John E. McFee

Analysis of an electromagnetic induction detector for real-time location of buried objects

Spatial response characteristics of an electromagnetic induction sensor as it passes over a metallic object are investigated using a simple analytical technique. In this low-frequency technique, one replaces a metallic object with equivalent electric and magnetic dipoles and then applies the principles of reciprocity to obtain the induced EMF in a sensor coil. Analysis is carried out for a sensor employing rectangular coils, and the object set is confined to a sphere and a prolate spheroid. The simple approach, which is illustrated with both numerical and experimental data, is found to be adequate to understand the effect on the response characteristics of parameters such as object depth, orientation, aspect ratio, and material properties. >

Determination of the parameters of a dipole by measurement of its magnetic field

A novel method to determine the parameters (dipole moment, orientation, and location) of a magnetic dipole is discussed. The method, which requires no a priori knowledge of the parameters, consists of measuring one or more components of the magnetic field of the dipole in a two-dimensional grid and performing a two-dimensional multiparameter nonlinear least squares fit to the data. Results of computer simulations which include the effects of electronic and sensor noise and motion of sensors are presented to show that reliable and consistent parameter estimation is obtained and that pseudoreal-time operation is achievable on a fast minicomputer. The simulations are valid for all major classes of magnetometer sensors. Further, application of the method to experimentally obtained data is discussed. Simulation and experiments show that for objects situated 60-200 cm from the plane of sweep of the sensors, which yield peak field values \sim 10-1000 nT, magnetic data must be accurate to within a few nT which is easily achieved using fluxgate sensors, and position accuracy of \pm 1 cm over a span of 700 cm is sufficient.

Standoff sensing of bioaerosols using intensified range-gated spectral analysis of laser-induced fluorescence

In atmospheric sensing, one application that has demonstrated several impressive successes over the last two decades is the light detection and ranging (LIDAR). Elastic LIDAR has shown an important capability in providing aerosol density and spatial distribution from a standoff position. However, it provides limited information on the material composition of the aerosol component. On the other hand, inelastic LIDARs (including laser-induced fluorescence and Raman LIDARs) measure the spectrally distributed returned signal that may contain important clues about the nature of the scatterers. In order to investigate the capability of these LIDARs in characterizing bioaerosols from a standoff position, Defence Research & Development Canada initiated a three-year program in spring 1999, named SINBAHD (Standoff Integrated Bioaerosol Active Hyperspectral Detection). The aim of the program was to investigate the sensitivity and discrimination capabilities of an inelastic LIDAR based on the intensified range-gated spectral detection of laser-induced fluorescence. An exploratory prototype based on this technique has shown sensitivity of a few living bioaerosol particles per liter of air for a range of 1.4 km at night. Furthermore, based on spectral signatures measured during open-air releases, good discrimination capabilities were obtained between Bacillius subtilis var globiggi (BG) and Erwinia herbicola (EH). These results agree well with a performance model using Raman returns from atmospheric nitrogen as a calibration tool.

Locating and identifying compact ferrous objects

A solution to the problem of magnetostatic location and identification of compact ferrous objects of arbitrary shape is presented. It is shown that, in practice, the inversion of the magnetostatic dipole field or field gradient is a necessary first step toward determining object location and identity. Several iterative and noniterative methods of determinating the dipole moment and location from field or gradient measurements are described and compared. It is shown that, given the dipole-moment estimates, it is possible to determine the identity of the dipole source in practical situations by pattern recognition. A unique prototype total field magnetometer which is capable of explicitly and accurately locating and identifying axially symmetric compact ferrous objects is described. It has performed well in preliminary tests using spheres and spheroids. >

Multisensor vehicle-mounted teleoperated mine detector with data fusion

The Improved Landmine Detector Project (ILDP) was initiated in Autumn 1994 to develop a prototype teleoperated vehicle mounted mine detector for low metal content and nonmetallic mines to meet the Canadian requirements for rear area mine clearance in combat situations and peacekeeping on roads and tracks. The relatively relaxed requirements, such as low speed and reduced detectability of completely nonmetallic mines, greatly increase the likelihood of success. The ILDP system consists of a unique teleoperated vehicle carrying a forward looking infrared imager, a 3 m wide down-looking highly sensitive electromagnetic induction detector and a 3 m wide down-looking ground probing radar, which all scan the ground in front of the vehicle. Scanning sensor information is combined using a suite of navigation sensors and custom designed navigation, spatial correspondence and data fusion algorithms. Suspect targets are then confirmed by a thermal neutron analysis detector. A key element to the success of the system is the combination of sensor information. This requires coordinated communication between the sensors and navigation system and well designed sensor co-registration, spatial correspondence and data fusion methodologies. These complex tasks are discussed in detail. The advanced development model was completed in October 1997 and testing and improvements are ongoing. Results of system performance during extensive field trials are presented. A follow-on project has been initiated to build four to six production units for the Canadian Forces by the year 2000.

Electrical Impedance Tomographic Imaging of Buried Landmines

A prototype confirmation landmine detector, based on electrical impedancetomography (EIT), which can operate under realistic environmental conditions, has been developed. Laboratory and field experiments demonstrated that it is possible to reliably reconstruct, on the scale of the electrode spacing (ES) (in width and depth), conductivity perturbations due to a shallow buried antitank mine or a similar object in a variety of soils (black earth, clay, sand) down to depths equal to the dimensions of the object (1-1.5 ES, equivalent to 14-21 cm for a 64-electrode 1 m times 1 m array). These represent the first EIT images of real landmines computed from measured data. Occasional problems were encountered with the electrical contact in very dry soils, with excessive insertion pressure being required for reliable electrode contact. However, poor contacts could be detected, and the offending probe was either reinserted or compensation was applied. A matched filter detection algorithm based on a replica of the object of interest was developed and shown to effectively reduce the false alarm rate of the detector. EIT is especially suited for wet lands and underwater, where other mine detectors perform poorly. Experiments in a water-and sediment-filled tank have demonstrated that detection of minelike objects in such an environment with a submerged array is feasible. These experiments represent the first EIT measurements of targets using an electrode array submerged underwater. EIT may also have an application in locating intact mines in the berms formed by mine-clearing equipment. The EIT sensor head could be made cheaply enough to be disposable and remotely inserted to improve safety

Time Domain Response of a Sphere in the Field of a Coil: Theory And Experiment

The time-harmonic solution for the anomalous vector potential due to a conducting permeable sphere in the field of a current-carrying loop is used to derive the corresponding step response. The step response is then used to obtain analytical expressions for the voltage induced in a second loop due to a chosen exciting current pulse train. The voltage induced in an actual system of coils is obtained by superposition. The effect of the measurement system is included in the analysis in order to experimentally verify the model. Measured responses of a number of aluminum and steel spheres at various distances from the coils are compared with theoretical predictions. The agreement between the two is generally good.

The Detection of Buried Explosive Objects

SUMMARYThere is a need for methods which can be used to detect buried objects, most prominently in the fields of archaeology, forensics, geophysics, submarine and mine detection. This article reviews methods of detecting and identifying small buried explosive-filled objects, with emphasis on mines and artillery shells. The approaches, which span a number of intertwined disciplines, are in many cases directly adaptable to these other applications of remote sensing.

Multisensor mine detector for peacekeeping: improved landmine detector concept (ILDC)

The Improved Landmine Detector Concept Project was initiated in Autumn 1994 to develop a prototype vehicle mounted mine detector for low metal content and nonmetallic mines for a peacekeeping role on roads. The system will consist of a teleoperated vehicle carrying a highly sensitive electromagnetic induction (EMI) detector, an infrared imager (IR), ground probing radar (GPR), and a thermal neutron activation (TNA) detector for confirmation. The IR, EMI and TNA detectors have been under test since 1995 and the GPR will be received in June 1996. Results of performance trials of the individual detectors are discussed. Various design configurations and their tradeoffs are discussed. Fusion of data from the detectors to reduce false alarm rate and increase probability of detection, a key element to the success of the system, is discussed. An advanced development model of the system is expected to be complete by Spring 1997.

Fast Nonrecursive Method for Estimating Location and Dipole Moment Components of a Static Magnetic Dipole

This paper presents a method of estimating the position and moment components of a static magnetic dipole based on measurements of the magnetic field component normal to an arbitrary plane in which measurements are obtained. The largest local maximum and smallest local minimum of the field component and the corresponding positions in the plane are all the data necessary to obtain the solution. The solution is accurate, nonrecursive, robust, fast enough to be carried out by a 16-bit microprocessor in a fraction of a second, and makes no assumptions about the orientation of the dipole. The method in most cases gives good results for total field data, too. Results of a computer simulation study, which was performed to evaluate the method using simulated but realistic data, are presented. It is found for both total and vector magnetic field data that accuracy of component estimates are nearly independent of magnetic sensitivity (for S/N ratios greater than 28 dB) and sensor positional errors in the plane of measurement (if less than 5 percent of the dipole depth). Percentage error in estimates of dipole position and moment components is approximately equal to the percentage error in sensor position orthogonal to the plane of measurement (if less than 5 percent of the depth).