Subsurface sensing and imaging problems arise in a variety of contexts: underground, underwater, inside the human body, and inside a cell or a collection of cells. All of these problems require reconstruction of internal structures or functions from a highly distorted probe or wave sampled outside of an obscuring surface. There is an emerging common framework of physics-based signal processing which will allow progress in any of these areas to be applied to create advances in the other areas. The recognition of the essential similarity of these problems and the development of the common framework is a key to the next generation of environmental and biomedical imaging systems.

When inspecting concrete structures with microwaves (radars, embedded microwave sensors, modulated scattering techniques, etc.) the dielectric properties of the concrete are considered as a ground truth data and must be known. During the past three years, extensive microwave near-field measurements of the reflection properties of concrete specimens with varying water-to-cement (w/c) ratios, sand-to-cement (s/c) ratios and coarse aggregate-to-cement (ca/c) ratios have been conducted. These experiments were conducted using open-ended rectangular waveguide probes radiating into a half-space of these concrete specimens. These measurements were conducted at S- (2.6 - 3.95 GHz) and X-bands (8.2 - 12.4 GHz). Moreover, an electromagnetic model, which took into account the presence of higher-order modes at the waveguide aperture, was also used to model this process. Finally, a root finding technique was applied to calculate the effective dielectric properties of the concrete specimens. This paper presents the results of these measurements and calculations as they related to determining the dielectric properties of concrete. Since concrete is a heterogeneous material, the results from many locations in a specimen are reported rendering effective dielectric properties showing the mean and standard deviation of the measurements and calculations at these frequency bands. The results of the dielectric constant can also be used to predict the reflection properties of concrete when using a standoff distance (i.e. non-contact measurements) or when using other types of microwave sensors.

By adapting a portable one-port analyzer which is normally used to test the integrity of cable feeds to antenna systems used in the communications industry, a permittivity probe has been constructed. To date, only an open-ended coaxial sensor has been used with this analyzer. Results from two applications are reported.

Much of today's applications in nondestructive testing by microwaves use an automatic network analyzer. As a result, there is a need for systems to reduce the cost of this kind of techniques. Fortunately, now we can benefit from the cost reduction of the microwave components, induced by the considerable development of the communication market, around 2 and 10 GHz. So, it seems reasonable to think that microwaves will take advantage of this new situation to assert themselves in this application field. In this context we conceive and develop original equipment competitive in term of price and reliability.

The quality of products depends on the moisture content. By means of the introduction of ISO 9000 the market of microwave moisture measurement instrumentation has been grown up to a great variety of possible applications in different areas of industry and agriculture. Moisture measurement systems have to determine the exact moisture content (MC) of a certain material, in order to allow the control of the dosage of water, the quality of products and the reduction of applied energy. The methods show the advantage of being non- destructive with high accuracy under severe environmental industrial conditions. The effect of disturbances, such as bulk density variations, material grain size or salt content, can be reduced by both suitable data processing and choice of measuring frequency.

This paper describes a new method for measuring moisture content distribution of timber using only attenuation at two microwave frequencies. The experiment showed that the setup of transmission type operating at 9 and 12 GHz measured moisture contents of some timbers of different kinds having various thickness and density without any knowledge about the thickness and the density. Four timbers of three species having different thickness and density were measured by the method. The result showed that the setup based on the method measured samples with the timbers mentioned above. The measurable range of moisture content was below 20% in dry basis and the mean value of error was 2.3%, and the maximum error was 7.7% using a calibration curve expressed with a third polynomial equation.

This paper introduces a microwave measurement setup using three microstrip antennas integrated with a microwave oscillator and Schottky-diode detectors used as bistatic microwave sensors. We have measured the microwave attenuation (water content) and the bistatic radar cross section (dimensions) of eggs having different time of storage (see Fig. 1). The measured attenuation for fresh egg is 8 - 10 dB at the frequency of 14.2 GHz, the variation with time is: dA [dB] equals -0.033T (days), thus nearly -1 dB/30 days. Our next program is to create a microwave equipment for automatic selection of old eggs. The basic arrangement and the results of our measurement are discussed.

Accurate characterization of the electrical activity of the heart is of great physiological and medical import. The only truly non-invasive sites to measure this activity are on the body surface, as in an ECG, but the signals are attenuated and distorted by the thorax volume conductor. Thus even an array of body surface electrodes records a remote observation of cardiac electrical activity. The goal of cardiac electrical imaging is to accurately and reliably reconstruct the source image sequence from this observed image sequence using a model of the volume conductor. Since the problem is ill-posed, we need to constrain solutions using prior knowledge of source characteristics. Because the source is quite complex and distributed in time and space, and reconstructions are sensitive to the choice and weighting of constraints, several groups, including ours, have recently introduced methods to impose multiple constraints. We will review the problem and some of these methods and describe recent work which uses a convex optimization framework to impose multiple constraints, combining traditional smoothing with preservation of sharp gradients in the reconstructed image sequences. We pay particular attention to efficient distributed computational implementations of these solution methods.

It has been shown previously that images collected at selected wavelengths in a sufficiently narrow bandwidth can be used to produce maps of the oxygen saturation of hemoglobin in the dermis. A four-wavelength algorithm has been developed based on a two-layer model of the skin, in which the blood is contained in the lower layer (dermis), while the upper layer attenuates some of the reflection and adds a clutter term. In the present work, the algorithm is compared analytically to simpler algorithms using three wavelengths and based on a single-layer model. It is shown through Monte-Carlo models that, for typical skin, the single-layer model is adequate to analyze data from fiber-optical reflectance spectroscopy, but the two-layer model produces better results for imaging systems. Although the model does not address the full complexity of reflectance of a two-layer skin, it has proven to be sufficient to recover the oxygen saturation, and perhaps other medically relevant information. The algorithm is demonstrated on a suction blister, where the epidermis is removed to reveal the underlying dermis. Applications for this imaging modality exist in dermatology, in surgery, and in developing treatment plans for various diseases.

The interaction of ultrasound with diffusive optical waves holds the promise of improved resolution and depth of penetration for non-ionizing, non-invasive medical imaging. Light offers the ability to make spectroscopic measurements, which can monitor metabolism but, while it can propagate deeply into the body at near infrared wavelengths, it is so strongly scattered in most tissues that imaging is achieved only with measures such as time-resolved or frequency-domain techniques. On the other hand ultrasound has the ability to penetrate deeply with considerably less scattering. We have proposed a mechanism for imaging through the body by mixing diffusive optical waves with focussed ultrasound to produce new, so-called virtual diffusive sources. In the present work, we consider various mechanisms of interaction and report some results suggesting that the interaction in a laboratory phantom is predominantly the result of particle motion rather than variations in the index of refraction of the background medium. This result may be important in development of techniques to maximize the strengths of the virtual sources.

A new infrared (IR) interferometric method has been developed in conjunction with low-loss, flexible optical fibers, sensors, and probes. This combination of fiber optical sensors and Fourier Transform (FT) spectrometers can be applied to many fields, including (1) noninvasive medical diagnostics of cancer and other different diseases in vivo, (2) minimally invasive bulk diagnostics of tissue, (3) remote monitoring of tissue, chemical processes, and environment, (4) surface analysis of polymers and other materials, (5) characterization of the quality of food, pharmacological products, cosmetics, paper, and other wood-related products, as well as (6) agricultural, forensic, geological, mining, and archeological field measurements. In particular, our nondestructive, fast, compact, portable, remote and highly sensitive diagnostics tools are very promising for subsurface analysis at the molecular level without sample preparation. For example, this technique is ideal for different types of soft porous foams, rough polymers, and rock surfaces. Such surfaces, as well as living tissue, are very difficult to investigate by traditional FTIR methods. We present here FEW-FTIR spectra of polymers, banana and grapefruit peels, and living tissues detected directly at surfaces. In addition, results on the vibrational spectral analysis of normal and pathological skin tissue in the region of 850 - 4000 cm^-1 are discussed.

In order to understand the potential of polarimetric borehole radar, field measurement was carried out at the Mirror Lake fractured-rock research site (NH, USA), in October 1998. The polarimetric borehole radar system can measure the full- polarimetry in a borehole by changing antenna arrangements. We developed a network analyzer based stepped-frequency radar. The radar penetration depth from the borehole in the reflection measurement was over 10 m, in the frequency range of 2 - 200 MHz. We could observe many clear reflections form fractures in each polarization state. Even in the raw data, we could observe the difference in the radar profile for different polarization states. Polarimetric future for the acquired data is analyzed and it was found that the depolarization effect is different in each fracture. We think this is closely related to the roughness of each fracture, and it is also related to physical properties of the fracture such as water permeability. Cluster analysis was tested for classification of fractures.

Magnetic detection offers one approach to complement optical, infrared, radar, and acoustic sensing for wide-area detection, localization and classification applicable for a large class of subsurface structures. Sensors incorporating Superconducting Quantum Interference Devices (SQUIDs) provide a promising approach for such wide-area searches. A SQUID- based sensor developed in the 1980s demonstrated an effective capability to detect, localize, and classify underwater magnetic targets such as sea mines and unexploded ordnance. This sensor incorporated low critical temperature (low Tc) superconducting components cooled by liquid helium to a temperature of 4 degrees Kelvin (K). In the late 1980s, a new class of 'high Tc' superconductors was discovered with critical temperatures above the boiling point of liquid nitrogen (77 K). Nitrogen refrigeration offers new opportunities for this superconducting sensor technology, including significant reduction in the size of sensor packages and reduction in the requirements for cryogenic support and logistics. As a result of this breakthrough, a high-Tc sensor cryocooled by liquid nitrogen has been developed and is being evaluated for mobile operation. In this paper, opportunities and approaches using magnetic sensors for subsurface structure detection will be addressed. Results from evaluation of this high-Tc SQUID sensor will be presented and its capability for these applications will be discussed.

We have developed a high sensitive and wide-band micro capacitive accelerometer for subsurface seismic measurements by using the micro machining technique which enables to fabricate three-dimensional micro structure and to realize small-size, low-cost and high performance sensors or actuators. The silicon capacitive micro accelerometers, which are used in automobiles for suspension control, brake control and air bags, are potentially be used for subsurface seismic measurements. However the sensitivity and the bandwidth of existing sensors are insufficient to detect seismic waves. The micromachining technique which we employed includes photolithography, anisotropic etching, reactive ion etching and anodic bonding. Problems in the fabrication process such as sticking of silicon mass on the electrode, buckling of spring beams, and a problem due to the squeezed film dumping effect have been resolved by improving the sensor structure and the silicon process. Finally, we have evaluated the characteristics of the sensor.

Waste disposals are covered with high density and highly saturated clay layers, which prevent rain penetration through waste into groundwater. These layers without monitoring are overdesigned to achieve long term performance. In case of a containment control, which means water content control, the construction can be less expensive and defects can be selectively repaired. We developed a new subsurface moisture sensor system, which locates a water content change of 2% with an accuracy of 4 m in a clay layer on a field of 2000 m². It consists of a network of unshielded high-frequency transmission lines, installed during the disposal construction. The electromagnetic propagation along these lines is affected by the dielectric coefficient of the surrounding media which is strongly related to its water content. Usually, soil moisture sensors with Time-Domain- Reflectometry (TDR) are much shorter than 1 m. By dielectric measurements on several soil types and by an advanced model of the effective line parameters in soil, we could design transmission lines with lengths up to 20 m. Additional coupling zones between lines are used to economize connecting cables and to enhance the space resolution. TDR methods have been found less accurate than appropriate frequency domain methods performed with a network analyzer. The demonstrated moisture distribution maps of this test field show small local and seasonal variation since 1997.

When wooden poles and cross-arms are exposed to weather, its surface undergoes changes such warping, loss of some surface fibers and surface roughing. As a result of weathering, its unprotected surface tends to absorb more moisture as compare to the surface of sound wood. After the rain, the moisture will sustain in the weathered wood for some time before it is dried out. There is a possibility to relate the quality of the wooden cross-arms with the amount of its moisture content (MC) at a particular time. This work deals with the effect of weather on the dielectric properties of wooden cross-arms. The samples from sound wood, incipient decay and decay woods were chosen for dielectric study with frequency ranging from 0.2 GHz to 20 GHz. It is found that the decay wood can absorbed more than 70% MC (dry-basis) as compare to sound wood of only about 30% MC. The results of the dielectric measurement are compared with the values predicted by dielectric mixture equations. From the above study, there is a great possibility of using microwave reflection method to determine the moisture content and condition of wooden cross-arms.

There are many anomalies associated with the application of ground penetrating radar (GPR) for the detection of hydrocarbon contamination. The aim of this paper is to investigate this problem by measuring the complex permittivity (i.e., the permittivity and conductivity) of contaminated soils over the GPR frequencies 100 - 1000 MHz. A coaxial cell, which is much larger than commercially available cells, is designed and calibrated to measure soil samples. The new system which is based on transmission line principle uses only one port and the far end of the cell is short-circuited by a metal plate, thus it is easy to load and hold the sample. Only the reflection coefficient is needed for data interpretation. In addition, there is no dielectric support for the inner conductor of the coaxial cell, therefore this system can offer good measurement accuracy. Measurements have been conducted on four samples obtained from two hydrocarbon contaminated sites with various degree of contamination. It is observed that the conductivity of the soil is associated with the contamination: the heavier the contamination, the higher the conductivity. This seems to agree well with on site measurement using EM conductivity and 2D resistivity imaging approaches.

Ground penetrating radar (GPR) has been used to explore the subsurface of the earth since 1929. Over the past 70 years, it has been widely used, misused and abused. Use includes agriculture, archaeology, environmental and geotechnical site characterization, minerals, groundwater and permafrost exploration, tunnel, utility, and unexploded ordnance location, dam inspection, and much more. Misuse includes mistaking above ground reflections for subsurface events or mapping things from off to the side as if they were directly below, synthetic aperture processing of dispersive data, minimum phase deconvolution, locating objects smaller than resolution limits of the wavelength in the ground, ignoring Fresnel zone limitations in mapping subsurface structure, processing radar data through seismic software packages without allowing for the differences, mapping the bottom of metal pipes from the top, claiming to see through thousands of feet of sediments, and more. GPR is also being abused as the regulatory environment changes and the radiofrequency spectrum is becoming more crowded by cellular phones, pagers, garage door openers, wireless computer networks, and the like. It is often thought to be a source of interference (though it never is) and it is increasingly interfered with by other radiofrequency transmitters.

Based on a discovery by Igor Zuykov, Environmental Investigations (EIC) has developed a novel portable subsurface imaging system, termed the RAP Plus^TM System, capable of generating detailed images of geological strata layers and underground man-made structures and objects. The system takes advantage of ever-present low-level random vibrations in the ground and detects resonances in these vibrations that naturally occur at the interfaces of strata layers and surface boundaries of structures. These resonances are detected using acoustic sensors in contact with the ground surface. No acoustical or RF signal needs to be injected into the ground, though a light tap on the surface near the sensor may enhance the processing of the received signal. The sensors detect the vibrations and convert them into electrical signals, which are amplified and detected. An algorithm based on the Fourier transform converts the signals from time into frequency domain. Another algorithm is then used to extract the information related to resonances and accurately determine the depth and shape of the strata and structure boundaries. This information is displayed as images in which the layers or structures appear in false colors. A significant advantage of this system that it is immune to electro-magnetic interference and even to strong vibration caused by passing vehicles.

Stepped frequency radar systems are gaining popularity in ground penetrating radar applications since these can be designed to be phase coherent, thereby taking advantage of coherent processing techniques for target detection and clutter rejection. One such system is the GeoRadar model 1000B GPR system. This system operates over the 100 - 1000 MHz frequency range with a step size of 2 MHz and a modulation frequency of 500 kHz. The magnitudes of the in-phase (I) and the quadrature (Q) channels of the received radar return signal are combined vectorially to yield the amplitude, which is then fast Fourier transformed (FFT-ed) and displayed as the depth profile. The original design of the system used a pair of log-spiral antennas for transmit and receive functions. Although log-spiral antennas are broadband and frequency- independent, their polarization, although linear at each frequency, rotates as the frequency is stepped. The images acquired under these conditions are similar to those obtained by circularly polarized antennas, making it difficult to detect long slender objects, such as pipes and cables, which can be better detected by linearly polarized antennas oriented in the same direction. In order to enhance the performance of the system, a fully polarimetric processing technique has been implemented. A set of four wideband linearly polarized antennas is used, two for transmit and two for receive, each of the two being orthogonally polarized. The system sequentially transmits, orthogonal polarizations, while the receiver simultaneously measures both reflected polarizations. The transmit polarization switching rate is high enough to ensure that the successive pulses are correlated, thereby appearing as if taken from the same location. Reflected data from all four linear combinations of transmit and receive polarizations are then coherently processed to yield the polarimetric image of the target. Sandbox test results indicate that the system is better able to image pipes and utilities.

In this paper the results of experimental investigation of buried objects are given. To find images of the objects the microwave multi-frequency diffraction tomography method was used. For image reconstruction of the objects we used a plane wave spectrum of diffracted field. A vectorial network analyzer measured real and imaginary parts of the diffracted field by its sin/cos converter. The wave incident near to the normal of the surface was used in our measurements at frequency range of 2.5 - 4.5 GHz. The experimental set-up allows to obtain the diffracted field data at 512 frequency points. It contains a mechanical scanner system that can move as well linearly as in 2-D space. In the inverse problem a complex function of space coordinates representing the normalized polarization currents was reconstructed. A module of the complex function is the image of the object under investigation. The reconstruction algorithm is based on Fourier inverse formulas. The images of dielectric and metallic objects of different shape and configuration were reconstructed. The results of investigation may be used for civil or medical science application.

Nondestructive evaluation of concrete structures is an important practical issue in the construction industry. Evaluation encompasses many issues including concrete constituent property, compressive strength and chloride contamination determination to name a few. To this end, a combination of modulated scattering technique and near-field microwave nondestructive evaluation technique, is used to determine its potential for evaluating dielectric properties of a hardened mortar specimen. This technique utilizes a small resonant PIN diode-loaded dipole scatterer embedded inside the mortar while using an open-ended rectangular waveguide probe operating at 7 GHz to detect this dipole which is modulated at low frequencies. The results of this preliminary experiment, and its future ramifications for nondestructive concrete inspection structures are provided in this paper.

For a vibration analysis of an object surface, the holographic interferometry measuring technique is one of the most efficient methods for non-contacting measurement of a microscopic displacement. Using this technique, it is possible to measure the vibration surface by means of an interferometric fringe pattern due to the vibration. In this fundamental study, for the time resolved displacement pattern measurement of the vibrating surface, the stroboscopic method was applied, and compared with the time average method. The results showed that the stroboscopic method is useful in time division pattern analysis of the microscopic displacement due to vibration of the object surface.

Tomographic methods of image reconstruction of two-dimensional cross-sections of cylindric objects are used. A microwave tomography system was created for imaging dielectrical cylindrical objects. It is shown that the image of object illuminated by an electromagnetic signal in the frequency band under consideration (f approximately equals 7.5 GHz) may be obtained by using first-order diffraction tomography method through Born approximation for the scattered field. The imaging system involves scanner with a rotating platform. In the test runs, a cylindrical object placed between the transmitter and the receiver antennas is scanned and rotated. Data processing is controlled by a computer system. Real and imaginary parts of scatted field are obtained by using vector network analyzer. Special software carries out image processing. Dielectric permittivity (epsilon) of the objects under investigation lies in the interval 1.1 less than (epsilon) less than 2. The surrounding media is air. It is shown that if (epsilon) satisfies the above condition and object size is some few wavelength (lambda) ₀ ((lambda) ₀ approximately equals 0.04 m), imaging process creates satisfactory results.

In this paper, the author discusses a Broadband Bionic Sonar Sensor System and a signal processing technique for detection and identification of underwater targets. This bionic sonar system with the resonance detection technique for detection and identification of underwater objects appears to mimic a dolphin's sensory system. The dolphin's sonar system transmits a very short broadband pulse. It detects and classifies a target by processing the modulation of the echo's (back scattering) broadband spectrum. This spectral modulation is directly related to the target's natural resonance. Using the G-Transform technique, the author has successfully showed that target resonance exists and it is unique to target size, shape, structure and material composition. Furthermore, this natural resonance exists in both (active sonar) acoustic echoes, back scattering and (passive sonar) acoustic scattering in acoustic noise background. Using trained neural networks, these targets' resonances/signatures can be correctly identified for the respective targets. It is conceivable that a broadband radar system, similar to a dolphin's sonar system, can be developed for targets and subsurface targets.

The subsurface conditions of a railroad track bed have a significant influence on track performance. The depth of the ballast, subballast, and subgrade layers, as well as the presence of water pockets, slurry and other foreign materials all affect track performance. Railroad tie integrity is also an important factor that needs to be monitored for optimum track performance. The possibility of using ground penetrating radar (GPR) to monitor various track parameters in a rapid, synoptic, and non-destructive manner is shown in several simulations presented in this paper. The pulse radar simulations are performed at two different frequencies (100 MHz and 400 MHz). The choice of the two frequencies is dictated by the combined need to perform high resolution probing near the surface (400 MHz), and to probe deeper into the substructure, albeit at lower resolution (100 MHz). The results of the simulation show that the 400 MHz system is better able to estimate the depth of the subsurface layers better than the 100 MHz system. Also, the higher resolution of the 400 MHz system makes it better suited to determine the condition of ties near the surface of the rail bed. However the 100 MHz system is able to better identify anomalies, such as water pockets at deeper depths, compared to the 400 MHz system. Preliminary field experiments at various test tracks also support the above conclusions. It is therefore recommended that a combination of GPR systems at both low and high frequencies be used for optimal monitoring of railroad bed and track condition.

Electromagnetic signals in the very low frequency range (VLF, below 10 kHz) can penetrate rock sufficiently far for miner's location with nearly no distribution of the field structure, if the near field condition is fulfilled. The relevant solutions of the Maxwell's equations for a magnetic dipole (transmitting antenna to be carried by the miner) in a dissipative medium then are independent of the medium. From these equations the magnetic field strength in any point of reception can be calculated. The key for the location method is the inverse distance function (locus curve), describing the possible location of the dipole from the measured field strength and direction in a point of reception (performed by the rescue team). At least two of these measurements give locus curves intersecting in two points, the correct solution can be found by means of an additional direction information calculated in the points of interest. A system has been built and successfully tested with miner's transmitters which can be activated by a coded signal on the rescue team, a receiver device measuring the field amplitude and field direction and a software package to calculate the miner's location on a notebook directly in the mine.

Geothermal is the cheapest resources given to human being by the earth. The best way of its regional investigation is thermal inferred remote sensing. Thermal inferred image is a precise distributive mapping of the temperature of all earth's surface, which generally reflects clearly the geothermal body directly exposed to the earth's surface. By means of the image processing of Taian suburb TM image, especially band 6, the information of geothermal resource is taken up. Combining geophysical method and survey temperature onsite, the geothermal range is enclosed. On basic of these above, the author investigates the geology of the earth's surface and analyses the underground water, put forward the mechanism of the geothermal formation in the area, conduced a set of RS ways of geothermal resource investigation.

Wave scattering from buried non-metallic mine-like targets is modeled using the finite difference frequency domain method to determine the feasibility of identifying mines from shape features. It is shown that for constant cross-sectional target area -- approximately 100 cm² -- the scattered fields of targets with widely varying shape but roughly the same height- to-width aspect ratio at 500 MHz are virtually indistinguishable regardless of burial depth. However, for GPR frequencies above 700 MHz the same selection of targets buried at a depth of 5 cm can indeed be discriminated by shape. It is deduced that while low GPR sensing frequencies may help to detect shallow anomalies, they do not supply any useful information about the shape details -- particularly the edges -- of the buried non-metallic mine-like targets. Higher frequencies provide significant additional information that can be used for target reconstruction. Based on the intuition generated from this scattering study, we develop a non-linear inverse scattering algorithm based on a low dimensional parameterization of the unknown object and the background. In particular, we use polynomials to represent the contrast in the real and imaginary parts of the object and background complex permittivities. The boundary separating the target from the unknown background is described using a periodic, quadratic B-spline curve whose control points can be individually manipulated. A greedy-type approach is used to minimize a regularized least-squares cost function in order to estimate the control point locations as well as the contrast expansion coefficients.

A Laser-induced acoustic technique has been demonstrated for the potential application of underground object detection. The high acoustic impedance mismatch between granular soil and buried mine (usually landmines are made from plastic, rubber and metal) gives good contrast for detection. We obtained 3-D images of underground objects by scanning a pulsed laser across the surface of the ground and keeping the acoustic detector at relatively fixed position with respect to the laser spots. A signal processing method is introduced by using de-convolution filter. When a laser pulse impinges on the surface of ground, a laser-induced acoustic pulse will be excited after a sudden increase of surface temperature followed a sudden volume expansion. We used a laser vibrometer as acoustic receiver and found that the acoustic pulse width depends on laser pulse width. Using 100 ns CO₂ laser, we obtained 4 to approximately 5 microsecond acoustic sound pulses.

Synthetic aperture radar (SAR) imaging technologies are increasingly finding applications in the geosciences and are becoming instrumental in enhancing the fundamental understanding of the physical processes pertaining to the earth and the environment. Of particular importance are studies conducted about the earth surface property characteristics using SAR remote sensing techniques. These studies are relevant for the better understanding of concepts such as soil moisture content, backscattering from crops, nearshore ocean surface currents, and subsurface imaging in hyperarid regions. In this work, impulse response or point spread functions are used as ambiguity functions in the efficient computation of raw data generation operations and efficient filtering techniques are presented for imaging kernel operations. The efficiency in these operations is obtained through the use of Kronecker products algebra as a basis for the mathematical formulation of multidimensional fast Fourier transform algorithms.

The detection of landmines and buried objects requires methods that can cover large areas rapidly while providing the required sensitivity to detect the optical and spectroscopic contrasts in soil properties that can reveal their presence. These conditions on contrast and coverage can be met by capturing images of the soil at wavelengths which are sensitive to the properties modified by the presence of buried objects. In this work we investigate both NIR scanning methods which may have some utility for the detection problem. For the scanning method, we acquire data point-by-point over a two- dimensional grid with a single emitter/detector probe. The soil (or sand) above a shallow buried object can be differentiated from the surrounding soil by detecting the difference in relative water content. Moist soil absorbs more near-infrared (NIR) incident light than dry soil. A light- emitting diode (LED) operating at 900 nm and a photodiode sensitive to NIR radiation formed the emitter/detector (ED) probe used in this study. The ED probe was mounted side-by- side and scanned over a surface in a two-dimensional grid as readings were collected point-by-point. The results indicated that this simple NIR emitter/detector probe discriminated between soils of varying water contents with an imaging resolution of 4 millimeters. To illustrate how imaging techniques can be used in this application, the fluorescence image of a landmine casing is presented. The results illustrate the potential of these two approaches for detection of landmines and buried objects.

The scattering mechanism of diffraction tomography is described by the integral form of the Helmholtz equation. The goal of diffraction tomography is to invert this equation in order to reconstruct the object function from the measured scattered fields. During the forward propagation process, the spatial spectrum of the object under investigation is 'smeared,' by a convolution in the spectral domain, across the propagating and evanescent regions of the received field. Hence, care must be taken in performing the reconstruct, as the object's spectral information has been moved into regions where it may be considered to be noise rather than useful information. This will reduce the quality and resolution of the reconstruction. We show how the object's spectrum can be partitioned into resolvable and non-resolvable parts based upon the cutoff between the propagating and evanescent fields. Operating under the Born approximation, we develop a beam- forming on transmit approach to direct the energy into either the propagating or evanescent parts of the spectrum. In this manner, we may individually interrogate the propagating and evanescent regions of the object spectrum.

Electromagnetic signals propagate underwater following a diffusion equation. Using similarities between the diffusion equation and wave propagation, a transformation between the diffusion and the wave propagation can be performed employing the equivalent wave field (EWF). Using EWF, it is possible to apply enhancements developed for wave propagation to diffusive propagation. Synthetic Aperture Radar (SAR) is one of the enhancements used in a conventional wave field. Using wide- band electromagnetic transmission and the EWF, it is possible to detect and localized underwater conductivity anomalies. SAR is known to improve the signal-to-noise ratio and the resolution. SAR together with EWF was applied to synthetic data of a conducting sphere embedded in an infinite ocean. Noise was added to the signal. A major improvement in the detection and resolution was demonstrated.

In two previous papers, the authors discussed a frequency- domain simulation of received signals in a focused array radar system (Rappaport and Reidy, SPIE 2747, April 1996, pp. 202- 213) illuminating a swath of flat terrain with one or more small plastic mines buried a few inches below the surface and a clutter background consisting of returns from random permittivity fluctuations, aggregates of rocks and the ground surface (SPIE 3392, pp 754 - 765, April 1998 and 'Near-field and timing effects in simulation of focused array radar signals from a mine in subsurface clutter,' SPIE, April 1999). The second paper emphasizes generalization of the algorithm to take proper account of the fact that distances between participants in the scattering processes are within fractions of wavelengths, enhancing the importance of near field effects in the modeling. In the present work, further generalizations are made which enhance the realism of the model. In particular, we investigate the role of surface height variations in changing the apparent propagation delay of the signal from a subsurface region. Since delay is the major discriminent in the processing of received signals in this system, this has a possibly significant effect on the simulated images. Still another generalization under investigation is that of inclusion of surface reflections from the interface's underside on the effective illumination of subsurface regions. Simulated image plots will be shown and compared with some previous results in order to assess the effects of these enhancements in the model.

The well-known six-port technology has found widespread applications in the measurements of microwave networks .Recently new applications were also suggested to use six-port junctions and six-port calibration techniques in communication receivers. In the existing six-port technology, the tested signal is mixed and compared with a standard carrier wave without modulation to extract its phase and amplitude information. In this paper, we use a modulated wave instead of the carrier to realize a RF comparator. In this case the output data of the six-port gives us the amplitude ratio and phase difference of the two input signals. Simulations using SPW (Signal Processing Workstation) software are performed to validate the proposed comparator. When two identical 64 QAM signals with different amplitude and phase are fed into the six-port comparator, the output takes a constant value related to the ratio of the input amplitudes and phase difference. If two different signals are used as the input, the output of the six-port will change with time. These changes may be used to equalize the input signals, to eliminate the distortion of one of the incoming signal by comparison it with the standard one, to identify signatures and so forth.

The application of six-port technology in the receiving part of a radar front-end is described in this contribution. This receiver allows measuring magnitude and phase of the radar signal without the need of down-conversion. According to the applications and the availability of the devices needed for assembling the prototype sensor the frequency of operation was set to 35 GHz. The structure of the six-port as well as the calibration and measurement algorithms are described in detail. The accuracy of the phase measurement is 2 - 10 degrees depending on the power level of the signal. Another key element of the proposed radar sensor is a direct frequency counter. With this circuit it is possible to measure the frequency of operation within 120 microseconds. The accuracy of the distance reading of the radar is directly related to the accuracy of the frequency measurement and has reached 0.1 mm.

Theoretical and experimental investigations of the radiation properties of a buried leaky coaxial cable (LCC), as a guarding radar system, are presented for various artificial and natural local inhomogeneous conditions along the cable system. The theoretical model of two coupling coaxial lines, outside which consists a sub-soil medium, is introduced to describe interactions of two modes, external and internal, which determine the working characteristics of such a leaky coaxial cable. The vertical and the horizontal distribution of radiation along and across the cable at height h above the ground are examined experimentally by means of a loop of the LCC buried in the sub-soil medium. The comparison between experimental data and theoretical predictions of radiation directivity of the LCC for the guarding radar system is presented. The possibilities of generating a complicated interference picture of such a radar pattern caused by different kinds of local inhomogeneities are discussed theoretically and studied experimentally.

In this paper, a stepped-frequency radar with frequency range from 0.8 GHz to 4 GHz is proposed to 'scan' the ground using a relatively long antenna array. This 1D array has a very narrow beam-width along the movement direction but a wide beam-width along the array axis, thus the radar can efficiently detect buried objects with reasonable accuracy and the algorithm for data interpretation can also be relatively simple and straightforward. Because of its excellent wideband performance, the tapered slot antenna is chosen as the array element for the system. The focus of our study is on the optimum design and performance evaluation of the antenna array. The optimizations of the element antenna, the spacing of elements, the length of the array and the arrangement of the transmitting and receiving arrays are investigated by means of analytical and numerical (finite-different time- domain) simulations. Both the near and far fields of the array are obtained in the time and frequency domains. The signal returns from some typical objects (such as a pipe and a land- mine) are also investigated. It's demonstrated that the proposed TSA array system is viable for efficient and accurate detection of subsurface objects.

Mine detection using active radar systems is the subject of a number of research programs both in the US and in Europe. This paper considers the environmental and operational drivers that influence the design of such radar systems. The prime system architectures are time domain and frequency domain configurations and each has its merits. The majority of the ground probing radars used at present are time-domain ultra- wideband radars and their characteristics are well established, while a smaller proportion of radars operate in the frequency domain, using FMCW, stepped frequency or noise modulation. The complexity and cost of the latter type of radar is at present greater than the time-domain radars but theoretically offers a better dynamic range. This paper considers the characteristics of these generic radar systems and the factors that need to be considered in system design. The performance of the antenna significantly affects the overall system and the paper qualitatively discusses this aspect. For GPR, an important technical challenge is associated with achieving a well-defined antenna footprint to maximize the signal to clutter ratio and consideration will be given, in the paper, to time domain array antennas for mine detection. The results of studies carried out during EU funded programs will be reported in this paper. The radar image of the mine depends not only on its construction and on geometry but also on the local environment, hence prior assumptions about the radar spatial signature of the mine may be ill- founded.

The (ARL), as part of customer and mission-funded exploratory development programs, has been evaluating low-frequency, ultra-wideband imaging radar for detection of buried mines and unexploded ordnance. As part of these programs, ARL has designed and built an instrumentation-grade radar measurement asset. In addition, ARL has established and conducted experiments at two nationally significant subsurface target test sites at Yuma Proving Ground, Arizona and Eglin Air Force Base, Florida. This paper reviews the significance of the test sites and recent modeling and data analysis results using the Eglin and Yuma data. It also discuses the difference in soil conditions from these data collections and the impact that they may have on mine radar cross section (RCS) and detection performance. Finally, new imagery is presented that shows positive contrast of low metal content mines at these test sites.

The detection of buried anti-personnel mines (APMs) is widely considered as a problem which may only be solved with a combination of two or more complementary sensors. We present processing and fusion results obtained from a multisensor data set, acquired with a pulse induction metal detector (MD), a pulsed ultra wide band ground penetrating radar (GPR) and a 3 - 5 (mu) thermal IR camera. Metal detector and GPR sensors were mounted on a rig for optimum control. Various types of soils, clutter objects and burial conditions were recorded. Anti personnel mines included minimum metal mines as well as mines with a significant metal content. We use a special projection to map a 3D GPR data cube, with time or depth as vertical coordinate, into a horizontal plane view 2D image. Object contours are then derived, based on an edge extraction method, followed by an automatic detection of circular shapes with a Hough-transform. In the association step, the stand-off IR image, the metal detector and GPR images and related detections are mapped onto a common cartesian grid on the ground surface. Detection results are fused on a decision level, using a Bayesian approach. Our results indicate that the GPR performance approximately matches that of the metal detector. With both sensors all metallic mines and around 60% of the minimum metal mines were detected. In the case of two false alarms per square meter combined detection probability clearly exceeds single sensor performance and reaches around 80%. Our fusion demonstrator, which incorporates all elements of the processing chain, has been implemented on the basis of MATLAB^TM.

This paper describes how Multiple Test Procedures (MTP) can be used to achieve control over the false alarm rate in a landmine detection system. The system is based on an impulse radar to detect objects buried underground. An impulse radar combined with a detector can detect both metallic and non- metallic objects. To be able to distinguish mines from nuisance objects the detector can be followed by a classifier. One drawback with this setting is that it is very difficult to control the false alarm rate of the complete detector/classifier system. The use of MTPs is a systematic way to control the false alarm rate.