The development of a feasible method for discriminating against metallic scrap, the predominant component of the false alarm rate in subsurface unexploded ordnance (UXO) investigations, has proved to be an elusive goal with current magnetic and electromagnetic methods. UXO site remediation costs are dominated by the false alarm rate, which frequently accounts for over 70% of the remediation costs. False alarms per UXO item are usually greater than 10 and sometimes as high as 100 or more. This project investigated a systematic approach to the physical description of measured fields in terms of the neglected near-field effects in magnetic data. The specific interaction of these near-field effects with the properties of metallic scrap has not been previously recognized. This interaction could provide a basis and methodology for discrimination against false alarms and these methods could reduce site remediation costs.
The objective of this research was to evaluate the feasibility of near-field physics as a means of discriminating against shallow scrap in magnetic UXO investigations.
Modeling the full three-dimensional (3D) geometrical complexity of a 80-mm projectile, 155-mm projectile and two pieces of irregular scrap provided the theoretical magnetostatic data required to perform the evaluation of near-field contributions to the total fields of these targets. Removal of the dipole field component was accomplished by fitting a standard prolate spheroidal model to the data and subtracting the resulting field. Residual fields were evaluated to determine if the non-dipole components are: measurable, of distinctive signature, enhanced in the scrap relative to the UXO and robust to typical noise sources. Additionally, practical sampling considerations for these fields were investigated to permit field collection and validation of the numerical results. Finally, field data were collected and analyzed to validate the numerical results.
Modeling results using the full 3D geometrical complexity of a 80-mm projectile, 155-mm projectile and two pieces of irregular scrap confirmed the richness of the magnetostatic-field (at depths of up to three body lengths) had not been previously exploited. Residual fields were obtained after subtraction of the fitted dipole fields at depths between 0.5 and 3.0 body lengths. These residual fields showed that the non-dipole components of scrap and UXO are measurable, distinctive in space and differ in amplitude, enhanced by as much as 10 times in the scrap relative to the UXO and robust to typical noise sources that interfere individually or in combination. Specifically, these residual fields were found to be of quadrupolar character in terms of spatial expression and decay rate.
The presence of such fields was predicted to arise in scrap, and possibly UXO, due to previously neglected broken symmetries. The quadrupolar components of scrap and UXO field residuals have not been previously identified. Residual signatures provide sufficient distinguishing characteristics to enable a practical discrimination methodology. For example, the signatures of an intact 155-mm projectile and a piece of scrap that is a substantial portion of its body were found to be noticeably different, raising the possibility of discriminating even between these closely related objects.
Practical sampling requirements were found to be more stringent than those commonly employed in field investigations and this led to a possible explanation for why prior field investigations have not detected these fields. Sampling of the residual fields requires consideration of the smallest target at the shallowest depth and may require tradeoffs between field collection time and data quality. Such tradeoffs can be made on a site specific basis. At present, a discrimination methodology using the quadrupolar-type field would be best applied as a cued technology. The unexpected identification of quadrupolar-type fields of UXO further enhances the method’s appeal as these fields potentially allow discrimination among different UXO, as well as between scrap and UXO. Field results from two UXO and two scrap items were found to be suggestive of quadrupolar-type residuals. However, positioning errors limited the extractable information and field validation of the numerical results remains an outstanding issue that may be resolved by further processing of existing data or collection of new data.
The results of this effort are promising for scrap discrimination as they indicate that non-dipole fields of modeled scrap are: measurable, stronger than the nondipole fields of modeled UXO, robust to noise sources and exhibit characteristic differences that serve to distinguish scrap from UXO at depths of up to a few body lengths. Additionally, because the modeled UXO also exhibit quadrupolar-type residuals and these residuals have characteristic signatures, it may also be possible to distinguish different UXO. It is recommended that these results be further investigated using additional forward modeling, development of new inversion and interpretation methods and, most importantly, field data collection to provide indisputable evidence for the predominance of the quadrupolar-type residual fields at up to a few body lengths.