This briefing focusses on some of the work forming the background of today's state of the ground probing radar art, and will attempt to draw conclusions about the prospects of this technology. We will cover some of the results achieved to date by many of the laboratories and organizations engaged in this work, including some work that is up to 20 years old, as well as work accomplished in the last few weeks. The spectrum of results is quite varied, ranging from the disappointing to the encouraging. We will attempt to identify some of the factors leading to or limiting success in this area and define some likely prospects for further applications. Finally, we will describe some of the research efforts that will be required to fully evaluate the feasibility of these hoped-for applications.

The Environmental Research Institute of Michigan (ERIM) has developed a unique ground- based, portable, synthetic aperture radar (SAR). This SAR images targets in their natural backgrounds without the expense of an airborne sensor and with higher performance (bandwidth, resolution) than existing airborne systems. A horizontal 36-foot long aluminum truss supports a rail and an antenna cartridge, which is moved along the rail to allow synthetic aperture focusing. The system is fully-polarimetric and has collected data over the frequency band of 400 - 1300 MHz resulting in a nominal resolution of 0.17 m in range and 0.5 m in cross-range. The low frequency range of the system allows for penetration of soil (to shallow depths) as well as foliage and the system has been used to collect images of buried and foliage- obscured targets. The ground imagery collected to date includes steel oil drums buried at depths of up to one-meter. Both the drums as well as the disturbances due to digging the holes are visible in the imagery. Foliage imagery includes portions of a Lear jet under a mature hardwood forest. Due to the low frequency and wide bandwidth of the sensor (400 - 1300 MHz), obscured objects are clearly visible in the SAR imagery. Other responses in the foliage imagery are due to the dihedral-like ground-trunk reflections.

The development status of two Stepped-Frequency Ground Penetration Radars (SFGPR) is presented. The equipment described includes a unit that operates over the 1 to 3 GHz RF band and is intended for shallow buried mine detection, and a unit that operates over the 150 MHz to 1 GHz RF band and intended for buried waste site characterization to depths of 7 meters. The SFGPR uses coherent high-range resolution radar processing methods to develop a radar- surface-position by slant range (depth) display. An extension of coherent synthetic aperture radar (SAR) imaging techniques that can yield 2-D and 3-D subsurface imaging is also presented. Results achieved to date are presented in 2-D for the high-frequency system and 3- D for the low-frequency system.

The United States Department of Energy's Special Technologies Laboratory (STL) staff has been actively involved with ground penetrating radar technology since 1968. STL has developed a stepped FM-CW Ground Penetrating Radar (GPR). The initial application of this unit was the detection and imaging of unexploded ordnance. Since this time the target types have been expanded to subsurface storage containers, pipes and utility cables. Additionally, some field tests have been conducted where nonmetallic targets were of interest. Four case studies on the detection of buried ordnance and hazardous waste storage containers are discussed in this paper. A brief description of the STL GPR unit is presented.

In order to investigate the detection of targets which are hidden by foliage, an experiment was designed which utilized a forest region located near Portage, Maine. The experiment was designed to address four issues. First, the properties of the backscatter or clutter which competes with the desired target were investigated. Second, the foliage induced attenuation that is experienced by the radar energy traversing the foliage were measured. Third, the ability of a synthetic aperture radar system to focus on a target obscured by foliage was investigated. Fourth, target signatures of foliage obscured and unobscured targets were measured. The forest region was investigated using two different airborne synthetic aperture radar (SAR) systems. A UHF wide-band SAR operated by SRI International was used as well as a L-, C-, and X-band SAR installed on a P-3 aircraft operated by the U.S. Navy. The SRI system was used to collect data over 16 square kilometers with repeat passes for verification of system performance. The P-3 system was used to collect over 50 square kilometers of data at three different depression angles with several repeat passes.

This paper investigates the attenuation and phase fluctuations of electromagnetic waves propagating through foliage. These fluctuations are important in determining how well an object obscured by foliage can be imaged with synthetic aperture radar. A theoretical model is developed to calculate the mean attenuation and the amplitude and phase fluctuations. The attenuation of average received field is obtained from the sum of absorption loss and scattering loss. The amplitude fluctuation of electromagnetic wave is calculated from the bistatic scattering coefficients using the radiative transfer theory. The phase fluctuation is obtained from the amplitude fluctuation assuming the phase of the fluctuation field is uniformly distributed from -(pi) to (pi) . The average received power is obtained from the sum of the power of average field and the power of fluctuation field. The attenuation is then obtained by comparing the radiated power from a source under foliage to the received power from a source in free space. Theoretical results are compared with experimental data collected by MIT Lincoln Laboratory during the 1990 Foliage Penetration Experiment. This theoretical model is also used to illustrate the polarization and angular dependencies of attenuation and phase fluctuations.

Radio waves may be used to image the electrical properties of rock for geological exploration and mining. The tomographic reconstruction of an attenuation image from radio wave survey data may be formulated as an ill-posed linear inverse problem for which two types of information are usually available: data collected by the remote sensing process and prior information on the expected form of the reconstruction image. The inversion is ill-posed since the data are incomplete and noisy and an inexact simplified linear model of radio wave propagation through rock is assumed. Such inverse problems have many 'solutions' which fit the observed data and we must choose one reconstruction as an answer. Bayesian image estimation provides a consistent framework for reconstructing an image from noisy and in- complete data. We discuss the choice of statistical distributions to represent our prior knowledge of the image to be estimated as well as the likelihood of this image relative to the measured data. We illustrate the approach by presenting rock attenuation and variance estimate images computed for a radio tomography survey in a base metal mine in Namibia.

Techniques have been developed which have been imaging optically opaque regions using an electromagnetic wave radar in order to estimate the location of the objects in those regions. One important application of these techniques is the detection of buried pipes and cables. In the case of underground radar, its image quality often becomes low because the nature of the soil is not uniform and an electromagnetic wave is attenuated in soil. Hence, the method which improves the quality of the radar images is required. In this paper, we point out that the quality of underground images can be improved significantly by means of the block migration method. In this method LOT (Lapped Orthogonal Transform) was applied. LOT is a new block transform method in which basis functions overlap in adjacent blocks, and it has a fast computation algorithm. In addition to above, we propose a method of estimating dielectric constant in soil using the processed images. The result of applying the block migration method to the underground radar images are presented. It points out the good capability for the image quality improvement and the application of LOT can improve the influence by blocking and the processing time. Also the dielectric constant in each block can be estimated accurately.

Plastic weapons and explosives concealed under clothing present unique challenges to conventional contraband detection technology. Nonmetallic, nonmagnetic objects cannot be detected by conventional systems that use low-frequency magnetic fields. Common techniques that can reveal these types of contraband involve X-rays or other ionizing radiation, which are inappropriate for use on the human body. We have found that millimeter-wavelength radiation is an excellent means for revealing objects hidden beneath clothing. Naturally occurring thermal radiation from the subject can be used in a process called passive imaging. An alternative process called active imaging uses one or more millimeter-wave sources to illuminate the subject. In this paper we describe the basic theory of millimeter-wave imaging. We present single-channel scanned images of both metal and plastic objects concealed beneath ordinary clothing. The clothing appears substantially transparent and the objects are distinguishable. The paper concludes with a discussion of prospective advances that should make multiple-channel real-time imaging systems practical.

We present a genetic algorithm (GA) for solving an ill-posed inverse problem from exploration geophysics, namely the estimation of a distribution of conductivities from a set of electrical current penetration depths. We formulate the inversion as a Bayesian inference problem and use a GA to efficiently sample the posterior parameter distribution. In particular, the conductivity distribution with maximum entropy relative to the observed data is estimated. The method is illustrated on an airborne electromagnetic data set collected over the Karoo, South Africa.

Underground objects are by nature often severely obscured although the general character of the intervening random media may be reasonably understood. The task of detecting these underground objects also implies that their exact location and or orientation is not known. To partially counter these difficulties, one may; however, be given a model of the target of interest, e.g. a particular tank type, a water pipe, etc. To set up a quality framework for solution of the above problem, this paper utilizes the paradigm of Bayesian decision theory that promises minimum error detection given that certain probability density functions can be found. Within this framework, mathematical techniques are shown to handle the uncertainties of target location and orientation, many of the random obscuration problems, and how to make best use of the target model. The approach taken can also be applied to other synergistic cases such as seeing through obscuring vegetation.

Electronic holography for imaging through biological tissue is described. A number of processes are given, including the holographic implementation of the first arriving light method, the broad source method, as well as variations on these methods. The equipment is discussed in some detail, including the camera-computer interactions.

We discuss dual-band infrared (DBIR) capabilities for imaging buried object sites. We identify physical features affecting thermal contrast needed to distinguish buried object sites from undisturbed sites or surface clutter. Apart from atmospheric transmission and system performance, these features include: object size, shape, and burial depth; ambient soil, disturbed soil and object site thermal diffusivity differences; surface temperature, emissivity, plant-cover, slope, albedo and roughness variations; weather conditions and measurement times. We use ground instrumentation to measure the time-varying temperature differences between buried object sites and undisturbed soil sites. We compared near surface soil temperature differences with radiometric infrared (IR) surface temperature differences recorded at 4.7 +/- 0.4 micrometers and at 10.6 +/- 1.0 micrometers . By producing selective DBIR image ratio maps, we distinguish temperature-difference patterns from surface emissivity effects. We discuss temperature differences between buried object sites, filled hole sites (without buried objects), cleared (undisturbed) soil sites, and grass-covered sites (with and without different types of surface clutter). We compared temperature, emissivity-ratio, visible and near-IR reflectance signatures of surface objects, leafy plants and sod. We discuss the physical aspects of environmental, surface and buried target features affecting interpretation of buried targets, surface objects and natural backgrounds.

Given multiple registered images of the earth's surface from dual-band infrared sensors, our system fuses information from the sensors to reduce the effects of clutter and improve the ability to detect buried or surface target sites. The sensor suite currently includes two infrared sensors (5 micron and 10 micron wavelengths) and one ground penetrating radar (GPR) of the wide-band pulsed synthetic aperture type. We use a supervised learning pattern recognition approach to detect metal and plastic land mines buried in soil. The overall process consists of four main parts: preprocessing, feature extraction, feature selection, and classification. We present results of experiments to detect buried land mines from real data, and evaluate the usefulness of fusing feature information from multiple sensor types, including dual-band infrared and ground penetrating radar. The novelty of the work lies mostly in the combination of the algorithms and their application to the very important and currently unsolved operational problem of detecting buried land mines from an airborne standoff platform.

First efforts by the Southern Regional Office of the U.S. Forest Service at locating gravel deposits using thermal imagery began in 1983. Of the available remote sensing methods, only the method using the Thermal Infrared Multi Spectral Scanner (TIMS) has produced a promising correlation with known subsurface gravel deposits. In 1988, The National Aeronautics and Space Administration (NASA) found the results of these efforts of sufficient interest to accept the study as a joint effort with the NASA Space Technology Laboratory at Bay St. Louis, MS under their Earth Observation Commercialization and Applications Program (EOCAP). That program continued through 1992. Over three million cubic meters of gravel bearing deposits were identified with thermal imagery between 1987 and 1991.

We discuss three-dimensional dynamic thermal imaging of structural flaws using dual-band infrared (DBIR) computed tomography. Conventional (single-band) thermal imaging is difficult to interpret. It yields imprecise or qualitative information (e.g., when subsurface flaws produce weak heat flow anomalies masked by surface clutter). We use the DBIR imaging technique to clarify interpretation. We capture the time history of surface temperature difference patterns at the epoxy-glue disbond site of a flash-heated lap joint. This type of flawed structure played a significant role in causing damage to the Aloha Aircraft fuselage on the aged Boeing 737 jetliner. The magnitude of surface-temperature differences versus time for 0.1 mm air layer compared to 0.1 mm glue layer, varies from 0.2 to 1.6 degree(s)C, for simultaneously scanned front and back surfaces. The scans are taken every 42 ms from 0 to 8 s after the heat flash. By ratioing 3 - 5 micrometers and 8 - 12 micrometers DBIR images, we located surface temperature patterns from weak heat flow anomalies at the disbond site and remove the emissivity mask from surface paint of roughness variations. Measurements compare well with calculations based on TOPAX3D, a three-dimensional, finite element computer model. We combine infrared, ultrasound and x-ray imaging methods to study heat transfer, bond quality and material differences associated with the lap joint disbond site.

We are developing a technique which will enable us to obtain high-contrast, high-spatial resolution, three-dimensional images in opaque objects. Our only constraint will be the radiation source and detector(s) located on the same side of the object. Our goal is to obtain images with a spatial resolution of approximately 1 mm at depths of 10 mm and approximately 3 mm at depths of 30 mm in materials of moderate density (brass, steel, etc.). Our technique uses a highly-collimated beam of monochromatic gamma rays and a slit collimated high- resolution, high-efficiency, coaxial germanium spectrometer. If the geometry is well known, the spectrum of Compton scattered radiation can be used to map out the density as a function of depth. By scanning the object in two dimensions, a full three-dimensional image of the electron density can be reconstructed. The resolution is, of course, dependent on the incident beam collimation and the energy resolution of the spectrometer. For our system, we anticipate a resolution of about 1 mm³. The apparatus, reconstruction algorithms, and current data verifying our predictions are presented here. Also included are the details on how our system can be modified to increase the efficiency by over two orders of magnitude.

Members of the Nondestructive Evaluation Section at the Lawrence Livermore National Laboratory (LLNL) are implementing the advanced three-dimensional imaging technique of x- and gamma-ray computed tomography (CAT or CT) for industrial and scientific obscured object evaluation. This technique provides internal and external views of materials, components, and assemblies nonintrusively. Our work includes building of CT scanners as well as data preprocessing, image reconstruction, display and analysis algorithms. These capabilities have been applied to a variety of industrial and scientific NDE applications. We have used CT to study various objects with obscured features at our laboratory ranging in size from 1 mm³ to 1 m³. In these studies, CT has revealed flaws (e.g. cracks, voids, and inclusions), internal and external dimensional information, differences in elemental composition or material density, and other important material characteristics. CT has also been used to localize, identify, and quantify radioisotopes within canisters. As illustrative examples, we describe how CT was instrumental in the analysis of concrete specimens, diesel engine thermocouple plugs, jet engine turbine blades, ballistic target materials, and radioactive waste canisters.

Coded aperture ECT is a non-contact non-destructive imaging which gives three dimensional emitter distribution. We have developed a new inversion algorithm for coded aperture ECT using multiple projections. To improve depth resolution of the three dimensional reconstruction, we use multiple projections obtained while varying the position of an area sensor along the depth direction. An optimum filter based on the three dimensional model is derived. It is shown that the depth resolution when five projections are used is twice as high as that when single projection is used.

Ultrasonic nondestructive evaluation techniques interrogate components with high frequency acoustic energy. A transducer generates the acoustic energy and converts acoustic energy to electrical signals. The acoustic energy is reflected by abrupt changes in modulus and/or density which can be caused by a defect. Thus defects reflect the ultrasonic energy which is converted into electrical signals. Ultrasonic evaluation typically provides a two dimensional image of internal defects. These images are either planar views (C-scans) or cross-sectional views (B-scans). The planar view is generated by raster scanning an ultrasonic transducer over the component and capturing the amplitude of internal reflections. Depth information is generally ignored. Examples of potential ultrasonic imaging applications are: inside liquid filled tanks, inside the human body, and underwater.

Neutron Elastic Scatter (NES) may be used for non-destructively assaying materials for the presence of narcotics, explosives, or other contraband. The technology relies on the high penetrating power of neutrons to reach through varying thickness of shielding materials, and also on the large probabilities for elastic scattering of neutrons. Elastic scattering probabilities are the largest of all neutron induced events, exceeding any single non-elastic process typically by a factor of ten or more. Indeed, usually the elastic scattering probability is larger than the sum of all inelastic processes.

Focusing an acoustic wave on an object of unknown shape through an inhomogeneous medium of any geometrical shape is a challenge in underground detection. Optimal detection and imaging of objects needs the development of such focusing techniques. The use of a time reversal mirror (TRM) represents an original solution to this problem. It realizes in real time a focusing process matched to the object shape, to the geometries of the acoustic interfaces and to the geometries of the mirror. It is a self adaptative technique which compensates for any geometrical distortions of the mirror structure as well as for diffraction and refraction effects through the interfaces. Two real time 64 and 128 channel prototypes have been built in our laboratory and TRM experiments demonstrating the TRM performance through inhomogeneous solid and liquid media are presented. Applications to medical therapy (kidney stone detection and destruction) and to nondestructive testing of metallurgical samples of different geometries are described. Extension of this study to underground detection and imaging will be discussed.

The United States Department of Energy's Special Technologies Laboratory (STL) has been actively involved with ground penetrating radar technology since 1968. A ground penetrating radar will utilize the reflective properties of various dielectric interfaces and nonhomogeneous materials in the soil to obtain target information. The production of total site characterization data will require implementation of advanced imaging techniques, data fusion, highly accurate decision, and delineation of subsurface objects. To meet these new technical requirements for high resolution data, STL is moving forward with advances to GPR technology with development of a stepped FM-CW Ground Penetrating Radar. This unit operates over a frequency range of 196 MHz to 708 MHz and produces phase-coherent data. It has a real-time display of data, saves the data to floppy discs and can also produce hard copies in the field. This system has successfully detected targets ranging from 60 mm projectiles to 500 pound bombs up to 15 feet deep. Additional field deployment of the GPR produced successful results on other metallic and nonmetallic targets. This paper outlines the theory of operation of a Stepped Frequency Modulated, Continuous-Wave (FM-CW) Ground Penetrating Radar, provides a technical description of the unit, data display format and presents some sample data sets.

The ability to detect and image buried objects has gained in popularity over the past decade. The use of new subsurface radar techniques and advanced signal processing has increased the probability of success. Paleontology and life science fields have benefitted from advances in ground penetrating radar technology. The United States Department of Energy's Special Technologies Laboratory (STL) staff has been using and developing ground penetrating radar instrumentation and imaging algorithms since 1968. STL has developed a stepped FM-CW Ground Penetrating Radar (GPR) that operates from 196 MHz to 708 MHz. Included is a brief technical description on this fully self-contained unit. Several sample data sets also are described for familiarization with the unique data format of this GPR. This paper describes how ground penetrating radar can be applied to paleontology and tunnel imaging, its limitations and several case study results.

One can envision many circumstances where radiography could be valuable but is frustrated by the geometry of the object to be radiographed. For example, extremely large objects, the separation of rocket propellants from the skin of solid fuel rocket motor, the structural integrity of an underground tank or hull of a ship, the location of buried objects, inspection of large castings etc. In our laboratory I have been investigating ways to do this type of radiography and as a result have developed a technique which can be used to obtain three dimensional radiographs using Compton scattered radiation from a monochromatic source and a high efficiency, high resolution germanium spectrometer. This paper will give specific details of the reconstruction technique and present the results of numerous numerical simulations and compare these simulations to spectra obtained in our laboratory. In addition I will present the results of calculations made for the development of an alternative single sided radiography technique which will permit inspection of the interior of large objects.

Several research programs aimed at infrared detection of underground and/or obscured objects were done by the Willow Run Laboratories of The University of Michigan in the 1960's. Airborne infrared line scanners were the major tools for field work. Examples discussed include: detection of concealed ice crevasses; delineation of volcanic thermal sub-structures; detection of small, hot fires under heavy canopy; imaging of underground voids leading to sink-holes in Florida; thermal imaging of ocean fronts; and thermal mapping of water-land boundaries beneath meters of snow.

The U.S. Army is interested in demonstrating a capability of detecting and discriminating tactical targets concealed in foliage. To investigate foliage and ground penetration phenomena, a fully polarimetric laboratory Synthetic Aperture Radar system has been built on a rooftop rail. The system uses impulse technology covering a bandwidth of 40 MHz to 1 GHz. The first image from the system showed the -3 db beamwidths to be 5 inches in range and 11 inches in cross-range measured to an 8-ft triangular plate corner reflector. This paper will briefly describe the measurement system and present images made of canonical targets in winter foliage.

For the past three years, the Airborne Environmental Surveys Division of Era Aviation, Inc. has continued the development and application of frequency-modulated, continuous wave radars and processes for detecting and mapping subsurface phenomena. Primary applications have focused on the detection of man-made objects in landfills, hazardous waste sites, and subsurface plumes of refined hydrocarbons. This paper presents a description of the current radar systems and peripherals and associated data processing. Examples of a recent survey are also presented.