The dependence of the electrical conductivity measured by time domain reflectometry on the water content of a sand

Abstract

We present paired measurements of the water content and electrical conductivity collected in a laboratory column packed with a homogeneous, clean sand over a wide range of water content and pore water electrical conductivity (EC) conditions. The EC was determined using the method of Nadler et al. [1991] from waveforms collected with two-rod time domain reflectometry (TDR) probes with and without baluns and with three-rod probes without baluns. Following Heimovaara et al. [1995], we calibrated the probes in saline solutions to account for the series resistance of the cable and connectors. The calibrated EC shows a nonlinear dependence on the water content that is well described by a simple power relationship [Archie, 1942]. Recognizing that calibration in saline solutions is impractical for some TDR probes, we demonstrate that the EC response can be calibrated directly using the results of drainage events, incorporating only a separate calibration of the cable resistance. None of the probe designs shows any clear advantage for EC measurement.