ARS | Publication request: Soil moisture sensing via swept frequency based microwave sensors

Submitted to: Sensors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 29, 2011
Publication Date: January 11, 2012
Citation: Pelletier, M.G., Karthikeyan, S., Green, T.R., Schwartz, R.C., Wanjura, J.D., Holt, G.A. 2012. Soil moisture sensing via swept frequency based microwave sensors. Sensors. 12(1):753-767.

Technical Abstract: Accurate measurement of moisture content is a prime requirement in hydrological, geophysical, and biogeochemical research as well as for material characterization, process control, and irrigation efficiency in water limited regions. Within these areas, consideration of the surface area and associated bound water content is becoming increasingly important for providing answers to many fundamental questions ranging from characterization of cotton fiber maturity, to accurate dielectric measurement methods of soil water content for hydrological assessment and efficient irrigation practices. One promising technique to address the increasing demands for higher accuracy water content measurements is utilization of electrical permittivity characterization of materials. This technique has enjoyed a strong following in the soil-science and geological community through measurements of apparent permittivity via time-domain-reflectometry (TDR), as well in many process control applications. However, many current applications require accuracies beyond that available from traditional TDR and not subject to inherent difficulties associated with waveform interpretation. The most logical pathway then becomes a transition from TDR based measurements to frequency-domain based network analyzer style measurements of absolute permittivity that will allow for removal of the adverse effects that high surface area soils and conductivity impart onto the measurements of apparent permittivity utilized in the traditional TDR approach. Unfortunately, network analyzer style measurements, while known for their accuracy, are an expensive alternative that typically precludes their use except for the most demanding research applications. Thus, a need exists for a low-cost high-accuracy frequency domain based measurement approach. This study assesses the performance of a new frequency swept microwave frequency domain instrument (SFI) that has promise to provide a low cost, high-accuracy alternative to the traditional and more expensive TDR. Permittivity measurements of soils obtained using the experimental swept frequency were compared with a high-end 12 GHz TDR/TDT system. For the study, three soils were selected: a sand, an Acuff loam, and an Olton clay-loam, to provide a range of soil types, conductivities, and dielectric losses by which to compare the performance of the SFI technology to the TDR measurements of permittivity across a range of saturation percentages.