Arthur D. van Rheenen

Detection and identification of explosives hidden under barrier materials: what are the THz-technology challenges?

We describe experiments where different explosives were hidden under common barrier materials, and THz radiation was used to detect and identify these explosives. Our THz system, a time-domain spectroscopy (TDS) system, is based on a femtosecond laser whose radiation is converted into THz radiation by a low-temperature grown GaAs photoconductive switch. A similar switch detects the reflected signal. The advantage of using a TDS system is that pulses reflected from the barrier and the actual explosive, arrive at different instances at the detector. This simplifies the separation of the barrier signature from the explosive signature, compared to a frequency domain system. However, partial temporal overlap between the two pulses makes it challenging to completely separate the spectral characteristics of the explosive from the characteristics of the barrier. Also, in addition to attenuating the THz-pulses, transmission through barrier materials may add spectral features to the reflected signal, hampering recognition of the explosive. On top of that, the explosive may have a rough surface, which reduces the strength of the reflected signal. In this contribution we shall address these issues and discuss strategies that may be used to face these challenges.

Comparison of terahertz technologies for detection and identification of explosives

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection.

Robust Identification of Concealed Dangerous Substances by Spectral Correlation of Terahertz Transmission Images

Terahertz (THz) images containing spectral information in each pixel are recorded in transmission mode using a fiber-coupled time-domain spectroscopy system. The images are acquired by mounting a sample holder on an x-y stage, which is stepped across the beam in the two transverse directions, while the transmitted THz waveform is captured. The materials under investigation consist of uncovered and hidden samples of an explosive (RDX) and simulants (lactose and tartaric acid). Spectral angle mapping is used to identify the materials in the THz images by comparing the spectrum in each pixel with a library of reference spectra for the different materials. We test the performance of several spectral characteristics derived from the measured transmission spectra. Robustness is studied by investigating the receiver-operating-characteristics (ROCs). The ROCs are used to find which of the spectral characteristics is most robust to different sample preparation conditions, without the need for extensive pre-treatment of the data, such as baseline correction. Simple theoretical considerations are used to support the experimental results.

Observations on military exploitation of explosives detection technologies

Accurate and timely detection of explosives, energetic materials, and their associated compounds would provide valuable information to military commanders in a wide range of military operations: protection of fast moving convoys from mobile or static IED threats; more deliberate countermine and counter-IED operations during route or area clearance; and static roles such as hasty or deliberate checkpoints, critical infrastructure protection and support to public security. The detection of hidden explosive hazards is an extremely challenging problem, as evidenced by the fact that related research has been ongoing in many countries for at least seven decades and no general purpose solution has yet been found. Technologies investigated have spanned all major scientific fields, with emphasis on the physical sciences, life sciences, engineering, robotics, computer technology and mathematics. This paper will present a limited, operationally-focused overview of the current status of detection technologies. Emphasis will be on those technologies that directly detect the explosive hazard, as opposed to those that detect secondary properties of the threat, such as the casing, associated wires or electronics. Technologies that detect explosives include those based on nuclear radiation and terahertz radiation, as well as trace and biological detection techniques. Current research areas of the authors will be used to illustrate the practical applications.

Measurements of IR propagation in the marine boundary layer in warm and humid atmospheric conditions

A multinational field trial (SAPPHIRE) was performed at the Chesapeake Bay, USA, during June 2006 to study infrared ship signature and atmospheric propagation effects close to the sea surface in a warm and humid environment. In this paper infrared camera recordings of both land and ship mounted sources are analyzed. The cameras were positioned about 4 m above mean sea level. Several meteorology stations - mounted on land, on a pier and on a buoy - were used to characterize the propagation environment, while sensor heights were logged continuously. Both sub- and superrefractive conditions were studied. Measurements are compared to results from earlier field trials performed in Norway during typical North-Atlantic atmospheric conditions (cool air with little water content), and differences between medium wave and long wave infrared are emphasized. The ship mounted source - a calibrated blackbody source - was used to study contrast intensity and intensity fluctuations as a function of distance. The distance to the apparent horizon is also determined. In addition, normalized variance of intensity for land based sources has been calculated for a number of cases and these values can easily be converted to refractive index structure constant C2n-values. Measurement results are compared to results from the IR Boundary Layer Effects Model (IRBLEM).

Identification of mixed substances using a random forest regressor to classify THz absorbance spectra

We report on the development and application of a random forest regressor that not only identifies but also estimates the relative concentrations of substances (one explosive and two simulants), both in one-substance and two-substance samples. Performance of the regressor is quantified using Receiver Operating Characteristics and the performance is contrasted with that of a simple Spectral Angle Mapping technique that worked well on single-substance samples [1-3].

Nondestructive testing of graphene/epoxy composites using THz waves

In earlier experiments [1] we found there was significant transmission of THz radiation through carbon-fiber enforced composites, despite that fact that the dc conductivity of the carbon fibers is expected to be good and hence should prevent penetration of electro-magnetic radiation. To study the relationship between absorption of THz radiation and electrical conductivity we performed measurements on samples with different concentrations of graphene in an epoxy matrix. We observed an increased absorption of THz radiation with increased graphene concentration. Our conductivity measurements (simple transverse DC measurements using tin foil as electrodes that cover the two sample surfaces) showed the typical increase of several orders of magnitude with graphene concentration. Although both the conductivity and the THz absorption increase with graphene concentration, there is no direct cause-and-effect relation between the two quantities. Careful analysis shows that even the highest dc conductivity values cannot explain even the lowest observed values for the THz absorption coefficient.

Detection metrics and ship [D]RI

Well-known detection metrics based on Johnson criteria or Target Task Performance (TTP) models were developed for land-based targets [1,2]. In this paper we investigate how (whether) we can apply these metrics to especially recognition and identification of ships at sea. Large sea targets distinguish themselves from land-based targets by their large aspect ratio, when seen broad side, and their relatively large and hot plume. We shall only address the second of these two issues here. First, however, we shall investigate how the simple Johnson approach to recognition and identification stacks up against a TTP approach. The Johnson approach has clear and simple criteria to measure the target task performance. To apply the TTP model N50 (V50) values need to be found through observer trials. We avoid these trials here but estimate the criteria based on a comparison of the models. From analysis of LWIR and MWIR recordings of a multipurpose ship running outbound and inbound tracks, we find little difference between the two metrics. As mentioned, we study the effect of the plume on task performance ranges, by considering two different estimates for the target contrast: the average contrast and the root of the squares of this contrast and the standard deviation of the contrast. We argue that the plume skews the recognition and identification ranges to much too optimistic values when the standard deviation is included. In other words, although the plume helps to detect the target, it does not help the recognition or identification task. It seems a more careful definition of the temperature contrast needs to be applied when these models are used.

MRTD: man versus machine

We present Minimum-Resolvable Temperature Difference (MRTD) curves obtained by letting an ensemble of observers judge how many of the six four-bar patterns they can “see” in a set of images taken with different bar-to-background contrasts. The same images are analyzed using elemental signal analysis algorithms and machine-analysis based MRTD curves are obtained. We show that by adjusting the minimum required signal-to-noise ratio the machine-based MRTDs are very similar to the ones obtained with the help of the human observers.

The FESTER field trial

An overview is given of the First European – South African Transmission ExpeRiment (FESTER), which took place in South Africa, over the False Bay area, centered around Simon’s Town. The experiment lasted from April 2015 through February 2016 and involved continuous observations as well as periodic observations that took place during four Intensive Observation Periods (IOPs) of 2 weeks each, which were spread over the year. The continuous observations aimed at a characterization of the electro-optical propagation environment, and included standard meteorology, aerosol, refraction and turbulence measurements. The periodic observations aimed at assessing the performance of electro-optical sensors in VIS / SWIR / MWIR and LWIR wavebands by following a boat sailing outbound and inbound tracks. In addition, dynamic aspects of electro-optical signatures, i.e., the changes induced by variations in the environment and/or target orientation, were studied. The present paper provides an overview of the trial, and presents a few first results.