Monitoring subsidence mechanisms using time domain reflectometry

Time domain reflectometry (TDR) is a pulse-testing technique that was developed by the telecommunications and power industry for use in locating breaks in cables. A voltage pulse is launched into coaxial cables or parallel pair wires by a TDR cable tester and a portion of the voltage is reflected back at any location were there is a change in the electrical properties. The traveltime can be converted to distance along the cable or wire by knowing the propagation velocity. The cable tester displays a waveform of reflection coefficient versus distance for the entire cable or wire, and changes in the waveform shape can be correlated with changes in the electrical property at every location where a reflection is detected. This technique has been adapted for monitoring changes in soil moisture, as well as deformation in rock and soil masses. Recently, the technique has been used to monitor changes in water levels in standpipe piezometers. Summaries are presented for studies in which TDR was used for monitoring subsidence over a high extraction coal mine and subsidence at an in-situ sulfur mine. In all these cases, TDR provided heretofore unrealized information about mechanisms associated with subsidence and also provided the capability to monitor subsurface displacements that occurred prior to surface subsidence.