Proceedings Article | 24 September 2002
KEYWORDS: Reflectometry, Liquids, Dielectrics, Remote sensing, Soil science, Hydrology, Reliability, Head, Multiplexing, Telecommunications
Time domain reflectometry (TDR) instrumentation is widely used in hydrology and soil science for accurate and flexible soil water content measurements. The most attractive advantages concerning the considered TDR measurement system are: good precision and accuracy, high reliability of the measuring head, an unique approach of pulsing a long coaxial probe and analysing the reflected voltage signature caused by changes in impedance, capability of multiplexing several probes, possibility of remotely acceding, controlling and electronically retrieving and transmitting data through existing telecommunications.
A time domain reflectometer transmits the incident signal, an ultra short rise time (200 ps), step voltage pulse, along the transmission line and records the travel time and magnitude of all reflected signals (echo) returning from the controlled system. Changes in capacitance, impedance, inductance and resistance causing electromagnetic discontinuities that reflect voltage can be located, particularly, for liquid level and soil dielectric properties monitoring purposes, discontinuities result from impedance changes produced by changes in the dielectric constant. Moreover, different systems are currently used to measure liquid levels in stocking tanks or vessels. The time domain reflectometry method used in this research has the purpose to monitor the behavior of the different liquid interfaces, detecting their levels. One of the the goals of present paper is to enhance field measurement capabilities; miniature pulsing and sampling cards have been used to create a smaller and more rugged time domain reflectometer, associated to the special steel probes working like a closed circuit radar, detecting any mismatch along the measuring lines. Measurements can be performed in a wide range of environmental conditions, independently from the nature or properties of the involved substances, giving information about their characteristics, such as volumetric content, dielectric constant, emulsions, dispersions, etc. Agreements with current TDR research data have been found. Furthermore, by characterizing several complex systems, the main objective of the present work is to develop an interpretation method based on the changes in the reflected TDR signal caused by the presence of hydrocarbons and their concentration in a soil.
