the radar signal, and by the highly site specific application of the technique. Multiple

reflections can complicate data interpretation.

Electromagnetic (EM) Induction Conductivity

The electromagnetic (EM) induction method uses alternating electric currents flowing

between a transmitter and a receiver coil to induce secondary magnetic fields in the

subsurface that are linearly proportional to ground conductivity up to approximately 100

mmhos/m (McNeill, 1980). The instrument reading is a bulk measurement of the apparent

formation conductivity calculated as the cumulative response to subsurface conditions ranging

from the ground surface to the effective depth of the instrument. The effective exploration

depths for commercially available equipment range from 3 to 60 meters depending on the

instrument orientation and the intercoil spacing. The EM technique has been applied to

mapping geologic deposits, locating subsurface cavities in karst environments, locating

subsurface trenches, mapping contaminant plumes, locating metallic conductors, mapping

saltwater intrusion, and locating buried drums, tanks, and subsurface utilities. By changing

the orientation and spacing of EM coils, it is possible to profile vertical changes in subsurface

conductivity, potentially allowing for vertical tracking of contaminant plumes. Like other

geophysical techniques, delineation of a particular subsurface feature from the bulk apparent

conductivity measurement requires a sufficient conductivity contrast in the subsurface.

When dry, soil and rock typically have low conductivities. In some areas, conductive

minerals like magnetite, graphite, and pyrite occur in sufficient concentrations to greatly

increase natural subsurface conductivity. Most often, conductivity is overwhelmingly

influenced by water content and by the following soil and rock parameters:

The porosity and permeability of the materials;

The extent to which the pore space is saturated;

The concentration of dissolved electrolytes and colloids in the pore fluids; and,

The temperature and phase state (i.e., liquid or ice) of the pore water.

In some cases, contaminants increase the electrolyte and colloid content of the

unsaturated and saturated zones. Examples of common ionic contaminants include chloride,

sulfates, the nitrogen series, and metals such as sodium, iron, and manganese. With the

addition of electrolytes and/or colloids, the ground conductivity can be affected, sometimes

increasing by one to three orders of magnitude above background values. However, if the

natural variations in subsurface conductivity are low, conductivity variations of only 10 to 20

percent above background may be observed.

November 1992

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