Earth Sciences Division
Nuclear Waste Program
Annual Report 1998-1999
A Probe for Measuring Saturation Changes in Rock Research
Objectives
comprised two electrical leads sandwiched between pieces of filter paper Rohit Salve, Tetsu K. Tokunaga In-situ measurements of hydrologic (Figure 1a). To maintain a consistent and Joseph S. Y. Wang p a ra m e t e rs at Yucca Mountain, geometry between probes, all electriContact: N evada, fo rm an integral part of cal leads were cut to the same length Rohit Salve efforts directed towards characterizaand meshed through a nylon fabric (510) 486-6416, r_salve@lbl.gov tion of flow and transport in the such that the distance between wires unsaturated zone of this potential site for high-level nuclear was the same for all sensors. Meshed nylon fabric was also used waste disposal. An important feature in this environment is the to provide a protective outer sleeve for the probes.The size of fractured, consolidated rock formation, which contains fractures the probes was determined by the type of application and of varying densities, orientations, spatial extents and apertures. ranged from squares with 0.02-m sides used for laboratory colBecause of this, unique problems arise with regard to instrument umn experiments to 0.075 m for borehole monitoring. placement and sampling techniques. While theories, methods Borehole Sensor Trays: To locate ERPs and psychrometers and experimental results developed from unconsolidated media in multiple locations along borehole walls, t rays we re designed to have initially formed the basis for the characterization of unsatup e rmit multiple arrays of sensors to be easily installed and rated consolidated media, re m oved in the boreholes, n ew techniques are now independent of bolehole orirequired (in addition to those entation. These trays we re established) to monitor fracfab ricated from PVC pipes. tured rocks. The outer diameter of each The primary objective of tray was selected to match this effort was to develop a the diameter of the borehole reliable tool for monitoring being instrumented (Figure the migration of a wetting 1b). Psychrometers we re f ront in boreholes located located in small diameter holes (~ 3 mm ID) dri l l e d within fra c t u red ro ck env ithrough the trays. ERPs were ronments. In this paper we attached to the outer surface p resent the design of of the trays with strips of Electrical Resistivity Probes Velcro.This assembly perm i t(ERPs) and Borehole Sensor ted ex t e n s i ve contact Trays (BSTs). We also include between the ERPs and the some results from ex p e riborehole wall while allowing ments conducted to evaluate the psychrometers to contact the ERPs in the laboratory the borehole wall through a and in boreholes within the small cav i t y. On each tray, Exploratory Studies Fa c i l i t y p s y ch ro m e t e rs we re (ESF) at Yucca Mountain. installed at typical distances Approach of 0.25-0.5 m along the borehole while ERPs we re Electrical Resistivity located at 0.25-m interva l s . Probes: The basic concept Two BSTs we re located defining the working of ERPs along each section of boreis that increasing water satuhole to permit sensors to be ration results in decreasing located on opposite sides.To e l e c t rical resistance. Probes i m p rove the contact were developed using a piece b e t ween probes and the of filter paper across which borehole wall, an adjustable changes in electrical re s i s twedge system was used. ance could be measured. The Results paper permits a flexible sensing surface that could be Figure 1. Design of electrical resitivity probes and borehole sensor trays. Water imbibition in a molded to the contours of basalt column: To test ERP the boreholes. These probes 59