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|Title:||Assessment of microgravimetry for UXO detection and discrimination|
|Authors:||Strategic Environmental Research and Development Program (U.S.)|
Butler, Dwain K.
Unexploded Ordnance (UXO)
|Publisher:||Geotechnical and Structures Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||ERDC/GSL TR ; 00-5.|
Abstract: A new capability to model the gravity anomaly field on the ground surface of buried spheroidal-shaped objects is developed. The modeling capability is applicable to predicting the gravity anomalies of objects such as buried UXO (prolate spheroid), land mines (oblate spheroid), underground storage tanks (prolate or oblate), landfills, and other localized features which can be approximated with a spheroidal geometry. The gravity anomaly signatures of ordnance items ranging from 105-mm projectiles to 16-in. projectiles and 2,000-lb bombs are modeled. Average lengths, diameters, masses, bulk density and density contrast are computed for 10 ordnance item types. The density contrasts of the ordnance items are assumed relative to a 2.0 g/cm³ soil, and range from ~ 1.5 g/cm³ for large bombs (e.g., 2,000-lb bomb) to ~ 5 g/cm³ for large projectiles (e.g., 16-in. projectile). Gravity anomaly signatures for these ordnance items are examined for maximum value (magnitude) and spatial wavelength to assess detectability. Relative to a nominal detection threshold (5 μGal), all the items are detectable at the shallowest depth, i.e., buried horizontally at a depth to center equal to half the diameter or just below the surface. Only five items, 1,000-lb bomb and larger, are detectable at depths ~ 0.5 m or greater, and only the 16-in. projectile is detectable at a depth of 1 m. The gravity anomalies of ordnance items will require measurement spacing of 0.25 to 0.75 m. An optimized survey detection (minimum) threshold (2 μGal) will approximately double the predicted detection depths for the ordnance items, but this minimum detection threshold will be difficult to achieve in the presence of geologic background anomalies. Results of a microgravity survey over a buried 155-mm projectile are presented. A positive gravity anomaly associated with the projectile is detected and the magnitude and wavelength are consistent with spheroidal model predictions. The gravity anomaly also indicates the general orientation of the projectile. The magnitudes of the measured and predicted anomalies are both ~ 4.5 μGal or approximately equal to the nominal detection threshold. The positive anomaly is superimposed on surrounding negative anomalies, and there are other closed anomaly features within the 3- × 3-m survey area. One of the additional anomaly features is positive and comparable to the ordnance item anomaly. The measurement spacing for the survey is 0.5 m, which is likely the highest spatial resolution microgravity survey ever conducted. In general, except for the largest ordnance items buried at very shallow depths, microgravity surveying is not a viable technique for detection and discrimination of buried UXO in real world settings.
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