Magnetic discrimination that will satisfy regulators ?

Discrimination, or the minimization of false alarms, is recognized as one of the major challenges in efficient UXO clearance. With appropriate historical knowledge as to what weapons were used at the site and geological conditions that are not too adverse, we believe that discrimination using magnetics can be structured in a way that would be very well regarded by the regulatory community. This conclusion is based on analysis of high quality total-field magnetic data collected over the Guthrie Road and Limestone Hills sites in Montana. These Montana case studies, also show that there is a significant difference between the magnetic signature of seeded UXO and that from live-site UXO which suggests that the measurement of discrimination performance on seeded sites may have disadvantaged magnetics in previous evaluations of this technology. The core of the discrimination method is an understanding of the remanent magnetic properties of live UXO and the inversion of individual anomalies for the best fitting magnetic dipole. Using the recovered dipole moment it is not possible to uniquely classify the item as ordnance/non ordnance, nor is it possible to unambiguously determine the ordnance type. Rather, the items can be placed in a list ordered by the likelihood that they are UXO. The ranking is based on how closely the recovered moment matches the predicted dipoles drawn from a library of ordnance items expected to occur in the area. Anomaly excavation is then prioritized based upon the discrimination ranking list. Data available after the validation of dig sheets by excavation in the Montana case study areas has demonstrated that all live ordnance items are recovered after less than 50% of anomalies are excavated. In the Guthrie Road case study a False Alarm Rate (FAR – defined as the number of false alarms divided by the number of UXO recovered) of 3.4 was achieved and in the Limestone Hills the FAR was 4.5. In both cases these excellent FAR results were achieved with 100% detection of the UXO. At Guthrie Road, this FAR result was obtained with all of the seeded targets detected. In a clearance operation a decision needs to be made regarding how far down the list the excavation stops. An initial estimate of the number of items to be recovered can be obtained by specifying a maximum remanent magnetization. This number can be modified depending on the yield of ordnance as the excavation progresses. Excavation of additional items further down the list is undertaken as part of a QC procedure. In summary, discrimination of magnetics, based on sound physical modeling and objective decision making criteria, is potentially capable of exceeding the requirements of regulators.