Phased Array Rotating Magnet Sensing of Subsurface Conductive Material

This paper describes a novel phased array system for generating low frequency magnetic fields for sensing and penetration into materials that prevent radio frequency interrogation techniques. Electronically geared, rotating permanent magnets that project controlled magnetic fields are key elements in a sensing-system that locates and distinguishes buried ferromagnetic and non-ferromagnetic objects. The sensing principle is for the projected oscillating magnetic fields to pass through an overlaying medium, such as soil or salt water, and then interact with subsurface metallic objects to produce measurable secondary fields. Rotating 12-pole permanent rare earth magnets at a rate of ~40 Hz broadcasts Extremely Low Frequency magnetic fields that oscillate at a frequency of 240 Hz. The interaction of conductive materials with the projected fields produces eddy currents that cause secondary magnetic fields that oppose the applied primary field. In this study, a compact array of four electronically geared, rotating multipole neodymium magnets project the oscillating magnetic fields. Magnetometers measure the combined primary and secondary fields at a rate of 490 Hz. The data are then read into a Raspberry Pi 4 Single-Board Computer (SBC), processing the data in near real-time to characterize the composition of subterranean objects. Processed readings extracted from the counter-propagated magnetic fields are fed into a neural network, training the program to discriminate between different types of sensed materials including copper, steel, iron, aluminum, and even lead. Additionally, variations of electronically geared rotating neodymium magnets are arranged and tested to maximize subsurface penetration. It is anticipated that the method extends to detecting other buried objects with ferromagnetic or electrically conductive components, such as landmines and buried explosives. Results from laboratory experiments and a field testbed are presented. Future plans are discussed.