UXO Characterization in Challenging Survey Environments Using the MPV
Dr. Nicolas Lhomme | Black Tusk Geophysics
Objectives of the Demonstration
Many Department of Defense (DoD) sites are contaminated by munitions and explosives of concern that are difficult to clean up because commercially available technologies are inadequate in forested areas and in rugged terrain. The Man Portable Vector (MPV) is a handheld technology designed for detection and classification of munitions in challenging survey environments. Following Environmental Security Technology Certification Program (ESTCP) projects MR-201005 and MR-201158, this MPV project extended the characterization of suitable conditions and expectable performance with live-site demonstrations at New Boston Air Force Station (NB), Former Waikoloa Maneuver Area (WK), Tobyhanna Artillery Range (TOAR), and Puako village in Waikoloa (PK). The project also paved the way for transition of the technology to the munitions response industry.
The MPV sensor is a handheld metal detector based on electromagnetic (EM) induction. The MPV sensor head comprises a 50-centimeter (cm) diameter vertical axis transmitter loop in which five receivers are placed in a cross pattern. The transmitter generates an energizing pulse that induces time-varying “eddy” currents within buried metallic targets. The MPV is equipped with vector receivers to accurately measure the three orthogonal components of the secondary field from these eddy currents, using a wide time range to best capture target-specific decay rates.
The standard MPV configuration is used to collect full-coverage, dynamic data along survey lines for digital geophysical mapping of the metallic contamination of an area. Detected anomalies can be subsequently selected for further investigation and classification by processing the dynamic data or acquiring additional data with the MPV. The properties of the buried objects can be reliably inferred by geophysical inversion if the object is energized by a complete set of transverse directions and if high-quality, accurately positioned data are available. The detection data alone may be sufficient for classification at sites with favorable environmental conditions. Otherwise, anomalies are revisited with the MPV for cued interrogation, where higher quality data are collected in stationary mode. With the standard MPV, the vertical-axis transmitter can generate transverse excitation by placing the sensor at multiple locations around an anomaly. This effect can also be achieved by using three orthogonal transmitters from a single location above the target, using a detachable set of two orthogonal, horizontal-axis transmitter coils placed on top of the MPV head. The latter method was validated at WK for its performance and became the new standard for static acquisition as it saved time and simplified data collection: one or two soundings are generally sufficient instead of five to six. Infield, near-real-time inversion of the data predicts target parameters, confirms data usability, indicates target location, and reduces the complexity of infield data interpretation.
The MPV head is tethered to a data acquisition system (DAQ) that regulates transmitter current and samples receiver signals. It draws power from high-capacity lithium-ion batteries that are mounted on a backpack with the DAQ. A computer tablet provides an interface for controlling the DAQ and the navigation system. The MPV position is either obtained using Real-Time Kinematic (RTK) Global Positioning System (GPS) or, at sites with obstructed view of the sky, a Robotic Total Station (RTS) optical ranger. The GPS rover or RTS prism is attached to the top of the MPV handling boom. An Attitude and Heading Reference System (AHRS) sensor provides the boom orientation to derive the MPV head geo-location.
The MPV technology was initially designed and laboratory tested by the Engineering Research and Development Center (ERDC) at the Cold Regions Research and Engineering Laboratory (CRREL) in Strategic Environmental Research and Development Program (SERDP) project MM-1443 (Kevin O’Neill and Benjamin Barrowes) with a prototype fabricated by G&G Sciences. Its field worthiness was demonstrated by BTG personnel in the ESTCP projects MR-201005 and MR-201158, after fabrication by G&G of a second-generation prototype with improved maneuverability and ruggedness. The technology was successfully demonstrated at Yuma Proving Ground and at three live sites (Camp Beale, Spencer Range, Camp George West) from 2010–2012. The same MPV unit was used for this project at NB and WK. In 2015, G&G fabricated a first production unit that was demonstrated at TOAR and PK.
The main performance metrics were the probabilities of detection and correct classification and the rate of coverage. The objectives were met at all sites. At NB, all seeds were found in the open field and the forest area and 95% of the targets of interest (TOI) were correctly classified while rejecting 40% of the clutter. At WK, the full site was covered, including rocky outcrops; however, one target was not detectable. All UXO were correctly classified, while clutter rejection was 81% for the standard cued method and 67% for the first-time use of the three-dimensional (3D) transmitter coils method. At TOAR, all ground was surveyed, all seeds were detected and all UXO were correctly classified while rejecting 83% of the clutter. At PK full coverage was achieved, all seeds were detected, and all TOI were correctly classified with 80% clutter rejection.
A secondary performance metric is production rate. In general, dynamic surveys took more time than expected because the challenges and delays associated with surveying in dense forest, operating RTS, or working in a residential area make the survey considerably slower than open field GPS operation. On average, detection surveys covered 0.2–0.5 acres per day, whereas cued data was more predictable at 100–170 targets per day.
During this project, there was significant progress toward making the MPV a production sensor. The manufacturer overhauled and standardized the sensor hardware, producing a sensor head that is now relatively sturdy and maneuverable for an advanced classification system. The DAQ is smaller, lighter, and requires fewer batteries. The cued interrogation process is made simpler and more robust through use of 3D transmitters and immediate data inversion, which predicts the target parameters and ensures that high quality data are acquired at the correct location. Field practices have been tested and refined under a wide range of conditions. Technology transfer to industry has also started, with involvement and training of commercial field crews at each site (CH2MHill, Environet, and Parsons).
The technology, including hardware and software support, is commercially available from the manufacturer, who is actively working on a long-term solution for manufacturing and support. Durability is adequate for an advanced EMI system—no failure or instrument-caused field delay to report as of 2017. Portability and maneuverability allow access to most man-trafficable areas and offer higher coverage rates than larger sensors. However, this also makes the MPV technology more complex to operate. The operator must take care to keep the sensor head in line with the survey path to avoid creating gaps and must monitor sensor height above ground to guarantee detection at depth. A high-accuracy AHRS sensor is indispensable because the MPV head is not always aligned with the direction of travel (the head can be rotated and tilted). These aspects also affect data processing: variation in ground clearance can introduce varying background noise in geologically active environments and positioning issues, gaps, or deviations from straight lines cannot unequivocally be traced to issues with the operator path or the positioning system. Field procedures, data quality checks, and processing algorithms have been developed over the course of this project to mitigate these effects.
Finally, these demonstrations have shown that the MPV can be used at live-munitions sites and fulfill the objectives of full-coverage mapping, detection, and reliable classification of unexploded ordnance (UXO)—even in challenging environments with terrain, vegetation, geologic background, and urban structures.
Points of Contact
Dr. Nicolas Lhomme
Black Tusk Geophysics
SERDP and ESTCP