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The Use Of Time Domain Reflectometry For Vertically Profiling The Water Content Through The Unsaturated ZoneNormal access

Authors: P.A. Ferre and D.L. Rudolph
Event name: 11th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Methods Emphasis: Developments In Direct Push Technologies
Publication date: 22 March 1998
Organisations: EEGS
Language: English
Info: Extended abstract, PDF ( 1.2Mb )

Standard, continuous-rod time domain reflectometry (TDR) probes, installed
vertically at the ground surface, measure the average water content from the ground
surface to the ends of the rods. Alternative probes have been described that allow for
vertical profiling of the water content beneath a single surface point. However, no single
probe design is optimal for all measurement needs Based on analyses of the properties of
published TDR probe designs, we make suggestions regarding the conditions for which
several probes are best suited. We suggest the use of two-rod, standard probes wherever
possible. Often, interval differencing of the responses of a series of vertically installed
probes of different lengths can provide an approximate measure of the water content
profile in a relatively homogeneous medium. Horizontally installed standard probes can be
used to improve the resolution of the water content profile if the disturbance caused by
their placement is acceptable. Two surface probe designs have been presented that can
measure the water content very near the ground surface. Two access tube designs have
been presented that can measure the water content over several preselected depth
intervals. However, probes with coated rods and probes that place the rods entirely within
an access tube will underestimate the water content if the water content varies along the
rods. In addition, the sample volume of these probes will vary with the water content of
the medium, becoming restricted to a region immediately adjacent to the coatings or
access tubes in higher water content conditions.

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