In time domain reflectrometry, electrical pulses travel down the cable, and the reflected energy is recorded by the TDR instrument. Any impedance change, such as cable end or cable damage, will cause some energy to reflect back toward the TDR instrument where it is displayed. The method can also be used to detect bridge scour, monitor pier and abutment movement during flood events, landslides and mass movements.
TDR uses electromagnetic impulses that travel along a cable and are reflected when they encounter an impedance change. This can occur at the end of the cable or at a break or imperfection. The TDR instrument measures the time that the signal takes to travel along the cable and return to the instrument. Knowing the speed of travel of the wave then allows the traveled distance to be calculated.
Cables that can be used with the TDR method are illustrated in figure below. In the case where two cables are shown impedance occurs between the two central conductors.
Cable configurations for use in the Time Domain Reflectometry method.
TDR instruments detect reflections from the cable ends or imperfections in the cable, as described above. Figure below shows the TDR signals from the ends of the cable and an imperfection in the center of the cable.
Time Domain Reflectrometry signals from cable ends and a thinner section of the cable.