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Electromagnetic induction surveys

Electromagnetic induction (EMI) instruments provide a rapid assessment of the soil's electrical conductivity. They can provide information that can be used for land resource assessment, salinity assessment, soil works, precision farming and property and catchment management.

The technology works on the basis that within an electromagnetic field, any conductive body carries a current. The instrument measures the apparent flow of electrical conductivity through the soil, called the soil's apparent electrical conductivity (ECa) measured in milliSiemens / metre, (mS/m). Each instrument has two coils (a transmitter and a receiver) that are at a fixed (EM38, EM31 and EM39) or a variable (EM34) separation. The instrument induces an electrical current into the soil, with the depth of penetration determined by the separation of the coils and the frequency of the current. ECa is affected by the soil's salt content and type, clay content and type, mineralogy, depth to bedrock, soil moisture, organic matter and temperature.

The 4 most common types of EMI instruments, are the EM38, EM31, EM34 and EM39. Although they all operate the same, they vary in the depth to which they read within the soil profile. All operate in both the vertical and horizontal mode (this determines the depth to which they read). A summary of this is given below:

EM38 - vertical mode (1.5m) horizontal mode (0.7m)

EM31 - vertical mode (6.0m) horizontal mode (3.0m)

EM34 - 6.0m to 60.0m

EM39 - used for logging down boreholes.

These depths are only indicative, as the depth of penetration of the electrical signal will be determined by the uniformity, or non-uniformity, of the soil. If the soil is very conductive near the surface then the signal will be dissipated and will not read to a greater depth.

Soil data is required to validate the EMI survey. Soil sampling sites need to be selected to represent the range of soil conductivity zones (low, medium and high) based on the range of ECa values as collected by the EMI instrument. Samples need to be collected to a depth that is indicative of the machines capability. If validating an EM31 survey, then it is necessary to sample to a depth of 6.0 metres. Soil samples need to be tested for a range of parameters depending on what the data is being collected for.