Soil moisture sensing via swept frequency based microwave sensors

• Published in: Sensors (Basel, Switzerland) Switzerland), 2012-01, Vol.12 (1), p.753-767
• Main Authors: Pelletier, Mathew G, Karthikeyan, Sundar, Green, Timothy R, Schwartz, Robert C, Wanjura, John D, Holt, Greg A
• Format: Article
• Language: English
• Subjects: Accuracy; Cotton; cotton moisture; Delay; Dielectric constant; Geology - instrumentation; Humidity; Hydrology; Loam soils; Measuring instruments; microwave moisture; microwave sensing; Microwaves; Moisture content; moisture sensing; Permittivity; Process controls; Propagation; saline; Salinity; Sand; Sensors; Signal Processing, Computer-Assisted; Soil (material); Soil - analysis; Soil moisture; TDR
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s120100753

There is a need for low-cost, high-accuracy measurement of water content in various materials. This study assesses the performance of a new microwave swept frequency domain instrument (SFI) that has promise to provide a low-cost, high-accuracy alternative to the traditional and more expensive time domain reflectometry (TDR). The technique obtains permittivity measurements of soils in the frequency domain utilizing a through transmission configuration, transmissometry, which provides a frequency domain transmissometry measurement (FDT). The measurement is comparable to time domain transmissometry (TDT) with the added advantage of also being able to separately quantify the real and imaginary portions of the complex permittivity so that the measured bulk permittivity is more accurate that the measurement TDR provides where the apparent permittivity is impacted by the signal loss, which can be significant in heavier soils. The experimental SFI was compared with a high-end 12 GHz TDR/TDT system across a range of soils at varying soil water contents and densities. As propagation delay is the fundamental measurement of interest to the well-established TDR or TDT technique; the first set of tests utilized precision propagation delay lines to test the accuracy of the SFI instrument's ability to resolve propagation delays across the expected range of delays that a soil probe would present when subjected to the expected range of soil types and soil moisture typical to an agronomic cropping system. The results of the precision-delay line testing suggests the instrument is capable of predicting propagation delays with a RMSE of +/-105 ps across the range of delays ranging from 0 to 12,000 ps with a coefficient of determination of r(2) = 0.998. The second phase of tests noted the rich history of TDR for prediction of soil moisture and leveraged this history by utilizing TDT measured with a high-end Hewlett Packard TDR/TDT instrument to directly benchmark the SFI instrument over a range of soil types, at varying levels of moisture. This testing protocol was developed to provide the best possible comparison between SFI to TDT than would otherwise be possible by using soil moisture as the bench mark, due to variations in soil density between soil water content levels which are known to impact the calibration between TDR's estimate of soil water content from the measured propagation delay which is converted to an apparent permittivity measurement. This experimental decision