Height-above-ground effects on penetration depth and response of electromagnetic induction soil conductivity meters

Abstract

Ground conductivity meters such as the Geonics EM-31 and EM-38 are used to measure apparent soil conductivity. Various agriculturally useful soil properties can be estimated from these readings. For most surveys the instruments are placed as close to the ground as possible. One of the advantages of increasing the instrument height is that the depth of penetration of the instrument increases. However, increasing the height of the instruments also reduces the magnitude of the apparent conductivity readings. If conductivity varies in a field, the decrease in amplitude will decrease the size of the conductivity differences. This makes it harder to discriminate between soils with different conductivities when the conductivity differences are small. Creating a map of conductivity ratios which divide conductivity values throughout a field by a typical conductivity in one area of the field avoids this problem. Conductivity ratio maps make it easy to map spatial variations in conductivity. Increasing the instrument height has little effect on the ratio maps, even at heights of 1 m. This technique is most useful under non-saline conditions when conductivity differences between soil types may be small.