Time Domain Reflectometry (TDR) is a remote-sensing electrical measurement technique that has been used for many years to determine the spatial location and nature of various objects. A coaxial cable is placed in the ground, and pulses are sent down the cable. When an earthquake or structural fault damages the cable, the readings on monitoring equipment attached to the cable will alert users to compromised structural integrity.
An early form of TDR, dating from the 1930s, which most people are familiar with is radar. Radar consists of a radio transmitter, which emits a short pulse of microwave energy, a directional antenna and a sensitive radio receiver. After the transmitter has radiated the pulse, the receiver then listens for an echo to return from a distant object. By measuring the time from the transmission of the pulse until the echo returns and knowing the speed of light, the distance to the reflecting object is calculated. Detailed analysis of the echo can reveal additional details of the reflecting object. Researchers soon recognized that there was a significant relationship between the dielectric properties of soil, rock and other materials and their moisture contents.
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