Publisher’s Note: This paper, originally published on 24 April 2020, was replaced with a corrected/revised version on 8 June 2020. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
Distributed acoustic sensing (DAS) systems using fiber optic (FO) cables are becoming commonplace in many perimeter security applications. They have the advantage of cost-effectively covering large geographic expanses without temporal or spatial gaps. Recent technology advances in commercial DAS systems have significantly improved system sensitivity and the ability to generate time-harmonic waveforms similar to microphones and geophones. These waveforms can be saved at regular channel intervals along the length of the cable. Relative to microphones and geophones, there are a number of DAS limitations, including generally lower system sensitivity, random signal fading,1 and strong longitudinal radiation pattern effects. The latter limits DAS systems for detecting seismic-acoustic waves propagating perpendicular to the cable. In this paper, we investigate the use of coiled bundles of FO cable. We show that coherently stacking channel waveforms in coils improves signal-to-noise ratio (SNR) by a root mean square (RMS) factor of 6 relative to noise in unstacked channels, and that the coils generate a more omnidirectional radiation pattern relative to straight cable segments. However, this is at the expense of decreased signal power and more complex installation methods. These developments are incremental steps in enhancing our ability to use DAS FO systems for tracking ground and air vehicles. In our test, we used a commercially available FO DAS system configured to monitor ground vehicles and low-flying aircraft. The experiment was conducted in the deep sandy soils of the New Jersey Pine Barrens.2 The two primary DAS components were a laser interrogator unit (IU) and acoustically sensitive FO cable (both manufactured by OptaSense). For redundancy, we used an additional IU from the Naval Research Laboratory. We buried 3,000 m of FO cable at an approximate 30 cm depth, collecting 2,900 channels of strain time series data. We constructed four arrays by coiling cable in discrete bundles. A single bundle could contain 16-24 m of cable wound on a 20 cm diameter jig. Across the entire emplacement, we made roughly 100 such bundles (i.e. 2/3 of the buried cable was wound into coils). As a result, the sensor covered a straight-line distance of approximately 1,000 m. This paper focuses exclusively on the FO array design and installation, as well as the processing methods and benefits of using coiled arrays. Our results indicate that these methods have significant merit for enhancing DAS air and ground vehicle detection and tracking
Acoustic, seismic, radio-frequency, optical, and other types of signals in complex real-world environments are randomized by processes such as multipath reflections from buildings and hills, surface scattering from rough terrain, and volume scattering by turbulence and vegetation. Bayesian classifier methods have the ability to incorporate physically realistic distributions for the random signal variations caused by these processes, and thus enable quantitative assessments of the uncertainty in the target classifications. This paper formulates a Bayesian classifier for problems involving strongly scattered signals with partially correlated features, as would be appropriate for situations involving observations of multiple signal features (e.g., spectral bands) at multiple sensor locations. In this case, the appropriate formulation of the likelihood function is a complex Wishart distribution. We simulate the classifier performance for two- and three-target problems involving multiple spectral signal features, for cases involving moderate and strong correlations between the signal features. The results illustrate the challenges of performing reliable classification based on a small number of samples of a strongly scattered signal, particularly when the target features are similar in strength. When there exist strong correlations between the feature data, full Bayesian classifiers decisively outperform naïve classifiers.
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