Preliminary results with the VERIS soil electrical conductivity instrument

• Note this work is in Progress
• This version 9th April 1999*

1. Introduction

 1a. Figure 1 How does it work?

1b. What do the numbers mean?

2. Preliminary surveys

2a Creek Field

3. Future Work

3a. Practical

3b. Research

4. Conclusion

5. Acknowledgement

6. References

ACPA recently purchased a VERIS. Here we report on its use in a couple of Australian conditions. The VERIS is a continuously recording version of the old 4-electrode resistivity probe long used in archaeology for finding buried structures.

A resistivity meter involves applying a voltage into the ground through metal electrodes and measuring the resistance to the flow of the electric current.

A typical system of resistivity survey consists of four equally spaced metal electrodes [a so-called Wenner array] inserted into the soil. An AC-power source supplies current flow (I) between the two outer electrodes and the resultant voltage difference (V) between the two inner electrodes is measured. The resistance of the soil is given by R = V / I. This needs to be standardised over a unit length. The resistance times the length (of the resistor in this case the soil) is called the resistivity (r) which is measured in ohm m. The equation is,

r = 2pd R = 2pd V/I, unit: [w m],

where d is the spacing between the electrodes (in m).

Alternatively, this can be expressed in terms of conductance (C = 1/ R, unit ohm^-1 = siemens) and conductivity (c = 1/ r, unit ohm^-1 m^-1 = siemens m^-1). The equation for the (soil electrical) conductivity (EC) is given by,

c = 1/ (2pd R) = I / (2pd V) unit: [S m^-1]

In the Veris 3100 Soil EC Mapping System the electrodes have been replaced by rotating discs which are placed 6cm into the soil. As the cart is pulled through the field, one pair of electrodes passes electrical current into the soil, while two other pairs of electrodes measures the voltage drop.

The system is set up to switch between two configurations, let’s call them configuration (A shallow) and (B deep)

As you can see Configuration A uses the four inner discs (2, 3, 4 & 5). The voltage is measured between the two innermost discs (3 & 4) which are d = m apart. In Configuration B the four outer discs (1, 2, 5 & 6) are used and the voltage is measured between discs 2 and 5. When the electrodes (discs) are d metres apart the conductivity is measured to a depth of roughly 1.5d metres.

A more thorough review of the typical signal contributions for a "Wenner array" (very similar in principle and to the Veris) revealed the following figure from John Milsom's 1989 book, Field Geophysics. It can be seen from this illustration that the signal contribution between different electrodes and through the various depths reached by the array is complicated. Indeed it appears that the signal contribution is ridiculously complicated when different regions in the array at the same depth contribute readings of opposite sign. However, Milsom points out that in relatively homogeneous soil with a short separation distance between the electrodes (as is the case with the Veris), the opposite signs returned near the electrodes "cancel quite precisely". Of greatest importance is the fact that despite the complexity of the physics, the array returns a signal which is the net result of relatively linearly weighted contributions through the signal depth. Each electrode contributes relatively equally, as does each depth within the soil profile. Subsequently, the Veris 0-300mm and 0-900mm readings should closely match the soil average EC within these soil volumes. The Veris achieves two separate depth readings by switching between the 6 available discs to increase or decrease the distance "d" separating the "excite" and "measure" discs.

(Click to enlarge)

The contour plots of the contribution made to the measured signal by each unit volume of soil. In this illustration the red regions have positive contributions and the blue regions have negative contributions.

Want to see what the signal contributions look like under the Veris? Click Here!!

It depends. The numbers generated by the VERIS should vary according to local variations in the soil electrical conductivity (EC). Soil EC depends especially on electrolyte concentration and its connectivity or continuity within the profile.

This then depends in turn on a number of factors, many of which are correlated in the field:

Moisture content will effect electrolyte concentrations within the soil profile and also have impact on the soil solution connectivity. A relatively wet profile will be more likely to exhibit uniform conductive properties than one which is approaching permanent wilting point where dry areas in the soil profile will act as insulating regions and inhibit EC.

Texture, especially the soil clay content will effect VERIS results. Clay, which has a large surface area has relatively more charge capacity than sand and silt and subsequently has greater ability to accommodate electrolytes. Additionally, it is typical for soil dominated by the clay fraction to retain more moisture than soil with a relatively higher percentage of sand.

Bulk density (to a small degree). To some extent the more compacted a soil is (higher bulk density) the more likely it will be that there remains good connectivity across the profile. This should result in slightly higher EC readings.

Temperature may also influence VERIS results.

One of the first paddocks investigated by ACPA researchers was "Creek". Although only preliminary analysis has been performed, images of the results from a VERIS survey, durum wheat yield and a previously collected bare soil colour aerial photograph indicate a strong spatial correlation between each information layer

The first practical work which we intend to perform involves the collection of data for a number of fields for which we already have existing layers of data. This existing information which includes yield map data, soil property maps and various remotely sensed data will be compared with the Veris data in an attempt to help us understand exactly what correlation we should expect between these relevant data layers under Australian conditions.

1. calibration of instrument with respect to factors which affect EC
2. comparison with EM38

 Conclusion

We believe that the Veris is a useful instrument which will find routine use in agriculture as more land users seek to characterise and manage their country at a more intensive scale

We would like to thank Veris Technologies Inc. especially Eric Lund and Colin Christy for their assistance with the acquisition of a Veris sensor and reference material. We also thank the University of Sydney, especially Professor David Siddle, Pro-Vice-Chancellor (Research), for providing the resources to purchase the VERIS instrument.

Milsom, J. 1989. Field Geophysics. Open University Press, Milton Keynes and Halsted Press, John Wiley & Sons, New York.

THE UNIVERSITY OF SYDNEY
© 2007 - Australian Centre for Precision Agriculture