Monday, 7 November 2005
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The Effect of Layered Heterogeneity on the Response of Neutron Moisture Probes.

Jason M. Keller1, Andy L. Ward2, Robert E. Schenter1, and Richard S. Wittman1. (1) Pacific Northwest National Laboratory - Battelle, PO Box 999, K9-36, Richland, WA 99352, (2) Pacific NW Nat'l Laboratory, PO Box 999, K9-33, Richland, WA 99352

The distribution of sedimentary facies control heterogeneity of hydraulic properties and difficulties in characterizing small-scale changes are known to contribute to model uncertainty. High-resolution neutron moisture logs can be useful stratigraphic indicators but the exact location of facies boundaries are difficult to interpret due to smearing of the probe response over the soil layers. We explore the effect of layering on neutron probe response with the neutron and gamma ray transport code, Monte Carlo Neutral Particles (MCNP). MCNP was used to simulate the neutron response in a 3-layered engineered porous medium and to quantify spatial sensitivity of the neutron probe as a function of moisture content. Predicted neutron response was in good agreement, with probe measurements accurately capturing the effect of the wetter layer sandwiched between two drier layers. Deconvolution of the simulated response produced a rectangular response whose boundaries coincide with the known layer interfaces. Spatial sensitivity analysis show a decline in sensitivity beyond a radius, r, with both r and the rate of decline depending on moisture content. The results suggest that with knowledge of the probe response, deconvolution of measured neutron response will assist in delineating the location of soil textural layer interfaces and in evaluating neutron moisture probe measurement volume. The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RL01830.

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