A TDR will only need one conductor. They inject a signal, either light (OTDR) or an RF pulse (ETDR), into one end of a conductor then measure the time that it takes for the echo to return to the unit and then calculate the distance to the anomaly. Either a break in the conductor or, in this case, the end. I have seen an OTDR (optical time domain reflectometer) demonstrated on a fiber optic run a few kilometers long. I it showed where every conector and splice was within a meter or two. I have never seen an electrical ETDR used on a ground rod, but I am sure that it would work similarly.
