Utilizing a Multiple Metal Detector Array for Locating Anomalies Indicative of Ferrous and Non-Ferrous OE/UXO

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Courtesy of Weston Solutions, Inc

ABSTRACT
An array of three Geonics Limited EM61-MK2 metal detectors was utilized to identify subsurface metallic anomalies at the Mare Island Naval Shipyard in Vallejo, California. The goal of the 120-acre survey was to locate anomalies indicative of ferrous and nonferrous ordnance and explosives/unexploded ordnance (OE/UXO) in dredge pond berms with particular attention to 20 mm high explosive (HE) rounds. The array spanned a total of 3.5 meters and the centers of the units were 1.25 meters apart. Prior to data acquisition, sensitivity tests were performed to establish the minimum distance between the array and the towing vehicle. Electromagnetic (EM) readings were collected every 0.20 seconds on each instrument equating to a distance of approximately 0.5 to 1 foot between readings along travel lanes. The instruments utilize a synchronization cable that allows the sensors to operate independently without interference. Real-time differential global position system (DGPS) information was collected simultaneously with the EM data for sub-meter location accuracy. Anomaly locations can be reacquired with a DGPS at any time for identification or excavation. Typical production rates using 1.25-meter lane spacing is approximately 6-8 acres in eight hours. A quality control test grid of buried inert ordnance, including 20 mm rounds, was surveyed with the array to help determine an optimal lane spacing and reading frequency. After calibration to the described settings and distances, the instruments imaged 20 mm rounds at depths between 1and 4 feet below land surface in multiple orientations.

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