|
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
|
