ESD
A Probe for Measuring Saturation Changes in Rock
function in the laboratory, six Significance of Findings ERPs (each a square with 0.02 m sides) were attached to the We were able to establish walls of a basalt column (0.20 that ERPs are effective for labom height, 0.06 m diameter) ratory scale experiments lookafter the column had been ing at water movement in the placed in a pool of water 0.01 rock matrix, in which both the m deep.As the column imbibed test bed and the sampling resowater, resistance changes along lution can be relatively small. the surface were continuously From field investigations, the measured. response of the ERPs compared Te m p o ral changes in re s i s twell with the performance of ance measured across the six psychrometers, suggesting that ERPs show the migration of a these probes (with their curwetting front along the basalt rent design) can be effectively column (Fi g u re 2a).The initially used as a qualitative tool to low resistance measured in the detect the arri val (or departwo lower ERPs reflects the ture) of wetting fronts in rock close proximity of the water matrix, fractures and faults. table.The diffe rence in the temUnlike psychrometers, these poral response observed in the probes are relatively inexpent wo higher zones suggests that sive, easy to maintain and have despite the wetting front re a cha low failure rate. This makes ing the higher probes,a gradient them part i c u l a r ly useful fo r in saturation developed with extensive down-hole monitorthe bottom of the column being ing applications in unsaturated wetter than the higher zones. fra c t u red ro ck env i ro n m e n t s Wetting front detection such as that found at Yucca following tunneling activiMountain. ties: At the ESF, plumes of water When installed on BSTs resulting from tunneling activialong with psych ro m e t e rs, ties (cutter head cooling, dust these ERPs provide an effective control, etc.) in the vicinity of a Figure 2. ERP responses observed in (a) laboratory and (b) field compilation of sensors, which new tunnel were tracked with experiments. provide data on relative wetERPs and psych ro m e t e rs. To ness and water potential changes at high spatial resolution.With monitor the plumes, a 30-m-long borehole (0.10 m ID) was constructed sloping downwards at an angle of 30° under the proERPs, the arrival (and departure) of a wetting front can be measposed path of the tunnel. Twenty-seven psychrometers and 54 ured at time intervals of less that a minute, while the spatial electrical resistivity probes located on nine BSTs were installed extent can be measured to a resolution of less than 0.10 m. in this borehole. Changes in water potential and electrical resistBackfill is not needed, increasing the accuracy of measurements ance were monitored along the entire length of the borehole and the relative ease of installing and removing sensors.The BSTs using psychrometers and ERPs located on BSTs as tunneling have the potential for further improving our abilities to measure activities progressed through the formation above. flow parameters in the unsaturated zone of fractured rocks. The ERPs responded in a pattern similar to that of the psyFunding chrometers located adjacent to the probes. For example, at a depth of 9.4 m where the water potential increased steadily from This work has been supported by the Director, Office of –400 m to –70 m, the corresponding ERP measurements followed Civilian Radioactive Waste Management, through Memorandum a similar pattern (Fi g u re 2b). Large fluctuations in water potenP u rchase Order EA9013MC5X between TRW Env i ro n m e n t a l tials in relatively short periods of time (~200 m in four days) we re Safety Systems, Inc., and the Ernest Orlando Lawrence detected by both types of pro b e s .The slower, m o re gradual Berkeley National Laboratory for the Yucca Mountain re c overy observed by psychrometers deeper in the fo rSite Characterization Project under U.S. Department of mation was also well tra cked by the ERPs (e.g., at 21.3 m; Energy Contract No. DE-AC03-76SF00098. Fi g u re 2c). http://www-esd.lbl.gov 60