TDR (time domain reflectometry) systems can directly detect shear or tensile displacement within a geologic body quickly, effectively and at a reasonable cost. Its digital character allows real time monitoring and analysis with telemetry or cable based data transfer, once again at a reasonable cost. These factors single out TDR and OTDR based networks as the method of choice for sub-surface ground dislocation detection systems.
TDR is based on transmitting a waveform with a known speed of propagation within a medium and watching for reflections of this transmission. As the velocity of propagation is known, the distance to the disturbance can be calculated.
The most familiar form of TDR is that of radar. Radio waves are transmitted in a predetermined direction at a specific frequency. Any object that interferes with this signal will cause reflections. The arrival time and characteristics of these reflections are measured by sensitive detectors, allowing determination of the distance to the object. Sonar utilizes the same principle, with the exception of the transmitted wave being that of sound within water.
TDR monitoring systems for ground motion detection are generally of two types.
- Standard TDR utilizes an electrical signal pulsed along a coaxial cable. Distortion of the dielectric in the cable, or complete cable shear, provide detectable return signals. This will detect kinks, shear, and tensile displacement.
- OTDR stands for optical time domain reflectometry. This is fiber optic based with a laser pulse being transmitted down line. Bends in the glass fiber provide points of detectable light loss and reflection, as do fiber breaks.
Both are reliable means of detecting displacement when the cable is encapsulated in a geologic material, with accuracy as to location of cable deformation to within about 0.3m (1 ft) down the length of the cable. Of course, the accuracy is also a function of cable length and quality. Extreme cable lengths show a reduction in accuracy.
A more extensive discussion of TDR and OTDR techniques is found in the PDF files listed to the right.