Near-Field Acoustic Imaging Using Fiber-Optic Distributed Acoustic Sensing and Beamforming Techniques.

Distributed acoustic sensors (DAS) detect mechanical vibrations along optical fibers with meter-scale spatial resolution, capturing the waves directly reaching the fiber. Deploying dense fiber nets in two or three dimensions for spatially-precise sensing over large regions is often impractical or unfeasible, highlighting the need for a method to monitor the propagating mechanical (acoustic) waves in regions far from the sensing fiber. Here, a beamforming-based method is proposed to image acoustic emissions around an optical fiber, including areas without fiber deployment. By combining DAS measurements from multiple sampled fiber locations (DAS channels) and using a near-field array signal processing approach, the technique can enable the development of an acoustic camera to generate 2D or 3D acoustic maps with meter-scale spatial resolution, covering areas spanning several square kilometers, even outside the optical fiber deployment region. The method includes a dedicated blind selection of high-quality DAS channels to address intrinsic DAS limitations (e.g., uneven longitudinal response and directional axial response) and external factors (e.g., poor fiber-ground coupling and multipath wave propagation). The approach allows the precise localization of vibration sources, and potentially of acoustic reflections, extending DAS capabilities to analyze the propagation of vibration waves in regions distant from the fiber.