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).

I. Introduction

This paper describes an instrument that is able to determine the vertical profile of the admittance of a sensor embedded in a soillike medium. Its novel design is the subject of a patent [1]. The purpose of this instrument is to obtain the dielectric permittivity and the conductivity of the medium at different heights and for various frequencies between 1 and 20 MHz. These variables are closely related to some medium characteristics, such as its water content, its salinity, and its structure [2]. They permit distinguishing different phases of a fluid [3]. The frequency scan can single out the influence of one of these characteristics and hence allows its determination [2]. The vertical profile gives information, such as the vertical water flows occurring in the soil. Dielectric measurements have the advantage over direct methods, such as double weighing that monitoring can be done in situ and continuously. In practice, the sensor determines the admittance of a capacitor that consists of two parallel cylindrical conductors, or electrodes, surrounded by the medium. The instrument and the medium are considered together as a capacitance in parallel with a resistance. The admittance is obtained from the voltage–current measurements between the two electrodes. Along their axis, the electrodes are divided into independent parts to achieve a vertical profile of the admittance.