The objective of this project was to demonstrate the performance of an electromagnetic (EM) system optimized for detection of small ordnance targets, such as 20 millimeter (mm) and 37mm projectiles, at depths greater than the standard “11x diameter” metric currently employed. The performance goal for this project was 20x the target diameter. Classification of targets was not an expressed objective of the project, except that considerable non-ordnance items of this size were expected to be detected by the system, so an approach for excluding these items from dig lists was needed. Classification of the sort demonstrated by cued instruments was beyond the expected capabilities of the data produced by this instrument. Ultimately, classification using inversion polarizabilities proved both possible and effective by utilizing bi-directional (orthogonal) surveying.
A total of four field operations were carried out during this demonstration. The first was a feasibility study using the one half of the existing airborne system without modification. The second was an initial shake-down of the modified system to determine the optimal field settings (base frequency, survey speed, platform configuration, transmitter power, vehicle offset, filter parameters, etc.). The third was a preliminary field test to ensure proper operability of the system at the specified settings before the fourth and final demonstration.
The initial feasibility study was conducted prior to the contract award. The feasibility study and initial shake-down were both conducted at the Camp Sibert Geophysical Prove Out (GPO) grid with the assistance of the U.S. Army Engineering Support Center, Huntsville. The preliminary field test was conducted at Battelle’s West Jefferson, Ohio Unexploded Ordnance (UXO) Test Grid. The final demonstration was conducted at the Aberdeen Proving Ground (APG) Geophysical Test Center.
This project met all of the original expectations for small target detection at depth, and greatly exceeded expectations in terms of discrimination and classification by combining orthogonal survey data. For the medium and large targets at normal depths (25mm-105mm down to 11x diameter), results were nearly perfect with a vertical receiver operating characteristics (ROC) curve. For the small targets at greater depths (20mm-40mm down to 20x diameter), all targets were detected at depths down to 20x diameter burial depth (probability of ordnance detection [Pd] 100%), and all except the 20mm targets could be discriminated with 100% probability of ordnance discrimination (Pdisc). The 20mm targets had Pdisc reduced to 0.87/0.90 (capped/uncapped), primarily at the greater depths. The ROC curve for these was very good, but not as vertical as for the medium/large targets. Classification of clutter (probability of clutter classification [Pcc]) was slightly lower than the corresponding Pdisc, reflecting a cautious approach to declarations. Pcc was 0.87 in the Blind Grid, and 0.52/0.61 in the Small Target Grid. This last metric was the only one that came close to falling below the design expectations of 0.60. All metrics surpassed those of the existing technology benchmark (EM-61 array). Originally intended purely as a detection tool and replacement for the standard EM-61 array, this system has demonstrated superior depth detection and comparable discrimination capabilities to other dynamic classification tools as demonstrated at the APG site.
The current prototype system is immediately available for implementation on small scale projects. Larger projects will require a more rugged platform to handle more difficult terrains for long periods. A commercial version of the system would require the design and manufacture of a simplified controlling console (the current console is primarily designed for airborne operations).
All data processing is currently handled within Geosoft, but a production version of the inversion software is required for routine work. Basic geophysical knowledge is required to operate and process the data. Any geophysicist familiar with the workflow for an EM-61 array can handle the basic processing. Analysis of the inversion results has largely been automated, but additional experience is required to optimize various parameters.
End users will be reluctant to accept results from this (or any) new technology on the basis of a single controlled-site demonstration. Additional demonstrations at live-sites will be required to mitigate those concerns. No potential regulatory restrictions have been identified for this technology.