites	3 - 10
quartzites	5 - 12

Table 3: Chargeability of minerals at 1% concentration in the samples (charging and integration times as per Table 2 above).

Material type	Chargeability (msec.)
pyrite	13.4
chalcocite	13.2
copper	12.3
graphite	11.2
chalcopyrite	9.4
bornite	6.3
galena	3.7
magnetite	2.2
malachite	0.2
hematite	0.0