Sensor beampattern and equivalent aperture in a distributed acoustic sensing system

Distributed acoustic sensing (DAS) turns a fiber-optic cable into a long, dense array of sensors for passive acoustic monitoring, which is well-suited for ocean acoustic measurements. DAS measures phase differences of the backscattered light from a laser pulse in an optical fiber to provide localized strain estimates. These strain estimates enable distributed sensing of external acoustic vibrations. The transfer function between the measured optical signal and the external acoustic quantities is factorized into four terms, describing the filtering effect of the acquisition gauge window and spatial averaging window and the acoustic wavenumber and direction of arrival. The resulting beampattern, i.e., the sensitivity as a function of frequency and angular direction, of individual DAS sensors is derived, and it is related to the sensor's equivalent spatial aperture. The shape of the spatial aperture determines the spatial coherence of DAS measurements in a diffuse acoustic wavefield, as demonstrated on publicly available data. The corresponding spatial coherence predicts the statistical characteristics of the speckle pattern in DAS.