Imaging Of Buried Objects Research Articles

Three-dimensional imaging of buried objects in very lossy earth by inversion of VETEM data

The very early time electromagnetic system (VETEM) is an efficient tool for the detection of buried objects in very lossy earth, which allows a deeper penetration depth compared to the ground-penetrating radar. In this paper, the inversion of VETEM data is investigated using three-dimensional (3-D) inverse scattering techniques, where multiple frequencies are applied in the frequency range from 0-5 MHz. For small and moderately sized problems, the Born approximation and/or the Born iterative method have been used with the aid of the singular value decomposition and/or the conjugate gradient method in solving the linearized integral equations. For large-scale problems, a localized 3-D inversion method based on the Born approximation has been proposed for the inversion of VETEM data over a large measurement domain. Ways to process and to calibrate the experimental VETEM data are discussed to capture the real physics of buried objects. Reconstruction examples using synthesized VETEM data and real-world VETEM data are given to test the validity and efficiency of the proposed approach.

Measurement of Attenuation and Speed of Sound in Soils

The potential application of this work is the detection and imaging of buried objects using acoustic methodology. To image buried artifacts, it is vital to know speed and attenuation of sound in the particular soil being examined because they vary in different soil types and at different moisture contents. To that end, our research involved six soils representing a range of properties expected to influence acoustic response. Clay ranged from 2 to 38%, silt from 1 to 82%, sand from 2 to 97%, and organic matter from 0.1 to 11.7%. Signals from an acoustic source were passed through soil samples and detected by an acoustically coupled hydrophone. From a total of 231 evaluations, we determined the acoustic attenuation coefficient and the propagation speed of sound in the soil samples as a function of four levels of soil moisture and two levels of compaction. Attenuation coefficients determined over frequencies of 2 to 6 kHz ranged from 0.12 to 0.96 dB cm−1 kHz−1 Lower attenuation tended to be in loose dry samples. Correlation coefficients were 0.35 (P = 0.01) and 0.31 (P = 0.03) between attenuation and soil water content and soil bulk density, respectively. Propagation speeds ranged from 86 to 260 m s−1 The correlation coefficient with speed was −0.28 (P = 0.05) for soil water content and −0.42 (P = 0.002) for total porosity. Given the acoustic properties, it is theoretically possible to detect an object down to ∼40 cm below the soil surface.

Synthetic aperture sonar imaging in a random ocean

The maximum achievable aperture of a coherent imaging sonar is normally considered to be limited by the horizontal coherence length of the medium, which is on the order of several tens to several hundred wavelengths. Synthetic aperture sonar results from the past several years, however, have clearly exceeded this limit by an order of magnitude or more. It appears that the horizontal coherence length alone does not describe some important aspects of the phase errors present in real data. In particular, we suggest that underlying structures in the phase errors are exploitable in processing algorithms and are responsible for these results. We give a number of numerical examples from sub-kHz to tens of kHz, including new results on subcritical imaging of buried objects.

Bistatic synthetic aperture imaging of buried objects from an AUV

During SACLANTCEN’s GOATS98 experiment an assortment of proud and buried objects were ensonified above and below critical angle by a parametric array and their scattered fields were observed bistatically on the nose array of a MIT Odyssey class AUV. Here the challenges of synthetic aperture sonar (SAS) processing for bistatic data sets are addressed and the detection of buried objects at subcritical ensonification angles is demonstrated. In addition, the differences between the measured resonant characteristics of spherical target scattering at below and above critical grazing angles is discussed.

SAR imaging of buried objects from MoM modelled scattered field

Synthetic aperture radar (SAR) images of resonant buried objects are modelled in the presence of ground surface clutter. The method of moments (MoM) is used to model scattered fields from a resonant buried conductor and clutter is modelled as a bivariant Gaussian distribution. A diffraction stack SAR imaging technique is applied to the ultra-wideband waveforms to give a bipolar signal image. A number of examples have been computed to illustrate the combined effects of SAR processing with resonant targets and clutter. SAR images of different targets show differences which may facilitate target identification. To maximise the peak signal-to-clutter ratio, an image correlation technique is applied and the results are shown.

Microwave imaging of buried objects for ground radar tomography

The detection and identification of buried inhomogeneities using electromagnetic waves are of great importance for nondestructive testing in geophysical, civil engineering, military, or humanitarian applications. The approach described here uses a linear inversion algorithm based on diffraction tomography to process the backscattered field signal generated by the buried inhomogeneities, measured over a probing area, for computation of a tomographic image of the ground. The technique has been extended to cater for time pulse (synthetic pulse or real pulse) illumination and to incorporate the near‐field radiation pattern transmitted by a broadband antenna. The backscattered field is measured at various locations with a receiving antenna for different positions of the transmitting antenna. The main advantages of such a method are the use of explicit formulas for solving the imaging problem and the possibility of rapid image display thanks to the use of Fast Fourier transform to implement the reconstruction algorithms on PCs. First, a number of tomographic reconstructions of buried objects are given for situations of practical interest. Results of numerical simulations with a Rayleigh incident pulse in the frequency band [0.3,3.0] GHz and the use of a bow tie antenna in the frequency band [0.3,1.3] GHz are presented. Second, tomographic reconstructions of buried objects are presented and discussed, to demonstrate the influence of object dielectric properties versus soil electromagnetic parameters.

Electromagnetic imaging of circular-cylindrical conductors and tunnels using a differential evolution algorithm

A technique is developed for the electromagnetic reconstruction of the location and radius of buried circular–cylindrical conductors or tunnels based on a differential evolution (DE) algorithm. Simulation results are presented which demonstrate that DE can offer a simple, yet efficient and robust method for the imaging of buried objects. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 330–334, 2000.

An image reconstruction method using GPR data

The spatial resolution of a ground penetrating radar (GPR) system using traditional interpretation methods is largely dependent on the isolations of echoes. It is difficult to identify weak echoes that are mixed with either direct waves or other larger echoes; especially, when the media is absorptive, the reflections cannot be identified from the tail of the direct wave. The authors present an iterative algorithm to reconstruct the image of the subsurface using time-domain waveforms. The forward modeling employed in the algorithm is derived from the transmission line matrix (TLM) method with a perfect matched layer (PML) absorbing boundary. One-dimensional (1D) inversion algorithm is briefly described, based on which, a two-dimensional (2D) inversion method is developed. The inversion algorithm proceeds iteratively in space and time step by step. The inversion algorithm is tested in cases in which both lD and 2D variations exist. The developed inversion method is used to reconstruct the images of buried objects using GPR data with echoes buried in the direct waves. The results show a superior resolution and stability.

3-D localization of buried objects by nearfield electromagnetic holography

We present a practical method of localizing underground objects with low‐frequency electromagnetic fields. The method uses spatial information from the field measured on the ground, and a nearfield “holographic” theory to reconstruct the field image of buried objects. Both numerical simulations and field tests demonstrate that buried metallic objects can be resolved and individually localized in 3-D space even if the measurement field shows only a single peak as might be indicative of a single object.

Atomic and mesoscopic scale characterization of semiconductor interfaces by ballistic electron emission microscopy

Ballistic electron emission microscopy (BEEM) is a powerful new low energy electron microscopy in materials physics for nondestructive local electronic characterization of semiconductor heterostructures. In this article, low energy electron imaging of buried semiconductor heterostructures will be explored, with particular emphasis on BEEM imaging of buried objects in metal–GaAs based heterostructures.

Infrared imaging of buried objects by thermal step-function excitations

Sudden heating or cooling of a surface, by turning on or off radiation flux on the surface, is used as a means of enhancing the detection capability of buried objects by thermal infrared imaging. An experimental analysis of a sand-surface temperature is presented as a function of a buried object's composition and depth, and it is compared with theoretical simulations. Object identification by means of the geometry of isothermal contours and the rate of change of the surface temperature with radiant flux is discussed.

Transient electromagnetic fields due to dipole antennas located near an infinite perfectly conducting cylinder

AbstractFor full‐fledged applications of radar imaging of buried objects by using electromagnetic waves, it is important to analyze theoretically and experimentally the fundamental problems such as the scattering of the electromagnetic wave by various objects and the electromagnetic wave propagation from the antenna above the ground into the ground.As the first step toward such a goal, this paper studies the transient electromagnetic fields scattered by an infinite perfectly conducting circular cylinder placed near a dipole antenna in free space. First, the transient electromagnetic fields are calculated rigorously for the case where one dipole antenna is placed in parallel to a conducting cylinder. Furthermore, geometrical optics approximation also is derived.From the computed results, it is shown that the transient scattered field can be interpreted as the total sum of the scattered fields corresponding to optical paths. It is shown also that the geometrical optics approximation is extremely accurate in the backscattering region important for the analysis of inverse scattering problems such as the detection of a buried object. Next, the transient transmission and reception responses are analyzed theoretically in the case where two dipole antennas are placed in parallel to the circular cylinder and its validity is confirmed by experiments.

Acoustical imaging of buried objects in bottom sediment by using focused ultrasonic transducer

The surveillance of the bottom sediment condition, especially detecting buried objects in sediment is important to assure safety in harbor construction work. Conventionally, however, a slant cylindrical object is usually detected as if there were two objects because the image is separated into two—each corresponds to its end—when using a usual plane type transducer. A test has been conducted in which, using a focused array transducer having elements arranged on a concaved surface, the detection characteristic of slant object has been found to be improved to a great extent as long as the object gradient is within a half of the transducer aperture angle. The frequencies used are 150 kHz when the object is scaled down to 1/10 and 15 kHz in the actual scale test. Two displays are used, one for the vertical plane, the other for horizontal plane. In the actual test, the cylindrical iron tube and others are buried in a sand‐box of 2 m deep having 4 × 3‐m2 area and apparent improvement has been observed.

Microwave holographic subtraction for imaging of buried objects

A simple technique for subtracting microwave holograms is presented. The technique is novel in that it superimposes a hologram over the complement of a second one obtained through an inverting amplifier with unity gain. Since the holograms contain mainly the wavefront information, only the difference of two dissimilar microwave fields is recorded. The technique is demonstrated by subtracting the effect of the front-surface reflections from the hologram of an object concealed behind a box of dry sand to retrieve the image of the object itself. This technique can have many applications, like in imaging of buried objects in dielectrics.