Ed Perfect

UTK earth and planetary sciences

High-tech water witching

A dowser scans the ground with a fresh-cut, y-shaped twig held taut in closed, upturned palms. When the stick turns down, water is said to lie below. What the twig is alleged to do for the dowser, neutrons do scientifically for soil hydrologist, Ed Perfect.

Sensitive to hydrogen, neutrons are attenuated (scattered and/or absorbed) as they pass through samples of moist soil. Patterns of attenuated neutrons create images of the distribution of water in pores throughout the soil. Images collected at varying water pressures reveal how air replaces water inside the sample during drying, or vice versa during wetting.

Soil scientists use hydraulic properties to determine the soil's water-holding capacity, the time it takes for soil to adsorb rainfall or irrigation, and the rate at which water moves through saturated or partially saturated soils. Traditionally, such properties are measured based on experiments run on samples in containers.

"But, inside the sample, it's essentially a black box," says JDRD team leader Ed Perfect. As a result, the data obtained must be combined with assumptions about the distributions of air and water in the sample in order to predict flow and transport.

A newcomer to neutron imaging, Perfect joined forces with LDRD team leader Hassina Bilheux to develop neutron-imaging techniques to improve predictions of the behavior of fluids in plant-soil groundwater systems. Ultimately, the two teams hope to build a community of users to benefit from a future neutron imaging facility at the Spallation Neutron Source at ORNL.

Currently, Perfect conducts neutron imaging studies at ORNL's HFIR facility and at the National Institute of Standards and Technology in Gaithersburg, MD. His JDRD study combines data from both two-dimensional (radiography) and three-dimensional (tomography) analyses. When plotted the data form a water-retention curve, from which he can glean those all-important hydraulic properties. The results will be used to strengthen mathematical models for predicting flow and transport in soil, and oil-water displacement in underground petroleum reservoirs.

Additional information about Maldonado can be found on his departmental web page,
Ed Perfect

Neutron radiography images of water (blue) displacing air (green) as a soil column is wetted up from below
(Time 1 = initial dry state,
Time 6 = final wet state). Note the filter paper at the bottom of the column which remains saturated with water throughout.