Frequently Asked Questions

How to convert conductivity to resistivity?

Are there any difference between LandMapper ERM-01 and other equipment available on the market?

How the equipment calculates electrical resistivity?

How to calculate geometrical coefficient K?

Does electrical resistivity correlate with any soil properties?

Can your equipment measure electrical resistivity/conductivity in soil and water samples?

How to determine the depth of the measurements?

How to convert conductivity to resistivity?

LandMapper ERM-02  was demonstrated at 2008 Joint Annual Meetings of GSA-SSSA-ASA-CSSA . Next FIELD DEMO will be during 22^nd SAGEEP (Symposium on Applications of Geophysics to Engineering and Environmental Problems), March 29-April 2, 2009 at Fort Worth, TX. See News.

Electrical resistivity (ER) is reciprocal to conductivity (EC) and results can be easily recalculated using formula EC=1/ER or vice versa ER=1/EC. Electrical resistivity is more convenient to use on non-saline soils, where conductivity can be mSm/m, while resitivity measures in 10²-10³ Ohm m range. 1000 Ohm m equals 1 mSm/m.

Are there any difference between LandMapper ERM-01 and other equipment available on the market?

LandMapper ERM-01 is based on pulsating DC current, which principle fully accounts for spontaneous polarization arising at the electrode contacts with soil. This allows to measure electrical resistivity fast (4 sec at one location) and with great accuracy. Geonics EM-series uses electromagnetic induction principle and AC current to measure electrical conductivity without any contact with the ground. Although such measurements are very fast they are subjected to electrical interference from nearby metals and sources of electricity. Besides, not a single modification of EM method can evaluate soil layer shallower than 0.5 m. Soil EC mapping systems supplied by Veris, Inc. imply the same four-electrode principle as Landmapper-ERM1, but their equipment is tailored to precision farming market and can be used only with motorized vehicle on arable fields. Landmapper-ERM1 is more compact and versatile and can be used anywhere where a person can walk, i.e. in forestry, wetland and road assessments, construction sites, golf courses, etc. Ground penetrating radar (GPR) evaluates profile differentiation of soil electrical conductivity in shallow subsurface, but its application is limited on soils with high conductivity, such as salty and clay soils (Liner and Liner, 1997).

How the equipment calculates electrical resistivity?

Landmapper applies a pulsating current (I) through electrodes A and B and measures arising potential (ΔU) on electrodes MN in well-known four-electrode principle described by van der Pauw and Wenner. Electrical resistivity calculated from the Ohm’s law as

where K is the geometrical coefficient of the array. The equipment calculates ER automatically if the user enters an appropriate coefficient into the device memory.

How to calculate geometrical coefficient K?

If you using the equipment with the supplied array or laboratory cuvette, the right coefficient are already calculated. However, if you want to use your own array, the geometrical coefficient can be calculated from the distances between electrodes as follows.

For any array:

For center-symmetric array:

Does electrical resistivity correlate with any soil properties?

Electrical resistivity is correlated with a number of soil properties, which influence density of mobile electrical charges. For saline soils these properties are the total salinity and sodium adsorption ratio (SAR). For non-saline soils the relationships have been developed with soil texture, stone content, cation exchange capacity, humus content, Ca+Mg, etc. It is important to understand that soil electrical resistivity or conductivity are complex characteristics of soil and should be used together with other conventional methods of soil analysis. The usual path is to delineate the soil survey area into regions with different resistivity and than sample those areas to determine what properties are causing these differences. In some cases only one or two soil properties primarily influence electrical resistivity, and field electrical methods can be used to evaluate those properties in-situ.

 Can your equipment measure electrical resistivity/conductivity in soil and water samples?

Yes. The optional 4-electrode cell can be purchased from us or obtained from AGIS, Inc., or other companies providing standard geophysical equipment, or can be constructed by user. Using cell you can measure electrical resistivity or conductivity in soil samples, soil pastes and suspensions, or water, juices, etc.

 How to determine the depth of the measurements?

 For the Wenner array, when all the electrodes are equally spaced (AM=MN=NB=a) the depth of the measurements is approximately equal to the distance between electrodes (a), for Schlumberger center-symmetric arrays (MN<<AB) the depth of the measuring is about ¼ to 1/6 of the AB distance (Barker, 1989). The depth of the measuring depends on electrical resistivity of the soil as well as from the distances among electrodes. Thus, the exact depth can be obtained through the computer interpretation of vertical electrical sounding (Landvisor-ERI1). The program for interpretation of the 1D sounding is supplied with the equipment.

References

Barker, R.D.  1989.  Depth of investigation of collinear symmetrical four-electrode arrays.  Geophysics.  54:1031-1037.

Liner, C. L., and J. L. Liner. 1997. Application of GPR to a site investigation involving shallow faults. The Leading Edge. November. 1649-1651.

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