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EM surveying is carried out by towing the EM machine on a wheeled frame 25cm off the ground across the soil surface, using our light-weight Polaris, usually at a swath width of 12m, although this can be varied to suit the project.
The EM soil electro-magnetic conductivity readings are then logged five times every second at two depths simultaneously- using Trimble RTK GPS to 2cm accuracy. This gives comprehensive data on soil characteristics at 0 to 50cm and 0 to 125cm depths. The RTK GPS also collects elevation, slope angles and other topography details so a 3D surface map can be built if required. This information can be very useful in determining avoidance zones or other areas that may require a different management strategy rather than only in accordance with soil type and soil water holding capacity.
An EM conductivity survey measures and maps the variability in apparent electrical conductivity within the soil profile through the use of sensors without any requirement for soil-to-instrument contact. The measured conductivity can then be linked to different soil characteristics such as stone and clay content for zoning differing management zones.
At Agri Optics we use EM technology across a range of different applications including pastoral farming, arable farming and viticulture. While it is most widely used in agriculture in NZ, EM can also be used for applications in archeology and general soil science due to its maximum exploration depths of 1.5m.
EM surveying works by emitting an electro-magnetic field into the soil. The strength of this magnetic field and how far it can travel through the soil profile is determined by the textural composition of the soil. The magnetic field is strongest about 30-40cm below the soil surface and has the ability to sense to a depth of about 1.5m when in the vertical mode. A receiving coil reads the induced currents in the soil and it is these currents that measure soil conductivity.
The usefulness of soil conductivity stems from the fact that sands have a low conductivity, silts have a medium conductivity and clays have a high conductivity. Consequently, conductivity (measured at low frequencies) correlates strongly to soil grain size and texture:
In addition to its ability to identify variations in soil texture, electrical conductivity has proven to relate closely to other soil properties that often determine a field’s productivity:
From an EM Survey with Agri Optics you are supplied two highly detailed maps of your soil variability (0-50cm & 0-125cm) as well as a digital elevation model, slope, aspect & landscape change among other topographic derivatives if you purchase the topography pack. We provide desktop software called “VA Gateway” and an online viewing portal so you can access your data and use it. We also supply you with a hard copy of your report and maps. These maps can then be further ground-truthed depending on their target application and used for zone management.
The data collected has many crucial management uses:
These are the standard units of measure of bulk soil conductivity. A Siemen is a measurement of a material’s conductance; expressing the value in mS/meter removes the volume from the equation—just as a material’s density is independent of its volume.
The advantage of a standard unit of measure is that it makes the data quantitative. Visual identification of soils can often determine colour differences, but cannot attribute quantitative values to those colours. An EM soil map that shows values of X mS/meter enables you to identify and manage other areas of the field with similar values.
To find out more, click here to download the EM survey flyer or check out the PDF below:
Phone: 0800 482 682
Level 1 167 Main Street,
Canterbury, New Zealand