Jennifer Holt

Multimode illumination in the terahertz for elimination of target orientation requirements and minimization of coherent effects in active imaging systems

Abstract. It is shown that with appropriate multimode illumination and modulation strategies, it is possible to achieve the high sensitivity of active illumination, with the elimination of the need for “strategic” angular orientation of the target and to do so while minimizing the impact of coherent effects such as speckle. It is also shown that very modest terahertz (THz) power levels correspond to very high brightness temperatures, even when this power is divided among the many modes of large enclosures. We also consider how technical advances in the THz will continue to expand the scenarios of applicability for these approaches.

Elimination of speckle and target orientation requirements in millimeter-wave active imaging by modulated multimode mixing illumination

Active imaging can provide significantly larger signal margins in the millimeter-wave spectral region than passive imaging, especially indoors-an important application for which there is no cold sky illumination. However, coherent effects, such as speckle, negate much of this advantage by destroying image clarity and target recognition. Moreover, active imaging demonstrations often use strategically chosen target orientations to optimally reflect power from the active illuminator back to the imaging receiver. In this paper we will discuss and show experimental results for a new active imaging approach that largely eliminates coherent effects and the need for optimized target orientation. The work described uses a synthesized harmonic multiplier chain to drive a 5 W extended interaction klystron at 218.4 GHz, a mechanical mode mixer to illuminate and modulate many modes, and a heterodyne receiver coupled into a 60 cm scanning mirror. Large signal margins were obtained in this ~50 m range work, showing paths to imaging at ~1 km, imaging with considerably less powerful illuminators, and the use of focal plane arrays.

Electromagnetic Induction and GPR Measurements for Creosote Contaminant Investigation

Multifrequency EM induction and GPR parallel dipole (co-pole) and orthogonal dipole (cross-pole) surveys were conducted to assist in the characterization of a former industrial site prior to it being remediated by the Ohio EPA and the U.S. EPA. The site has been a major concern to both agencies for the past decade due to high concentrations of creosote present in clay-rich surficial soils, resulting from many years of wood treating at the site. Information provided on the approximate extent of contamination at the site and the locations of several contaminant-filled structures determined through the use of quadrature phase EM data and cross-pole GPR data served as the basis for an efficient, comprehensive and cost-effective site remediation plan. Geophysical data interpretations were confirmed through exploratory trenching and soil sampling subsequent to the completion of this study. This study demonstrates the potential for mapping the extent and variation with depth of resistive compounds under circumst...

COMBINING MULTIPLE GEOPHYSICAL DATA SETS INTO A SINGLE 3D IMAGE

Traditional geophysical interpretations of multiple data sets have been carried out by interpreting the data on an individual basis and painstaking comparison of the overlapping registration of anomalies on different types of data. This paper illustrates a visualization approach to combining multiple data sets. The individual data sets can consist of any quantified data on a two dimensional (2D) grid (e.g., EM, gravity, magnetics, digital coded geology, topographic maps, model responses, etc.), or three dimensional time-dependent data (e.g., seismic, or GPR). The approach is based on computer software that automatically registers the coordinates of the data sets to the same base map. The interpreter then assigns a vertical position in a 3D block and a color scale to each individual data set. The resulting 3D block is displayed on the screen for further manipulation of opacity and color scales to provide an optimum image for the interpretation of the fused data sets. The interactive interpretation phase is further enhanced with an ability to generate cross section slices and smaller 3D blocks that highlight individual anomalies. Multiple data sets that are handled in this manner provide the interpreter with the optimum environment for visual comparison and interpretation of diverse and complex data sets. One of the keys to the interpretation of multiple data sets is the ability to manipulate both the color assignments and the opacity so that individual features can easily be seen from one data set to the other. In addition, the digital display must be completely interactive to allow interactive control of the color, the opacity, and the view. A good interactive display also allows the interpreter to easily select and switch between block views or slices, with the capability to view a small portion of the block. In short, the interpreter needs to have full control of the 3D block of data to change any display parameter. The final display is the interpretation.

Range resolved mode mixing in a large volume for the mitigation of speckle and strategic target orientation requirements in active millimeter-wave imaging

In spite of many reports of active millimeter-wave imaging in the literature, speckle and requirements for cooperative target orientation significantly reduce its practical usefulness. Here we report a new technique, range resolved mode mixing (RRMM), which significantly mitigates both of these issues. It also provides a three-dimensional (3D) image. RRMM accomplishes this by combining multimode illumination (which eliminates the requirement for cooperative target orientation) with range resolution (which provides statistical independence of speckle patterns for averaging and the 3D image). The use of a 5W extended interaction klystron amplifier results in large signal margins in the 50 m scale atrium of the Physics Department at Ohio State University. It appears that there are a number of scenarios out to a range of 1 km for which this approach is useful to provide 3D images, with minimal speckle, and no requirement for cooperative target orientation.

Infrared/Terahertz Double Resonance for Chemical Remote Sensing: Signatures and Performance Predictions

Single resonance chemical remote sensing, such as Fourier-transform infrared spectroscopy, has limited recognition specificity because of atmospheric pressure broadening. Active interrogation techniques promise much greater chemical recognition that can overcome the limits imposed by atmospheric pressure broadening. Here we introduce infrared - terahertz (IR/THz) double resonance spectroscopy as an active means of chemical remote sensing that retains recognition specificity through rare, molecule-unique coincidences between IR molecular absorption and a line-tunable CO2 excitation laser. The laser-induced double resonance is observed as a modulated THz spectrum monitored by a THz transceiver. As an example, our analysis indicates that a 1 ppm cloud of CH3F 100 m thick can be detected at distances up to 1 km using this technique.