Time-Domain Reflectometry (TDR) is widely used to measure soil water content, bulk electrical conductivity, and rock mass deformation. TDR measurements are nondestructive and offer excellent accuracy and precision.
The principal components of a TDR system are the CSI datalogger, TDR100 Reflectometer, SDMX50-series coaxial multiplexers, interconnecting cabling, and TDR probes. To control the TDR100, the CR10X and CR23X use Instruction 119 and the CR800, CR850, CR1000 and CR3000 use the TDR100 instruction. The CR10X operating system (OS) should be version 1 rev. 13 or higher and the CR23X OS should be version 1 rev. 10 or higher. All CR800, CR850, CR1000, and CR3000 operating systems (OS) are compatible with the TDR100.
Typically the system is powered with a user-supplied, deep-cycle battery that is recharged by a 20 watt solar panel. Installations that have access to ac power may be able to use the PS100 sealed rechargeable battery for a CR800, CR850, CR10X, or CR1000 installation, or the rechargeable battery incorporated in a CR3000 or CR23X's base.
PCTDR software is used during system setup and troubleshooting to display volumetric water content and electrical conductivity, switch multiplexer channels, collect waveform and derivative data files, and determine probe constant values needed for electrical conductivity measurements. PCTDR is included with the TDR100.
The system components are:
The TDR100 Time-Domain Reflectometer is the core of the Campbell Scientific Time Domain Reflectometry system. The TDR100 (1) generates a very short rise time electro-magnetic pulse that is applied to a coaxial system which includes a TDR probe for soil water measurements and (2) samples and digitizes the resulting reflection waveform for analysis or storage. The elapsed travel time and pulse reflection amplitude contain information used by the on-board processor to quickly and accurately determine soil volumetric water content, soil bulk electrical conductivity, rock mass deformation or user-specific, time-domain measurement. Use the link at the right to learn more about the TDR100.
The SDMX50-series multiplexers are eight-channel coaxial switching devices used in our TDR System. Three levels of switching allows up to 512 soil water content or rock mass deformation cables to be connected to one TDR100. The multiplexers are controlled by a CR10X or CR23X datalogger during automated measurements. The multiplexers can be controlled by the TDR100 when using PCTDR or connected to a PC.
Three multiplexer models are available: the SDMX50, SDMX50LP, and SDMX50SP. All provide reliable and programmable channel selection, but are packaged differently to allow flexibility for a range of installation methods. Explore the differences between our three models using the links provided on the right.
The reflectometer, datalogger, multiplexer and power supply should be housed in an environmental enclosure to protect the equipment from weather, condensing humidity, and dust. Campbell Scientific offers the ENCTDR100 for this purpose. It can house the datalogger, datalogger’s power supply, TDR100, and SDMX50SP (the SDMX50 includes its own enclosure and the SDMX50LP is intended to be mounted in a separate user-supplied enclosure). The ENCTDR100 includes interconnecting SDM and coaxial cabling, grounding wires, desiccant, humidity indicator, and hardware for mounting the enclosure on a pole, tripod mast, or tower leg.
The TDR probes act as a wave guide. Impedance along the rods varies with the dielectric constant of the surrounding soil. Because the dielectric constant of soil primarily depends on the amount of water present, soil volumetric water content can be inferred from the reflected measurements. Soil bulk electrical conductivity is determined from the attenuation of the applied pulse.
Campbell Scientific has two soil probe models available. Both models consist of three pointed, large-diameter rods and a large epoxy head allowing use in rugged environments. The models only differ in their connector cables, and are selected based on the desired cable length.
Rock Mass Deformation Measurements
Time Domain Reflectometry is used to detect deformation within a rock mass. The system is commonly used to monitor deformation associated with landslide, mining, and construction activities. A length of coaxial cable, grouted into a borehole, serves as the system’s sensor. Electronic pulses are sent down the cable; reflected pulses are related to deformation of the cable or to pre-established reference points (crimps). Areas of offset in the resulting trace depict zones of extension or shear along the cable.
TDR was applied to measurement of rock mass deformation by Panek and Tesch (1981) and O’Connor and Dowding (1984). A coaxial cable is emplaced in a borehole and grouted into place. Crimps, at measured intervals along the cable, partially reflect the transmitted signal and provide a more accurate scale for correlation of deformational zones to depth (Dowding, Su, and O’Connor, 1989). Crimps appear as small negative polarity events along the trace of the waveform. Events that offset the waveform indicate deformational zones; the polarity of the offset indicates whether a zone is experiencing tensile or shear deformation.
The Campbell Scientific TDR system is optimized for remote use. A CR10X or CR23X datalogger is housed in a weather-resistant enclosure along with a TDR100 Reflectometer. The datalogger, using Instruction 119, controls operation of the reflectometer; power is supplied only during measurement. This significantly reduces power demands. The datalogger digitally stores waveform data from a sequence of cable tests, thus allowing the system to operate in an unattended mode. Data are retrieved via telecommunications. It is also possible to reprogram the datalogger through telecommunications allowing zones of interest to be thoroughly examined. Multiplexers controlled by the datalogger allow the system to test as many as 512 cables.
Because of the specificity of preferred coaxial cable to a particular installation, as well as the shipping costs and logistical difficulties associated with transporting long lengths of coaxial cable, Campbell Scientific recommends that customers acquire suitable coaxial cable from a local electrical parts supplier.