Environmental applications of Sub-Audio Magnetics

Abstract

Sub-Audio Magnetics (SAM) is a method by which a total-field magnetic sensor may be used to simultaneously acquire both total magnetic intensity (TMI) and total-field electromagnetic induction (TFEMI) responses from buried metal. In situations where public safety is involved, regulators, such as the U.S. Environmental Protection Agency, can mandate the use of both magnetic and electromagnetic sensors for the detection of potentially hazardous items such as unexploded ordnance (UXO) and metal drums. SAM has now completed a program of evaluation and certification as a state-of-the-art technology, acceptable for application in the litigation-sensitive environmental hazard remediation market. During its certification program, SAM was applied to controlled U.S. test sites at the Aberdeen Proving Ground, Maryland, McKinley Range, Alabama, Waikoloa, Hawaii, and at the Newholme facility in Australia. It was also applied to situations in Montana where live UXO was present. In these demonstrations, a direct comparison was made between SAM results and common, state-of-the-art alternative geophysical detecting technologies. Based on demonstrations at these sites, it has been possible to quantify the key performance parameters: level of effort, detection performance as a function of depth and target size, false-alarm rejection achieved through dual-mode sensing, and the cost-benefit ratio of SAM compared with contemporary sensor technologies such as the Geonics EM61 and EM63 electromagnetic detectors, and G-tek's TM-5EMU electromagnetic detector and TM-4 total field magnetometer. SAM has proven to be very successful in detecting a wide range of metallic UXO items to depths exceeding achievable detection by all contemporary sensors. In the Montana live sites, where the smallest item of UXO was a 76 mm projectile, false-alarm rates were reduced from 7 per UXO, using TMI only, to 0.7 per UXO using combined SAM TMI and TFEMI data, without diminishing the probability of UXO detection. Survey cost per hectare using SAM was determined to be less than one quarter that of conducting TMI plus EMI surveys separately. The application of SAM to extremely magnetic lava flows at Waikoloa in Hawaii showed that SAM cannot yet compete with the ground-balancing capability of the TM-5EMU in this environment. Advanced signal-processing development may reduce this present limitation.