When an artillery round undergoes a low-order detonation during live-fire training or an unexploded ordnance clearance operation, up to 25% of the round's energetic contents are scattered over a small, localized area, sometimes less than 100 m 2. Training-range fate and transport models require an accurate representation of the particle-size characteristics of the material left behind from low-order detonations. This study investigated using laser diffraction particle size analysis to characterize 26 samples collected from four low-order command-detonated 81 mm mortar bodies filled with IMX-104. The refractive index of IMX-104 was estimated using an iterative recalculation technique on a Horiba LA-960 that yielded 1.845 0.01i. Of the 25 triplicate analyses conducted using this value, 12 passed the USP <429> measurement standard with 9 of the remaining samples found to have had a reduction in particle size during analysis that caused artificially high coefficient of variance values. The cumulative percent of particle sizes determined by laser diffraction and sieve stack differed by 0%-21.9% (median = 0.2%-7.2%). In addition, the higher resolution results of the laser diffraction particle size analysis, especially for particles smaller than 0.5 mm, make it the preferred method of analysis. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.

Discrete deep targets are a significant challenge for most surface-based geophysical techniques, even when considering high property contrasts. Generally, surface-based geophysical methods lose lateral and vertical resolution with depth as a result of poor measurement geometry and increased signal attenuation. The poor measurement geometry can be overcome through use of cross-borehole methods, but lateral localization is still needed for optimal borehole placement. As such, a relatively small, deep void located near the maximum depth of investigation is very unlikely to be detected. Yet, secondary features associated with these voids can be exploited for enhanced detection performance. When voids are located below the groundwater table a significant amount of dewatering and pumping is required to make them a functional passageway. This dewatering not only removes water from the void space but also the surrounding formation, resulting in a much larger, if more diffuse, secondary target: an induced groundwater table gradient. Many geophysical sensing methods are sensitive to subsurface moisture content. Here we implement a two-dimensional (2D) joint-petrophysical mixing-model, using inverted electrical resistivity tomography and inverted seismic refraction models to sense changes in the groundwater table gradient. Results are validated using depth to bedrock, groundwater-surface water information, ground-penetrating radar, and time-domain reflectometry methods. Our initial proof of concept is applied to a shallow area with a significant soil moisture gradient, through different surface soil types and bedrock. The 2D joint-petrophysical mixing-model results were used to generate an estimate of air, moisture, and matrix percent fractions in the investigation area, providing a clear delineation of the groundwater surface.

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