Airborne EM Data Complement Magnetics In An Unexpected Way

In the fall of 2008, low-altitude airborne geophysical surveys were carried out at a military installation. The primary goal of the surveys was detection and mapping of infrastructure, buried wastes, and other buried and surface metallic objects within a 1738 hectare area. The project employed two Battelle airborne detection systems, the VG16 vertical magnetic gradient system and the TEM-8 transient electromagnetic system.

Airborne electromagnetic (EM) methods are often used as a reconnaissance tool in mineral exploration. With the improvement of airborne transient EM (TEM) systems and interpretational techniques, more and more quantitative information can be obtained from airborne EM data. Thus we pose the question is it possible to develop airborne TEM technology to be as effective in orebody detection as ground TEM techniques in the future? Recently, we developed a fast approximate solution of transient EM response to a thin target buried under a thin conductive overburden. Using this solution, the signal-to-geological-noise ratios are computed for an airborne and a ground moving-loop configuration over a range of values of overburden conductance, target conductance and target depths. The results indicate that airborne TEM systems have potentially the same resolving capability as low or moderate power ground TEM systems for discrete targets. The key design features required to achieve this potential capability in airborne TEM systems are large system bandwidth (i.e. range of time windows) and large transmitter moment.

The Very Early Time Electromagnetic (VETEM) system, the High-Frequency Sounder (HFS), and the Tensor Magnetic Gradiometer System (TMGS) were developed by the U.S. Geological Survey (USGS) for applications other than unexploded ordnance (UXO). However, features of these systems may make them adaptable to UXO applications. VETEM is a time-domain electromagnetic (EM) induction system that utilizes full waveform recording. The HFS is a flexible frequency-domain EM system. The TMGS uses four sensitive 3-axis fluxgate magnetometers so that gradients of field components are recorded. Tests were conducted in air using a selection of inert UXO objects ranging from 20 mm to 105 mm in diameter. A nulled overlapped antenna arrangement was used for both VETEM and the HFS. It was found that this antenna configuration resulted in distinctly different responses to horizontal UXO objects aligned along the direction of traverse versus horizontal and perpendicular to the traverse. Vertical orientation of UXO objects resulted in smaller amplitude signals and an interesting slightly asymmetric “crater” shape response in plan view. The HFS response was a strong function of frequency, which is good for discrimination. The TMGS data are the most sensitive to ferromagnetic objects and the multi-component data hold the promise of accurate location, including depth, to UXO objects using a dipole mapping algorithm. Plans include increased sensitivity and multicomponent capability for the EM systems, advanced algorithms to enhance UXO discrimination for EM and TMGS systems and field evaluations at standard UXO test sites of the Strategic Environmental Research and Development Program (SERDP). Introduction UXO is one of the most pressing problems facing the Department of Defense and other government agencies that have lands that were once used for military training and are now closed or closing and being returned to civilian use. Cleanup of all UXO-contaminated lands using existing methods would be prohibitively expensive, so a great deal of effort is being directed to finding better ways to detect, locate and identify buried UXO. It is not sufficient to merely detect buried metal objects, because many of these objects are not UXO and pose no hazards. In many cases of range cleanup 70% or more of the cost consists of locating and removing harmless metal including soda cans, broken plow blades and fragments of ordnance that exploded as designed. Among the primary geophysical methods for detecting and classification are various time and frequency domain EM systems and magnetometer systems. Since the USGS developed some prototype EM and magnetic systems for applications other than UXO, and because some of these prototype systems have some unusual characteristics, we proposed to test, evaluate, and modify three of these systems, the VETEM system, the HFS, and the TMGS. We are now in the process of carrying out this work and this paper describes some of the results from laboratory tests.

Ground penetrating radars (GPR) are widely used for detection of underground buried objects. Synthetic aperture radars (SAR) are common instruments for land remote sensing. GPRs with synthetic aperture radar technique (GPR-SAR) are popular instruments to obtain image of the buried objects. To know about dielectric contents, position and shape of the buried target objects, ordinary GPR-SAR is insufficient, because angle between transmit and receive antenna is fixed. Therefore, information is lacked, mainly about microwave propagation velocity of soil which affects measurement accuracy of position of various buried objects. Multi-angle GPR-SAR, a new method, is proposed and can change angle between transmit and receive antennas. In the multi-angle GPR-SAR, a lot of angle points between the antennas can be set, and much more detailed information can be obtained related to buried objects and surrounding soil. A new indoor experiment instrument of full-polarimetric, multi-angle and bi-static GPR-SAR have been developed, which enables us to evaluate possibility of imaging various already-known buried objects. Outline of the instrument is introduced, and polarimetric calibration and imaging capability are verified.

The ground surface of the Earth is usually mapped using microwave radar, and in particular synthetic aperture radar. A different technique is presented for the imaging of isolated ground surfaces, which may range from the order of a metre to a few kilometres in diameter, using radiowaves generated from antennas on the ground surface of the Earth. The technique is first formulated using groundwave propagation theory to solve the forward electromagnetic scattering problem of the isolated surface. Conjugate gradient methods are then used to solve the inverse problem to obtain the normalised surface impedance distribution in an iterative manner, with the establishment of a cost function of the measured and scattered fields around the surface. Numerical simulations at 0.5 MHz of two ground features, namely dry ground and dry–wet mixed ground surfaces confined within a diameter of 500 m and surrounded by seawater, show that the distributions of both real and imaginary parts of the normalised surface impedance can be well reconstructed using the technique. It is also shown that the Polak–Ribiere–Polyak conjugate gradient method gives a faster rate of convergence than the Fletcher–Reeves method. The simulation at 300 MHz of dry ground with a buried metallic object confined within a diameter of 50 cm shows that the metallic object and its position can be identified from the images of normalised surface impedance distribution. The study demonstrates a new technique for the tomographic imaging of isolated ground surfaces which may be useful in ground surface mapping, remote sensing and monitoring, and detection of buried metallic objects.

This paper deals with the detection of buried objects using a parametric sonar working at grazing angles below the critical angle beneath which, according to the Snell theory, no penetration into the sediment is possible. Two experiments were performed in October 1995 and March 95 in France at the GESMA test site in order to assess the feasibility of detecting buried objects in sub-critical angle conditions. The TOPAS sonar from Simrad was used. Proud (unburied) objects were also considered in order to compare the results between the high frequency and the low frequency. For the proud objects it is shown that, for this type of bottom type, the peak signal to bottom reverberation ratio (PSSR) is 9 dB higher for the low frequency than for the high frequency. For the buried objects their detection is shown to be possible even if the grazing angle (16 degrees) is lower than the critical angle (28 degrees). Comparing the theoretical target strength (TS) to the measured TS only a few dB loss is observed, corresponding to both the absorption and the attenuation into the sediment, which is very promising. Almost no detection was observed with the high frequency. While the sonar was moving on the rail it was possible to perform synthetic aperture sonar processing with the parametric array resulting in significant improvements. In particular, all the objects were precisely located within the minefield and gains of 10 dB and 8 dB were obtained for the proud objects and the buried objects respectively.

The discrimination of the scattered wave from an object buried in shallow ground from that of the rough surface is a difficult task with present ground penetrating radar (GPR) systems. Recently, a new approach for this classical problem has been proposed and its effectiveness has been verified. This new method is based on the angular correlation function (ACF) of the scattered wave observed at two or more different incident and scattered angle combinations. It has been shown that the angular memory signatures of rough surfaces are substantially different from those of typical man-made targets and by choosing the appropriate incident and scattered angles, the surface scattering can be minimized whereas the scattering from the target is almost unchanged. The authors present detailed numerical studies of the ACF of the scattered wave from rough surfaces with and without a buried object. To obtain the ACF, the three averaging methods: realization, frequency and angular averaging, are tested numerically. It is shown that a single random rough surface of moderate extent can exhibit memory effect by using frequency averaging. Frequency averaging with a wide bandwidth is also effective for suppressing fluctuation in ACF and is most useful for practical applications. Numerical simulations indicate that even when the ratio of scattered intensities with and without the buried object is close to unity, the corresponding ratio of ACF magnitude can be more than 10 dB. Thus, using the ACF is superior to using the radar cross section (RCS) in the detection of buried objects.

Flexible steel plates are generally transported by rollers; however, the contact between the rollers and the steel plate degrades the surface quality of the plate. To solve this problem, noncontact transportation of steel plates using electromagnetic force has been proposed. However, ultrathin flexible steel plates can easily fall owing to deflection. A magnetic levitation system using electromagnets installed in the horizontal direction has also been proposed to improve the levitation performance of a conventional system. However, it is difficult to control vibrations with such a system because flexible steel plates are elastically deformed into complex shapes by gravity. Therefore, an electromagnetic levitation system was proposed, wherein electromagnets were installed near the edge of the steel plate such that it could be controlled with noncontact grip, such as by allowing one side of the steel plate to hang. This system is expected to improve levitation stability because the moment of inertia increases with vertical levitation and simplifies the control system. In addition, this system actively uses gravity acting on a steel plate to decrease its deflection. The use of gravity to suppress deflection is novel. In this study, the feasibility of magnetic levitation using the proposed system was investigated using magnetic field analysis. Its usefulness was investigated experimentally using a constructed magnetic levitation system. In addition, it was found that a magnetic levitation system that maintains the standing position generates a peculiar vibration.

The paper considers electric devices, based on the magnetic systems with an open core. It is usually used in various industries, in agriculture, household, transportation, medicine in the form of magnetic energy concentrators. For example, some devices can be used for water cleaning from contaminants, or as magnetic catchers in oil refinery industry, in some iron separating structures, and linear propellers [1–4, 8]. Special nozzles are used for concentrator magnetic field strengthening. The aim of the paper is to study imitation models of two electric devices structures with open magnetic system (concentrator) and hybrid magnetic system (an iron separator and a concentrator). The specific tasks include: designing the structures, their physical, mathematical and imitational models; obtaining magnetic field patterns, power characteristics as a result of physical and numerical experiments. It is shown that the hybrid model of the iron separator provides processing of substantial volume of the material being refined by the magnetic field. Numerical calculations were made by the finite element method [5–7] and with the help of ANSYS and ELCUT programmes.

The knowledge of the total magnetization direction has always been a drawback in among geological and geophysical exploration when using magnetic data. Remanent magnetization, present in almost all magnetic bodies, can significantly alter this direction. Usually the reduced to pole transform is applied to magnetic data but, without the knowledge of the total magnetization direction the data will not be reduced to pole correctly. Consequently, the interpretation will be erroneous. The need of a more precise and accurate interpretation always leads us to new methods and techniques. In this paper a method to determine the magnetization direction was tested and analyzed. The methodology is based on the cross-correlation of the vertical gradient and the total gradient of the reduced to pole anomaly. The method is only applicable to isolated anomalies and showed consistence and efficiency when applied to synthetic and real data, resulting in a more symmetrical and centered anomaly. Introduction A variety of interpretation techniques of magnetic data requires the knowledge of the total magnetization direction (TMD). The presence of remanent magnetization can influence this direction and consequently affect the interpretation and modeling of magnetic bodies. The total magnetization direction of a magnetic body is the vector sum of the induced magnetization and the remanent magnetization. The induced magnetization is aligned with the magnetic field of the earth, and in most cases it is the dominant magnetization and the only magnetization assumed when treating the magnetic anomaly. However, the remanent magnetization can be strong and alter significantly the direction of the total magnetization leading to erroneous interpretation. This paper is a reproduction and analyses of a technique used to determine the total magnetization direction of magnetic bodies. The proposed method, described by Dannemiller and Li, 2006, is based upon correlation between two quantities used in magnetic data interpretation: the vertical gradient and the total gradient of the reduced-to-pole (RTP) field. The method was tested on synthetic data and on real magnetic anomalies. Method The process of reducing to the pole transforms a total field anomaly, which is asymmetrical out of the pole, into a symmetrical anomaly generated as it would occur in the pole. The reduction to pole can only be correctly applied if the total magnetization is known. Numerically, this transformation (RTP) can be done using an estimate of the magnetization direction, but the result will not be an anomaly reduced to pole. The anomaly will still be asymmetrical when compared to that reduced to pole. So, the problem consists in finding the magnetization direction that, when applied to the reduction to pole, will generate the most symmetrical anomaly possible for our magnetic causative body. The methodology is based on the cross-correlation of two well known quantities used in interpretation of magnetic data: the vertical gradient and the total gradient of the reduced to pole anomaly. Once calculated the vertical and total gradient of all possible direction of magnetization, defined by the pair magnetic inclination and declination (IM, DM), the correct reduced to pole transformation will be the one that reaches the highest correlation between the two quantities. The cross-correlation can be computed using the following formula:

Electromagnetic Induction sensor (Metal Detector) has wide application areas for buried metallic object searching, such as detection of buried pipes, mine and mine like-targets, etc. In this paper, identification of buried metallic objects was studied. The distinctive features of the signal were obtained, than classification process was performed. Identification process was realized by utilizing k-Nearest neighbor and Neural Network Classifiers.

A main problem of effective landmine and UXO decontamination is efficient and reliable detection and localization of suspicious objects in reasonable time. This requirement demands for fast sensors investigating large areas with sufficient spatial resolution and sensitivity. Ground penetrating radar (GPR) is a suitable tool and is considered as a complementing sensor since nearly two decades. However, most GPRs operate in very close distance to ground in a rather punctual method of operation. In contrast, synthetic aperture radar (SAR) is a technique allowing fast and laminar stand-off investigation of an area. TIRAMI-SAR is imaging radar at lower microwaves for fast close-in detection of buried and unburied objects on a larger area. This allows efficient confirmation of a threat by investigating such regions of detection by other sensors. For proper object detection sufficient spatial resolution is required. Hence the SAR principle is applied. SAR for landmine/UXO detection can be applied by side-looking radar moved on safe ground along the area of interest, being typically the un-safe ground. Additionally, reliable detection of buried and unburied objects requires sufficient suppression of background clutter. For that purpose TIRAMI-SAR is using several antennas in multi-static configuration and wave polarization together with advanced SAR processing. The advantages and necessity of a multi-static antenna configuration for this kind of GPR approach is illustrated in the paper.

An analytical study of environmental and clutter effects on microwave radiometers used for the detection of buried objects is presented. To simplify the analysis, it is assumed that the soil/target medium has a constant physical temperature versus depth, so that Kirchhoff's law can be applied to determine emissivities, and a simple layered medium geometry is used to model a buried target. Changes in brightness temperatures which result due to the present of a buried target are illustrated for varying soil dielectric properties, radiometer frequencies, and target depths, and are contrasted with changes in brightness temperatures which can occur when no target is presented due to slight soil moisture or soil temperature variations. Brightness temperature clutter due to a small surface roughness is also analytically modeled, through application of the small slope approximation for the homogeneous medium case and the small perturbation method in the presence of a subsurface layer, and it is shown that surface clutter effects can be mitigated through proper choice of sensor polarization and observation angle. Particular attention is given to the relationship between passive and active microwave sensors; results demonstrate that these two can provide complementary information. Finally, the use of wideband radiometric measurements are discussed as a means for reducing environmental clutter effects and improving detection algorithms.

Inconsistent and unacceptable probability of detection (PD) and false alarm rates (FAR) due to varying environmental conditions hamper buried object detection. A 4-month study evaluated the environmental parameters impacting standoff thermal infra-red(IR) detection of buried objects. Field observations were integrated into a model depicting the temporal and spatial thermal changes through a 1-week period utilizing a 15-minute time-step interval. The model illustrates the surface thermal observations obtained with a thermal IR camera contemporaneously with a 3-d presentation of subsurface soil temperatures obtained with 156 buried thermocouples. Precipitation events and subsequent soil moisture responses synchronized to the temperature data are also included in the model simulation. The simulation shows the temperature response of buried objects due to changes in incoming solar radiation, air/surface soil temperature changes, latent heat exchange between the objects and surrounding soil, and impacts due to precipitation/changes in soil moisture. Differences are noted between the thermal response of plastic and metal objects as well as depth of burial below the ground surface. Nearly identical environmental conditions on different days did not always elicit the same spatial thermal response.

Borehole conductivity logs provide an in situ measurement of the electrical conductivity of the subsurface. Despite the measurements being a proxy for the true earth structure, they are often used as ground truth when inferring subsurface electrical conductivity boundaries between lithologies. Borehole conductivity measurements are therefore commonly used to plan and benchmark electromagnetic (EM) surveys and to establish the credibility of a given inversion technique. A consequence of the diffusion physics of EM prospecting is that not all subsurface features present in a conductivity log can be resolved by an EM system, nor can they be recovered by a subsequent inversion. Quantification of the ability of an EM system to determine layer boundaries in the subsurface is therefore an issue meriting investigation. We have developed a reversible-jump Markov chain Monte Carlo (RJMCMC) method to segment borehole conductivity logs at the scale recoverable by a given EM system as the foundation for an objective comparison between the inversion results and conductivity logs. A common consequence of RJMCMC inversions for EM problems is that few layers are required to fit the data. Similarly, we find that a borehole log blocked at the scale sensed by an EM system consists of a limited number of segments. Segmentation of borehole conductivity logs is determined by the physics of EM prospecting and by factors such as base frequency, number of gates, system geometry, and noise levels. For a survey line intersecting a borehole near Carnarvon, Western Australia, we see that different inversion schemes result in images of the subsurface that are consistent with a borehole conductivity log segmented according to the mechanics of the EM system and accounting for the physics of EM prospecting.