REMOTE DIODE SHORTING IMPROVES MEASUREMENT OF SOIL-WATER BY TIME DOMAIN REFLECTOMETRY

We describe a new time domain reflectometry (TDR) technique that increases the reliability of soil water content measurements. Remotely switched shorting diodes in combination with differential detection techniques greatly increase the effective amplitude of problem reflections and significantly reduce background noise. Reliable detection of the two reflections that define the apparent dielectric constant of soil was achieved in all laboratory and field conditions. This, for example, enables measurement of volumetric water content in saline or layered soils and allows the use of long connecting cables. Additionally, increased signal-to-noise ratios should allow the development of significantly less complex and expensive measurement systems. A three-rod probe with a single diode was used to evaluate reflections from the air-soil surface; reflected signal levels were increased 10:1 for short cables and 56:1 for a probe located at the end of a 100-m cable. A three diode probe was employed to demonstrate performance in a simulated layered soil; the correct time of arrival of the reflection from the rod end was readily measured even though the reflection was attenuated by a factor of 2500:1. A strip-line two-segment probe was used to measure water content in two adjacent sand layers, providing a 23:1 signal-to-background noise ratio. For all these probes, waveform subtraction was used to remove virtually all background noise.