It is possible to obtain estimates in situ of volumetric
soil moisture content (SMC) as a function of depth using measurements of radio
wave reflection at the soil surface at several discrete frequencies. The
feasibility of the method was demonstrated through computer simulation by
deriving empirical relationships between the number of frequencies, the
frequency range, and the number of soil layers for which the SMC is estimated.
The SMC profile was obtained to a depth of 1.5 m by inverting, through
function minimisation, a simulation of the reflection coefficient from layered
dielectric materials. The number of soil layers in which SMC could be resolved
was found to be twice the number of frequencies used. The required bandwidth
increased with the number of soil layers from 20 MHz for 6 layers to 140 MHz
for 16 layers. Within some assumptions about the nature of the radio wave
propagation, the theoretical accuracy of the estimate depended only on the
quantisation error introduced by having to consider discrete layers of uniform
soil with finite thicknesses. The method may have potential for enabling
sensitive measurements of SMC profiles at lower cost and lower complexity than
methods which use analysis of steps or pulses, such as time-domain
reflectometry, and should be well suited for routine use by horticulturists
and soil researchers.
Time-Domain Terahertz Imaging of Layered Dielectric Structures With Interferometry-Enhanced Sensitivity
