One of the advantages of a real time monitoring system is the capability to employ automatic warning systems for detection of ground motion.
The base assumption for all such TDR warning systems is that the system can discriminate an "event" or physical change in the cable's characteristics, from noise. As TDR traces can be quite noisy, especially with longer cable lengths, physical damage, and environmental effects, this relatively simple task can become quite a challenge.
The best approach, at present, appears to be that of describing two levels of alert:
- The first level of alert is a warning. This is triggered by what appears, as delineated by algorithm, to be a deflection of the cable (a kink). While seemingly obvious, detection of such low level events can be difficult. Small changes in ground motion, system variables, and external interference tend to vary the magnitude, and thus detectibility, of kinks. On one day the spike on the trace is obvious, on the next it may be gone, only to re-appear the following day. If such levels of displacement are critical to the application, it is advised that the alerts generated by algorithm be checked by a qualified person utilizing a quality TDR instrument.
- The second level of alert is cable short/shear. The truncation or shorting of a cable is a fairly traumatic and relatively easily detectible event. Once again, however, when this alert is issued by the system, it should be verified by an operator with a TDR.
Event alerts can be displayed in a variety of fashions - flashing lights and sirens at the source of the disturbance, paging of operators, etc. All are dependent upon the sophistication and requirements of the client.
For those who are interested, two traces analyzed by trace alert software are to the right. These are animated GIFs, shown as a function of time. Trace1 is a relatively noisy cable whereas Trace2 is better behaved.