AUTOMATED TIME DOMAIN REFLECTOMETRY MONITORING
The MLTDR-WW-50 is a datalogger designed to automate monitoring of up to 8 coaxial cables embedded in a slope or borehole for deformation monitoring. It contains a control module, TDR interface and, optionally, an 8 channel coaxial cable multiplexer.
It may be equipped with an AVW200 interface to read Vibrating Wire gages. These gages are typically used to monitor pore pressures along the length of the installed TDR cable. The interface provides connection for 2 Vibrating Wire gages, a Canary Systems MicroMux or MiniMux multiplexer may also be installed to expand the measurement capability to 4 or 16 instruments.
A 50AHr lead-acid battery provides power, and is recharged either through a 40W solar panel or an AC power connection.
Communications are possible via a wireless or wired network interface. For wireless networks, a 9dBi directional antenna is supplied, including a lightning surge protector. A Lantronix WiBox is used for the network interface.
THEORY OF OPERATION
Time Domain Reflectrometry (TDR) is increasingly being used in geotechnical applications for deformation monitoring in soils and concrete. It provides accurate location information for faults and can provide indication of the static or dynamic nature of the faults. It is typically used to monitor slope movement of embankments including highway cuts, rail beds, bridge abutments and open pit mines.
TDR technology was originally developed by the telecommunications industry as a method for detecting faults, or breaks, in cables. The principle is similar to radar, where a signal is broadcast and distant objects cause a portion of the signal to be reflected back towards the generator. By measuring the time between transmission and reception, and knowing the speed at which the signal is traveling, accurate determinations of the distance can be calculated.
In TDR applications the signal is an electrical pulse broadcast down a coaxial cable, and changes in the impedance of the cable, either inductive or capacitive, cause reflections to be returned to the signal generator. The resultant reflections are measured and presented as a function of time, the time is translated to distance based on the known propagation velocity of the cable being utilized. In geotechnical applications the coaxial cable typically takes the form of a rugged coaxial cable with a diameter of 12mm (0.5”). The cable is usually grouted into a borehole drilled into the embankment to be monitored. Any movement of the embankment will crimp the cable, and the reflections indicate approximate magnitude of the crimp and the location. The resolution is related to the number of samples of data being collected for the waveform.
Data returned by the electronics is referred to as a reflection coefficient, or percentage of reflection, and is in a range of 1 (open circuit) to -1 (short circuit). A value of zero indicates no reflection. Typically, with very long cables there is some absorption of the signal down the length of the cable which shows as an increasing reflection coefficient. Deformations or abnormalities in long cable lengths will still result in an increase or decrease in the reflection coefficient at those locations. A sample data output is provided below.
All distances are “apparent”, not absolute. A key configuration parameter of the monitoring system is the velocity of propagation (Vp), this is the percentage of the speed of light that the electrical signal travels down the cable. Vp typically ranges between 0.65 and 0.90 but these are approximate values supplied by the manufacturer. In-situ modification of the cables, using a few crimps applied to the cable at measured distances during installation, can be useful to help establish more accurate distance measurements. Contact Canary Systems for more information on this technique.