Monday, November 13, 2006 - 2:45 PM

Modeling the Permittivity of Saline Solutions and Porous Media using a Coated Coaxial TDR Probe.

Hiruy Abdu1, David A. Robinson2, and Scott Jones1. (1) Utah State University, 4820 Old Main Hill Ags 160, Logan, UT 84322-4820, (2) Stanford University, Dept of Geophysics, 397 Panama Mall, Stanford, CA 94305-2215

The application of time domain reflectometry (TDR) in the measurement of water content in the near subsurface is inhibited in samples with high electrical conductivity (EC). TDR waveforms show a loss of signal amplitude in a saline environment due to the conductive nature of ionic solutions. Recent studies have shown that coated TDR probes allow the determination of permittivity in solutions with high EC, but extracting the permittivities of the medium of interest from the TDR signal requires advances in modeling. The objectives of this study were: (i) to measure effective permittivities in highly saline solutions and porous medium using coated probes and (ii) to integrate appropriate models describing a system comprising of a coated coaxial probe containing a solution or porous medium. A 12.5 cm long coaxial TDR probe with a center-rod sheathed with two different thicknesses of PTFE tubing (εr=2.1) was connected to a Textronix 1502B cable tester. The TDR wave acquisition program WinTDR was used to collect a 1004-point wave trace each for the saline solution with EC ranging from 1 to 250 dS m-1. A transmission line model describes the effect of the coatings on the apparent dielectric permittivity of the medium. The measured permittivity was fitted to the predicted permittivity from the coated probe model to obtain a correlation and to extract the medium dielectric permittivity. Measured solution permittivities were well described by the coated probe model with R2 values of 0.88 and 0.87 for the 254μm and 508μm PTFE coatings respectively.  We have also designed a guide for optimal selection of coated TDR probe geometries. The motivation of the study is to facilitate accurate water content determination in saline environments using coated probes.