Rapid, Accurate Mapping of Soil Salinity by Electromagnetic Ground Conductivity Meters

Abstract

The saturated paste extract method is widely accepted as an accurate indication of the degree of salinization of a soil sample. This method basically measures the electrical conductivity of a carefully prepared sample of the soil and, while accurate, is time consuming to carry out because of the preparation procedure. It is also a “point-sample” method, leaving doubt as to whether the measurement is representative. To overcome these limitations extensive investigation into the use of conventional in situ direct current (DC) resistivity techniques has been carried out to determine conditions under which this technique accurately measures the degree of soil salinization. However, conventional DC resistivity measurements are also relatively slow to carry out. Instruments using inductive electromagnetic techniques are now widely used to map terrain conductivity. These devices, which electromagnetically induce small currents in the ground, measure the magnetic field strength generated by these currents to determine terrain conductivity. They are well suited to assessing soil salinity since they respond to more conductive (and thus more saline) soils and, furthermore, do not require electrical contact with the ground. Exploration depth of these devices is determined by the spacing between the transmitter and receiver coils; commercially available instruments measure from a depth of approximately 1 m to several tens of meters. Their chief advantage is that large areas can be surveyed quickly, and thus in considerable detail. A significant disadvantage is that variation of conductivity with depth is not well resolved. Because of various factors other than soil salinity which can affect the bulk soil conductivity, saturated paste extract samples must still be taken, but now at much reduced intervals and at locations now known to be statistically significant. The theory of operation of electromagnetic ground conductivity meters is briefly described, comparative survey results (with both saturation paste extract and DC resistivity measurements) are shown, and survey results over the depth range referred to above are presented.