A TIME DOMAIN REFLECTOMETRY BOREHOLE PACKER FOR USE IN ROCK

Over the previous two decades time domain reflectometry has been developed for measuring moisture content in soils, and has been widely used by soil scientists. However, its application for monitoring moisture content in rocks is less well developed, because of the difficulty in inserting the waveguides to the required depths and without air gaps. Previous attempts to solve this problem have focussed on the use of short borehole packers (with TDR waveguides embedded in their walls) that are inserted in uncased boreholes on a temporary basis. Data is presented which shows that such a system produces significant errors when used to collect time series moisture content data, due to the repeated repositioning of the TDR waveguides between subsequent readings. Furthermore, open boreholes may act as preferential flow paths for water. A new borehole packer system is presented in which a long packer with multiple TDR waveguides is permenantly installed by filling with grout. The waveguides are interrogated sequentially using a multiplexor, allowing the option of automatic data collection using an on-site datalogger. A laboratory-derived calibration and analysis of the spatial sensitivity of the new device is presented.

L. J. West & P. Winship
School of Earth Sciences
University of Leeds
Leeds

Y. Huang & M. Nakhkash

Department of Electrical and Electronic Engineering
University of Liverpool
Liverpool, England