ALLTEM Multi-Axis Electromagnetic Induction System Demonstration and Validation
Objectives of the Demonstration
Unexploded ordnance (UXO) is one of the most pressing problems facing the U.S. Department of Defense (DoD) and other government agencies that have lands that were once used for military training and are now closed or closing and being transferred to civilian or non-DoD government use. Cleanup of all UXO-contaminated lands using existing methods would be prohibitively expensive. A great deal of effort is being directed to find better ways to detect, locate, and identify buried UXO. It is not sufficient to merely detect buried metal objects because many of these objects are not UXO and pose no hazards. In many cases of range cleanup, 70% or more of the cost consists of locating and removing harmless metal including soda cans, broken parts from agricultural equipment, and fragments of ordnance that exploded as designed. Various time- and frequency-domain electromagnetic induction (EMI) systems and magnetometers are among the primary geophysical methods for detection and classification.
This project follows SERDP project MR-1328. In that project, one prototype magnetic system, the Tensor Magnetic Gradiometer System (TMGS) and two prototype EMI instruments, the Very Early Time-domain ElectroMagnetic (VETEM) system and the High Frequency Sounder, were evaluated. Subsequent to the evaluations, it was decided that a new multi-axis EMI system should be designed, built, and tested. Specifically, it was recommended that a multiple component, On-Time Time-Domain Electromagnetic system (ALLTEM) using a triangle current wave excitation should be built and that the data should be analyzed by time-domain methods. This system is able to record the earth’s electrical conductivity response at much later times, which has decayed to essentially zero. The new system is named ALLTEM because it is a time-domain electromagnetic (TEM) system in which the transmitter current is on all of the time. In addition, analytical methods should be developed for extracting as much target information as possible from ALLTEM with the aim of identifying the targets and discriminating between UXO and harmless metal scrap.
An advanced multi-axis EMI system, ALLTEM has been specifically designed for detection and discrimination of UXO. ALLTEM uses a continuous triangle-wave excitation that measures the target step response rather than the more common impulse response. Ferrous and nonferrous metal objects have opposite polarities. The system multiplexes through all three orthogonal (Hx, Hy, and Hz axes) transmitting loops and records a total of 19 different transmitting (ALLTEM transmitter [Tx]) and receiving (ALLTEM receiver [Rx]) loop combinations with a spatial data sampling interval of 15 cm to 20 cm. Performance objectives for this project included upgrading the acquisition electronics and building a new sensor and cart out of lightweight fiberglass facing over plastic honeycomb core material (NOMEX). These upgrades were accomplished as well as upgrades to the data processing, inversion, and classification algorithms within Geosoft’s Oasis montaj platform. Data analysis was streamlined within Oasis montaj as routines specifically developed for ALLTEM - these included importing survey data; gridding; noise analysis for threshold determination; automatic selection of targets; batch inversion of selected targets using prolate, oblate, and ellipsoidal spheroids; and automatic statistical classification of inverted targets as clutter or targets of interest.
The ALLTEM proved itself able to acquire data in the high temperatures of YPG in May, 2009 and in the swamp lands of APG in February, 2010. Surveys were conducted in a “race track” manner with the sensor moving at 1 m per second (m/s) along survey traverses separated by 0.5 m. The line spacing is typically determined by the smallest munitions of interest. Demonstrations were successfully completed at YPG and APG standardized test sites with good scores. Ongoing data analyses indicates that ALLTEM is able to detect anomalous features and to automatically classify targets as being items of interest or not and then to discriminate between individual known munitions types.
An early limitation on data acquisition was the loss of Global Positioning System (GPS) radio signals at YPG, which is common to many UXO investigations. This issue was resolved by logging the raw GPS data at the base station and at the rover on the moving ALLTEM sensor, post-processing the two data sets, and then reintegrating the results back with the ALLTEM data. An attitude heading and reference system (AHRS) has also been integrated into the acquisition stream and is used to generate more precise locations of sensor locations. The other limitation on data acquisition is whether the tractor pulling the sensor can get over large rocks, boulders and vegetation and rough, irregular, or steep terrain. The ALLTEM cart now has a Lexan skid-plate that glides over most objects.
Technology transition efforts are in the proposal stage as Battelle Engineering and the U.S. Geological Survey (USGS) have developed plans for a phased transition of ownership and operation from USGS to Battelle over the next 2 years. This would involve application of the ALLTEM at three ESTCP Live Site demonstrations. The first site would be surveyed and the data processed by the USGS with Battelle looking on, the second site with both groups cooperatively working together on surveying and data processing, and the third site would be surveyed and the data processed by Battelle with the USGS acting in an oversight and advisory capacity.
An ALLTEM built from scratch would cost approximately $60,000. Survey costs, based on recent field exercises at Camp Stanley Storage Activity near San Antonio, TX, are about $3,000 per acre, including mobilization/demobilization, data acquisition, data analysis, and interpretation.
Points of Contact
Dr. Theodore Asch
U.S. Geological Survey
SERDP and ESTCP
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