Yogadhish Das

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.

Effects of soil electromagnetic properties on metal detectors

This paper presents an analysis, based on existing work in geophysics and nondestructive testing, of the effects of soil electromagnetic properties on the functioning of metal detectors widely used to detect buried landmines. The host soil is modeled as a half-space having real and frequency-independent electrical conductivity but frequency-dependent complex magnetic susceptibility. The analysis technique has been applied to three examples of soil of practical importance, namely, nonconducting soil with frequency-independent susceptibility, nonconducting soil with frequency-dependent susceptibility, and nonmagnetic soil with constant conductivity. Simplifications are made to clearly explain a number of previous field and experimental observations, for example, the greater influence of magnetic properties than of electrical conductivity on the performance of metal detectors. Results also show that soil magnetic properties affect continuous wave and pulsed-induction detectors differently. The effect that electrical conductivity and magnetic susceptibility of the host soil have on the signal produced by a target is investigated by computing the response of a buried small metallic sphere. Computations show that in some cases, which could represent practical landmine detection scenarios, the signal from the soil can dominate that due to the target, making it hard to detect the target. Further, it is shown that magnetic soil can alter a targets spectral response, which implies that, contrary to present practice, object identification techniques should take into account the electromagnetic parameters of the host medium.

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.

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.

Articulated robotic scanner for mine detection: a novel approach to vehicle-mounted systems

Conventional vehicle-mounted mine detector system employ an array of sensor elements to achieve a detection swath. Some systems employ more than one type of sensor technology. These systems, while being very useful, are often expensive, complex and inflexible. A human operator, on the other hand, sweeps a mine detector from side to side while moving forward to cover ground. The operator can follow the ground profile with the detector head close to the ground without hitting the ground or any objects on it. She can also vary the width of sweep to suit a particular situation, and is usually not limited by terrain. In this paper we present the concept and early prototype of a system that incorporates the advantages of the two methods described above while minimizing the disadvantages of both. For example, it will have the flexibility of a manual system with the rapid and safer mechanized scanning of the vehicle-mounted system but at a reduced cost, size and overall system complexity, when compared to existing approaches. Our approach uses an articulated robotic device capable of automatically moving mine detection sensor over natural ground surfaces including roads and tracks in a manner similar to a human operator. The system can also easily be used to place a confirmatory point sensor at a specific location if needed. The early prototype, which incorporates only a metal detector for a mine sensor, implements ground following by using a laser range finder and four ultrasonic sensors.

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.

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).

Modeling the Response of Electromagnetic Induction Sensors to Inhomogeneous Magnetic Soils With Arbitrary Relief

A general model to compute the response of an electromagnetic induction sensor to a magnetic soil, in both time and frequency domains, is developed. The model requires modest computational resources and can be applied to arbitrary soil inhomogeneities and relief, and to arbitrary sensor coil shapes, orientations, and positions. Central to the model is the concept of a head sensitivity map, which can be used to characterize the sensor head as a function of the shape, size, and position of the sensor coils. Two further concepts related to the head sensitivity are presented, which are the zero equisensitivity surface and the volume of influence. We demonstrate that these concepts aid the understanding of the detector behavior. The general model is based on the Born approximation, which is valid if the soil magnetic susceptibility is sufficiently small. A simpler model, which is only valid for homogeneous half-space soils but does not require the Born approximation, is also developed. The responses predicted by both models are shown to be in good agreement with each other and also with available analytic solutions. Comparing the two models also enabled an expression for the error incurred when using the Born approximation to be established. We shown that, for most soils of relevance to mine clearance, the corresponding error is negligible.