Ancillary Data: Electromagnetic (EM)

The term "electromagnetic" (EM) refers to geophysical techniques where induction coils or magnetometers are used to measure the magnetic fields associated with current flow in the earth.

Among geophysical methods, EM techniques have broadest range of instrumental systems. Generally, EM methods can be classified as either time-domain (TEM) or frequency domain (FEM) systems.

FEM systems use either one or more frequencies, whereas TEM systems make measurements as a function of time. In terms of ground based surveys, most EM instruments used are FEM systems (i.e. EM38 and EM31).

EM methods can be either active or passive. In most environmental and agricultural applications an active method is used.

An EM instrument consists of two coils. These are located at either end of the device. One is a transmitter coil, while the other is a receiver coil.

To operate an EM instrument requires an alternating electrical current to be introduced into the transmitter coil.

This alternating current induces a primary magnetic field to pass perpendicular to the coil orientation. If the instrument is held in the vertical mode of operation, the primary magnetic field enters the soil. In essence this magnetic field oscillates into and out off the soil as a function of the direction the electrical current flows in the transmitter coil.

As the current passes through the soil, small eddy currents are generated through any conductir (i.e. conductive material).

There are a number of conductors including:

  1. amount of negative charge on the clay particle;
  2. clay content;
  3. concentrations of salts in the soil solution; and,
  4. soil moisture content
  5. temperature, and
  6. bulk density.

The production of these eddy currents in the soil then induces the creation of another magnetic field, termed the secondary magnetic field. This secondary magntic field is then detected by the receiver coil which is located some fixed distance frrom the transmitter coil.

The strength of the secondary magnetic field is a function of these various conductors; whereby the more conductive the soil, the more likely it is that the soil contains greater amounts of negative charge on the clay particle, clay content, soluble salts, etc.

EM instruments actually measure the ratio of the two magnetic fields (i.e. primary and secondary); whereby the physical property of interest is the apparent soil electrical conductivity (i.e. ECa).

Most instruments measure ECa in millisiemens/metre (mS/m). However, the S.I. unit of ECa measurement is siemens/metre (S/m).

Geonics limited pioneered the development of commercial EM instrumentation. In the first instance Geonics developed the EM34 instrument for ore body identification. This was followed by the shallower measuring EM31 and then the EM38.

To improve the efficiency of field measurement of EM instruments various researchers and extension personnel have mounted EM instruments onto small platforms (e.g. quad bikes, tractors). In most cases Global Positioning Systems (GPS) allow the data collected to be georeferenced and used to produce rapid and repeatable information at the field level.

With the ECa data generated, soil-sampling sites can be strategically selected to determine the reasons for the spatial variation in ECa, and hence soil properties, across a given field.

These measurements can be used for targeting the location of soil samples sites to calibrate the EM instrument. Once a suitable relationship is established between one or a number of soil attributes a map can be produced. As such measures of ECa, add value to the limited soil information collected using traditional soil samplingmethods.

The range of application of groundbased EM methods is large. Initial applications were for mineral ore exploration and structural mapping of gas deposits. Increasingly

EM methods are finding application in environmental geophysical exploration and natural resource management. Applications include:

Mineral exploration
Mineral resource evaluation
Location of geological faults
Geological mapping
Groundwater surveys
Mapping contaminant plumes
Permafrost mapping
Environmental Contaminated land mapping
Landfill surveys
Natural Resource Management
Precision Agriculture
Salinity assessment and management
Mapping spatial distribution of clay content and cation exchange capacity
Soil moisture
As such, EM instruments have been used extensively to determine the spatial distribution of soil attributes including clay content, moisture, soil salinity, nutrient status, depth to a clay layer, identification of high sodium adsorption ratio and exchangeable sodium percentage and determination of organic carbon fraction.

All of these studies were based on the initial calibration, which determines the soil attributes that most contribute to the response of the instrument.


Natural resource management for cotton growing regions

Launch now

© Copyright UNSW 2007 | Terms of use | Privacy Policy