My research in this area has focused on soil moisture measurement and on the physics of freezing soils. In the late 1980's I began working with time domain reflectometry, benefiting greatly from help provided by Clarke Topp and Frank Dalton. This culminated in the first automated, multiplexed TDR system, described in Baker & Allmaras (1990). The initial system used an analog Tektronix TDR coupled to a Campbell Scientific 21X datalogger (see photo below). The major challenge was writing a CSI program that could analyze the waveform to determine pulse travel time. When the Tektronix digital TDR became available we coupled that to a computer, which allowed more sophistication in waveform analysis, including simultaneous determination of volumetric water content and bulk electrical conductivity.
Subsequently, Egbert Spaans and I began using TDR in frozen soils research, taking advantage of the fact that liquid and frozen water have very different dielectric permittivities. In Egbert's dissertation research we developed the theory for using TDR in frozen soils Spaans & Baker, (1995) and used it to explore similarity between the moisture characteristic and freezing characteristic curves Spaans & Baker, (1996).