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We report on a method to measure the longitudinal position of an acoustic burst, or spark, using an all-fiber Sagnac interferometer (SI). A focused pulsed ∼10 ns laser is used to ionize air creating the spark. This ionization point is situated near a coil of test fiber connected to the SI but acoustically separated from the bulk of the SI. A fast Fourier transform (FFT) performed on the temporal signal of the SI shows a series of s from which the position of the acoustic signal is determined. Over an SI length of ∼35 km, longitudinal positions are determined to within tens of meters, corresponding to a best run percent uncertainty of 0.07%.