A New Device for In Situ Measurement of an Impedance Profile at 1–20 MHz

This paper describes a new instrument dedicated to determining the vertical profile of the admittance of a sensor embedded in a medium, such as soil. The instrument consists of two conductive parallel cylinders with a separation between axes on the order of 10 cm to scan a large volume of the medium. The device works at frequencies in the range from 1 to 20 MHz to increase the sensitivity to capacitance. The high frequency and the large size of the sensor bring about drawbacks like electronic instabilities and parasitic impedances. Accurate design and modeling of the electronic circuit have minimized these parasitic effects. The calibration procedure to test the model and adjust its parameters, as well as its results, is also detailed. These improvements permit reducing the uncertainty of the measure of admittance to less than 3% over the working range (0.001-0.1 S).