The Johns Hopkins University Applied Physics Laboratory (APL) has developed a prototype metal detection survey system that will increase the search speed of conventional technology while maintaining high sensitivity. Higher search speeds will reduce the time to clear roads of landmines and improvised explosive devices (IED) and to locate unexploded ordnance (UXO) at Base Realignment and Closure (BRAC) sites, thus reducing remediation costs. The new survey sensor system is called the moving belt metal detector (MBMD) and operates by both increasing sensor speed over the ground while maintaining adequate sensor dwell time over the target for good signal-to-noise ratio (SNR) and reducing motion-induced sensor noise. The MBMD uses an array of metal detection sensors mounted on a flexible belt similar to a tank track. The belt motion is synchronized with the forward survey speed so individual sensor elements remain stationary relative to the ground. A single pulsed transmitter coil is configured to provide a uniform magnetic field along the length of the receivers in ground contact. Individual time-domain electromagnetic induction (EMI) receivers are designed to sense a single time-gate measurement of the total metal content. Each sensor module consists of a receiver coil, amplifier, digitizing electronics and a low power UHF wireless transmitter. This paper presents the survey system design concepts and metal detection data from various targets at several survey speeds. Although the laboratory prototype is designed to demonstrate metal detection survey speeds up to 10 m/s, higher speeds are achievable with a larger sensor array. In addition, the concept can be adapted to work with other sensor technologies not previously considered for moving platforms.
This paper describes a prototype three-dimensional electromagnetic induction (EMI) sensor system that has the potential to measure directly the multiple components of buried metal targets' magnetic polarizability tensor without the need to invert spatial data from single-axis EMI sensors. This novel sensor is called a three-dimensional steerable magnetic field (3DSMF) sensor system. The 3DSMF sensor is a high-time resolution, wide-bandwidth time-domain EMI system combined with a 3-axis magnetic field generator (3AMFG) and magnetic field receivers. The 3AMFG differs from previous 3-axis magnetic field generators in a number of ways: the projected magnetic field is relatively uniform in space and is steerable. These two features offer the potential to greatly improve target classification. This paper discusses the 3DSMF sensor system design philosophy and modeling results.
This paper describes a spatial scanning time-domain electromagnetic induction (EMI) sensor and presents results from recent field experiments with buried metal and low-metal content (LMC) anti-personnel (AP) and anti-tank (AT) plastic-cased land mines. The EMI sensor is an modified version of the Electromagnetic Target Discriminator (ETD) sensor developed for the US Army CECOM/NVSED by the Johns Hopkins University Applied Physics Laboratory. The spatial scanning ETD sensor has demonstrated the ability to measure metal target decay times starting approximately 6 ms after the transmitter current is turned off and with metal target decay time constants as short as 1 ms. The sensor antenna sweeps 80 cm over a target area and makes time-decay measurements at 14.5 mm intervals. In addition to metal target signatures, the paper describes coincident void and metal signatures from LMC land mines. The detection of coincident void and metal signatures is shown to be an important classification technique for LMC land mines.
This paper describes a prototype electromagnetic induction (EMI) sensor system designed specifically to measure the horizontal component of a metal target's eddy current time decay signature. Instead of creating a vertical magnetic field from a horizontal loop transmitter configuration used by most EMI metal detectors, the prototype transmitter geometry has been designed especially for creating a horizontal magneti field (HMF). One of the potential advantages of the HMF sensor is the relatively uniform magnetic field that is created over a large volume. A second potential advantage is that, compared to a conventional loop antenna, the magnetic field intensity falls off slowly with distance from the plane of the sensor. These two advantages potentially make the HMF sensor well suited for detection and classification of metal targets buried deeply in the ground (e.b., unexploded ordnance, UXO) or from a vehicle-mounted mine detector sensor. Preliminary modeling of the antenna and laboratory data from a time-domain version of the HMF sensor are presented.
This paper presents wide bandwidth, time decay responses from low metal content (LMC) mines, LMC mine simulates, and ground voids. Measurements were collected both in the laboratory and in the field. The target time decay responses were measured with the Johns Hopkins University Applied Physics Laboratory developed Electromagnetic Target Discriminator (ETD) sensor developed for the US Army CECOM/NVSED. The ETD sensor has demonstrated the ability to measure metal target decay times starting approximately 3 to 5)mus after the transmitter current is turned off and metal target decay time constants as short as 1.4)mus.
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