Transient electromagnetic response of a halfspace with exponential conductivity profile and its applications to salinity mapping

Abstract

Soil salinity profiles frequently display monotonically increasing or decreasing salt concentrations with depth, z. This salt concentration is strongly correlated with the conductivity, s, of the ground and frequently can be represented by an equation s (z) = s1exp(±|b|z), where s1 and b are the parameters that define the conductivity profile. For exponentially decreasing conductivity with depth, the late time coincident loop response is identical to that for a slab of thickness 1/b and conductivity s1. For exponentially increasing conductivity with depth, expressions can be found for the voltages recorded by a modified INPUT or an airship system. The INPUT system must be modified by having the measurement of the voltages made over delay times longer than 1 ms. Calculations show that these voltages are sensitive to variations in the parameter b and they are sufficiently large to be measured by an airborne system. The parameter s1 is difficult to resolve for late times. The particular numerical example chosen shows that the normalized voltage Vr is not very sensitive to s1 but is more sensitive to the parameter b, while the reverse is true for Vz. The parameter b is the more important of the two, as it shows the rate at which salt concentration varies with depth. Accordingly some idea of the salinity of the soil may be obtained from these EM methods.