Magnetic Ordinance Detection By Christopher Fenton

Goals Analyze feasibility of magnetic ordinance detection methods, specifically with IED detection in Iraq in mind If feasible, build working prototype Successfully detect something metallic

Different Approaches to Object Detection Traditional Metal Detectors Ground-Penetrating Radar Magnetic Detectors

Magnetic Detection Approaches Balanced-Loop Detects change in B-field over time Covers large areas Magnetometers Measures absolute B-field Covers small areas

Balanced Loops First use of Magnetic “Indicator Loops” for harbor defense in 1915 by British in WWI, adopted by U.S. in 1942 during WWII Can only detect moving magnetic disturbances Typically large and immobile (>1.6 km^2) Abandoned for harbor defense in favor of SONAR following WWII

Balanced Loops in Action Old detector station in Nahant, MA

Magnetometers First invented in 1833 by Carl Gauss Can detect magnitude and direction of magnetic field Small and lightweight Still used for geological surveying and “Magnetic Anomaly Detectors”

Magnetometers in Action Magnetometer Array used for UXO detection Circuit model of sensor used in MicroMag3 (Sensor inductance changes with external B-field) MicroMag3 3-axis Magnetometer

Approach: Magnetometer Array Sensors are small (~1”x1”), cheap ($50) and easy to handle > Even small loops are several m^2 Insensitive to scanning speed and tilt > For loops, tilt and speed need to be precisely monitored Arrays can be scaled to arbitrary width for wide-area scanning > Magnetometers give point measurements, but can be expanded to cover wide areas like loops do

The MAGNETube

MAGNETube Setup 3 x MicroMag3 3-axis SPI magnetometers Sensors mounted 15” apart Calibrated so Earth’s B-field = 1 = 0.48568G 2 x Picaxe 18X microcontrollers Expandable through “daisy-chaining” 1 Laptop running “Listener” software and outputting to CSV file for analysis in Microsoft Excel®

Setup A B C

How is the magnitude computed? 1. X, Y, and Z values for all 3 sensors are sent to laptop 2. Calibration offset is subtracted from each direction 3. Magnitude = √(X^2 + Y^2 + Z^2) 4. Magnitude is scaled from 150-200 range to approximately equal “1” in Earth’s B-field 5. Sensor: 1=.48568 Gauss in Los Angeles

Test 1: 80 lbs of Iron Location: Erdem’s Apartment Target: 80 lbs of iron weights in a plastic trashcan

Test 1: 80lbs of Iron Possibly due to misalignment of sensor during test Conclusion: Readily detectable if directly above pile, drops off quickly

Test 2: 4” Brass Artillery Shell

Test 2: 4” Brass Artillery Shell Background: 12” above ground Test 2: 12” above ground Conclusion: Brass has no magnetic signature. Only bolts were detectable, and only then at close range.

Test 3: Neodymium Magnets (high sensitivity simulation) Large 3”x6” Neodymium magnet

Test 3: N.D. Magnet Conclusion: Magnet is easily detectable at a reasonable range

Test 4: Attenuation in Water

Test 4: Submerged N.D. Magnet Conclusion: Water has no attenuation effect on magnetic field

Future Improvements Use faster microcontroller with on-board FPU (~3X improvement in sampling rate) Add wireless serial link for easier calibration and field-use Experiment with distortion detection vs. simple magnitude detection Use higher-sensitivity magnetometers and higher-density array Compare vs. traditional metal detector

Conclusion Undocumented hardware failure-modes can be extremely difficult to fix Magnetic detection appears to be a valid method (and is apparently in-use) A simple array can be constructed for less than $250 With more time, the current design could be greatly improved