Gabriel P. Kniffin

Terahertz material detection from diffuse surface scattering

The potential for terahertz (THz) spectroscopy to detect explosives and other materials of interest is complicated by rough surface scattering. Our previous work has demonstrated that by averaging over diffuse observation angles and surfaces, spectral features could be recovered from laboratory measurements and numerical computer simulations. In addition to averaging, a low-pass cepstrum filter was used to reduce noise due to the random rough surface. This paper expands on these concepts by using the cepstrum of both the random rough surface and the material properties of the target material to choose an optimal cutoff frequency for the filter. The utility of these techniques is evaluated using laboratory measurements and Monte Carlo simulations for many sets of random surface realizations. The Kirchhoff Approximation is used to quickly model diffuse scattering from dielectric materials with gradually undulating rough surfaces when the incident and diffuse scattering angles are near the surface normal. Th...

Model-Based Material Parameter Estimation for Terahertz Reflection Spectroscopy

One important yet commonly overlooked phenomenon in terahertz (THz) reflection spectroscopy is the etalon effect-interference caused by multiple reflections from dielectric layers. Material layers are present in many THz applications from the monitoring of pharmaceutical tablet coating thickness to the detection of drugs, explosives, or other contraband concealed beneath layers of packaging and/or clothing. This paper focuses on the development and implementation of a model-based material parameter estimation technique, primarily for use in reflection mode THz spectroscopy that takes the etalon effect into account. The technique is adapted from techniques developed for transmission spectroscopy of thin samples and includes a priori information; namely, the assumption that a materials complex refractive index behaves consistently with the Lorentz dispersion model. The inclusion of the multiple returns provides additional information about the material and its structure, alleviating the need for short time windows in the fast Fourier transform (FFT) processing, which can limit spectral resolution. In addition, the parametrization of the materials dielectric function offers the potential for material classification based on these estimated dispersion model parameters. The parametric model-based method is validated by comparison with results from a conventional, non-parametric method applied to transmission mode data before being applied to reflection mode data for further validation with transmission mode results. Tests are also conducted to evaluate the parametric techniques robustness against measurement noise and ambiguity in sample thickness.

Measurement and modeling of rough surface effects on terahertz spectroscopy

Recent improvements in sensing technology have driven new research areas within the terahertz (THz) portion of the electromagnetic (EM) spectrum. While there are several promising THz applications, several outstanding technical challenges need to be addressed before robust systems can be deployed. A particularly compelling application is the potential use of THz reflection spectroscopy for stand-off detection of drugs and explosives. A primary challenge for this application is to have sufficient signal-to-noise ratio (SNR) to allow spectroscopic identification of the target material, and surface roughness can have an impact on identification. However, scattering from a rough surface may be observed at all angles, suggesting diffuse returns can be used in robust imaging of non-cooperative targets. Furthermore, the scattering physics can also distort the reflection spectra, complicating classification algorithms. In this work, rough surface scattering effects were first isolated by measuring diffuse scattering for gold-coated sandpaper of varying roughness. Secondly, we measured scattering returns from a rough sample with a spectral signature, namely α-lactose monohydrate mixed with Teflon and pressed with sandpaper to introduce controlled roughness. For both the specular and diffuse reflection measurements, the application of traditional spectroscopy techniques provided the ability to resolve the 0.54 THz absorption peak. These results are compared with results from a smooth surface. Implications of the results on the ability to detect explosives with THz reflection spectroscopy are presented and discussed. In addition, the Small Perturbation Method (SPM) is employed to predict backscatter from lactose with a small amount of roughness.

Performance metrics for depth-based signal separation using deep vertical line arrays

A recent publication by McCargar and Zurk [(2013). J. Acoust. Soc. Am. 133(4), EL320-EL325] introduced a modified Fourier transform-based method for passive source depth estimation using vertical line arrays deployed below the critical depth in the deep ocean. This method utilizes the depth-dependent modulation caused by the interference between the direct and surface-reflected acoustic arrivals, the observation of which is enhanced by propagation through the reliable acoustic path. However, neither the performance of this method nor its limits of applicability have yet been thoroughly investigated. This paper addresses both of these issues; the first by identifying and analyzing the factors that influence the resolution and ambiguity in the transform-based depth estimate; the second by introducing another, much simpler depth estimation method, which is used to determine the target trajectories required for observation of the interference pattern and the array requirements for accurate depth estimation.

Measurement and modeling of terahertz spectral signatures from layered material

Many materials such as drugs and explosives have characteristic spectral signatures in the terahertz (THz) band. These unique signatures hold great promise for potential detection utilizing THz radiation. While such spectral features are most easily observed in transmission,real life imaging systems will need to identify materials of interest from reflection measurements,often in non-ideal geometries. In this work we investigate the interference effects introduced by layered materials,whic h are commonly encountered in realistic sensing geometries. A model for reflection from a layer of material is presented,along with reflection measurements of single layers of sample material. Reflection measurements were made to compare the response of two materials; α-lactose monohydrate which has sharp absorption features,and polyethylene which does not. Finally,the model is inverted numerically to extract material parameters from the measured data as well as simulated reflection responses from the explosive C4.

Terahertz Non-destructive Evaluation of Layered Media with the Maximum Likelihood Estimator

Terahertz (THz) non-destructive evaluation (NDE) has shown great promise for applications in manufacturing, security screening, and medical imaging. However, interpretation of the THz A-scans is complicated by a number of factors. Other researchers have applied signal processing techniques that improve object boundary detection, but still rely on human interpretation without a statistical framework that provides automated detection. This work presents a statistical signal processing algorithm to detect boundaries/defects in THz NDE data based on techniques that have traditionally been used to detect targets in radar/sonar applications. It is demonstrated that this method provides better signal-to-noise ratio (SNR) in the processed waveform along with a mature mathematical framework for analysis of measured data.

Parabolic Equation Methods for Terahertz 3-D Synthetic Aperture Imaging

Terahertz (THz) imaging has shown great potential for nondestructive evaluation of a wide variety of manufactured products composed of nonpolar dielectric materials. Recent work by the authors and others has demonstrated the ability to produce three-dimensional (3-D) THz tomographic images of such objects using a single dataset collected in a 2-D synthetic aperture configuration. However, the irregular surface topographies of many objects of interest introduce refractive effects that distort the tomographic images of the objects interior. This paper presents a novel physics-based approach to correct these refractive effects using parabolic equation methods common to undersea acoustics, seismology, and recently adapted for ground-penetrating radar and biomedical ultrasound. The ability of this method to estimate the thickness of a curved dielectric layer in a specially designed 3-D printed test object is demonstrated using experimental data. The error due to approximations made in the methods derivation is also analyzed and used to select simulation test cases to examine the effect of the approximation error on the accuracy of the resulting thicknesses estimates.

Terahertz scattering from contaminants embedded in textile rope and sling materials

Terahertz Time Domain Imaging (THz-TDI) is a new technology that is rapidly being explored by researchers around the world with particular interest in non-destructive evaluation (NDE) applications for manufacturing, security screening, and medical imaging. This work expands the scope of THz NDE by investigating its potential as a new and innovative method for NDE of nylon ropes and slings, which are used throughout all branches of the military as well as the civilian construction, transportation and shipping industries. Ropes and slings are exposed to harsh environmental conditions as they are dragged through sand and dirt or exposed to seawater, and their strength is compromised when grains of sand or sea salt become deeply embedded between the rope strands and wear away at its nylon threads.

Deep ocean vector sensor array performance metrics

Recent work in passive sonar has drawn interest in the potential for vertical line arrays (VLAs) deployed below the critical depth—the depth inthe deep ocean at which the sound speed below the channel axis reaches the sound speed near the surface. Such arrays can take advantage of propagation via the reliable acoustic path (RAP), which has been shown to improve thesignal-to-noise ratio (SNR) of received signals from sources at or near the surface at moderate ranges. The potential of these deep ocean VLAs hasspawned further interest in the design of vector sensor VLAs that would allow azimuthal rejection and additional array gain over conventional pressure sensor VLAs. This work will present simulation results that explore and quantify the performance of such vector sensor VLAs deployed in thedeep ocean in terms of surface area coverage, detection probability, andoperational lifetime. The potential use of these deep ocean vector sensorarrays to estimate source depth using the depth-harmonic interferencebet...

Terahertz Non-destructive Evaluation of Textile Ropes and Slings

Terahertz (THz) technology has shown great promise for non-destructive evaluation (NDE) applications in manufacturing, security screening, and medical imaging. This work expands the scope of THz NDE, demonstrating its use as a new and innovative method for NDE of nylon ropes and slings. The strength of ropes and slings is compromised when grains of sand or sea salt become deeply embedded between the rope strands and wear away at its nylon threads. Until now, inspections of nylon rope has been limited to subjective observations of the surface because the textile material is opaque to visible light. In this paper, the authors present THz imaging as a new NDE modality with the capability to map the 3D distribution of the sand and salt contaminants located within textile ropes and slings. The first THz tomographic images of salt and sand embedded within nylon webbing are presented and a design concept for a prototype THz rope scanner is introduced and discussed. Military and civilian organizations alike would benefit from this innovative approach, as it will protect personnel and cargo from previously unseen hazards.