Electromagnetic Terrain Conductivity
Using a combination of electromagnetic instruments, a fast, accurate and cost-effective system of mapping soil conductivities is available. Electromagnetic terrain conductivity surveying is carried out using the Geonics Ltd. Model EM-31 and EM-34 terrain conductivity instruments.
The EM-31 is an electromagnetic instrument consisting of a transmitter coil and a receiver coil at opposing ends of a 3.66 m boom. Together with the electronics package, the instrument is one person portable and has an effective penetration depth of six metres. The instrument is calibrated to adjust for the reduction of reading due to the instrument being held 1 metre above the surface.
The EM-34 is a two man version of the EM-31, designed to achieve greater penetration depths. The system consists of a separate receiver and transmitter, and using 3 inter-coil spacings, readings down to 60 metres are possible.
Both the EM-31 and EM-34 are operated at audio frequencies and make use of the effect that earth conductivity information is contained in the behavior of the phase of the received EM field. This phase information is calibrated to read directly in milliSiemens/metre (mS/m), units of conductivity. They can operate over a range of conductivities from a few mS/m to thousands of mS/m.
High rates of production are possible with both of these instruments as they are easily portable. In practice, grids are established, usually with a minimum of 5m intervals between stations and lines. In some cases, the integrated GPS system may be used for automated positioning information. If both instruments are being used together, a detailed image of the near surface and deeper soils is ascertained when each of the separation distances for the EM-34 are utilized.
Both instruments are relatively unaffected by metallic objects and power lines, but care should be taken nearby such objects. The advantages of using these instruments are improved conductivity resolution and the ability to operate over terrain where current injection problems occur.
The digitally recorded data is downloaded and plotted. The process is quick and easy to interpret. The usual presentation would be a plan view of the site with contours of the recorded conductivity. If varying EM-34 separations were used, multiple maps would be produced to ease interpretation between depths.
The figure below illustrates the results of an EM 31 survey to detect possible elevated conductivities due to the presence of any chlorides and dissolved salts in the earth materials and shallow groundwater. A wide band of elevated soil conductivity is seen running west to east from the salt shed. Outlining the extents of these elevated soils is important in the remediation process.