One of the recommendations of the 2003 Defense Sciences Board (DSB) Report on unexploded ordnance (UXO) was to immediately assess the scope of ordnance contamination of roughly ten million acres of land on formerly used defense sites (FUDS) and base realignment and closure (BRAC) sites and rapidly ascertain what percentage of this acreage actually contains UXO. Airborne technologies are well-suited for acquiring data over sites comprising thousands of acres and assessing the degree of UXO contamination. In particular, helicopter-based magnetometry has been shown to be effective in detecting individual UXO objects of a range of sizes. However, because magnetic field strength falls off as one over the cube of the distance (1/R3), the increased sensor height of helicopter-based sensors makes detection of objects smaller than 60 mm very difficult. The Geonics EM61 pulsed induction sensor, frequently the sensor of choice for ground-based UXO detection, has an even steeper (1/R6) falloff, and for this reason has not been routinely employed in helicopter-based UXO detection. Further, there are limits to the safe terrain-following of helicopter-based systems. For these reasons, ground-based digital geophysical mapping (DGM) systems have a role to play in wide area assessment (WAA), both for close-in detection of object boundaries, as well as in validation and verification of the results from airborne surveys.
The technology used for this project—the Vehicular Simultaneous Electromagnetic Induction (EMI) and Magnetometer System (VSEMS), formerly known as the Simultaneous Multisensor Surface Towed Ordnance Locator System (STOLS)—is a ground-based vehicle-towed array that collects total field magnetometer and EM61 data simultaneously in a single survey pass. The benefits of using a concurrent multisensor towed array are that (1) many sites contain surprises in the form of unexpected munitions or explosives of concern (MEC) or MEC-related activity, so choosing a sensor because of its detection characteristics may result in missing unexpected objects and (2) since most common geology (“hot rocks”) doesn’t show up on the EM61, the presence of a confirming electromagnetic (EM) signature can be used as a highly effective geologic false alarm reduction tool. The use of VSEMS at the Kirtland WAA site yielded both of these advantages.
The objective of this project was to use the VSEMS to collect magnetometer and EM61 digital geophysical mapping (DGM) data on pre-planned transects (generated by another contractor running Visual Sampling Plan [VSP] transect-planning software) to refine bombing target locations, extents, and edges in support of WAA.
The system collected more than 350 acres of concurrent magnetometer and EM61 Mk2 data at the Former Kirtland Bombing and Gunnery Range. In terms of surveying desired transects and 100% geophysical survey areas, locating evidence of bombing targets, and reporting results next-day, the system performed very well. Productivity did not reach the 12.5 acre/day metric due to a number of factors, chief among them being GPS issues.
Specific demonstration results include:
- Data from the system helped to delineate edges of the three known bombing targets (N2, N3, and New Demolitions) and helped to discover previously unknown bombing targets around N3.
- The probability of false alarm of 13.5% exceeded the 5% metric due to no-find digs caused by geology, but this can be reduced to 1% by requiring a confirming signature on the EM61 in order to exclude geology.
- The production rate, averaged across both transect surveys and 100% geophysical surveys, and averaged across all days in the field, was just under 10 acres per day.
Terrain: Smooth, grassy areas that have already been run over by heavy equipment are far more vehicularly navigable than rocky or stumpy areas, and lower coverage rates engender higher survey cost. This is particularly true due to the proof-of-concept nature of the fiberglass towed platform, which had no suspension and thus had to be treated gently.
GPS Coverage: The major surprise in terms of productivity was the degree of difficulty maintaining GPS coverage across the site. Because the site was physically large, the demonstrators employed high-power, 35-watt, long-haul GPS radios in the UHF band, as these allow a range as high as 6 miles. However, UHF radios have only a finite number of channels, and they do not automatically frequency-hop between channels. The channel would be jammed whenever a construction crew in the valley below began using a GPS. In the future, the team would be sure to also have 900 MHz spread spectrum frequency hopping radios to eliminate jamming problem.
Forward Rate of Advance: Though the terrain was friendly at Kirtland, forward rate of advance was limited by the somewhat fragile towed platform. This has since been replaced with a newly designed carbon fiber platform with an engineered suspension. In addition, the 10 Hz EM61 update rate imposes speed constraints, as data quality degrades with increased down-track data separation.
Expert On-Site Presence: The Principal Investigator has accompanied VSEMS on all of its surveys. This ensures a minimum of downtime and the delivery of a high-quality product but carries with it a high cost. Due to improvements to the system, this expert, on-site presence is less and less necessary over time.