Electromagnetic Induction Techniques - Part 8

By PG Cook and BG Williams

Electromagnetic induction (EM) techniques are used in recharge/discharge studies by providing measurements of the apparent electrical conductivity of soil profiles. This booklet provides a summary of EM techniques that are most widely used. A number of case studies are presented which demonstrate the applications of the techniques to field problems.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Aug 1, 1998
• ISBN: 9780643106093

Book preview

1  THE ELECTRICAL CONDUCTIVITY OF A SOIL

The electrical conductivity of a soil sample is a function of the soil structure, texture and mineralogy, as well as its water content and salt content (Nadler and Frenkel 1980; Shainbergr et al. 1980).

Sauer et al. (1955) represented the electrical conductivity of a soil by a three element model consisting of:

1.  Conduction through the soil solution

2.  Conduction through or along the surfaces of particles in contact with one another, and

3.  Conduction through alternating layers of particles and solution (Figure 1).

Figure 1.    Three element model of electrical conductivity.

Nadler and Frenkel (1980) and Shainberg et al. (1980) have shown that when the electrical conductivity of the soil solution exceeds about 4 dS/m, the bulk soil conductivity (ECa) becomes directly proportional to the soil solution conductivity and may be approximated by

where ECwis the electrical conductivity of the soil solution, θv is the volumetric water content, T is the tortuosity, and ECs is the solid phase contribution to the apparent electrical conductivity.

This simple model has been validated by a range of empirical studies (e.g. Rhoades and Ingvalson 1971; Gupta and Hanks 1972). At low ECa values, however, the more sophisticated models of Sauer et al. (1955) and Rhoades et al. (1989) should be considered.

Electrical Conductivity of Soil Solution ECw

Pore water may be extracted from soil samples by a variety of centrifuge and non-polar liquid displacement techniques. However, these are time consuming and the more usual technique is to measure the electrical conductivity of a water extract of the soil (e.g. a 1:5 soil:water dilution) and then adjust that value for the initial soil water