Thursday 3 May 2018

EM Values - What the data is telling you

Today we've got the second part of a 4 part series on EM Surveying and all it's uses. This week we've been into the EM Surveying over on the West Coast (check out our Facebook page if you want to see more) and it's certainly an important part of the job being out there doing the survey and seeing the physical aspects of the job to help make sense of the data and what it's telling you. Today we look at what the EM data does tell you...

An electro-magnetic (EM) sensor generates a constant electro-magnetic field that penetrates into the soil profile. It measures the bulk electrical conductivity of the soil profile. As we conduct an EM survey the sensor is taking readings at two different depths simultaneously. These two depths are known as the ‘Shallow EM’ and the ‘Deep EM’. The depths the DualEM reads depends on the height the machine is off the ground. With our EM setup we are reading the soil profile depth of 0-50cm for the shallow EM and the deep EM at a soil profile of 0-125cm. So the deep EM values are the same as the shallow plus another 75cm deeper. This is why the deep EM readings are always higher than the shallow as it is reading that extra 5cm.
Figure 1: Shallow EM survey values varying from 2-20 EM units (mS/m)

Figure 2: Deep EM of the same area with values ranging from 14-30 EM units (mS/m)
In this survey the same features are showing in the shallow EM and deep EM results, however sometimes this is not always the case the deeper profile can have a different underlying soil type that the shallow EM doesn’t pick up but the extra 85cm of deeper soil does and it changes the overall structure.

Generally speaking and depending on what part of the country you are in and the time of year the survey is carried out amongst other things, we would class a range in EM in the shallow profile of 1-3 units as low variability, 4-8 units as moderate variability and over 8 units range as high variability in the shallow layer/soil profile. In the deep EM/soil profile layer a range of 1-6 would be low variability, 6-15 moderate variability and over that high. It is often dangerous to generalise like that, but it gives you an idea of the type of ranges we look at, and as previously stated there are a lot of other factories that determine if the readings are low, medium or high variability. You also have to look at the distribution of the values as well, if the majority of the values are within a certain range and a few rogue values outside that but on a minimal area of the total, then the range in variation may not be as much as it first looks. How much the variability is costing you in terms of blanket irrigation applications compared to variable rate irrigation applications be it water, seed or fertilizer is a subject for another day!

For more information on EM Surveying please contact us at Agri Optics NZ Ltd.

Chris Smith.facebook